G proteins at Higher Level: An AHL statement: the receptors that work through G proteins.
Practise this as you read
- Outline how a G protein-coupled receptor passes a signal on.
- State the role of GTP.
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The big idea: A G protein-coupled receptor crosses the membrane. When its ligand binds, it activates a G protein on the inside: the G protein releases GDP and binds GTP. The active G protein then switches on an enzyme or channel, and switches itself off by breaking GTP down to GDP.
The points to remember
- The receptor crosses the membrane; the G protein sits on the inside.
- Inactive G protein: GDP bound.
- Ligand binds, receptor changes shape, G protein swaps GDP for GTP: active.
- The active G protein switches on an enzyme (or channel).
- GTP is broken down to GDP: the G protein switches off.
Remember it as: GDP off, GTP on.
Real examples. Humans have hundreds of different G protein-coupled receptors. Receptors for adrenaline, for many smells in the nose and for light in the eye (rhodopsin in rod cells) are all of this kind.
Trap: The G protein is not the receptor: it is a separate protein inside the cell that the receptor activates.
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How it is asked: Expect an MCQ on the GDP and GTP swap, or an Outline question on how a G protein-coupled receptor passes the signal on.
IB-style questionOutline[4 marks]
Outline how a G protein-coupled receptor passes a signal into a cell.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.