The big idea: Proteins sit in or on the bilayer. Integral proteins are embedded in it; peripheral proteins are attached to one of its surfaces.
The points to remember
- Integral proteins are embedded in one or both lipid layers.
- Some integral proteins pass right through both layers: they are transmembrane proteins.
- Integral proteins have hydrophobic parts where they touch the core, which hold them in place.
- Peripheral proteins are attached to one surface, outer or inner; they do not enter the core.
- Peripheral proteins are often attached to integral proteins or to the phosphate heads.
Remember it as: Integral: in it. Peripheral: on it.
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The big idea: Membrane proteins are very diverse: different shapes, different places in the membrane, different jobs.
Moving things across
- Channel proteins: a pore for ions or water
- Pump proteins: use ATP
- Carrier proteins: change shape
Signals and recognition
- Receptors bind hormones such as insulin
- Glycoproteins let cells recognise each other
Other jobs
- Enzymes, such as those on gut cells that digest sugars
- Adhesion: holding cells together
Real example: the insulin receptor: The insulin receptor is an integral protein that passes right through the plasma membrane: insulin binds on the outside and the signal is passed inside.
Most transport proteins and receptors are integral, because they must reach both sides of the membrane.
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How it is asked: Paper 1A shows a drawn membrane with two structures lettered and asks what they are. Paper 2 can ask you to distinguish the two kinds of protein.
A membrane diagram shows X: a protein with a branched chain on the outside, passing through both layers; and Y: a protein resting on the inner surface. What are X and Y?
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And a two-mark Distinguish:
Distinguish between integral and peripheral proteins.
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See the answer broken into clear points, with guidance on how each point earns marks.