eukaryotic cells

Structure to function in eukaryotic cells
The cell surface membrane and the membranes which form organelles in
eukaryotic cells all have the same basic structure, known as the fluid
mosaic model. Such membranes provide control of the entry and exit of
substances into cells and organelles and such control is a result of the
phospholipid bilayer and membrane proteins.
In eukaryotic cells, such membranes divide the cytoplasm into multiple
compartments (organelles). Organelles allow different functions to occur
efficiently and simultaneously in different parts of the cell. For example,
the outer double membrane of the mitochondrion separates out those
reactions which occur in mitochondria from those in the general cytoplasm.
Furthermore, the internal membranes of the mitochondria allow the enzymic
reactions of the Kreb’s cycle to be kept quite separate from the electron
transfer chain reactions (ETC). This is essential since both sets of reactions
have different enzymes, hence different pH optima. By splitting up the
cytoplasm of the mitochondria, the membranes which form the crista allow
enzymes and substrates to be concentrated and pH to be optimised.
Membranes can therefore be said to compartmentalise the interior of
eukaryotic cells.
The relationship between structure and function can be described in terms
of the whole cell or in terms of the individual organelles of the eukaryotic
cell.
Table 1 describes the structure and function of a motor neuron cell which is
commonly featured in examination questions. The structure and function
of eukaryotic organelles is described overleaf.
Cell Structure Function
Motor Neuron
Long axons
Axon contains axoplasm
For rapid transmission of nerve impulse. Synapses, where
two nerves join, is the slowest part of transmission, so
the longer the axon, the fewer the synapses and the faster
the impulses transmitted.
Allows transport between cell body and axon
Nodes of Ranvier between
Schwann cells
High phospholipid content in
membrane of Schwann cell
Allows Na+ /K+ pump to operate which sets up resting
potential. Schwann cells of myelin sheath speed up the
impulse because they increase the surface area for
transmission of current.
Provides electrical insulation.
Synaptic knob at end of
dendrite contains:
(i) many mitochondria
(ii) numerous vesicles
To provide ATP for active refilling of synaptic vesicles.
For modification and release of chemical transmitters
across the synapse.
Many dendrites To allow communication with other neurons.
Cell body contains:
(i) Nucleus
(ii) dense groups of ribosomes
and endoplasmic reticulum
called Nissl granules
Provides the genetic code for the production of
neurotransmitter substances, e.g. acetylcholine and
enzymes, eg. cholinesterase.
For production and transport of proteins and
neurotransmitters.
axon may be a metre or more in length
nodes of
Ranvier
axon
direction
of nerve
impulse
Table 1.
Exam Hint - Structure to function questions are very commonly set
on all A level Biology syllabuses. They are one of the syllabus areas
where all candidates should be capable of gaining the highest marks.
Once the functions of organelles have been memorised, candidates
should become confident at interpreting the function of unknown cells.
Nissl
granules
nucleus
dendrites
Structure to function…