The phospholipid bilayer is an excellent wall, and a wall on its own is useless. Everything a membrane actually does — letting glucose in, pumping sodium out, hearing a hormone, sticking to the cell next door — is done by a protein sitting in it.
📚 What you need to know
Membrane proteins fall into two groups: integral and peripheral.
Integral proteins are embedded in the bilayer. They are partly hydrophobic, so they are amphipathic like the phospholipids themselves.
Integral proteins may cross both layers (transmembrane) or sit in just one.
Peripheral proteins are hydrophilic and sit on the surface, attached to integral proteins or to the phosphate heads.
Membrane proteins carry out transport, reception, catalysis, adhesion and cell-to-cell recognition.
Channel proteins form a pore; carrier proteins change shape to move a substance across.
Every transport protein is specific to one ion or molecule, which is how the cell controls what enters and leaves.
The protein content of a membrane varies with its job — mitochondrial and chloroplast membranes have the most, because of their electron carriers.
Integral or peripheral?
The difference comes straight from the last page. The middle of the bilayer is hydrophobic, so any protein that sits inside it must have hydrophobic regions of its own — hydrophobic R groups on the outside of that part of the protein, facing the tails.
Proteins that have those hydrophobic regions can embed themselves in the membrane: these are integral proteins. Proteins that are hydrophilic all over cannot, so they stay on the surface: these are peripheral proteins.
The channel on the left is transmembrane; the block beside it is integral but only reaches into one layer. Both are integral.
This is your protein structure knowledge paying off. A transmembrane protein has a band of hydrophobic R groups around its middle, so it is stable sitting in the tails, and hydrophilic R groups at both ends, so it is stable sticking out into the water. The membrane does not choose where proteins go — the amino acid sequence does.
What membrane proteins do
Six jobs come up again and again. You are not expected to memorise a list of protein names, but you should be able to name a job and give an example.
Notice how many of these you have met before. Membrane proteins are where several topics meet.
Channels and carriers
Transport is the job you will be asked about most, so it is worth separating the two protein types properly. Both are integral, both are specific, and they work in completely different ways.
Feature
Channel protein
Carrier protein
Shape
Fixed — a permanent pore through the protein
Switches between two shapes
How it works
The substance passes through the hole
The substance binds, the protein changes shape, the substance is released on the other side
Typical cargo
Ions and water
Larger polar molecules such as glucose and amino acids
Can it be gated?
Yes — part of the protein can swing across to close the pore
Not gated, but only works when the correct substance binds
Used in
Facilitated diffusion only
Facilitated diffusion and active transport
Gated channels are worth a second look. Part of the protein on the inner surface of the membrane can move to open or close the pore, so the cell is not stuck with a permanent hole. This is how nerve cells control exactly when sodium ions are allowed to rush in, which is what makes a nerve impulse possible.
Specificity, again. A transport protein binds its substance the same way an enzyme binds its substrate: the binding site has a shape and a set of R groups that only fit one thing. Same principle, different job.
Not every membrane has the same proteins
The protein content of a membrane matches what that membrane is for. A membrane that only needs to be a barrier has few proteins; a membrane that has to do chemistry is packed with them.
Inner mitochondrial membrane and the thylakoid membranes of chloroplasts have the highest protein content of any membrane, because they are crowded with electron carriers.
Myelin, which insulates nerve axons, has very little protein — it is there to block current, not to move things.
The plasma membrane of a typical cell sits in between, with a broad mixture of transport, receptor and recognition proteins.
If you are ever asked to compare two membranes, do not just say “one has more protein”. Say why: the inner mitochondrial membrane carries out oxidative phosphorylation, which needs electron carriers and ATP synthase, and all of those are proteins.
Worked examples
WE 1
Distinguishing the two types
Distinguish between integral and peripheral membrane proteins. (3 marks)
Point 1: position
Integral proteins are embedded in the phospholipid bilayer, whereas peripheral proteins sit on its surface.
Point 2: chemistry
Integral proteins are amphipathic, with hydrophobic regions that hold them in the core; peripheral proteins are hydrophilic.
Point 3: attachment
Peripheral proteins are attached to integral proteins or to the phosphate heads, rather than to the tails.
integral = inside the bilayer; peripheral = on the surface“distinguish” needs both sides of each point — use “whereas” to force yourself to do it
WE 2
Explaining a position in the membrane
Explain why a transmembrane protein has hydrophobic amino acids in the middle of its chain but hydrophilic ones at each end. (3 marks)
Point 1: the middle
The middle of the protein sits in the hydrophobic core of the bilayer, among the fatty acid tails.
Point 2: why that matters
Hydrophobic R groups there are stable next to the non-polar tails, which anchors the protein in place.
Point 3: the ends
Both ends stick out into watery surroundings — the cytoplasm and the extracellular fluid — so hydrophilic R groups are stable there.
the R groups match whatever surrounds each part of the proteinthis question is really testing tertiary structure; say “R groups” and you are on the right track
WE 3
Interpreting a difference between membranes
A student finds that the inner membrane of a mitochondrion contains about 75% protein by mass, whereas the myelin sheath around an axon contains about 20%. Suggest a reason for this difference. (2 marks)
Step 1: what the mitochondrion does
The inner membrane carries out the electron transport chain, which needs many electron carrier proteins and ATP synthase.
Step 2: what myelin does
Myelin is an insulator. Its job is to be a barrier, and a barrier is made of lipid, not protein.
protein content matches the function of the membranewhenever you see a percentage comparison, answer in terms of function, not just numbers
💡 Exam tips
Say embedded in for integral and attached to the surface of for peripheral. Vague positions do not score.
Not all integral proteins go all the way through — only transmembrane ones do.
Explain protein position using hydrophobic and hydrophilic R groups. That is the mark-scheme answer.
Channel = fixed pore; carrier = changes shape. Learn that one line and you can answer most transport questions.
Carriers are used in both facilitated diffusion and active transport. Channels are not used in active transport.
Have one example ready for each function — a named example often carries a mark of its own.
⚠ Common mistakes
Thinking integral means “goes all the way through”. It means embedded; some integral proteins sit in one layer only.
Saying channel proteins change shape. That is a carrier. Channels have a fixed pore, although gated ones can open and close.
Describing proteins as “floating on top” of the membrane. Only peripheral proteins are on the surface.
Forgetting specificity. Each transport protein handles one substance; that is what gives the cell control.
Assuming every membrane is identical. Protein content varies enormously with function.
Mixing up receptors and channels. A receptor binds a signal; a channel lets something through. Some proteins do both, but say so deliberately.
Up next: Membrane Transport — we have the wall and we have the doors. Now let us watch things actually go through.
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