What are the main differences between prokaryotic and eukaryotic cells?

S294 Cell biology
Book 1, Chapter 2
Copyright © 2012, Second edition 2014, Third edition 2017 The Open University
Generating Diversity
Contents
Chapter 2 An introduction to cell diversity 2
2.1 Introduction 2
Summary of Section 2.1 4
2.2 How cells are studied: microscopy and cell culture 4
Summary of Section 2.2 17
2.3 Prokaryotic cell diversity 17
Summary of Section 2.3 20
2.4 Eukaryotic cell diversity 20
2.5 Final word 38
2.6 Learning outcomes 39
Chapter 2 An introduction to cell diversity
2.1 Introduction
You have already learnt that all cells are composed of the same kinds of molecular
building blocks and share some common features. Despite these common features,
cellular diversity is enormous, both between different types of organism and within
individual multicellular organisms.
� From your study of Chapter 1, what are some of the common properties of cells?
� Some common properties of cells are that they:
l use the same kinds of carbon-based macromolecules as basic components
(proteins, lipids, carbohydrates and nucleic acids)
l use DNA as their genetic material, which they ‘decode’ to make proteins
l are enclosed by a membrane
l require a constant supply of energy.
You may have included another attribute of living organisms, the ability to grow and
reproduce, but note that within adult multicellular organisms some of the individual
specialised cells have lost their ability to divide; for example, most mature nerve cells
(also known as neurons) are unable to divide.
In Chapter 1, you also learnt something about the main features of ‘typical’ prokaryotic
and eukaryotic cells.
� What are the main differences between prokaryotic and eukaryotic cells?
� Eukaryotic cells have a number of specialised organelles each enclosed by its own
intracellular membrane. In eukaryotic cells the DNA is separated from the cell
cytoplasm because it is enclosed by a nuclear membrane, forming a large organelle
called the nucleus. Prokaryotic cells have no membrane-bound organelles, and their
DNA is not separated from the cell cytoplasm.
� Aside from the nucleus, name two other cell organelles in eukaryotes, and state their
main functions.
� You may have thought of mitochondria and chloroplasts. The mitochondria generate
most of the cell’s supply of ATP (Section 1.2.1). Chloroplasts are the sites where
energy from light is used to convert carbon dioxide into organic compounds in the
cells of photosynthetic eukaryotes (Section 1.2.4).
Eukaryotic cells contain a number of other organelles, which you will learn about in
Chapter 3.
When considering cells, both prokaryotic and eukaryotic, it is important to consider their
environment. Few cells exist as isolated entities; most are part of a cellular community,
which therefore forms an important aspect of a cell’s environment. As described in
Chapter 1, the nature of cell communities varies. Single-celled organisms, for example
bacteria, yeasts and some protists, frequently live in colonies. A widely used definition of a
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colony is that it is a group of individual organisms that are linked together either by living
extensions of their bodies (e.g. cytoplasmic strands) or by non-living material that they
have secreted. This definition is rather imprecise, but implies that although there may be
communication between members of a colony, there is little or no difference between
them; individual cells in a colony are functionally equivalent and could survive and form a
new colony if separated from other colony members.
Some simple organisms are considered to be multicellular; that is, unlike colonies, they
are composed of different types of cells. Examples include animals such as sponges,
which are essentially aggregates of a small number of different types of cells
(Section 1.2.5). In some cases, however, it can be difficult to be certain if an organism
should be classified as colonial or multicellular. You will encounter some more examples
of this distinction later in the chapter.
The different types of cell in more complex multicellular organisms are specialised to
perform particular functions, such as movement, photosynthesis or secretion. Different
molecules, particularly but not exclusively proteins, play an important role in these
specialised functions. In plants, for example, some cells have the molecular apparatus
that allows them to carry out photosynthesis. In animals, muscle cells synthesise specific
proteins that enable them to contract, while non-contractile cells, such as skin cells, do not
synthesise these proteins. The differential expression of proteins is therefore fundamental
to the characteristic properties of specialised cells, as you will discover throughout this
module.
In addition to differences in the biochemical properties of the various cell types in
multicellular organisms, the shapes of different cell types also vary. In humans, for
example, red blood cells are small and disc-shaped, whereas nerve cells (neurons) have
long processes, called axons, some of which extend very long distances, for example
from the spinal cord to the muscles of the toes. The structure or form (i.e. the shape and
appearance) of cells is known as cell morphology, and plays an important role in cell
function, as you will see.
� What is the name of the process by which cells become specialised?
� The process by which cells become specialised is known as differentiation
(Section 1.2.5).
Differentiation is a complex but fascinating process, which continues to be the subject of
intense research. You will learn more about it in Chapter 1 of Book 3.
In the complex multicellular eukaryotes, different cell types tend to be organised into
distinct groups or ‘tissues’ according to their function. The most complex organisms have
evolved highly organised arrangements of different types of cells and tissues into organs
and organ systems that perform specific functions. Examples include the vascular system
(a system of vessels for transporting fluids) of plants, and the digestive system of animals.
Different tissues and organ systems are not described in any detail in this module, which
instead focuses on just a few examples to illustrate the organisation and diversity of cells.
A final point to note in this introductory section is that, apart from dormant cells such as
those found in seeds and spores (agents of dispersal, typically associated with
reproduction), all living cells are continually active. In addition to the obvious examples of
physical activity exhibited by muscle cells and by motile cells such as sperm cells, at the
molecular level all cells are highly dynamic. They continuously take up nutrients from their
environment and use these as a source of energy and raw materials for synthesising new
molecules; they transport molecules to different locations within the cell and eliminate
waste molecules; and if conditions are right, many cells grow and divide. Some can
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change their shape, and all cells respond to changes in the environment and interact in
various ways with other cells, by processes collectively known as ‘cell communication’, or
‘cell signalling’, which you will learn about in Book 2, Chapter 4. All these different
processes involve constant movement of molecules, multiple coordinated biochemical
events and in some cases major structural rearrangements within the cell. As well as
these ‘housekeeping’ processes that take place in all cells, some specialised reactions
occur only in particular
cell types.
In this chapter, some examples of cell diversity are considered. You will see that, despite
having the same molecular ‘building blocks’, cells can be very different indeed in their
structure; and you will learn throughout the module how, as a result of biochemical and
structural specialisations, cells differ in their function. In order to fully appreciate the
details of cell structure and function, it is important to have some basic understanding of
how the properties of cells are studied. The next section will introduce some of the
methods that are used to study cells. You will learn about more of the techniques that are
widely used in cell and molecular biology throughout this module.
Summary of Section 2.1
l Despite their underlying uniformity of molecular and intracellular organisation, cells
are extremely diverse in structure and in function, live in diverse environments and
utilise diverse energy sources.
l Cells form communities. Some unicellular organisms form colonies. In multicellular
organisms, different cells are specialised to perform different functions.
l In complex multicellular organisms, cells of a similar type are often organised into
tissues. In the most complex animals, different cells and tissues are often found
together in an organ or an organ system which is specialised to perform a specific
function(s).
l Except when in a dormant state (e.g. spores), all living cells are dynamic. They
interact with their environment in order to obtain a source of energy and molecular
building blocks, and respond to environmental changes. Individual cells, particularly
in multicellular organisms, receive and respond to ‘signals’ from other cells. Many
cells move within their environment. All these processes require a myriad of
coordinated biochemical events within the cell.
2.2 How cells are studied: microscopy and cell
culture
There is not sufficient space here to describe all the many techniques that are used in the
study of cells, but some key techniques are introduced as appropriate throughout the
module. Here, two techniques are outlined: microscopy and cell culture.
In the context of cell morphology, one technique has been of fundamental importance,
and that is microscopy. Almost all cells are very small, too small to be seen with the naked
eye, so it was only when lenses and microscopes were developed that cells were
discovered, and the study of the cellular organisation of organisms began. The first simple
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microscopes were little more than individual glass lenses; they were rather like very small,
but powerful, magnifying glasses.
The first ‘compound’ microscopes (so called because they contain several lenses) were
made early in the 17th century. Robert Hooke (1635–1703) has been called the greatest
scientist of the 17th century. Alongside significant contributions to a range of science and
technology disciplines, including biology, chemistry, physics, geology, architecture and
astronomy, he devised the compound microscope and illumination system shown in
Figure 2.1a, one of the best such microscopes of the time. With it he observed a diversity
of objects, including insects and sponges, and he recorded them with accurate drawings
and beautifully detailed notes. When Hooke examined thin slices of cork under a
compound microscope in 1655, he noticed small rectangular-shaped structures
(Figure 2.1b). He wrote:
…I could exceedingly plainly perceive it all to be perforated and porous…these
pores, or cells, … were indeed the first microscopical pores I ever saw, and
perhaps, that were ever seen, for I had not met with any Writer or Person, that
had made any mention of them before.
Because they reminded him of monks’ cells, he named these structures ‘cells’. Although
what Hooke was observing were the cell walls in dead cork tissue, he had effectively
discovered plant cells and gained a first understanding of the basic structure of plant
tissue.
(a) (b)
Figure 2.1 (a) Robert Hooke’s light microscope. (b) The cell walls of cork drawn by
Hooke.
2.2.1 Observing small objects
You are already aware of the wide range of sizes among different organisms, from ants to
elephants for example, but it can be difficult to appreciate just how small individual cells
and cell organelles are. A typical prokaryotic cell, for example, is about one micrometre in
diameter and no more than a few micrometres long. If you are not familiar with these very
small units of measurement, you should now work carefully through Box 2.1 before
continuing with the chapter.
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Box 2.1 Units used for measuring the size of cells
Because cells are so small, they need to be measured in much smaller units than those you
may be familiar with. In science, the units used for measurement are known as SI units,
which is an abbreviation for ‘Système Internationale d’Unités’ (International System of
Units). You will be familiar with the basic SI unit for length, the metre (abbreviated to m),
and will know that different prefixes are used to denote multiples of a metre. For example, a
‘kilometre’ (km) is one thousand metres, while a ‘centimetre’ (cm) is one-hundredth of a
metre and a ‘millimetre’ (mm) is one-thousandth of a metre.
� How many millimetres are there in one centimetre?
� A metre is made up of 100 cm, or 1000 mm; so there are 10 mm in 1 cm.
To get down to the scale of cells, a unit is needed that is one-thousandth of a millimetre.
This unit is the micrometre; abbreviated to μm (μ is the Greek letter mu) and sometimes
referred to as a micron.
� How many micrometres (μm) are there in one metre?
� If there are 1000 μm in 1 mm, and 1000 mm in 1 m, there will be
1000 × 1000 = 1 000 000 μm in 1 m. So there are 1 million (or 106) μm in 1 m.
The prefix ‘micro’ strictly speaking means one-millionth, but it is also used more generally,
as in words like microbe, to mean very small.
An even smaller unit, called a nanometre, abbreviated to nm, is more appropriate for
describing the size of subcellular components such as cell organelles, which you will learn
about in Chapter 3. A nanometre is one-thousandth of a micrometre.
In summary,
1 m = 100 cm = 1000 mm = 106 μm = 109 nm
1 cm = 10 mm (1/100 m or 10−2 m)
1 mm = 1000 μm (1/1000 m or 10−3 m)
1 μm = 1000 nm (or 10−6 m)
1 nm = 1/1000 μm (or 10−9 m)
Eukaryotic cells are generally larger than most prokaryotic cells. Animal cells typically
measure about 10–50 μm in diameter while the diameter of a mature plant cell is typically
around 50–100 μm; however, some cells in eukaryotes can be very large indeed. For
example, the giant nerve cells of squids have axons that can be nearly 1 mm in diameter
(i.e. about 1000 times the diameter of a typical bacterium) and are also very long,
extending up to a metre in length. The more typical axons of a large vertebrate are much
thinner, at around 2 μm in diameter, but they may be several metres in length (for
example, those in the legs of large vertebrates, that connect the feet with the spinal cord).
� Taking into account the endosymbiotic theory for the evolution of organelles in
eukaryotic cells (Section 1.2.4), what would you expect the size of a mitochondrion
to be?
� You would expect mitochondria to be similar in size to typical bacteria, i.e. 1 μm
diameter and a few μm in length.
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In animal cells, mitochondria are indeed usually about 1 μm in diameter (although their
length can be much greater than their diameter). You will learn more about the shape and
size of mitochondria in Chapter 3.
A schematic illustration comparing the relative sizes of some organisms, cells, organelles,
molecules and atoms is shown in Figure 2.2.
Figure 2.2 The relative sizes of cells, organelles, molecules and atoms, arranged on a
logarithmic scale. The ranges of structures visible with the light and electron microscopes
(Section 2.2.2 and Chapter 3) are also shown. Globular proteins are compact proteins
with a roughly spherical shape. (Note that the illustrations are not drawn to scale.)
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2.2.2 How a light microscope works and what can be
seen: resolution
There are nowadays many different types of microscope, ranging from the basic to the
very complex. Which structures can be seen using a microscope depends upon its
magnifying power, and this in turn depends upon the lenses and the type of light used. A
simple explanation of how a basic light (or optical) microscope works can be found in
Box 2.2.
A logarithmic scale can be helpful when showing a wide range of values on
the same graph. In Figure 2.2, each unit on the scale is 10 times greater than
the previous unit.
Box 2.2 How a light microscope works
Figure 2.3 shows a very simple conventional light microscope, accompanied by a
schematic diagram illustrating how light passing through the microscope forms an image in
the eye of the observer.
A beam of light from a light source is focused on the specimen by passing it through a
‘condenser’ lens. In ‘standard’ microscopes, the light and condenser are located beneath
the specimen. After passing through the specimen (such light is described as being
transmitted through the specimen), the light then passes through an ‘objective’ lens, which
magnifies the image and passes it to the eyepiece lens (or two lenses, if it is a binocular
microscope), which again magnifies the image and focuses it into the eye (or to a computer
screen or digital camera). Typically each of these three lenses (condenser, objective and
eyepiece) is actually composed of several lenses. Usually the microscope has several
different objective lenses, allowing a choice of different levels of magnification.
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eye
light
source
eyepiece
objective
condenser
stage
specimen
Figure 2.3 A conventional compound light microscope with a diagram illustrating how
the microscope focuses light on a specimen and transmits it through the objective and
eyepiece lenses to the observer.
Although high magnification can be achieved using a compound light microscope, what
can actually be seen also depends on another factor, called resolution. The resolution
(sometimes known as ‘resolving power’) of a microscope is the smallest distance by
which two objects are separated and can still be seen as being separate (i.e. the two
objects can be resolved; they do not appear as a single object). Visible light is part of the
spectrum of electromagnetic radiation, which includes radio, microwave, infrared, visible
light, ultraviolet, X-rays and gamma rays. Like all electromagnetic radiation, light behaves
as a series of waves, and the distance between each wave, the wavelength, is constant
and determines its properties. The wavelength of visible light (between about 400 and
750 nm) determines the maximum resolution of a light microscope; that is, it is not
possible to see detail that is much smaller than the wavelength of the light. The best
resolution possible for a standard microscope using visible light is about 200 nm (0.2 μm).
So, even if very high magnification lenses are used, very small structures, including many
organelles, cannot be distinguished (or resolved) from surrounding structures using a light
microscope.
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� Could two closely adjacent but not overlapping small spherical structures that are (a)
3 μm and (b) 0.05 μm apart be distinguished using a light microscope?
� The structures that are (a) 3 μm (i.e. 3000 nm) apart could be distinguished
(resolved), but structures that are (b) 0.05 μm apart (i.e. 50 nm) could not be
resolved.
The so-called ultrastructure of cells, which includes the fine structure and details of
intracellular structures, including organelles, must be studied using electron microscopes,
which instead of light, use a beam of electrons which has a much shorter wavelength than
visible light. You will learn about electron microscopy, and about organelles, in Chapter 3.
2.2.3 Light microscopy of cells and tissues
Small organisms, such as bacteria and many protists, and also individual eukaryotic cells
such as blood cells or cultured cells, can be viewed under a light microscope simply by
placing them between two glass surfaces. Typically a microscope slide and thin glass
‘coverslip’ are used for this.
For larger organisms, or for parts of organisms such as the stem of a plant, or a sample of
a tissue or organ taken for research or diagnostics (known as a biopsy), the process is not
so straightforward.
� Can you think of a problem in studying cells within a tissue such as the stem of a
plant, or the muscle of a vertebrate?
� The tissue may be very thick, so it may be difficult for light to pass through it.
The study of the organisation of complex plant and animal tissues by microscopic analysis
of tissue sections is known as histology (from the Greek word histos, meaning ‘tissue’ or
‘web’). In order to allow the cells in thick tissue samples to be studied, the samples are cut
into very thin slices, known as sections. For light microscopy, tissue sections are typically
between about 5 and 50 μm thick. This tissue sectioning is carried out using special
equipment, usually after the tissue sample has either been frozen (when it is sectioned
using a cryostat), or after it has been embedded in a supporting material such as wax
(when it is sectioned using a microtome).
A further complication in the study of animal tissue samples is that they are very easily
damaged, and they cannot be stored for long before they decompose. So, to preserve
their structural integrity, they are usually immediately preserved (or ‘fixed’) in chemical
fixatives when they are removed from the animal. Alternatively, for some studies, pieces
of tissues may be preserved by rapid freezing, for example in liquid nitrogen.
Most animal tissues are translucent when cut into thin sections, so early microscopists
found it difficult to discern structural detail. During the 19th century, the use of chemicals
to fix and stain samples was developed and stains were identified that bound to particular
cellular components or to particular types of cell. The use of chemical stains to study
tissues is known as histochemistry, and is described in Box 2.3.
Box 2.3 Histochemistry: the use of chemical stains to identify
cells and some cell components
Two examples of histochemical stains that are used to identify cells and their components
are outlined here. The first is the Gram stain, which is routinely used as one of the
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procedures for identifying different types
of bacteria.
The Gram stain is named after its inventor, the Danish physician Christian Gram. It was
devised in 1884 as a method for detecting bacteria in animal tissues. The staining
procedure starts with a heat-fixed smear of bacteria on a glass microscope slide. The
smear is first stained with a dye called crystal violet and a mordant such as a dilute solution
of iodine. (The mordant traps the dye inside cells by forming large complexes.) This
procedure stains all the bacteria a deep purple. The smear is then treated with an organic
solvent such as acetone or alcohol, which dissolves away the purple stain. However, some
bacteria, referred to as ‘Gram-positive’, resist decolourisation and remain purple. This
difference in response to the Gram stain arises from the structure of the outer layers of the
bacteria, which you met in Section 1.2.3 and will learn more about in Chapter 3. The smear
is then ‘counterstained’ with a red dye such as safranin. The bacteria that were purple (the
Gram-positives) remain purple because the red dye does not show up, while the
decolourised bacteria, the Gram-negatives, take up the safranin and appear red when
viewed under a microscope. A typical Gram-stained slide of a mixed bacterial population is
shown in Figure 2.4.
10 μm
Figure 2.4 The Gram stain, which allows visualisation of bacterial cells, is used to
distinguish between different groups of bacteria. The image shows mixed Grampositive
(purplish) and Gram-negative (pinkish) bacteria.
The second example of histochemical staining uses a combination of chemicals to visualise
subcellular compartments, namely the nuclei and cytoplasm of eukaryotic cells. One of the
chemicals is haematoxylin, which binds to negatively-charged molecules such as those
with many phosphate groups, and so stains nucleic acids and is used to visualise nuclei
(Figure 2.5); while the chemical eosin binds to positively-charged molecules, including
many cytosolic proteins, and so is used to stain the cytosol.
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100 μm
nucle i
cilia
epithelia l
cells
Figure 2.5 Image showing part of a section of guineapig trachea (the airway that
connects the mouth with the lungs). The section has been stained with haematoxylin
and eosin. The nuclei of the cells are stained dark purple, the cytoplasm of different
cells is stained different shades of pink/purple, depending on their contents. The
epithelial cells that line the trachea are clearly visible, as are cilia, which are present on
the surface of many of these cells and which move, assisting the removal of unwanted
material from the airways to the mouth.
These and many other stains have been used with great effect for many years to study the
organisation of tissues, both in specimens from healthy individuals and in samples from
diseased tissues.
Much valuable information about cells and tissues has been obtained by histological
techniques such as those outlined above, but during the past 40 years or so, more
sophisticated microscopes and more specific labelling techniques have been developed
and are now widely used to identify particular molecules (often but not always proteins)
within cells. Particularly useful in this respect are the large Y-shaped proteins called
antibodies which are produced by the immune system of vertebrate animals in response
to invasion by foreign material, e.g. infection by bacteria or viruses. You will learn some
more about the immune response in Book 3, Chapter 2.
An antibody recognises and binds specifically to one particular molecule (or part of a
molecule). This specificity makes antibodies very useful tools and they are used
extensively in the techniques of immunohistochemistry and immunocytochemistry
(‘immuno’ comes from the term immune response). Immunohistochemistry is the
localisation of specific molecules within tissue sections, whereas immunocytochemistry
is the labelling of cell preparations, such as a cell culture or cell suspension (the cyto
denoting ‘cell’). The two techniques are also sometimes known as ‘immunolabelling’,
summarised in Box 2.4.
Box 2.4 Immunolabelling: using antibodies to identify molecules
in cells and tissues
Antibodies have the property of recognising and binding in a highly specific manner to a
particular target molecule, termed an antigen. So, when an antibody is applied to a fixed
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tissue section or cell sample, it only binds to the cells that contain that particular antigen. It
is then necessary to detect where the antibody has bound, which usually involves adding
another, ‘secondary’ antibody that recognises the first antibody (the ‘primary’ antibody)
bound to the antigen (Figure 2.6a). If the secondary antibody has been ‘labelled’ with a
chemical such as a fluorophore, which emits fluorescent light of a particular colour when
illuminated with light at specific wavelengths, the localisation of the bound primary
antibodies can be viewed using a specialised fluorescence microscope. Alternatively,
antibodies can be labelled with enzymes that, with the use of an appropriate substrate,
produce a coloured reaction product at the site of antibody binding, which can therefore be
seen using conventional light microscopy.
This ‘indirect’ immunolabelling approach is very convenient because researchers can
perform double (Figure 2.6b), and even triple, labelling to simultaneously detect multiple
molecules in the same sample using two (or three) different primary antibodies followed by
appropriate secondary antibodies – labelled with either fluorophores that emit fluorescent
light of different wavelengths (Figure 2.7a), or different enzyme–substrate combinations
that give different-coloured reaction products (Figure 2.7b). By choosing appropriate
antibodies, it is possible to, for example, distinguish between different cell types in a tissue
on the basis of the particular proteins that the cells express.
labe lled
‘s e conda ry’
antibody
mole cule s
two s e conda ry
antibody
mole cule s labe lled
with diffe rent
fluorophore s
two diffe rent
unlabe lled prima ry
antibody
mole cule s
labe lled ‘s e conda ry’
antibody mole cule s
bind to prima ry
antibody mole cule
labe lled s e conda ry
antibody mole cule s
bind to spe cific
prima ry antibody
mole cule s
unlabe lled
‘prima ry’
antibody
mole cule
bound to antigen
(a )
(b)
antigen A antigen B
Figure 2.6 (a) Indirect immunolabelling: unlabelled ‘primary’antibody molecules bind
to the antigen. Labelled ‘secondary’ antibodies recognise and bind to species-specific
sequences on the primary antibody. (b) Double immunolabelling: two different antigens
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can be localised in the same specimen using two primary antibodies raised in different
species. Two different species-specific secondary antibodies are then applied, each
coupled to a different fluorescent or coloured marker.
In addition to microscopes that are used to view tissue sections and cultured cells growing
as a single layer, more specialised microscopes called confocal microscopes are also
widely used in research laboratories. Confocal microscopes (Figure 2.7c) allow images of
fluorescent labelling to be captured at several different levels within a sample, and so allow
very detailed analysis and even three-dimensional reconstruction by computer of labelled
cells and thin tissues. With the continued development of such technology, microscopy
remains one of the most versatile and widely used techniques in cell biology.
(a)
(c)
(b)
20 μm 50 μm
Figure 2.7 (a) Double immunolabelling of cultured cells (a keratinocyte cell line).The
cells have been exposed to two primary antibodies, followed by appropriate secondary
antibodies labelled with different fluorophores. One primary antibody binds to one type
of cytoskeletal protein (keratin, red) and the second primary antibody binds to one type
of cell junction protein (desmoplakin, green). Other cytoskeletal and cell junction
proteins are present, but not labelled, because the primary antibodies are highly
specific. (b) Double immunolabelling of hormone-producing cells in the rat pancreas.
The figure shows immunolabelling using two antibodies: one that recognises insulin,
the other that recognises another pancreatic hormone, glucagon. The binding of the
two antibodies is visualised indirectly by the subsequent application of two different
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secondary antibodies that have been chemically coupled to enzymes that produce
different-coloured reaction products; cells that contain insulin are stained blue, while
those that contain glucagon are stained brown. (c) An example of a confocal
microscope, being used to analyse cultures of nervous system cells. Supporting cells
(green) and neurons (red) can be seen.
2.2.4 Cell culture: the study of intact and living cells
Finally in this section, the advantages of using culture techniques in cell biology research
are considered, focusing on the analysis of cultured cells by microscopy.
If tissues or cells are removed from an organism and provided with a suitable liquid growth
‘medium’ containing all the nutrients that they normally require for their metabolism,
suitable conditions of temperature and pH, and (in some cases) an appropriate surface, or
substrate, on which, or in which, to grow, then most cells remain alive for some time, and
in many cases, grow and divide. Cells from many species have been grown successfully
in culture.
Single-celled organisms such as bacteria (and yeasts) can be grown in suspension in a
liquid medium or on the surface of nutrient agar plates. Agar, a gelatinous substance, is
first made up as a liquid to which appropriate nutrients are added. The agar is then
allowed to solidify in Petri dishes (sometimes referred to as ‘plates’), allowing samples of
bacterial suspension to be spread on the surface. Spreading bacterial samples onto agar
containing different nutrients and seeing if they grow to form colonies (Figure 1.11a) is a
simple and convenient way to characterise the type of bacteria in an unknown sample.
In contrast to single-celled organisms and other cell types (e.g. blood cells) that typically
exist as independent cells in a liquid environment, the cells that form part of an animal
tissue usually require a solid support, or substrate, to adhere to. The first cell culture
experiments utilised glass dishes to grow small pieces of tissue (hence the term ‘in vitro’,
from the Latin, meaning ‘in glass’ – in contrast to ‘in vivo’, meaning ‘in the living’). The
success of the technique was found to depend on the size of the tissue pieces; because,
in the case of isolated animal tissue samples, the blood supply is necessarily lost, so
gases can only pass by diffusion from the growth medium into the cells of the isolated
tissue. Cells in the centre of larger chunks of tissue are thus vulnerable to lack of oxygen
and accumulation of carbon dioxide. Since these early experiments, however, methods
have been refined, and enzymes and gentle mechanical agitation are frequently used to
carefully break down the extracellular molecules that hold the cells of a tissue together, so
that individual cells can be separated and obtained as a suspension. The cells are then
‘plated’ onto appropriately treated glass coverslips or specially-made plastic dishes. As
soon as the cells are plated, they are provided with a synthetic liquid culture medium
containing all the necessary nutrients, and placed in an incubator that has the appropriate
temperature and gaseous conditions for the cells.
The animal cells grown in such cultures usually flatten and form a single layer and so are
clearly visible under the light microscope, allowing living cells to be examined. However,
since animal cells are translucent, special optics that allow some cell components to be
visualised are needed in order to see the living cells clearly, as shown in Figure 2.8. One
such method is phase contrast microscopy, which uses the difference in the way light
passes through different parts of the specimen to increase the contrast of the image,
allowing some cell components, such as the cell membrane and nucleus, to be seen.
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(a) (b)
50 μm 50 μm
Figure 2.8 Living cells in culture viewed by (a) standard light microscopy and (b) phase
contrast microscopy. In (a) almost no detail of cell structure is visible, whereas in (b) the
cell membranes and nuclei are clearly visible, and some detail of the cytoplasm can be
seen. The box indicates the same cell viewed by the two types of microscopy.
For the study of complex tissues, three-dimensional culture ‘models’ are increasingly
used to more closely mimic the tissue of origin. Often such cultures are prepared using
supportive gels made from proteins such as collagen, or synthetic materials. Cell
suspensions or mixtures of cells are introduced into the gel before it sets, so their growth
can be studied in a three-dimensional environment.
The ability to grow certain cell types in cell culture offers many advantages for cell
biologists. One advantage is the ability to readily assess the direct effects of exogenous
agents added to the culture. The use of cultured animal cells avoids the complexity of
whole animal studies, in which it is frequently difficult to discriminate the direct effects of
an agent on a particular type of cell or tissue from secondary effects arising from an action
of the agent on a different part of the animal. Although the effects seen in culture may be
different from those seen in vivo (i.e. in the living organism), cell culture offers an
extremely useful initial procedure with which to screen the possible harmful effects of new
drugs and other chemicals; so a second advantage is to reduce the numbers of animals
used for drug testing. A third advantage is that the external environment of the cultured
cells can be manipulated very precisely: for example, by the addition of specific agents,
such as signalling molecules, to the culture medium. This approach has proved invaluable
to biologists who are interested in understanding cellular processes such as cell
movement (Book 2, Chapter 5), and the factors that control cell proliferation and longevity
(Book 3, Chapter 1), to name but a few. A further advantage is that the cultured cells can
also be manipulated experimentally, to alter their expression of particular genes
(Chapter 6 in this book), allowing the roles played by specific gene products to be studied.
Most primary cells (cells derived from a fresh tissue sample) are only able to divide a
certain number of times in culture, so it can be difficult to accumulate sufficient numbers to
work with. For studies where large numbers of cells are required, a more convenient
source are cell lines, consisting of homogeneous populations of cells typically derived
from tumours. Such cell lines are ‘immortal’; unlike primary cells, they have the ability to
continue dividing indefinitely (Book 3, Chapter 1) and can be propagated as required from
frozen stocks, without the need to obtain fresh tissue samples. Such cells do not behave
exactly like ‘normal’ cells, but allow much useful work to be done. Some primary cell
cultures can be ‘immortalised’ in vitro by various means such as the introduction of a
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particular type of modified virus. This procedure allows a theoretically unlimited source of
a particular cell type that is more like its normal counterpart in an animal than a tumour cell
line.
Finally, living cultured cells can be viewed under the microscope using video or time-lapse
microscopy, so their movements and interactions can be examined. For example,
microscopy of intact cells has revealed that mitochondria are more complex than
previously realised, undergoing both fusion and fission, and moving around within cells.
Summary of Section 2.2
l Much of the current understanding of the chemical nature and organisation of cells
and how they function has come from microscopy and cell culture techniques.
l Light microscopy provides valuable information about the organisation of tissues, but
is limited by its low resolving power.
l Coloured cell stains are used to better visualise cell structure by light microscopy, in
a technique known as histochemistry.
l Labelled antibodies are used in immunohistochemistry and immunocytochemistry to
localise specific molecules within cells and to distinguish between cell types.
l Cell culture allows living cells to be studied. Individual cells can be observed and the
effects of specific molecules on cells and cell processes, such as cell division, can be
analysed.
2.3 Prokaryotic cell diversity
Prokaryotes usually have a relatively simple structure, as outlined in Chapter 1. Most
Archaea and many Bacteria are round (cocci, Figure 2.9a), rod-shaped (Figure 2.9b) or
thread-like (filamentous, Figure 2.9c), but some Bacteria have more complex specialisations
(Figure 2.9d). Several bacterial phyla include species that are multicellular. For
example, some cyanobacteria form chains of cells including specialised cells that ‘fix’
nitrogen, while the actinobacteria commonly form branched, multicellular filaments that
spread over a substrate and then send up aerial branches which produce and release
single-celled spores.
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(a)
1 μm
1 μm 1 μm
1 μm
(b)
(c) (d)
Figure 2.9 Coloured scanning electron micrographs (SEMs) illustrating common shapes
of different bacterial species. (a) The coccus Streptococcus pneumoniae occurs in the
respiratory tract of healthy individuals but can become pathogenic, causing conditions
including pneumonia, pleurisy, peritonitis and meningitis. (b) The rod-shaped E. coli
bacterium. Escherichia coli is found in the lumen (central space) of the intestines of
healthy individuals. There are different strains of E. coli, some of which produce toxins,
which can cause severe diarrhoea. (c) Spirillum volutans, which has a corkscrew or spiral
shape. These bacteria typically live in water containing organic material, such as stagnant
ponds, and require low concentrations of oxygen. Most species of Spirillum are not
pathogenic, but some are, for example Spirillum minus causes the so-called rat-bite fever.
(d) The bacterium Caulobacter crescentus (so-called because of its crescent shape) is
found in freshwater, soil and seawater. Caulobacter has two very different forms: a mobile
(‘swarmer’) cell with a hair-like flagellum for swimming, and an immobile reproductive form
with an adhesive stalk that enables it to attach to surfaces (seen here).
Some bacterial species have structures extending from the cell membrane; these are of
two types, flagella (singular, flagellum, Figure 2.10a) and pili (singular, pilus,
Figure 2.10b). Bacterial flagella are relatively long, distinctive structures that are capable
of movement and are the means by which some bacteria can ‘swim’ through their
aqueous environment. Some types of bacteria have a single flagellum, some have several
and others none at all. You will learn how flagella cause movement in Book 2, Chapter 5.
Pili are much shorter and thinner structures than flagella and are involved in adhesion of
bacteria to various substrates, for example to eukaryotic cells during infection. Special
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long ‘sex’ pili are also involved in the transmission of genetic material between different
bacterial individuals during mating, a process known as conjugation, which you will learn
more about in Chapter 5 of this book.
(a) (b)
1 μm 1 μm
Figure 2.10 (a) The predatory bacterium Bdellovibrio, which lives on other Gramnegative
bacteria, showing the flagellum. (b) Higher magnification image of Escherichia
coli, an inhabitant of the human intestine. The short hair-like appendages around the
bacterium are a type of pili known as fimbriae, structures associated with bacterial
adhesion to surfaces. This specimen is in the early stages of cell division.
Finally, it is interesting to note that although most single-celled prokaryotes are small,
there are exceptions. For example, among the spirochaetes, which have a characteristic
shape rather like a coiled spring (Figure 2.9c), there are some that are up to 3 μm wide
and over 100 μm long. These giant spirochaetes all live symbiotically (symbiosis means
‘living together’) within invertebrate animals, notably in the guts of wood-eating termites.
Even these monster microbes are dwarfed by the Gram-negative coccoid bacterium
Thiomargarita namibiensis (sulfur pearl of Namibia), the largest bacterium known at the
time of writing (Figure 2.11). Members of this species were found off the coast of Namibia,
Africa in sediments on the sea floor. This environment is very low in oxygen and these
bacteria use nitrate instead of oxygen during ATP synthesis (Book 2, Chapter 3).
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0.2 mm
Figure 2.11 Thiomargarita namibiensis (sulfur pearl of Namibia). These bacteria are
sometimes known as ‘sulfur pearls’ because they digest sulfur-containing compounds,
producing sulfur, which is what gives them a ‘pearl-like’ appearance. They often occur in
chains, as shown here, and although most are between
0.1 and 0.3 mm across, they can be up to 0.75 mm.
� How, using a microscope, could you determine that these organisms were
prokaryotes and not eukaryotes?
� Using a light microscope, the absence of a nucleus, which is characteristic of
prokaryotes, would be evident.
You will learn more about the internal or subcellular organisation of prokaryotes in
Chapter 3 of this book.
Summary of Section 2.3
l Prokaryotes, particularly Archaea, do not exhibit great structural diversity, although
they do have a range of sizes, shapes and some structural specialisations.
l Two structural specialisations associated with the cell membrane of some bacteria
are flagella, which are involved in movement, and pili, which are involved in adhesion
and transfer of genetic material.
2.4 Eukaryotic cell diversity
The evolution of diverse cell types has enabled eukaryotic organisms to survive and
reproduce in a range of environments. Selective pressures, including predation and
competition for nutrients, would have led to the evolution of diverse structural
specialisations that facilitate the acquisition of food and water and the ability to move
(e.g. towards nutrient-rich areas and away from predators). In large multicellular
organisms, the evolution of cellular specialisations has allowed, for example, transport of
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gases and nutrients throughout the organism, or enabled signalling, for example of
environmental changes, from one part of the organism to another.
2.4.1 Protists
The majority of protists are unicellular, although a small number are multicellular. There
are some 30 phyla that are regarded as protists, including some that you may be aware
of, such as algae (of which there are a number of different phyla), amoebae, slime moulds
and diatoms (Figure 1.8). Examples that may surprise you are the marine algae, more
commonly known as seaweeds, including some species that are able to reach very large
sizes. For example, the giant kelps off northwest America can reach 50 m in length. Most
protists live in (and during evolution diversified in) aqueous environments, and some are
parasites, such as Giardia lamblia, which is a pathogen of the human gut. Because of
their diversity, there is no ‘typical’ protist and no characteristic cell features that can be
considered to be representative of the protists.
Among the unicellular protists, selective pressures have led to the evolution of very
diverse cell shapes. As well as very simple single-celled species, such as Amoeba
proteus (Figure 2.12), there are very complex single-celled protists. This diversity is the
result of the evolution of different mechanisms for movement, feeding, protection and
support. In many of these organisms, different parts of an individual cell are specialised to
perform specific functions.
In aqueous environments, one result of selective pressures is the evolution of large size in
single-celled organisms, which helps them to resist predation by animals that filter feed
(strain small food particles from water) or engulf their prey (see below). An increase in
size, however, poses some problems.
� Suggest a problem for single-celled organisms that are very large.
� One problem is the absorption of sufficient nutrients; another is excretion of wastes.
For example, a large spherical cell has a small surface area to volume ratio (that is, a
small surface area compared with its volume), so absorption of nutrients from the
surrounding environment is limited and diffusion of the absorbed nutrients to the centre of
the cell is slow. Similarly, disposal of wastes (excretion) from the centre of a large cell
poses a problem. These selection pressures have resulted in many protists that have
evolved flattened, lobed shapes, which increase their surface area to volume ratio
(Figure 2.12a).
Many protists also have the ability to engulf food particles by a process known as
endocytosis, made possible by the possession of a flexible cell surface and the ability to
extend lobes of cytoplasm known as pseudopodia (Figure 2.12a and b). The flow of
cytoplasm into the pseudopodia is thought to involve particular proteins that are part of the
cell ‘skeleton’ or cytoskeleton; you will learn more about this important group of proteins
in Chapter 3. The ability to extend pseudopodia in this way also allows a form of
movement known as amoeboid movement, often described as ‘crawling’ or ‘creeping’, by
amoebae (and some animal cells) when on a solid surface. Pseudopodia are extended
and the remainder of the cell follows behind.
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nucleus
(a) (b)
pseudopodia
food particle
(in the process of
being engulfed)
100 μm
100 μm
Figure 2.12 (a) An example of a structural specialisation that confers functional
advantages. Amoeba proteus has amoeboid movement, enabling the organism to move
and engulf food (see text above figure). (b) Light micrograph of Amoeba proteus. Several
pseudopodia can be seen. The pseudopodia are seen extending to move. The main part
of the cell is coloured red and pink in this image.
Another problem of increased size is transport and communication within the cell, from
one area to another, particularly from the nucleus to more distant parts of the cell. Large
unicellular protists have therefore evolved strategies to overcome this problem; some
species have very large nuclei, others have more than one nucleus. These specialisations
reduce the distance between the nucleus and other parts of the cell.
Another advantageous strategy for single-celled organisms, which has already been
mentioned, is the grouping of cells together into colonies (Figure 2.13).
� Can you think of an advantage of colony formation?
� Colonies are larger in size than individual organisms, which may reduce predation by
filter feeders.
There are examples of colony-forming protists among the Chlorophyta (a division of the
eukaryotic green algae, not to be confused with the prokaryotic blue–green algae, i.e. the
cyanobacteria, mentioned in Section 2.3). Chlorophytes are photosynthetic eukaryotes,
some of which are unicellular, some colony-forming and others multicellular
(Figure 2.13d).
Chlamydomonas (Figure 2.13d (i)) is a unicellular genus of the Chlorophyta; its members
have a flagellum, while members of the closely related Gonium genus (Figure 2.13d (ii))
form a small, disc-shaped colony of several cells, which is able to move through water
because individual cells possess multiple flagella that all point in the same direction. All
the cells in a Gonium colony are the same, and can individually give rise to a new colony.
The size of Gonium colonies helps them to escape predation by filter-feeding organisms.
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(a)
(d)
(b) (c)
(i) (ii) (iii)
(iv) (v) (vi)
10 μm
10 μm 10 μm 10 μm
10 μm 10 μm
10 μm 10 μm 20 μm
Figure 2.13 Colonial protists. (a) A star-shaped colony of the diatom Asterionella
formosa. Cells grouped this way sink more slowly than do single cells. (b) A green alga
with four-celled colonies, Scenedesmus sp. (c) A colony of the green alga Pediastrum.
The colony has a flat circular shape. (d) Examples of volvocine chlorophyte species
varying in cell number, colony volume and degree of specialisation: (i) Chlamydomonas
reinhardtii, a unicellular species; (ii) Gonium pectorale, a flat or curved sheet of 8–32
undifferentiated cells; (iii) Eudorina elegans, a spherical colony of 16–64 undifferentiated
cells; (iv) Pleodorina californica, a spherical colony with somatic cells and reproductive
cells; (v) Volvox carteri; and (vi) Volvox aureus, which both contain a mix of reproductive
and somatic cells. Where two cell types are present (iv–vi), the smaller cells are somatic
cells and the larger cells are reproductive cells.
Members of the chlorophyte genus known as Volvox (Figure 2.13d (v) and (vi)) form large
groups of several thousand cells. Volvox cells are embedded in a gelatinous matrix that
forms a hollow sphere in which individual cells are connected by their cytoplasm. Beating
of individual cell flagella is coordinated, allowing the group to move. The cells are
dependent on each other; if the group is disrupted, individual cells cannot divide
independently and eventually die. However, such groups of Volvox cells also include
several specialised reproductive cells which are able to form complete new mini-colonies
inside the sphere or the ‘parent’ colony.
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� Based on the definitions in Section 2.1, are Volvox colonial or multicellular
organisms?
� They are multicellular, because they contain specialised reproductive cells which can
form new colonies.
The Chlorophyta are very interesting organisms, not least because they have been used
to study the genetic changes that have occurred during the evolution of multicellularity
(Figure 2.13a). A selective advantage for the multicellular Volvox is that the ‘daughter’
cells are protected inside the ‘parent’ group of cells. Another important advantage for
larger Volvox is that they can store essential nutrients and minerals such as phosphate
that they have absorbed into the matrix between their cells.
The problems associated with increasing cell size, described above, probably resulted in
the evolution of true multicellularity, in which individual organisms consist of a number of
different cell types, specialised to perform different functions.
Summary of Section 2.4.1
l Many protists are unicellular, and some have evolved large size, diverse structures
and specialisations that enable them to feed and move effectively, and avoid
predation.
l Other protists have evolved a colonial mode of life, which has also allowed them to
feed effectively and avoid predation.
2.4.2 Fungi
Fungi are probably less familiar to you than plants and animals, so first take a moment to
consider some of the organisms that you would describe as fungi.
You are likely to have thought of cultivated mushrooms and the ‘wild fungi’, including
brackets (shelf fungi) on trees and the great variety of ‘toadstools’. These structures are
all in fact fruiting bodies of fungi, reproductive structures that produce and release spores.
In addition, you might have thought of the microscopic and often disease- or decaycausing
fungi, such as mildews, rusts and moulds, and the yeasts that are used in making
wine, beer and some kinds of bread.
Like plants, fungi have diversified mainly on land and they occur in soil and on the surface
of, or inside the tissues of, other organisms (living or dead) virtually everywhere. The fact
that you rarely see fungi except as the occasional toadstool or fuzzy patches of mould
reflects the nature of the fungal ‘body’ and their mode of life. The majority of fungi consist
of microscopic filaments or hyphae (singular, hypha) which grow at their tips and branch
repeatedly (Figure 2.14). Hyphae have rigid cell walls in which the main structural
component is chitin, a polysaccharide that is also found in the outer skeleton of insects.
The mass of hyphae is called a mycelium (plural, mycelia).
Figure 2.14a (ii) and (iii) show that hyphae may have partitions or septa (singular, septum)
which divide up the hyphae into cell-like compartments. However, the ‘cells’ may have one
or several nuclei and the septa may be perforated, which allows nuclei and cytoplasm to
move along the hyphae. In some cases (Figure 2.14a (i)) there are no septa at all and this
state is described as coenocytic (from the Greek words for ‘shared’ and ‘vessel’).
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Figure 2.14 (a) Diagram illustrating types of fungal hyphae: (i) non-septate (coenocytic);
(ii) septate with one nucleus per compartment; (iii) septate with many nuclei per
compartment. (b) Coloured scanning electron micrograph showing hyphae of the fungus
Trichophyton interdigitale growing on human skin.
� From Figure 2.14, should fungi be described as unicellular or multicellular?
� Neither term provides a precise description of the situation in fungi but multicellular is
how fungi are usually described.
Some fungi spend part or all of their life cycle as a unicellular or yeast form (Figure 2.15a
and b). The yeast, Saccharomyces cerevisiae, which is used in baking, wine and beer
making and as a ‘laboratory’ organism, is generally considered to be unicellular, but it has
been reported that under certain conditions it can form rudimentary hyphae. This example
again highlights that much still remains to be learnt about the biology even of well-known
organisms, and that the classification of cells and organisms is still often controversial.
1 μm
(a) (b)
1 μm
Figure 2.15 (a) Diagram showing the usual unicellular structure of yeast, Saccharomyces
cerevisiae. (b) Coloured SEM showing Saccharomyces cerevisiae (brewer’s, or baker’s,
yeast) cells. These cells occur singly. Some cells can be seen to have small
protuberances; these are ‘daughter’ cells, which are formed by budding off from the
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larger ‘mother’ cells.
The hyphal branches of a fungal mycelium have an enormous surface area and here lies
the clue to the fungal way of life. Fungi are heterotrophic absorbers; that is, they utilise
pre-existing organic molecules (Chapter 1 and Book 2, Chapter 3) as an energy source,
and obtain these molecules by absorption from their environment. Because of their rigid
walls, they cannot engulf particles and instead, their hyphae secrete enzymes, which
break down large insoluble organic molecules, releasing soluble products that can be
absorbed to provide nourishment. The importance of fungal activity in breaking down
dead organic matter cannot be overstated, because it is central to the process of
decomposition, whereby mineral nutrients (e.g. nitrate and phosphate) are cycled within
ecosystems. Many fungi also live in partnership with plants or photosynthetic algae,
obtaining organic nutrients from these photosynthetic organisms and usually supplying
inorganic nutrients such as phosphate ions in return. This symbiotic mode of life is very
ancient: some early fossil plants from over 400 Ma ago have been found with fossilised
fungal partners and it has been suggested that fungal symbionts played a major role in the
invasion of land by plants; perhaps an example of coevolution. Two species that are in a
coevolutionary relationship exert a selective pressure on each other; therefore each
species affects the evolution of the other.
Whatever their mode of life, however, the basic mycelial structure of fungi remains much
the same. So although, like bacteria, fungi show great metabolic diversity in the
substrates they use and diversity of reproductive structures, the structural diversity of the
feeding stage of the life cycle is limited.
Summary of Section 2.4.2
l Classification of fungi as unicellular or multicellular organisms is difficult for many
species because of their cellular organisation, but most fungal species are generally
considered to be multicellular. Some species (the yeasts) are considered to be
unicellular, but classification of these species is also difficult and somewhat
controversial, because their growth behaviour may change under different
conditions.
l The majority of fungi form filamentous hyphae with rigid cell walls. This arrangement
is known as the mycelium. Hyphae may be divided up by septa and may be
multinucleate.
l Fungi play an essential role in the breakdown of organic material in the environment.
2.4.3 Plants
Plants are multicellular eukaryotes, adapted primarily to life on land. They are
photosynthetic organisms (Section 1.2.1) that evolved from green algae. Mature plants
are non-motile and have cells with rigid walls strengthened by a polysaccharide known as
cellulose. Most, but not all, have an upright leafy shoot (the photosynthetic part), nongreen
underground parts for anchorage and absorption (roots) and a vascular system to
conduct water and nutrients around the plant. The first plants to evolve were the
bryophytes, which are small and non-vascular and remain close to the ground; an
example is the mosses. In contrast, plants with a vascular system are able to grow
upwards and attain much larger sizes, and so compete effectively for the light needed for
photosynthesis. The angiosperms, or flowering plants, form by far the largest plant group,
with the greatest number of species, and this section will focus on cells of flowering plants.
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One of the most distinctive features of plant cells is actually an extracellular structure, the
cell wall, which is rigid and surrounds the cell membrane, conferring shape and support. It
is composed predominantly of cellulose. Small channels in the cell walls allow the
passage of water, ions and small molecules from cell to cell. The thickness of the cell wall
is one feature that distinguishes different types of plant cells. Another is the shape of the
cell, and a third is whether or not the cell is alive; together with living cells, dead cells play
important roles in providing support and transport vessels in plants.
There are three main tissue types in flowering plants; these are:
l ground tissue, which provides packing and support, and also energy storage, and
includes the majority of photosynthetic cells (palisade cells), which are located in the
interior of leaves
l vascular tissue, which enables transport of water and nutrients within
the plant
l dermal tissue, which is the outer cell layer, and provides protection, and controls
uptake of water, nutrients and gases, in different parts of the plant.
There are several cell types in each of these different plant tissues and some ground
tissue cells are also found in the vascular tissues. Some of the different types of plant cells
and tissues are shown in Figure 2.16.
(a) (b)
(c) (d)
100 μm 100 μm
100 μm 100 μm
epidermal cells
xylem
vascular tissue
phloem starch grains
palisade cells parenchyma cells
Figure 2.16 Light micrographs illustrating examples of different plant cells.
(a) The outer layers of a leaf showing epidermal cells (dermal tissue) at the surface of the
leaf and palisade cells (ground tissue), where most photosynthesis takes place. (b) The
deeper layers of a leaf. (Palisade cells and parenchyma cells are both types of ground
tissue.) (c) A section of a root showing the vascular tissues (xylem and phloem)
surrounded by ground tissues. (d) Parenchyma cells (ground tissue) from a potato tuber,
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specialised for storage. The blue-stained objects are starch grains.
Ground tissue
One type of plant cell, usually classified as a type of ground tissue cell, forms the basic
structural element of all plant tissues. These cells are known as parenchyma cells and
they form the bulk of stems, roots and leaves. Parenchyma cells are quite variable in size
and shape (e.g. Figure 2.16b, c, d) but a common feature is that they have thin cellulose
walls, so are readily deformed by pressure from adjoining tissue. Adjacent cells are linked
by pores through their cell walls called plasmodesmata (singular, plasmodesma), which
are lined by a plasma membrane and have a strand of cytoplasm in the middle. Small
molecules usually pass freely through these pores, so their number and distribution can
play a key role in cell-to-cell transport and communication.
Vascular tissue
There are two types of vascular tissue in plants: xylem, which transports water and
dissolved ions from the roots around the rest of the plant (Figure 2.16c); and phloem,
which transports the products of photosynthesis around the plant. The arrangement of the
two tissue types varies between different plants and in different parts of the same plant
(e.g. stems and roots), but they are often situated close together. The cells of the vascular
tissues are arranged end-to-end and form tube-like structures, arranged in bundles.
Dermal tissue
Plant dermal tissue, also known as the epidermis, is a layer of cells known as epidermal
cells (Figure 2.16a) that covers the entire plant. It is usually a single cell thick. Epidermal
cells have thickened external cell walls and in shoots are covered by a layer of waxy
material called cutin forming a protective layer, the cuticle.
The epidermis of leaves has pores known as stomata (singular, stoma) (Figure 2.17),
which open and close, thereby facilitating the gas exchange that is essential for
photosynthesis to occur; carbon dioxide must be able to enter leaves and oxygen must be
able to leave. Water is also lost from plants via open stomata; some water loss is
necessary to ensure continuous transport in the xylem of water and mineral nutrients from
roots to leaves. Stomata occur most frequently on the lower sides of leaves and in other
protected areas such as infoldings of stem surfaces. They are bounded by specialised
epidermal cells called guard cells, which change shape in response to internal and
external stimuli (including light, heat and carbon dioxide concentration), so opening and
closing the pore, and thereby regulating gas exchange and water loss.
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Figure 2.17 (a) Surface view of a leaf epidermis showing a stoma with a pair of guard
cells. (b) Coloured scanning electron micrograph showing stomata on the lower leaf
surface of a garden rose (Rosa sp.). The leaf surface is covered by epidermal cells,
among which are stomata which are bounded by two guard cells. Guard cells open and
close the stoma, allowing gas exchange when the stomata are open, and preventing
moisture loss when closed.
Summary of Section 2.4.3
l Plant cells have a rigid cell wall composed predominantly of cellulose. This helps to
support the plant.
l The three main types of tissue are ground tissue (packing, support, storage),
vascular tissue (transport) and dermal tissue (protection and uptake of water,
nutrients and gases).
2.4.4 Animal cells
Animal species exhibit a very wide range of complexity and cellular specialisation. Some
animals are very simple, containing only a few different cell types, or not very highly
specialised cells. Others, notably the vertebrates, comprise probably the most diverse
and also the most highly specialised cells of all living organisms. This section will focus on
vertabrate, particularly mammalian, cells.
At first, the early histologists studied parts of animals and categorised animal tissues
according to their function; so the major tissue types were classified as nerve
(communication), muscle (movement), epithelial (barrier) and connective (support and
storage) tissues. Additional categories were blood or lymphoid cells, germ cells
(reproduction), and glandular (endocrine) tissue, which is essentially a very complex type
of epithelial tissue. In vertebrates, however, most tissues are compound in nature; that is,
they contain a mixture of these six major cell types (for example, muscle contains muscle
fibres, blood vessels (which themselves comprise several tissues), nerves and connective
tissue), so there is now a somewhat less rigid classification of tissues. Nevertheless, it is a
useful skill to be able to recognise some of the different cell types typically present in a
tissue or organ.
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� Why might such a skill be useful in medicine?
� A knowledge of the typical or ‘normal’ arrangement and relative abundance of cells in
a tissue allows detection of ‘abnormal’ cells or arrangements of cells, which may
occur in certain diseases such as cancer or neurodegenerative diseases.
Identification of cells in a tissue specimen also allows deduction about the function of the
tissue, which might, for example, be of interest in working out the physiology of a newly
discovered animal. Cells with similar functions often (but not always) have a similar
appearance, even in animals that are only distantly related. Some examples of animal
tissues are shown in Figure 2.18, and a summary of different cell types is given in
Table 2.1.
(a) (b)
(c)
capillaries
connective
tissue
blood vessel with
red blood cells
muscle fibre
bundle
nucleus
leukocyte red blood
cells
(d)
10 μm 100 μm
100 μm 10 μm
Figure 2.18 Transmitted light micrographs of some animal tissues. (a) Skeletal muscle,
sectioned longitudinally (along the muscle) and stained with haematoxylin and eosin.
Bundles of muscle fibres can be seen with their nuclei stained dark purple. Connective
tissue and small blood vessels with red blood cells are visible. The individual myofibrils
that make up each muscle fibre (Book 2, Chapter 5) cannot be seen at this magnification,
but their cross-striations can be distinguished, because in each myofibril the crossstriations
run in register to those of the neighbouring myofibrils. (b) Human adrenal gland,
sectioned and stained with haematoxylin and eosin. This image shows the cells located
near the centre of the gland (the adrenal medulla), which secrete the hormones adrenalin
and noradrenalin. The cytoplasm of these cells is stained deep pink, their nuclei are
purple. Small capillaries, containing red blood cells, are also just visible at this
magnification. (c) Human adrenal gland, sectioned and stained with haematoxylin and
eosin. This image shows the cells located near the edge of the gland (the adrenal cortex),
which secrete steroid hormones, e.g. cortisone. The steroid-secreting cells appear pale
because the cholesterol that fills their cytoplasm has been dissolved away by the
chemicals used to fix the tissue. The pale steroid-secreting cells are arranged in ovalshaped
clumps, enclosed by thin strands of connective tissue through which run wide
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irregularly shaped capillaries, full of bright pink red blood cells. (d) Human blood smear,
stained with Leishmann’s stain. Many red blood cells (salmon pink) and three leukocytes
are visible. Leukocyte nuclei are stained blue/purple, their cytoplasm is granular and a
very pale purple colour. Two of the leukocytes have lobed nuclei (these are neutrophils),
the other is a lymphocyte and has a round nucleus.
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Table 2.1 Summary of the principal types of mammalian cells. Note that
some cells, such as leukocytes, can be classified into more than one
group (blood and immune system). Endocrine cells and other glandular
cells are frequently considered to be a specialised type of epithelial cell.
Adipose tissue cells are often classified as connective tissue. For
detailed explanations, see text.
Cell type Examples Functions Special features
epithelial cells epidermis (outer layer of
skin); lining of intestine,
blood vessels and lungs;
cells of glands (e.g. salivary
glands, mammary glands)
protection,
barrier,
absorption,
secretion
form sheets of closely
linked, polarised cells*
hormoneproducing
(endocrine)
cells
pancreas; adrenal gland
(Figure 2.18b and c)
widespread
communication
produce and secrete
chemical messengers into
the circulation
muscle cells smooth muscle of internal
organs, such as intestine
and blood vessels
movement,
e.g. peristalsis
contain contractile proteins;
are linked together by ‘gap
junctions’ (Chapter 3)
skeletal muscle of limbs
(often known as striated
muscle because it has a
striped appearance)
(Figure 2.18a)
movement of
limbs
contain contractile proteins;
form a syncytium of long
multinucleate fibre-like cells
cardiac (heart) muscle contraction of
heart
contain contractile proteins
nerve cells
(neurons)
neurons of brain and spinal
cord; small groups of
neurons (ganglia) in body
rapid and
specific
communication
are polarised cells with long
processes; have special
membrane properties that
allow electrical signalling
support cells
(often classified
as connective
tissue cells)
bone cells (osteoblasts and
osteoclasts), cartilage,
fibroblasts
provide support
and help to
organise tissue
structure
fibroblasts produce much of
the extracellular material
(Chapter 3)
adipocytes
(often classified
as connective
tissue cells)
adipose tissue around
certain organs and under
skin
energy storage,
protection
have cytoplasm mostly
composed of lipid
blood cells red blood cells (RBCs)
(Figure 2.18d)
oxygen
transport
RBCs contain haemoglobin
which binds oxygen;
mammalian RBCs lose their
nucleus when mature
immune system
cells
leukocytes (several types)
(Figure 2.18d)
defence B lymphocytes and plasma
cells produce
antibodies; macrophages
ingest pathogens, etc.
germ cells eggs, sperm reproduction are haploid (contain half the
normal number of
chromosomes, Chapter 4)
* Polarised cells are explained in Section 2.4.5.
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2.4.5 Mammalian cell diversity: an example
As an example of the diversity of animal cells found in tissues and organ systems, this
final section will consider some of the different types of cell found in the mammalian small
intestine. Figure 2.19 is a photomicrograph that shows a section through the gut wall in
which different layers of cells can be identified.
Fig 2.20
Fig 2.23
Fig 2.22
Fig 2.21
100 μm
Figure 2.19 Light micrograph of a section of rat small intestine, stained with Alcian blue,
haematoxylin and eosin. The labelled boxes indicate areas of different cell types and
these are illustrated in more detail in Figures 2.20–2.23.
Each of the different cell types within the gut has a role to play in gut functions. Smooth
muscle cells contract, causing a wave of constriction of the gut wall (known as peristalsis),
which moves food along the intestine. Connective tissue cells provide support. Epithelial
cells are a varied group; most are involved in the absorption of nutrients, but some
produce digestive enzymes, some secrete mucus, which aids passage of contents along
the gut, and others are specialised to secrete hormones into the bloodstream. Blood
vessels, the larger of which are actually composed of several cell types, transport
absorbed nutrients to the rest of the organism. Cells of the immune system (not visible in
Figure 2.19) defend against damage by ingested pathogens. Nerve cells (neurons)
coordinate the activities of the other cell types.
Epithelial cells
This section continues with a closer look at the structural and functional differences
between some of these cells, starting with the epithelial cells which form a barrier, or
interface, across which some substances are secreted and nutrients are selectively
absorbed. What structural and molecular properties of the epithelial cells confer these
particular functional characteristics? The barrier properties arise because the epithelial
cells are tightly packed next to each other as a distinct cell layer (Figure 2.20a). This
packing occurs because of the type and arrangement of structural molecules within the
cells, which results in the formation of special close contacts between them.
The absorptive properties of epithelial cells arise because of the presence and
arrangement of specific proteins, called transporters, in their plasma membranes. There
are many kinds of transporters; those involved in absorption of nutrients are located only
in the part of the cell membrane that comes into contact with food (known as the apical
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surface; the other boundaries, which contact adjacent cells and connective tissue are
known as the basolateral surfaces). So, the membrane of these epithelial cells is
polarised; that is, its properties are different on one side of the cell compared with the
other (Figure 2.20b). Absorption is facilitated by the presence of finger-like projections on
the apical surface. These projections are known as microvilli, and they increase the
surface area that is in contact with the ingested nutrients. The properties of a tissue or cell
are therefore determined not only by the particular molecules that they contain, but also
by the arrangement of these molecules within the cell. You will learn more about the
properties of intestinal epithelial cells in Chapter 3.
Figure 2.20 (a) Light micrograph showing epithelium from rat small intestine stained with
haematoxylin and eosin, and also Alcian blue, which primarily stains
mucopolysaccharides. Cytoplasm is pale pink; nuclei are dark purple/black. The bright
blue areas are mucus present in some of the epithelial cells. EC = epithelial cells; CT =
connective tissue; BV = blood vessel. (b) Simplified schematic diagram showing some
properties of intestinal epithelium. The epithelial cells form a barrier because they are
closely packed together, and linked by specialised proteins (Chapter 3). The cells are
polarised; the surface of absorptive cells that is in contact with the nutrients in the gut
lumen possesses microvilli, which increase the surface area available for absorption,
while the other surfaces do not. Also shown (not to scale) is the uneven distribution of
transporter proteins in the membrane. Different types of transporter are present in the
apical and basolateral membranes.
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Smooth muscle cells
The smooth muscle cells (Figure 2.21) are contractile; that is, their shape can change,
either shortening (contraction) or lengthening (relaxation). Coordinated contraction and
relaxation of many smooth muscle cells together results in the intestinal movements
known as peristalsis. How are the movements of the separate muscle cells coordinated,
and what is the nature of the molecules that produce this movement? Again, specific
proteins and structures are involved in these processes. One type of structure, known as
a gap junction (Chapter 3), allows electrical and chemical communication between the
cells that coordinates their contraction; other proteins cause a change in shape of the
muscle cells during contraction (Book 2, Chapter 5).
Smooth muscle is under involuntary control, while skeletal muscle is under
conscious or voluntary control.
Figure 2.21 (a) Light micrograph showing smooth muscle from rat small intestine stained
with Alcian blue, haematoxylin and eosin. (b) Simplified schematic diagram showing some
properties of smooth muscle cells. Smooth muscle cells are closely packed and linked by
gap junctions. Muscle cells also contain specialised proteins that mediate contraction (not
shown).
Nerve cells
Next, consider the nerve cell, or neuron (Figure 2.22). Small groups of neurons are
situated within the gut wall, in small linked clumps known as ganglia. The neurons are not
identical; they are diverse and have a number of different functions in the gut. All,
however, are involved in conveying information to other cells. Some extend long
processes into the surrounding smooth muscle, where they activate the smooth muscle
cells, stimulating them either to contract or relax; others have processes that extend to the
epithelium, or to other neurons. Neurons, like intestinal epithelial cells, are polarised. The
functional properties of neurons are reflected in structural specialisations which are,
again, the result of the presence and arrangement of specific proteins in different parts of
the cell.
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ta rge t ce ll
(e .g. smooth mus cle )
􀀀
􀀀
􀀀
nucleus
axon
dendrites
(a)
(b) (c)
100 μm 100 μm
Figure 2.22 (a) Simplified diagram showing some of the structural features of neurons.
Neurons are specialised to transmit electrical signals rapidly, often over long distances.
Typically they receive information at processes known as dendrites, and transmit
information to their target cell, which may be a smooth muscle cell (as illustrated here), an
epithelial cell or another cell type, along a long cellular process known as an axon (not to
scale). Neurons are polarised cells: different membrane proteins are found in different
regions of the neuronal membrane. (b) Light micrograph showing a small group of
neurons in the rat small intestine stained with Alcian blue, haematoxylin and eosin. (Note
that the processes of the neurons are not visible in this preparation.) (c) Fluorescence
micrograph of a neuron from rat small intestine, labelled with a fluorescent antibody.
Fibroblasts and leukocytes
Other cell types present in the gut include connective tissue cells, called fibroblasts, and
leukocytes (Figure 2.23). These cells also have specific functions: fibroblasts provide
support and secrete molecules that form the extracellular matrix (Chapter 3); leukocytes
(sometimes referred to as white blood cells) are involved in defence against ingested
pathogenic microbes. At a first glance, these two cell types perhaps do not have such
interesting structural specialisations as some of the other cells that have been described
here, but they each contain and secrete special proteins that determine their functions.
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Figure 2.23 (a) Light micrograph showing fibroblasts and connective tissue from rat small
intestine stained with Alcian blue, haematoxylin and eosin. A blood vessel is also visible in
cross-section (lower right). (b) Simplified diagram showing some of the properties of
fibroblasts. Fibroblasts have an irregular shape, and are often difficult to discern by light
microscopy. They produce and secrete molecules into the extracellular space, forming the
extracellular matrix and connective tissue fibres (composed of collagen and elastin).
These five cell types (epithelial, smooth muscle, neuron, fibroblast and leukocyte) have
been used here as examples of cellular diversity and will be referred to again in Chapter 3.
There are, of course, several other types of cell in the gut, and many more in other organs.
As you have seen, different types of animal cells differ not only in shape, but also
importantly, in the molecules that they contain. Many structural molecules and also
enzymes involved in core metabolic reactions are common to all the cells of a particular
organism, but different types of cells also contain additional molecules, usually proteins,
that enable them to perform specialised functions. You will remember that proteins are
coded for by the genetic material of the cell, DNA, which is situated in the nucleus of
eukaryotic cells.
� What is the name used for the units of DNA that encode different proteins?
� They are called genes.
You will recall that although all the cells of an organism contain the same genetic
information, only some genes are expressed (transcribed and translated into proteins) in
any particular cell type. Put another way, the different cell types of an organism express
different genes. It is this differential gene expression that gives rise to the structural and
functional differences between cells. How gene expression is controlled is an important
topic, which you will return to in Chapter 6.
You will be beginning by now to appreciate the complexity of cell interactions in just one
part of an animal.
� Give two examples of cellular interactions that occur in the gut.
� (1) Coordinated smooth muscle contraction involves interactions between individual
smooth muscle cells. (2) Smooth muscle cells are stimulated to contract or relax by
the action of neurons.
All animal cells interact with adjacent and nearby cells, and in addition some specialised
cells have evolved that provide communication over greater distances: for example,
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neurons, and also hormone-secreting (endocrine) cells. You will learn more about cell
signalling and communication between cells in Book 2.
Once a cell has received a signal from another cell, it usually responds in some way. For
example, smooth muscle may contract in response to nerve stimulation; other cells, such
as some types of epithelial cells may undergo cell division, which often occurs in response
to factors secreted by other cells. It is important to realise that biochemical changes
underlie all cellular responses. Muscle contraction is brought about by changes in the
conformation and activities of specific proteins within the smooth muscle cell (Book 2,
Chapter 5). Cellular responses to signals often involve a chain, or cascade, of biochemical
events within the cell (Book 2, Chapter 4), often culminating in changes in gene
expression. Cell division, for example, requires coordinated changes in either the activity
or synthesis of a large number of proteins (Book 3, Chapter 1). Cell interactions thus also
play an essential role in the regulation of gene expression, and thereby help to determine
the behaviour, properties and appearance of cells, as you will see throughout the module.
Summary of Sections 2.4.4 and 2.4.5
l The cells of animal tissues can be classified as epithelial, muscle, nervous,
connective, blood and immune system, endocrine (hormone-secreting) or germ cells.
Some cells, such as the hormone-secreting cells of the intestinal epithelium, fall into
more than one of these categories.
l Different types of cells are specialised to perform different functions. These
differences are possible because different types of cells express different proteins,
and have differing shapes and structural specialisations.
l Mammalian tissues and organs typically comprise a mixture of different cell types;
the intestine contains cells of all categories, except germ cells.
l In animals, interactions between cells play crucial roles in the development and
functions of both individual cells and organ systems.
2.5 Final word
This chapter has introduced the diversity and complexity in cell structure and function, and
has begun to consider how this diversity can arise from an underlying uniformity of
organisation. You have also been introduced to some of the techniques that are widely
used to study cells, and you will encounter more examples of data obtained using these
techniques during this module, and in your future study of biology.
Diversity in shape, size, function and other cell properties has been illustrated here with
examples from some better-known, well-studied organisms. It should be borne in mind,
however, that cell biologists have not made detailed studies of all types of cells; and the
structure and function of many organisms, and how they should be classified
taxonomically, remain uncertain. As in other areas of biology, there is much that remains
to be discovered about cellular diversity. In order to understand how cells perform their
many functions, in the next chapter you will delve more deeply into the internal
organisation of cells.
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2.6 Learning outcomes
2.1 Describe, using examples, the diversity of cells in different organisms.
2.2 Outline some of the techniques used in the study of whole cells, and interpret simple
data obtained using these techniques.
2.3 Describe the different types of cell found in the mammalian intestine, and how their
structure relates to their function.
2.4 Interpret and take measurements from annotated images of different cell and tissue
types.
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