3.2.3 Transport across cell membranes

Every cell is wrapped in a phospholipid bilayer. Five mechanisms move substances across it: simple diffusion, facilitated diffusion, osmosis, active transport, and co-transport. Bulk transport via vesicles handles material too large for any of those. Each mechanism has its own protein involvement and its own ATP requirement.

The fluid mosaic model describes the membrane as a phospholipid bilayer with embedded proteins, cholesterol, and surface carbohydrates.

The cell-surface membrane is described by the fluid mosaic model: a phospholipid bilayer with proteins and cholesterol embedded throughout. Fluid because the phospholipids and proteins move laterally within each leaflet. Mosaic because the proteins are distributed unevenly across the sheet like tiles in a patchwork.

Hydrophilic heads out, hydrophobic tails in. Phosphate heads inside the bilayer or fatty acid tails on the outside is the most common inversion error and scores zero on bilayer-arrangement marks.

Cholesterol restricts phospholipid movement. The credited verbs are restricts, less fluid, less flexible, and stabilises. Strengthens alone is rejected.

Pitfall — Membrane-level adaptations

Membrane adaptations are membrane-level features.

When a question asks about cell-surface membrane adaptations, the credited answers are more channel proteins, more carrier proteins, and microvilli, which are folds of the membrane itself that increase the surface area for transport. Each adaptation must be paired with its mechanism. "More channel proteins so that more polar molecules cross by facilitated diffusion" scores; "more channel proteins" alone does not.

Whole-cell or organ-level features are rejected: villi, thin membranes, mitochondria, good blood supply, thin epithelium. Microvilli are membrane folds at the μm scale; villi are gut-wall projections at the mm scale. The two are not the same thing.

Passive transport runs down the concentration gradient with no ATP, by simple or facilitated diffusion.

Passive transport moves molecules down a concentration gradient, from high concentration to low, with no ATP. Two forms operate in parallel. Simple diffusion takes small non-polar molecules directly through the bilayer. Facilitated diffusion takes polar, charged, or larger molecules through specific channel or carrier proteins that provide a route through the hydrophobic core.

Write down the concentration gradient. Along the gradient and with the gradient are not credited.

Name channel protein or carrier protein by exact type. Transport protein or protein alone scores zero. Facilitated diffusion never uses ATP; citing ATP in a facilitated-diffusion answer is an explicit reject.

Osmosis moves water down a water potential gradient, never a water concentration gradient.

Osmosis is the movement of water molecules across a partially permeable membrane, from a region of higher water potential (less negative) to a region of lower water potential (more negative). Water potential is the credited term at A-level. Water concentration or concentration of water is an explicit reject, even when the rest of the mechanism is described correctly.

Water potential, not water concentration. The reject applies even when the underlying mechanism is correctly described. The marker scans for the term.

For animal cells gaining water, write volume increases, not pressure increases. Turgidity is plant-only; animal cells have no cell wall and cannot become turgid.

Active transport and co-transport drive carrier proteins against the gradient using ATP.

When a molecule needs to move against the concentration gradient, from low concentration to high, it requires active transport: a carrier protein and energy released by ATP hydrolysis. Co-transport exploits the ion gradient that one active-transport pump establishes to drive a second molecule's uptake, without directly consuming ATP at the second step.

Active transport uses carrier proteins only. Naming channel proteins for active transport is among the most consistently flagged errors on this topic, and channel proteins are an explicit reject here.

1. The molecule binds to a carrier protein on the lower-concentration side of the membrane.
2. ATP is hydrolysed, releasing energy that drives a conformational change in the carrier protein.
3. The shape change moves the bound molecule across the membrane against its gradient, and the molecule is released on the higher-concentration side.

Write energy released from ATP hydrolysis. Energy produced and ATP produced are both rejected; both phrasings imply ATP synthesis, which is the wrong direction. The cell hydrolyses ATP during transport, it does not produce it.

1. ATP hydrolysis at a separate pump establishes the driver ion gradient. Sodium-potassium pumps in the basal membrane of the ileum epithelial cell actively pump Na⁺ out, lowering intracellular Na⁺ concentration.
2. Na⁺ and glucose bind simultaneously to the same co-transporter carrier protein on the gut-lumen side of the cell. Same side, same time.
3. The carrier protein undergoes a conformational change, moving Na⁺ down its gradient into the cell and carrying glucose with it against its own gradient.
4. Glucose accumulates inside the cell, lowers the water potential, and water enters by osmosis. Glucose then moves into the blood by facilitated diffusion.

Co-transport requires simultaneous binding of both molecules to the same carrier, on the same side, at the same time. Sequential binding, or describing the second molecule as following Na⁺ down the gradient passively, misses the mechanism mark.

Pitfall — Co-transport four-step chain

The co-transport chain is four steps, and steps 1 and 4 are the consistent drops.

Step 1, the ATP-hydrolysis pump that establishes the ion gradient, is dropped because students forget that co-transport depends on ATP indirectly. Step 4, the osmosis link, is dropped because students stop at "glucose enters the cell" without continuing the chain to water uptake.

On any co-transport question that asks about volume change, the chain must end at osmosis. 2025 P1 Q05 mastery on the canonical question was below 10%.

Bulk transport moves large material in by endocytosis and out by exocytosis, both requiring ATP.

For material too large to cross by diffusion or through protein channels, the cell uses bulk transport via vesicles. Endocytosis moves material in. Exocytosis moves material out. Both require ATP for the membrane folding, vesicle pinching, and vesicle fusion involved. Neither uses a carrier protein, so neither shows single-molecule selectivity in the way active transport does.

Write cell-surface membrane for endocytosis and phagocytosis. Bilayer alone is not credited. The process involves the whole membrane apparatus, not just the lipid bilayer.

Key terms

• water potential
• osmosis
• co-transport
• active transport
• facilitated diffusion
• carrier protein
• channel
• bilayer
• ATP
• gradient
• cell-surface membrane
• membrane permeability