Membrane proteins at Higher Level: The same statement as SL. At HL it links back to protein structure: the non-polar amino acids of an integral protein sit in the hydrophobic core, and forward to the channels, pumps and adhesion molecules of this topic.
Practise this as you read
- Identify integral and peripheral proteins on a membrane diagram.
- Explain what holds an integral protein in the membrane.
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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: cytochrome c: Cytochrome c is a small peripheral protein on the outer face of the inner mitochondrial membrane. It carries electrons between two large integral protein complexes.
Most transport proteins and receptors are integral, because they must reach both sides of the membrane.
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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