The big idea: A chain folds so that each R-group ends up where it is most stable: non-polar ones away from water, polar ones in contact with it.
The points to remember
- In a soluble globular protein, the chain folds so non-polar R-groups cluster in the core.
- Polar R-groups end up on the surface, where they form hydrogen bonds with water.
- That is why such proteins dissolve in water: in blood plasma or in the cytoplasm.
- The hydrophobic core also helps hold the fold together.
Remember it as: Oil in the middle, water-lovers on the skin.
Real example: myoglobin: Myoglobin, the oxygen store in muscle, has almost all its non-polar R-groups on the inside and its polar R-groups on the outside, so it dissolves in the watery cytoplasm of muscle cells.
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The big idea: A membrane is the opposite of water inside: its middle is hydrophobic. Proteins that sit in it have a matching hydrophobic belt.
The points to remember
- An integral protein is embedded in a membrane, crossing the phospholipid bilayer.
- The part inside the membrane has non-polar R-groups, in contact with the hydrophobic fatty acid tails.
- The parts outside have polar R-groups, in contact with water on each side.
- So hydrophobic regions hold the protein in the membrane, the way the core holds a soluble protein together.
Remember it as: A non-polar belt for the tails, polar ends for the water.
Soluble globular protein
- Non-polar R-groups in the core
- Polar R-groups on the surface
- Dissolves in water
Integral membrane protein
- Non-polar R-groups where it crosses the membrane
- Polar R-groups at the ends
- Stays embedded in the bilayer
Real example: aquaporins: Aquaporins, the water channels in kidney cells, cross the membrane several times. Each crossing has a band of non-polar R-groups that sits among the fatty acid tails.
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How it is asked: Paper 1A gives a protein whose polar and non-polar R-groups are placed, and asks what can be deduced. Paper 2 can ask how integral proteins stay in the membrane.
In a globular protein, nearly all the non-polar R-groups point inwards and the polar R-groups are on the outside. What can be deduced?
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And a three-mark Explain on integral proteins:
Explain how the distribution of amino acids allows an integral protein to stay embedded in a membrane.
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See the answer broken into clear points, with guidance on how each point earns marks.