B2.1 Membranes and membrane transport

The question describes ions being moved against their concentration gradient (from low to high concentration outside the cell). This is the defining feature of active transport.

Pump proteins carry out active transport by hydrolysing ATP. The energy released causes a conformational change in the protein, physically moving the bound ion through the membrane and releasing it on the other side, regardless of the gradient direction.

Because movement is against the concentration gradient, it is thermodynamically unfavourable — energy must be supplied. ATP hydrolysis provides this energy, distinguishing active transport from all passive processes.

The correct answer is that the pump protein hydrolyses ATP, using the released energy to drive conformational changes that move calcium ions against their electrochemical gradient. This precisely matches the mechanism of active transport via pump proteins.

The question asks how a pump protein can move ions against a concentration gradient — which mechanism makes this possible?

"The pump protein forms an open hydrophilic pore..." describes a channel protein, not a pump protein. Channel proteins only allow passive, downhill movement — they cannot move substances against a gradient.

"...releases them on the other side without any energy input, because the process is thermodynamically spontaneous" is incorrect. Moving ions against a gradient is not spontaneous — it requires energy input. This option contradicts thermodynamics.

"The pump protein acts as a receptor, triggering a signalling cascade..." describes a receptor function, not a transport mechanism. This does not explain how calcium ions physically cross the membrane against their gradient.

The remaining option — ATP hydrolysis driving conformational changes to move ions against their gradient — is the correct description of pump protein action in active transport.

The protein is described as: (1) not embedded in the bilayer, (2) attached to the cytoplasmic (inner) face, and (3) associating with phospholipid heads and hydrophilic regions of other proteins. These are the hallmarks of a peripheral protein.

Peripheral proteins are not embedded in the hydrophobic core of the bilayer. They attach to the surface — either inner (cytoplasmic) or outer (extracellular) — via non-covalent interactions with lipid head groups or integral proteins. Their attachment is often reversible.

The description specifies the cytoplasmic face, meaning this protein is on the inner surface. This confirms it is a peripheral protein on the inner surface, not an extracellular glycoprotein.

The correct answer is a peripheral protein on the inner surface of the membrane, matching all given descriptors precisely.

The question asks which protein type matches the description: surface-attached, cytoplasmic face, not embedded in the bilayer.

"An integral protein with a single transmembrane domain" is embedded in the bilayer, with at least one hydrophobic region interacting with the fatty acid tails. The described protein does not enter the hydrophobic core, so this is incorrect.

"A glycoprotein on the extracellular surface" is found on the outer face and has carbohydrate chains attached. The described protein is on the cytoplasmic face with no mention of carbohydrate chains, so this is incorrect.

"A transmembrane protein spanning the full width of the bilayer" would have regions projecting into both the cytoplasm and extracellular space, and would be embedded in the hydrophobic core. This contradicts the description of surface attachment only.

The only matching option is a peripheral protein on the inner surface, which correctly describes a non-embedded protein attached to the cytoplasmic face of the membrane.