Описание
contribute to the cellular membranes, and also
the production of steroids. Smooth ER is associated with detoxifying reactions, particularly in
the liver, and is also the site from which components are transported to other organelles by
vesicular transport.
The proportions of rough and smooth ER may
reflect the specialisation of the cell, e.g. in the
adrenal cortex, which is responsible for the
production of steroid hormones, the smooth ER
accounts for most of the cell volume.
The Golgi apparatus
The Golgi apparatus modifies many cellular
proteins, through the addition of carbohydrates.
It resembles a series of sac-like structures –
cisternae – which receive proteins from the
rough ER by vesicular transport. As proteins are
transported between the different cisternae, they
receive carbohydrate modifications; transport may
be forwards or backwards between compartments,
before the proteins are packaged into vesicles for
secretion or transport to specific organelles. This
process, trafficking, relies on the presence of
specific signals in the proteins themselves, which
target them to their final destination.
Mitochondria
Mitochondria synthesise ATP and other phosphate
compounds that power cellular reactions
through the catabolism of a variety of metabolic
compounds.
Structure
Mitochondria are double-membrane organelles.
The outer membrane is smooth whereas the inner
membrane possesses a series of projections –
cristae – that impinge on the interior of the mitochondria, known as the matrix.
Mitochondria contain a primitive genome that
encodes some of the proteins not found in the
nuclear genome that are necessary for their function. This, and their replication independently of
the nucleus of the cell, may reflect their endosymbiotic origin. However, many of the proteins that
mitochondria require are solely encoded in the
main genome in the nucleus of the cell.
Unlike the main human genome, the mitochondrial genome is always inherited entirely from the
mother; mitochondria in sperm do not contribute
to the zygote.
DEFINITION Endosymbiotic theory of
the origin of mitochondria
The mitochondrial genome has been sequenced
and has many features consistent with a bacterial
genome. It has been suggested that, at some
point in the past, a bacterium became engulfed by
a primitive cell and remained within it, resulting in
a symbiosis between the two organisms. It is
thought that this symbiotic bacterium evolved into
the mitochondria of today.
Function
Mitochondria meet the energy needs of the cell.
In the matrix the TCA (tricarboxylic acid) cycle
occurs, which supplies the electron transport
chain with the reduced co-factors necessary
to generate ATP, the main energy currency of
the cell.
The number of mitochondria present in the cell
reflects its energy need; muscle cells and neurons
have high energy demands and frequently have
many mitochondria, whereas cells with lower energy requirements may possess very few.
CLINICAL Myoclonic epilepsy with
ragged red fibres
Myoclonic epilepsy with ragged red fibres (MERRF)
is a rare condition characterised by myoclonus,
epilepsy and ataxia, usually manifesting in the teens
or student age. Further symptoms include muscle
weakness, deafness, dementia and seizure. The
cause has been traced to a decrease in the function
of the electron transport chain resulting from
mutation in a mitochondrially-encoded protein. As
such, the condition is maternally inherited, although
the mother may express a weaker phenotype than
her child.
Ribosomes
Ribosomes are minute organelles, around 20 nm
in diameter, that synthesise amino acids. They
are found free floating in the cytosol, or on
the rough ER, where the proteins produced are
destined for secretion or transport to another
organelle.
Cell biology 3
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