IB Biology SLTopic 2 — Cell Membranes & TransportPaper 1 & 2Structure and function~10 min read
Membrane Proteins
The phospholipid bilayer is a good wall, but a wall with no doors, no doorbell and no name plate is not much use to a cell. Everything the membrane does beyond simply blocking things is done by proteins stuck into it — and where a protein sits depends entirely on which parts of it are hydrophobic.
📘 What you need to know
The bilayer provides the barrier. Extra functions are carried out by proteins in the membrane.
Integral proteins are amphipathic (partly hydrophobic) and are embedded in the bilayer, spanning both layers or just one.
Peripheral proteins are hydrophilic and are attached to the surface of integral proteins, or to the membrane by a hydrocarbon chain. They can be inside or outside the cell.
Protein content varies with the job of the cell. Mitochondrial and chloroplast membranes have the highest, because of their many electron carriers.
Transport proteins: channel proteins form pores; carrier proteins change shape to move a substance across.
Each transport protein is specific to one ion or molecule, so the cell controls what enters and leaves.
Other jobs: receptors, immobilised enzymes, cell adhesion and cell-to-cell recognition.
Integral or peripheral
The rule is simple. A protein sits where its own surface chemistry lets it sit.
Integral proteins have a band of hydrophobic amino acids around their middle. That band is comfortable among the fatty acid tails, so the protein sits inside the bilayer. Because their ends are still hydrophilic and stick out into the water, integral proteins are amphipathic — just like the phospholipids around them.
Peripheral proteins are hydrophilic all over. They cannot enter the hydrophobic core at all, so they stay on the surface, attached to an integral protein or anchored by a hydrocarbon chain.
Notice that an integral protein is amphipathic for exactly the same reason a phospholipid is. The two fit together because they are built on the same principle.
Protein content is not fixed. A membrane that mainly separates things needs few proteins. The inner membrane of a mitochondrion is stuffed with them, because it is packed with electron carriers doing respiration. If a question tells you what a membrane does, you can predict how protein-rich it will be.
The jobs proteins do
The pump and the channel both move ions, but only one of them can push against a concentration gradient. That difference is the whole of the next page.
Transport: channels and carriers
Channel proteins are pores. They let charged substances such as ions pass through the membrane, and some are gated — part of the protein on the inside surface can move to open or close the pore, so the cell controls the flow of ions.
Carrier proteins have a binding site. The substance attaches, the protein switches shape, and the binding site now opens onto the other side of the membrane. Pumps and electron carriers work this way.
A channel is a doorway; a carrier is a revolving door. Both let things through, but only the revolving door has to physically move — which is why carrier proteins, not channels, are the ones used for active transport.
Function
What the protein does
Example
Transport — channel
Forms a pore that specific ions pass through; some are gated
Sodium ion channel; voltage-gated potassium channel
Transport — carrier
Binds a substance and changes shape to move it across
Sodium–potassium pump; electron carriers such as cytochrome
Receptor
Binds a signalling molecule, which triggers reactions inside the cell
Insulin receptor; neurotransmitter receptors
Immobilised enzyme
Integral protein with its active site exposed on the membrane surface
Maltase in the small intestine lining
Cell adhesion
Attaches a cell to neighbouring cells so tissues hold together
Adhesion proteins between cells in a tissue
Cell-to-cell recognition
Acts as a marker or antigen so other cells can identify it
Glycoprotein antigens, including the ABO blood group markers
Anchoring
Holds the membrane to structures inside or outside the cell
Anchor proteins linked to the extracellular matrix
🤔 Why specificity is the point
Each transport protein only handles one type of ion or molecule, because the binding site or pore has a shape and charge that suit that one substance. That sounds like a limitation, but it is what gives the cell control. If the cell wants more glucose in, it puts more glucose transporters in the membrane. Nothing else comes in with it.
Worked examples
WORKED EXAMPLE
Explain why integral proteins are described as amphipathic. [3]
Point 1
They are embedded in the bilayer, so part of the protein is surrounded by hydrophobic fatty acid tails.
Point 2
That region has hydrophobic amino acid R groups on its surface, so it is stable there.
Point 3
The parts sticking out into the cytoplasm or extracellular fluid are hydrophilic, so the protein has both types of region.
Hydrophobic middle, hydrophilic endsLink it back to R groups if you can — it shows you understand why, not just that.
WORKED EXAMPLE
The inner membrane of a mitochondrion has a much higher protein content than the membrane of a lysosome. Suggest why. [2]
Point 1
The inner mitochondrial membrane carries out aerobic respiration, so it contains many electron carriers and pumps.
Point 2
A lysosome membrane mainly acts as a barrier holding enzymes in, so it needs far fewer proteins.
Protein content matches the number of jobs the membrane has“Suggest” means apply the idea to an unfamiliar case. The reasoning is the mark.
WORKED EXAMPLE
A membrane protein is hydrophilic across its whole surface and is found attached to the inner face of the membrane. Identify the type of protein and explain your answer. [2]
Step 1: use the surface chemistry
No hydrophobic region means it cannot sit among the fatty acid tails.
Step 2: name it
It must stay at the surface, attached to an integral protein or anchored by a hydrocarbon chain.
A peripheral proteinThe location is a clue, but the hydrophilic surface is the actual reason.
💡 Exam tip
Integral = embedded and amphipathic. Peripheral = attached to the surface and hydrophilic. Learn the pair together.
Channel proteins have a fixed shape; carrier proteins change shape. That single difference answers a lot of questions.
Say transport proteins are specific to one ion or molecule — it is often a separate mark.
Give a named example whenever you name a function. “Receptor, e.g. the insulin receptor” beats “receptor”.
An immobilised enzyme is an integral protein with its active site exposed on the surface.
Integral proteins can span both layers or just one. Diagrams often show both, so do not assume they always go all the way through.
⚠ Common mix-up
Saying integral proteins are hydrophobic. They are partly hydrophobic, which is what amphipathic means.
Swapping channel and carrier. Pore that stays open or gated = channel. Binds and changes shape = carrier.
Thinking peripheral proteins are only found inside the cell. They occur on both faces.
Assuming all membranes have the same proteins. Content depends on the job of that particular membrane.
Confusing receptors with transport proteins. A receptor binds a signal; it does not carry the molecule across.
Writing that channel proteins use ATP. They do not. Movement through a channel is passive.
Up next: Membrane Transport — diffusion, osmosis, facilitated diffusion and active transport, and how to tell instantly which one a question is describing.
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