How do membrane proteins move substances across the cell membrane and generate a membrane potential?
Membrane proteins: the phospholipid bilayer and fluid mosaic model, integral and peripheral proteins, transport by channels, carriers and pumps, the sodium-potassium pump, and the generation of the resting membrane potential.
An SQA Advanced Higher Biology answer on membrane proteins, covering the phospholipid bilayer and fluid mosaic model, integral and peripheral proteins, transport by channel and carrier proteins, active transport and the sodium-potassium pump, ligand-gated and voltage-gated ion channels, and the resting membrane potential.
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What this key area is asking
The SQA wants you to describe the structure of the cell membrane, distinguish the kinds of membrane protein, and explain how channels, carriers and pumps move substances across the membrane. The central examples are the sodium-potassium pump and how the unequal distribution of ions it creates produces the resting membrane potential, which underlies nerve and muscle activity.
The membrane and its proteins
Transport across the membrane
Small non-polar molecules cross the bilayer directly by simple diffusion down their concentration gradient. Ions and polar molecules cannot, and need protein help.
Movement against a concentration gradient is active transport and requires energy from ATP.
The sodium-potassium pump
The resting membrane potential
When the cell is stimulated, voltage-gated channels open and the potential reverses (depolarisation) and then recovers (repolarisation), which is the basis of the nerve impulse.
Examples in context
Example 1. The synapse. At a synapse, neurotransmitter binds ligand-gated channels on the next neuron, opening them and letting ions flow to change the membrane potential. This shows a ligand-gated channel converting a chemical signal into an electrical one.
Example 2. Glucose uptake by co-transport. In the gut, the sodium gradient set up by the pump drives glucose into cells through a co-transporter that carries sodium and glucose together. The example shows how the energy stored in an ion gradient is used to power the transport of another molecule.
Try this
Q1. State the difference between an integral and a peripheral membrane protein. [1 mark]
- Cue. Integral proteins span the bilayer through hydrophobic regions; peripheral proteins attach to the surface.
Q2. Explain why ions need transport proteins to cross the membrane. [2 marks]
- Cue. Ions are charged and cannot pass through the hydrophobic core of the bilayer, so they need channels or carriers.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style4 marksDescribe how the sodium-potassium pump moves ions across the membrane and explain why it requires ATP.Show worked answer →
A 4-mark answer needs the direction of movement, the conformational change and the energy requirement.
The sodium-potassium pump is a carrier protein that actively transports three sodium ions out of the cell and two potassium ions into the cell per cycle. Both ions are moved against their concentration gradients.
The pump binds sodium inside the cell, is phosphorylated by ATP, and changes conformation to release sodium outside and bind potassium. Removal of the phosphate returns it to its original shape, releasing potassium inside.
ATP is needed because both ions are pumped against their concentration gradients, which is active transport and cannot happen by diffusion alone.
Markers reward (1) three sodium out and two potassium in, (2) against the gradients, (3) ATP phosphorylates the pump causing a conformational change, and (4) active transport needs energy.
SQA AH style3 marksExplain how the resting membrane potential is established and maintained.Show worked answer →
A 3-mark answer needs the unequal ion distribution, the pump and the leak channels.
The sodium-potassium pump sets up concentration gradients with more sodium outside the cell and more potassium inside. This unequal distribution of ions, together with the membrane being selectively permeable, creates a potential difference across the membrane, with the inside negative relative to the outside.
Potassium leak channels allow potassium to diffuse out down its gradient, which is the main contributor to the negative resting potential, while the pump continually restores the gradients.
Markers reward (1) the pump creates unequal sodium and potassium distribution, (2) the membrane is selectively permeable so a potential difference forms, and (3) potassium leaking out makes the inside negative.
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Sources & how we know this
- SQA Advanced Higher Biology Course Specification — SQA (2019)