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Cell structure and organisation
Despite their variety, cells do have certain common characteristics which enable them
to be recognised as cells.
The organisation in an individual cell as revealed by the electron microscope is known as
its ultra-structure.
Specific parts making up this organisation are called organelles (like organs to an
organism), each with a specific structure and function within the cell.
[Note that the cells conforming to this organisation are called eukaryotic cells, i.e. they
have distinct membranous organelles, including a membrane- bound nucleus. Those
without such structures are known as prokaryotes. Similarly, organisms made up of
eukaryotic cells are known as eukaryotes and those made up of prokaryotic cells are
prokaryotes.]
Organelles within the cytoplasm of eukaryotic cells are membranous structures, whose
membranes can be referred to as internal membranes.
Whilst the cell surface membrane (plasma membrane) acts as a boundary to the cell
itself, the internal membranes are also important as:
Cell structure
1. Plasma membrane
6. Ribosomes
11. Nuclear membrane
2. Cytoplasmic matrix
7. Lysosomes
12. Nucleus
3. Pinocytotic vesicle
8. Vacuoles
13. Nucleolus
4. Mitochondria
9. Golgi apparatus
5. Endoplasmic reticulum
10. Centrioles
The organelles
Mitochondria
A mitochondrium is the site of biochemical reactions that take place during the release
of energy from food (respiration). The energy is in a form (called ATP) which can be used
easily for all other cell functions. Each mitochondrium has a double membrane; a smooth
outer membrane, an inner membrane which is folded into extensions called cristae, with a
fluid filled inter-membrane space. The innermost part of the mitochondrium is called the
matrix, and it contains numerous chemical compounds, including enzymes needed for
aerobic respiration, ribosomes (see later notes) and several copies of its own genetic
material called mitochondrial DNA (mtDNA). The mtDNA allows the mitochondirum to
replicate during cell division as well as other times.
The reactions of respiration take place in the matrix and on the inner membrane, where
the cristae increase ther surface area available.
The numbers of mitochondria found in a cell can give us clues as to its function. Active
tissues tend to have a high number of mitochondria as well as lots of tightly packed
cristae.
The size of mitochondria can vary: cylinders of 10 μm by 0.2 μm, or spheres of 0.5 – 5 μm
diameter.
Endoplasmic reticulum (ER)
It spreads through the whole of the cytoplasm as a 3D network of parallel flattened
sacs, called cisternae, all the cavities are inter-linked and this is what is known as the ER.
It also links with the nuclear membrane and may link to the Golgi body. The outside of
the ER may be encrusted with ribosomes, in which case it is called the rough endoplasmic
reticulum (RER), otherwise it is known as the smooth edoplasmic reticulum (SER).
The RER’s membranes are lined with ribosomes and their function is to produce proteins,
the RER’s role in this is to isolate and transport the proteins once they have been made.
The RER has a very large surface area to giving space for the synthesis, and making
possible their storage and transport, within the cell and from the inside to the outside of
the cell. RER occurs extensively in protein-secreting cells, e.g. those lining the gut and in
hormone producing glands.
The SER have membranes lacking ribosomes and their function is to synthesise and
transport lipids and steroids. SER occurs extensively in the testes and liver.
Again, by examining the amount of rough and smooth ER in a cell, we can get an idea of its
functions.
Ribosomes
They are amongst the smallest structures found in the cytoplasm (20 – 25nm) and were
only discovered with the advent of the electron microscope. They are composed of
ribosomal RNA and protein and are the most numerous structures in the cell. They may be
bound to the ER in which case they synthesise extracellular proteins, or free where they
are thought to produce proteins required for internal cellular use. They are important on
protein synthesis where they move along mRNA in succession.
Golgi Body / apparatus.
This looks like a dense area of cytoplasm, which has only been observed clearly since the
advent of the electron microscope. It is similar in structure to ER but is more compact.
It is made up of stacks of parallel, flattened membrane pockets, which link but not join
with the ER. These sacs are continually being formed on one side (from vesicles off the
ER) and pinched off (to form vesicles containing secretions) on the other. These vesicles
may then go on to fuse with the cell surface membrane to release the secretions.
A variety of different processes are assoiated with this organelle:
 packaging and secretion of secretory enzymes
 secreting carbohydrates (for plant cell walls)
 producing glycoproteins (e.g. mucin – an important component of mucus)
 formation of lysosomes
Cells which are involved in the secretion of substances contain particularly large
amountsof Golgi apparatus.
Lysosome
Lysosomes contain and isolate digestive enzymes from the remainder of the cell. They
appear as dark, spherical bodies around 500nm in diameter, in the cytoplasm of most
cells, they frequently fuse with each other and with membrane bound vacuoles containing
either food or an obsolete organelle. The enzymes then break down the contents into
smaller molecules that can often be reused.
Lysosomes can also self-destruct, if an entire cell is damaged or running down the
lysosomes may rupture releasing their enzymes and destroying the whole of the cell.
Nucleus
It is the largest and most prominent organelle of the cell (it can be seen with a light
microscope), and its function is to control the cell’s activities and to retain the
chromosomes.
It is bound by a double membrane, called the nuclear membrane, which has a pore in it to
allow the transport of mRNA and nucleotides.
The cytoplasm-like material within the nucleus is called the nucleoplasm. It contains
chromatin, which is made up of coils of DNA bound to protein. During cell division the
chromatin condenses to form the chromosomes. Within the nucleus are one or two
spherical bodies, each called a nucleolus. They are extra dense areas of almost pure DNA.
These are thought to manufacture RNA, and assemble ribosomes.
Centrioles
Pairs of centrioles are found near the nucleus lying at right angles, they are hollow
cylinders (300-500nm long and 150nm diameter) made up of bundles of 9 tubules. They
arise in distinct region of the cytoplasm known as the centrosome. At cell division they
migrate to opposite poles of the cell where they synthesise the microtubules of the
spindle.
Chloroplasts
Make own notes on stucture and function of chloroplasts.
Vacuoles
A vacuole is a fluid-filled sac bound by a single membrane.
In animal cells vacuoles:
- are not a permanent feature, they tend to be formed and lost as
needed
- are small vesicles and may occur in large numbers
- may be formed during phagocytosis (they are formed around the pathogen)
- act as contractile vacuoles (they allow control of the water content of the
cytoplasm)
In plant cells:
- they are a permanent feature
- there is usually one large central vaculoe and the single membrane around it is
called a tonoplast
- they are filled with cell sap (a solution of various substances e.g. salts and sugars,
in water)
- vacuoles function as storage sites and provide an osmotic system which functions in
support of young tissues
- it is the solution in the cell sap which causes the movement of water into the cell
by osmosis, causing the cytoplasm to be pressed against the cell wall, keeping the
cells turgid (firm).
Cellulose cell wall
This is only found in plant cells. It consists of cellulose microfibrils embedded in a
polysaccharide matrix.
Its main functions are:
- to provide strength and support
- to permit the movement of water from cell to cell.
Plasmodesmata
The cell wall is interrupted at intervals by narrow pores carrying fine strands of
cytoplasm which join the living cells to one another. The plasmodesmata contain
endoplasmic reticulum which is therefore continuous from cell to cell and facilitate the
movement of materials between cells.