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Chapter 3 - Cell
CHAPTER 3 - Cell
3.1 CELL STRUCTURE
The cell is the fundamental unit of life. All organisms, whatever their type or
size, are composed of cells. The modern theory of cellular organization states that:
1. All living organisms are composed of cells
2. All new cells are derived from other cells.
3. Cells contain the hereditary material of an organism which is passed from
parent to daughter cells.
4. All metabolic processes take place within cells.
3.2 Cell membranes
(plasmalemma)
Both the plant and the animal cell have a cell membrane surrounding the cytoplasm. The
membrane consists of an inner and an outer layer, with two layers of fat molecules,
known as phospholipids, between them.
There is no living cell that is not surrounded by a cell membrane, and most
cells have membranes inside them too. The membrane around the outside of a cell is
called the cell surface membrane. Fig. 3.1, 3.2, 3.3 shows the structure of a cell
surface membrane. This representation of its structure is called the fluid mosaic
model.
Structure of the Cell membranes
A cell membrane is formed from a phospholipids bilayer. Interspersed amongst the
phospholipids molecules are cholesterol molecules. Cholesterol molecules help to
make the membrane more fluid at low temperatures and less fluid at high
temperatures.
Floating amongst the phospholipids and cholesterol molecules are many
globular protein molecules, many of which span from one side to the other. These
proteins tend to be arranged with hydrophilic parts of their chains (that is parts
containing amino acids with hydrophilic R groups) on the outer surfaces of the
membrane, and hydrophobic parts within the membrane amongst the hydrophobic
tails of the lipids. Many of these proteins act as pores or transporters, allowing
substances to pass from one side of the membrane to the other.
Most of the protein molecules and many of the lipid molecules have short
carbohydrate chains attached to them. These molecules are called glycoproteins and
glycolipids. The carbohydrate chains are all on the outer surface of the membrane.
Glycolipids and glycoproteins help to stabilise membrane structure by forming
hydrogen bonds with water molecules outside the membrane.
Why this structure is called a fluid mosaic? All of these molecules are in
constant motion, vibrating and bumping into each other and changing place within
layer. So the membrane behaves rather like a fluid-although it does not flow away
into its surroundings! The mosaic part of the name refers to the mosaic pattern
protein molecules which you would see if you looked down on the surface on the
membrane.
Membranes inside cells have a very similar structure to cell surface
membranes. However, the relative proportions of the different kinds of molecules
can vary considerable. For example, the membranes around mitochondria have
hardly any carbohydrate attached to their lipids and proteins.
Function
The cell membrane is part of the living cell and is selectively permeable allowing only
certain substances to enter and leave the cell. This prevents the cell from poisoning
itself since harmful substances are prevented from entering the cell.
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Chapter 3 - Cell
Fig 3.1
Fig 3.2
Fig 3.3
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Chapter 3 - Cell
3.3 Cytoplasm
The term cytoplasm is often used to refer the background material inside cell,
within which all the organelle such as mitochondria and ribosome’s are found.
Cytoplasm is mostly water, with a variety of other molecules dissolved or
suspend in it. Many of these are proteins, especially enzymes. These include simple
ions such as sodium, phosphates and chlorides, organic molecules such as amino
acids, ATP and nucleotides, and storage material such as oil droplets. Many
important biochemical processes, including glycolysis, occur within the cytoplasm. It
is not static but capable of mass flow, which is called cytoplasmic streaming.
3.4 Ribosomes
Ribosomes appear as small black dots. They are usually in clusters called
polyribosomes. Some ribosomes are found free in the cytoplasm, while others are
attached to the outer surfaces of the membranes of the rough endoplasmic
reticulum. Ribosomes are also found inside mitochondria and chloroplasts.
The function of ribosomes is to provide a platform on which protein synthesis
takes place and to help with several stages of this process.
3.5 Endoplasmic reticulum
‘
Endoplasmic’ means ‘inside the cytoplasm’, and ‘reticulum’ means ‘network’.
The endoplasmic reticulum (often abbreviated to ER) is a network of membranes
running through the cytoplasm of every cell. The membranes enclose spaces called
cisternae (Fig.3.7), which form an interconnecting channel throughout the
cytoplasm.
Some parts of the endoplasmic reticulum have ribosomes attached to the
cytoplasmic side of the membranes. This is called rough endoplasmic reticulum (or
RER). These ribosomes synthesise proteins, which are to be secreted from the cell,
to form lysosomes or to become part of the cell surface membrane. As these proteins
are being made, some of them are passed through pores in the endoplasmic
reticulum membrane into the cisternae, while others become part of the membrane
itself, The cisternae then break into small vesicles and carry these proteins to the
Golgi apparatus (also sometimes called the Golgi body) (Fig.3.7)
Other parts of the endoplasmic reticulum have no ribosomes attached. These
are called smooth endoplasmic reticulum (or SER). The cisternae of SER tend to be
more tubular, in contrast to the flattened sacs of RER. SER has many different
functions, including the synthesis of cholesterol and of steroid hormones such as
testosterone, and the breaking down of toxins such as drugs.
The functions of the ER may thus be summarized as:
1. Providing a large surface area for chemical
2. Providing a pathway for the transport of materials through the cell
3. Producing proteins, especially enzymes (rough ER)
4. Producing lipids and steroids (smooth ER)
5. Collecting and sorting synthesized material.
6. Providing a structural skeleton to maintain cellular shape (e.g. the smooth ER
of a rod cell from the retina of the eye).
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Fig 3.7
3.6 Golgi apparatus
The Golgi apparatus (or Golgi body) is a stack of curved cisternae with several
smaller vesicles entering and leaving it (Fig. 3.8 and 3.9). Vesicles containing newly
synthesized proteins break off from the rough endoplasmic reticulum, and travel
towards the Golgi apparatus where they fuse with its convex face. Here the proteins
are ‘finished off’ and packaged before being exported from the cell. They may, for
example, have carbohydrates added to them to form glycoproteins. The proteins are
concentrated within the Golgi cisternae; in pancreatic cells secreting insulin, for
example, the insulin is concentrated so much in the Golgi that it crystallizes.
When the protein is ready, small vesicles break away from the concave face
of the Golgi apparatus and move towards the surface of the cell. They fuse with the
cell surface membrane and release their contents to the outside. The membranes of
the vesicles, which were originally part of the rough endoplasmic reticulum
membrane, become incorporated in the cell surface membrane.
Not all the proteins dealt with in the Golgi are destined for export. In animal
cells some of the vesicles released from the convex face become lysosomes. Plant
cells do not have lysosomes. In plant cells, some of the Golgi vesicles take proteins
to the large vacuole where they are released into the cell sap.
In plant cells the Golgi is also involved in processing carbohydrates. The
polysaccharides, which will make up the matrix (background material) of the cell wall
are first made in the endoplasmic reticulum, then assembled in the Golgi apparatus,
then carried to the cell surface inside secretory vesicles. The whole process, from
endoplasmic reticulum to cell surface, can take as little as 20 minutes.
Fig 3.8
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Chapter 3 - Cell
Fig3.9
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Chapter 3 - Cell
3.7 Lysosomes
Lysosomes are tiny vesicles found in most animal cells but not in plant cells.
They are surrounded by a single membrane. They have no structure inside them, but
simply contain a variety of hydrolytic (digestive) enzymes in solution (Fig 3.10).
Lysosomes are formed as buds which break away from the Golgi apparatus.
Their main function is to fuse with other vesicles in the cell which contain something
which needs to be digested, for example a bacterium which has been brought into
the cell by phagocytosis or a worn-out mitochondrion which needs to be destroyed.
The enzymes in the lysosome then digest the contents of this vesicle, producing
soluble substances which can be absorbed into the cytoplasm. In plant cells these
functions are carried out by enzymes in the vacuole.
Fig 3.10
3.8 Microbodies (peroxisomes)
Microbodies are small spherical membrane-bound bodies. Apart from being
slightly granular, they have no internal structure. They contain a number of
metabolically important enzymes, in particular the enzyme catalase, which catalyses
the breakdown of hydrogen peroxide. Hence these Microbodies are sometimes called
peroxisomes.
Hydrogen peroxide is a potentially toxic by-product of many biochemical
reactions within organisms. Peroxisomes containing catalase are therefore
particularly numerous in actively metabolizing cells like those of the liver.
3.9 Storage granules
Every cell contains a limited store of food energy. This store may be in the
form of soluble material such as the sugar found in the vacuoles of plant cells. It may
also occur in insoluble form, as grains or granules, within cells or organelles.
Starch grains occur within chloroplasts and the cytoplasm of plant cells.
Starch may also be stored in specialized leucoplasts called amyloplasts. Glycogen
granules occur throughout the cytoplasm of animal cells. They store animal starch or
glycogen. Oil or lipid droplets are found within the cytoplasm of both plant and
animal cells.
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Chapter 3 - Cell
3.13 Mitochondria
Mitochondria are quite large organelles which can be seen with a good light
microscope. To see detail in their structure, an electron microscope is needed;
Mitochondria are very variable in size and shape. It has two membranes, separated
by an intermembrane space. The outer membrane is relatively smooth, but the inner
membrane is folded to form projections called cristae. Between the cristae is the
matrix, which fills the rest of the space inside the mitochondrion. The matrix also
contains ribosomes and DNA, which are used to make some of the mitochondrion’s
own proteins.
Mitochondria are the site of the aerobic stages of respiration, Krebs cycle and
oxidative phosphorylation.
Fig 3.15
3.14 Nucleus
The nucleus is the part of the cell, which contains DNA.
Within the nucleus (Fig 3.16 and 3.17) there is often a particularly darkly
staining region called the nucleolus. Here, ribosomal RNA is made by transcription
from DNA. The small and large subunits of ribosomes are assembled in the
nucleolus. They leave the nucleus through pores (Fig 3.16) before being assembled
into complete ribosomes in the cytoplasm.
The nucleus is surrounded by a double membrane. The content of the nucleus is
known as the nucleoplasm. It contains two important structures: the nucleolus and
the chromosomes. The nucleolus consists mainly of nucleic acids. The chromosomes
are the carriers of hereditary characteristics which are carried from one generation to
the next. Before the cell divides these chromosomes are visible as a network of
threads known as the chromatin network.
The functions of a nucleus are:
1. To contain the genetic material of a cell in the form of chromosomes.
2. To act as a control centre for the activities of a cell
3. To carry the instructions for the synthesis of proteins in the nuclear DNA.
4. To be involved in the production of ribosomes and RNA.
5. In cell division.
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Chapter 3 - Cell
Fig 3.16
Fig 3.17
3.15 Nuclear envelope
The nuclear envelope is made up of two membranes, with a narrow space
between them. The outer of these two membranes links up with the endoplasmic
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Chapter 3 - Cell
reticulum. Indeed, in some cells it is exactly like a piece of rough endoplasmic
reticulum complete with attached ribosomes.
These two membranes have many gaps in them which are called nuclear
pores. The gaps are relatively large much bigger than the protein pores in the cell
surface membrane. They are large enough to allow partially assembled ribosomes
from the nucleolus to pass through, as well as messenger RNA on its way out of the
nucleus and enzymes such as DNA polymerase on their way in.
3.16 Cell wall
The cell wall only occurs in plant cells. It lies outside the cell membrane. It consists
of insoluble cellulose and has three layers: the primary cell wall closest to the
cytoplasm, the middle lamella, and an outside secondary cell wall. Pits occur in
the cell wall which allows the exchange of substances with adjacent cells through fine
protoplasmic threads called plasmodesmata.
Although small molecules and ions can pass easily through cell walls, plant cells need
a faster and more reliable way of allowing larger substances to move between
adjacent cells. This is done through plasmodesmata.
Function
The cell wall protects the contents of the plant cell. It give rigidity to the plant cell.
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Chapter 3 - Cell
3.18 Vacuole
A vacuole is a membrane-bound organelle that usually contains liquid, All
cells have vacuoles, but plant cells differ from animal cells in that their vacuoles are
very large, permanent, and usually occupy a position fairly near the centre of the
cell, In a mature plant cell, up to 90% of its volume may be taken up by the vacuole,
The membrane surrounding a plant cell vacuole is often known as the tonoplast.
The fluid inside the vacuole is known as cell sap.
Plant cell vacuoles contain many different substances in solution in water;
these include sugars, storage proteins, pigments (coloured substances) and
enzymes.
Functions
the colours of some flower petals are caused by pigments held inside vacuoles in
their cells, Some plants store sucrose in their vacuoles, either temporarily or for
much longer periods; the sugar which we obtain from sugar beet, sugar cane and
many fruits comes from vacuoles, In many plants the vacuoles perform the same
functions as lysosomes in animal cells and contain digestive enzymes.
In unicellular animals, example, Amoeba, the vacuoles are known as contractile
vacuoles and function as excretory organs. Food ingested by Amoeba forms a hollow
called a phagosome or food vacuole. Vacuoles thus store water and organic and
inorganic substances. They maintain the rigidity of the cell and give mechanical
support by exerting an outward pressure on the cell wall of the plant cell (turgor
pressure). They assist in intracellular support.
T
3.19 Plastids
Plastids are found only in plant cells, not in animal cells. Plastids are
organelles surrounded by two membranes. In this they are similar to mitochondria.
Another similarity with mitochondria is that they contain DNA and ribosomes, which
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suggest that, like mitochondria, they have probably evolved from what, were
originally symbiotic prokaryotes.
Three types can be distinguished
– The chromoplast such as carotenoids which give colour to carrots and ripe
fruits;
the leucoplasts which are organelles for the storage of starch, oil
and protein granules;
The chloroplast which are essential for photosynthesis. Chloroplast
contains green pigments known as chlorophyll.
Fig 3.21 shows structure of a chloroplast. They are large organelles,
surrounded by a double membrane. Inside the chloroplast is a third system of
membranes, forming many tiny flattened sacs called thylakoids. In places these
thylakoids are stacked on top of each other to form grana. Grana are linked by
extensions of some of the thylakoids, forming long membrane-bound tubes called
intergranal lamellae. These entire membranes lie in a matrix called the stroma.
The thylakoid membranes contain chlorophyll molecules, which give the whole
chloroplast – and the whole leaf – its green colour. Also in these membranes are the
molecules involved in the light-dependent reactions of photosynthesis, including
photophosphorylation. These reactions involved the ejection of electrons from some
of the chlorophyll molecules when light hits them; the electrons are then passed
along a chain of carrier molecules in the thylakoid membrane. This process provides
energy for synthesising ATP.
The stroma contains the enzymes required for the Calvin cycle, in which
carbohydrates are made from carbon dioxide and water. The most abundant of these
enzymes is ribulose bisphosphate carboxylase, usually known as Rubisco. It is not
only the most abundant enzyme in the world, but actually the most abundant
protein. Up to one-quarter of the total protein in a leaf is Rubisco. In1993, it was
estimated that there were 10kg of Rubisco in the world for every person on Earth.
If the plant makes more carbohydrate in photosynthesis than it needs, then
some may be temporarily stored as starch inside the chloroplasts. The starch forms
granules, which may take, up a large amount of space in the stroma. The stroma
also contains lipid droplets, DNA and ribosomes.
Fig 3.21
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Chapter 3 - Cell
Centrioles
In animal cells there is a specialised patch of cytoplasm called the centrosome. This is
found near the nucleus. Inside this ‘patch’ are two cylindrical bodies called centrioles.
Function:
Centrioles play an important role in cell division.
PROKARYOTIC CELLS
3.20 Structure of a prokaryotic cell
Prokaryotic cells (pro-‘before’, karyo – ‘nucleus’) were probably the first forms
of life on earth. Their hereditary material, DNA, is not enclosed within a nuclear
membrane. This absence of a true nucleus only occurs in two groups, the bacteria
and blue-green bacteria. There is no membrane-bound organelle within a prokaryotic
cell, the structure of which is shown in Fig 3.22
Fig 3.22
Fig 3.22 shows the structure of a bacterium. Like all bacteria it is single-celled and
has no nucleus. Such cells are called prokaryotic cells, all bacteria, including bluegreen, are prokaryotes. Plant, animal and fungal cells, which do have nuclei, are
eukaryotic cells.
Prokaryotic cells are usually smaller than eukaryotic cells, they are similar in
size to a mitochondrion or a chloroplast. Indeed, mitochondria and chloroplasts
probably were prokaryotic cells, which came to live inside the larger eukaryotic ones
many millions of years ago.
All prokaryotic cells are surrounded by a cell wall, which gives support and
protection to the cell and is made of a variety of polysaccharides. These
polysaccharides, however, are very different from those in plant cell walls;
prokaryote cell walls do not contain cellulose, for example. Bacterial cell walls
contain large amounts of substances known as peptidoglycans, which, as their
name suggests, are made up of molecules in which peptides and sugars are
combined. These form long, branched, cross-linked chains and make the wall very
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Chapter 3 - Cell
strong. Cell walls are very important to bacteria. They stop them from bursting when
they absorb water and help to protect them from invasion by viruses. If you can
damage the cell wall you can kill the bacterium. The antibiotic penicillin inhibits the
enzymes which help to form the cross-links between the peptidoglycans.
Many bacteria have a thick layer of jelly-like material surrounding them called
a capsule. The capsule is made of polysaccharides which absorb water to form a
slimy material. (Bacterial capsules make up a high proportion of the plaque which
can collect on you teeth.) The capsule protects the bacterium from attack by viruses,
and from antibodies. For example, the bacterium pneumococcus exits in two forms,
one with a capsule and one without. The one with capsule is a dangerous pathogenic
(disease-causing) organism, able to infect a person’s lungs and cause severe
pneumonia. The one without a capsule is easily destroyed by the immune system,
and does not cause disease at all.
Beneath the cell wall is a cell surface membrane. This has a very similar
structure to that of eukaryotic cells, being made up of a phospholipid bilayer in which
protein molecules float. (Some bacteria, known as Gram-negative bacteria, have
another membrane) In photosynthetic bacteria, there may be extensive membrane
systems inside the cell, sometimes closely associated with the cell surface
membrane. These membrane systems hold the molecules which are involved in
capturing light energy.
The cytoplasm often contains large numbers of ribosomes. Like the slightly
larger ribosomes of eukaryotic cells, these are made of ribosomal RNA and protein
and are the sites of protein synthesis.
The DNA of bacteria is a single, large, circular molecule. This is unlike the
DNA of eukaryotes, which is linear rather than circular and is usually made up of
several molecules each of which forms a chromosome. The chromosomes of
eukaryotic cells are complex structures involving proteins called histones as well as
DNA. Prokaryotic DNA does not form chromosomes, and although it does have
proteins associated with it, these are not histones. There is no nuclear envelope in a
prokaryotic cell so the DNA lies free in the cytoplasm.
Prokaryotic cells do not have a cytoskeleton that is they do not have
microtubules or intermediate filaments supporting the structure of the cell.
Some prokaryotic cells have a flagellum, which is used for movement. These
flagella have no Similarity in structure with those of eukaryotic cells and are unique
to prokaryotes both in their structure and the way they work. While eukaryotic
flagella throw waves along themselves, bacterial flagella actually rotate like the
propellor of a boat. At the base of the flagellum is a true motor with a rotating
bearing-the smallest motor known.
Table 2-Comparison of prokaryotic and eukaryotic cells (Fig 3.23)
Prokaryotic cells
Eukaryotic cells
No distinct nucleus; only diffuse area(s)
of nucleoplasm with no nuclear
membrane
No chromosomes – circular strands of
DNA
No membrane – bound organelles such
as chloroplasts and mitochondria
Ribosomes are smaller
Flagella (if present) lack internal 9+2
fibril arrangement
A distinct, membrane –bound nucleus
Chromosomes present on which DNA is
located
Chloroplasts and mitochondria may be
present
Ribosomes are larger
Flagella have 9+2 internal fibril
arrangement
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Chapter 3 - Cell
No mitosis or meiosis occurs
Mitosis and /or meiosis occurs
Fig 3.23
3.21 Structure of the eukaryotic cell
Eukaryotic cells (eu – ‘true’, Karyo – ‘nucleus’) probably arose a little over
1000 million years ago, nearly 2500 million years after their prokaryotic ancestors.
The development of eukaryotic cells from prokaryotic ones involved considerable
changes, as can be seen from table 2. The essential change was the development of
membrane-bound organelles, such as mitochondria and chloroplast, within the outer
plasma membrane of the cell. The presence of membrane-bound organelles confers
four advantages:
1. Many metabolic processes involve enzymes being embedded in a
membrane. As cells become larger, the proportion of membrane area to
cell volume is reduced. This proportion is increased by the presence of
organelle membranes.
2. Containing enzymes for a particular metabolic pathway within organelles
means that the products of one reaction will always be in close proximity
to the next enzyme in the sequence. The rate of metabolic reactions will
thereby be increased.
3. The rate of any metabolic pathway inside an organelle can be controlled
by regulating the rate at which the membrane surrounding the organelle
allows the first reactant to enter.
4. potentially harmful reactants and / or enzymes can be isolated inside an
organelle so they won’t damage the rest of the cell
Fig 3.24
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Chapter 3 - Cell
Fig 3.25
Difference between plant and animal cells
The major difference between plant and animal cells are given in Table 3
Similarities between plant and animal cells
Both have a cell membrane surrounding the cell
Both have cytoplasm
Both contain a nucleus
Both contain mitochondria
Both contain endoplasmic reticulum
Both contain ribosomes
Table 3 - Difference between plant and animal cells
Plant cells
Animal cells
Have a cellulose cell wall present (in
addition to the cell membrane)
Plastids,
e.g.
chloroplasts
and
leucoplasts, present in large numbers
Mature cells normally have a large
single, central vacuole filled with cell
sap
Tonoplast present around vacuole
Cytoplasm normally confined to a thin
layer at the edge of the cell
Nucleus at edge of the cell
Cell wall absent – only a membrane
surrounds the cell
Plastids absent
Vacuoles, e.g. contractile vacuoles, if
present, are small and scattered
throughout the cell
Tonoplast absent
Cytoplasm present throughout the cell
Nucleus anywhere in the cell but often
central
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Chapter 3 - Cell
Lysosomes not normally present
Centrioles absent in higher plants
Often regular in shape
Starch grains used for storage
Only some cells are capable of division
Few secretion are produced
Lysosomes almost always present
Centrioles present
Often irregular in shape
Glycogen granules used for storage
Almost all cell are capable of division
A wide variety of secretions are
produced
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