AQA Biology Essay A-Level
The functions of enzymes and their importance in organisms
Plan:
Phosphodiesterase in low-light visual phototransduction.
Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) in the flow of the Calvin cycle.
ATP hydrolase in facilitating reaction coupling.
Many enzymes working together in the prokaryotic replication of DNA.
One enzyme that is of considerable importance in low-light visual phototransduction in humans is
phosphodiesterase. For visual phototransduction in low-light situations, photoreceptor cells in the
human retina, known as rods, use a photosensitive pigment – rhodopsin. Rhodopsin is a complex
molecule formed by the reversible combination of scotopsin (a G protein-coupled receptor) and
retinal (an aldehyde derivative of Vitamin A). Retinal exists in two isomeric forms and, as scotopsin
absorbs a photon, 11-cis retinal undergoes an isomeric change to all-trans retinal. The
photoisomerisation changes the conformation of scotopsin, leading to the activation of the
transducin G-protein and the hydrolysis of cyclic guanosine monophosphate (cGMP) via the
activation of phosphodiesterase. As a result, the absorption of a single photon by scotopsin will
cause phosphodiesterase to hydrolyse ~4800 cGMP molecules (each photoisomerised rhodopsin
molecules activates 800 transducin molecules which can each activate one phosphodiesterase
molecule which, in turn, hydrolyse 6 cGMP molecules). cGMP nucleotides work to keep Na + and Ca2+
ion channels open and their absence causes closure of these channels, leading to hyperpolarisation
of the rod’s disk membrane. This process prevents action potentials being fired to the synaptic knobs
of the rods and subsequently reduces the release of glutamate across the synaptic cleft as there are
also no calcium ions available to facilitate movement of vesicles containing the neurotransmitter to
the pre-synaptic membrane. Glutamate is an inhibitory neurotransmitter for on-centre bipolar cells
and an excitatory inhibitor for off-centre bipolar cells. Thus, as a photon is absorbed via scotopsin,
on-centre bipolar cells can become depolarised, and the off-centre bipolar cells will hyperpolarise.
Without phosphodiesterase, no hydrolysis of cGMP would occur, leading to no reduction in
glutamate. Without a reduction in glutamate, the antagonistic polarisation phenomena in the
bipolar cells would not occur and spatial processing of visual input derived from the photoreceptor
cells would not be realised, as the relevant action potentials would not propagate to the visual
cortex through the optic nerve. If early humans were unable to spatially process information, they
would likely have succumbed to interspecific competition and, as they became overwhelmed by
The functions of enzymes and their importance in organisms
Plan:
Phosphodiesterase in low-light visual phototransduction.
Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) in the flow of the Calvin cycle.
ATP hydrolase in facilitating reaction coupling.
Many enzymes working together in the prokaryotic replication of DNA.
One enzyme that is of considerable importance in low-light visual phototransduction in humans is
phosphodiesterase. For visual phototransduction in low-light situations, photoreceptor cells in the
human retina, known as rods, use a photosensitive pigment – rhodopsin. Rhodopsin is a complex
molecule formed by the reversible combination of scotopsin (a G protein-coupled receptor) and
retinal (an aldehyde derivative of Vitamin A). Retinal exists in two isomeric forms and, as scotopsin
absorbs a photon, 11-cis retinal undergoes an isomeric change to all-trans retinal. The
photoisomerisation changes the conformation of scotopsin, leading to the activation of the
transducin G-protein and the hydrolysis of cyclic guanosine monophosphate (cGMP) via the
activation of phosphodiesterase. As a result, the absorption of a single photon by scotopsin will
cause phosphodiesterase to hydrolyse ~4800 cGMP molecules (each photoisomerised rhodopsin
molecules activates 800 transducin molecules which can each activate one phosphodiesterase
molecule which, in turn, hydrolyse 6 cGMP molecules). cGMP nucleotides work to keep Na + and Ca2+
ion channels open and their absence causes closure of these channels, leading to hyperpolarisation
of the rod’s disk membrane. This process prevents action potentials being fired to the synaptic knobs
of the rods and subsequently reduces the release of glutamate across the synaptic cleft as there are
also no calcium ions available to facilitate movement of vesicles containing the neurotransmitter to
the pre-synaptic membrane. Glutamate is an inhibitory neurotransmitter for on-centre bipolar cells
and an excitatory inhibitor for off-centre bipolar cells. Thus, as a photon is absorbed via scotopsin,
on-centre bipolar cells can become depolarised, and the off-centre bipolar cells will hyperpolarise.
Without phosphodiesterase, no hydrolysis of cGMP would occur, leading to no reduction in
glutamate. Without a reduction in glutamate, the antagonistic polarisation phenomena in the
bipolar cells would not occur and spatial processing of visual input derived from the photoreceptor
cells would not be realised, as the relevant action potentials would not propagate to the visual
cortex through the optic nerve. If early humans were unable to spatially process information, they
would likely have succumbed to interspecific competition and, as they became overwhelmed by
