Generalized Animal Cell
http://waynesword.palomar.edu/lmexer1a.htm#e
The major cellular organelles
The Nucleus
As we have seen, the major difference between prokaryotic and eukaryotic
cells is the presence, in the latter, of a nucleus. The nucleus is also the most visible
organelle in the eukaryotic cell. The nucleus is involved in the control of the cell and in
reproduction. The nucleus is encapsulated in a double membrane called the nuclear
envelope. This double membrane is not continuous but is perforated by a number of
pores. These pores allow the, controlled, transfer of substances into and out of the
nucleus. Messenger RNA (mRNA), as we shall see later, is generated in the nucleus from
where it passes out into the cytoplasm and where, subsequently, it controls the synthesis
of proteins.
Perhaps most significantly, the nucleus contains DNA which carries the
genetic "code" for the individual organism. We shall discuss the structure and role of
DNA later but, for now, we can visualise DNA as a large molecule consisting of two
linked chains. The chains are comprised of four types of nucleotide and it is these,
relatively simply units, that determine our genetic make up. Usually the cell's DNA is
dispersed throughout the nucleus but, during cell division, the DNA, with its associated
proteins is clearly seen as chromosomes. Each organism has a specific number of
chromosomes (although mutations can occur which affect this), humans, for example,
have 23 pairs of chromosomes (gamates, the sex cells, eggs or sperm, contain 23
chromosomes).
Ribosomes
The ribosomes are responsible for the production of proteins and are
manufactured as sub units within the nucleus before emerging into the cytoplasm. These
sub units come together and form either free or bound ribosomes. The free ribosomes
work within the cytoplasm and generate proteins that are used within the cell itself.
Bound ribosomes are attached, at least temporarily, to the endoplasmic reticulum and
produce proteins that are either used by specific organelles or are transported out of the
cell. Some specialist cells, such as those of the pancreas, have developed to secrete
digestive enzymes and such cells have high numbers of bound ribosomes. There appears
to be no difference between free and bound ribosomes and interchange between the two
is possible.
The Endoplasmic Reticulum
On the cellular scale, the endoplasmic recticulum (ER) is
huge, accounting for about half the total membrane area of the cell. The ER is continuous
with the outer of the two membranes of the nuclear envelope and so only a single
membrane separates it from the nuclear contents. There are two type of ER, the rough
endoplasmic reticulum and the smooth endoplasmic reticulum.
The rough endoplasmic reticulum (RER)
The rough endoplasmic reticulum gets its
name from its grainy appearance. This apparent roughness is cause by the presence of
ribosomes which are attached to the outer surface. These ribosomes manufacture proteins
which are destined for use outside the cell, hence RER is found extensively in those cells
specialised in producing proteins (such as enzymes and hormones) for extra-cellular use.
The smooth endoplasmic reticulum (SER)
The smooth endoplasmic reticulum lacks
the ribosomes of the RER and is involved in the production, metabolism, storage and
distribution of non-protein compounds such as lipids, carbohydrates and steroids. Cells
involved in such activities are rich in SER. Liver cells containing SER are involved in the
breakdown of glycogen resulting in the release of glucose, an important mechanism
controlling the level of sugar in the bloodstream. A further function of the SER, also seen
in the liver, is the detoxification of drugs and poisons. The functions of the liver are
reviewed later in this course.
The Golgi Apparatus (GA)
The Golgi Apparatus was given its intriguing name after
its discoverer, Camillo Golgi (1898), and is an organelle which remained mysterious for
many years after its discovery. The Golgi Apparatus receives the products of the ER
(notice their proximity to one another) and prepare them for use. The products of the ER
are received by one side of the Golgi Apparatus (the cis side) and are then modified.
From here they products are progressively transferred to the other side (the trans side)
where they are "packaged" and sent on their way. Enzymes within the Golgi Apparatus
modify the proteins they receive from the ER and then each is packaged in its own
vesicle for distribution. Some substances are stored, others are sent on their way out of
the cell via the plasma membrane.
In addition to this modify and distribution role the
GA also synthesizes molecules of its own such as polysaccharides.
Lysosomes
Lysosomes are the Terminators of the cellular world. Lysosomes are sacs of
powerful enzymes that can hydrolyse (or destroy) all manner of substances including
proteins, fats, polysaccharides and nucleic acids. The lysosomes are tough guys, they can
destroy a whole host of molecules and are clean up the merchants of cells. Fortunately
their activity is pH restricted and they operate best in slightly acidic conditions and so
they only work within their own environment (if there was a leak from the lysosome into
the cytoplasm, the enzymes would not function very well). Lysosomes can help "clean
up" a cell by digesting a damaged organelle or can eliminate whole cells by a similar
process. In single cell organisms, the lysosomes are essential for feeding as digestive
vacuoles encase the nutrient and these then fuse with lysosomes to complete the
digestion. Human white blood cells operate in a similar way to destroy infectious agents.
Lysosomes maybe the tough guys of the cell but without them we would suffer, indeed
there are some fatal human genetic disorders where lysosome activity is missing or
altered in some ways.
Vacuoles
The term vacuole refers to membrane lined sacs within the cell. The vacuoles
perform a variety of functions including, as we have just seen, digestion. In plant cells the
vacuoles can be very large and can occupy much of the cell. These vacuoles serve to hold
water (maintaining cell rigidity), to store nutrients, to hold products of metabolism and to
aid rapid growth. In petals the vacuoles contain pigments that give the flower its colour.
In aquatic protists, vacuoles are used to collect and eject excess water that enters the cell
under osmotic pressure.
Mitochondria
The mitochondria (and, as we shall see, the chloroplasts) are organelles
that provide the basis for the endosymbiotic theory of evolution of eukaryote cells. The
mitochondria are the power stations of the cell, converting organic compounds and
oxygen into energy. We need not worry at this stage about the intricacies of this process,
suffice to say that mitochondria use enzymes to generate ATP which is the energy store
of the cell. One of the fascinating aspects of mitochondria (and chloroplasts) is that they
contain DNA, not only that, but they also grow and reproduce independently within the
cell. This has led many to believe that both mitochondria and chloroplasts were originally
separate prokaryotes that were taken into the early eukaryotic cell, perhaps as
photosynthetic cyanobacteria. Once inside the cell a symbiotic relationship developed
and we now have the situation where both "host" (the original eukaryotic cell) and
"invader" (the cyanobacteria or equivalent) are inseparable.
Chloroplast
Chloroplasts are found in plant and algae cells and are involved in
photosynthesis. In some ways, the chloroplast do the opposite of mitochondria, in that the
convert energy (from light) into organic compounds. The chloroplasts contain pigments,
chlorophyll and carotenoids which absorb light. The chlorophyll is responsible for the
green colour of plants and it is this which uses light into produce carbohydrates.
That concludes our brief tour of the structure of the cell. We have seen that cells are the
building blocks of life and, although small, are incredibly intricate. We have seen that the
cells themselves contain minute organelles that are highly complex and perform a myriad
of functions. We have seen that cells can be highly specialized, adopting shapes and
varying cellular composition to suit their role. We have not had time to examine in depth
the workings of individual organelles but, hopefully, you will now appreciate some of the
complexities of that most simple entity, the cell.