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emmz
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im doin the aqa a2 biology exam on thrsday but i just cant see any logic to the following:

rod cells contain a pigment called rhodopsin and are sensitive to the intensity of light. they respond by the following reaction

light
rhodopsin - opsin + retinal

opsin open ion channels in the membrane which results in an action potential so u can see (vision).

in bright light though the rod cells cannot function coz rhodopsin is constantly being broken down.

wouldnt this mean though that if it is constantly broken down then opsin will keep making an action potential therefore enabling good vision frm the rod cells?
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hornblower
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(Original post by emmz)
im doin the aqa a2 biology exam on thrsday but i just cant see any logic to the following:

rod cells contain a pigment called rhodopsin and are sensitive to the intensity of light. they respond by the following reaction

light
rhodopsin - opsin + retinal

opsin open ion channels in the membrane which results in an action potential so u can see (vision).

in bright light though the rod cells cannot function coz rhodopsin is constantly being broken down.

wouldnt this mean though that if it is constantly broken down then opsin will keep making an action potential therefore enabling good vision frm the rod cells?
Erm, a few questions:

Are you doing AQA Biology specification A?
Is this from a previous paper?
If it is, then you can download the marking scheme from the AQA web site.

I'm doing this exam as well - started revision today. Good luck anyway.

J.
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Ben.S.
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(Original post by emmz)
im doin the aqa a2 biology exam on thrsday but i just cant see any logic to the following:

rod cells contain a pigment called rhodopsin and are sensitive to the intensity of light. they respond by the following reaction

light
rhodopsin - opsin + retinal

opsin open ion channels in the membrane which results in an action potential so u can see (vision).

in bright light though the rod cells cannot function coz rhodopsin is constantly being broken down.

wouldnt this mean though that if it is constantly broken down then opsin will keep making an action potential therefore enabling good vision frm the rod cells?
In the dark, a so-called 'dark current' flows through vertebrate photoreceptors - carried iinwards by sodium and calcium ions, through nucleotide-gated channels in the cell membranes, and outwards by potassium ions. This makes photoreceptors less polarised than normal cells as fewer sodium ions are 'kept out'. These channels are called 'nucleotide gated' because they are kept open by the presence of the nucleotide, cyclic GMP - or, cGMP. Light causes the isomerisation of 11-cis retinal to all trans-retinal, which starts a series of reactions known as a G-protein cascade. The end product of this cascade is an enzyme which removes these nucleotides from the sodium and calcum channels, causing the channels to close and the inward dark current (carried by sodium and calcium ions) to stop flowing. This causes the photoreceptor to HYPERPOLARISE, not depolarise in response to light. It's sort of the opposite of what normally happens - channels open, current flows inward, cell depolarises. In this case it's - light causes channels to close, inward current stops, cell hyperpolarises.

Ben
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waighty
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(Original post by emmz)
im doin the aqa a2 biology exam on thrsday but i just cant see any logic to the following:

rod cells contain a pigment called rhodopsin and are sensitive to the intensity of light. they respond by the following reaction

light
rhodopsin - opsin + retinal

opsin open ion channels in the membrane which results in an action potential so u can see (vision).

in bright light though the rod cells cannot function coz rhodopsin is constantly being broken down.

wouldnt this mean though that if it is constantly broken down then opsin will keep making an action potential therefore enabling good vision frm the rod cells?
In bright light, all of the rhodopsin is broken down into opsin and retinal. It is not that rod cells cannot function in bright light; it is that cone cells "take over" as they have better acuity so we can see in colour. Cones cannot operate in the dark so only rods work and these have poor acuity so can only see in black and white, hence this is why we can't see colour in the dark.
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