Active Transport, Endocytosis and Exocytosis

Diffusion and osmosis move molecules down their concentration gradients, requiring no energy. But cells frequently need to move substances against gradients — accumulating nutrients, expelling waste, maintaining ion concentrations, or taking in large particles. These tasks require energy, supplied as ATP. This lesson covers OCR A-Level Biology A specification point 2.1.5 (e)(iii) and (iv) — active transport and bulk transport by endocytosis and exocytosis.

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1. Active Transport

Key Definition — Active Transport: The movement of molecules or ions across a membrane against their concentration gradient, using energy from ATP hydrolysis and specific carrier proteins.

Active transport differs from facilitated diffusion in three key ways:

Feature	Facilitated diffusion	Active transport
Direction	Down gradient	Against gradient
ATP required	No	Yes
Proteins involved	Channels and carriers	Carriers (pumps) only
Saturates at V_max	Yes	Yes
Affected by respiratory inhibitors?	No	Yes

The last point is important: a compound such as cyanide (which blocks the electron transport chain and so stops ATP production) will halt active transport but not diffusion. Experimental evidence of ATP dependence is a common data-response question.

1.1 The Carrier Protein Mechanism

An active transport carrier protein works as follows:

1. The substrate (e.g. Na⁺ ion or glucose molecule) binds to a specific site on the carrier facing one side of the membrane.
2. ATP is hydrolysed by the carrier; the phosphate group is transferred to the protein (phosphorylation).
3. Phosphorylation causes a conformational change — the carrier "flips" so the binding site now faces the opposite side of the membrane.
4. The substrate is released on the far side (often in a region where its concentration is already higher).
5. The phosphate is released; the carrier returns to its original conformation, ready to bind the next substrate.

Each cycle moves a fixed number of substrate molecules and consumes ATP.

1.2 Worked Example — the Sodium-Potassium Pump (Na⁺/K⁺ ATPase)

Perhaps the most important active transport carrier in animal cells is the sodium-potassium pump. In each cycle it moves:

• 3 Na⁺ ions out of the cell
• 2 K⁺ ions into the cell
• Using 1 molecule of ATP

This creates the resting potential of neurones (more negative inside), the gradients needed for the sodium-glucose co-transporter in the gut, and ultimately drives nerve impulses and muscle contraction. About 30% of a resting cell's energy budget goes on this one pump.

graph LR
  A[Na/K pump cycle] --> B[3 Na+ bind inside]
  B --> C[ATP hydrolysed]
  C --> D[Conformation change]
  D --> E[3 Na+ released outside]
  E --> F[2 K+ bind outside]
  F --> G[Phosphate released]
  G --> H[Conformation reverts]
  H --> I[2 K+ released inside]

1.3 Co-Transport (Secondary Active Transport)

Sometimes the energy stored in a gradient set up by active transport is used to move another molecule against its gradient. This is co-transport.

Classic example: sodium-glucose co-transport in ileum epithelial cells.

• The Na⁺/K⁺ pump actively removes Na⁺ from the epithelial cell into the blood, keeping cell Na⁺ low.
• Na⁺ in the lumen of the gut then flows down its gradient into the epithelial cell through a co-transporter — and drags a glucose molecule with it, even when glucose is already more concentrated inside.
• Glucose then leaves the basolateral side of the cell by facilitated diffusion via GLUT2.

Co-transport is not driven directly by ATP — but it depends absolutely on the gradient maintained by active transport, so it is sometimes called secondary active transport.

1.4 Other examples of active transport

• Proton pumps in plant root hair cells — actively pump H⁺ out; the returning gradient drives uptake of mineral ions.
• Calcium pumps (Ca²⁺ ATPase) in muscle cells — return Ca²⁺ to the sarcoplasmic reticulum after contraction.
• Proton pumps in parietal cells of the stomach — secrete H⁺ to acidify stomach contents.
• Mineral ion uptake by plant roots against gradients.

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2. Bulk Transport — Why Endocytosis and Exocytosis?

Some substances are too large to cross via channels or carriers — proteins, polysaccharides, whole microbes. For these, cells use bulk transport, which involves invagination or outgrowth of the plasma membrane to form vesicles.

• Endocytosis — bulk uptake into the cell.
• Exocytosis — bulk release from the cell.

Both require ATP (to drive membrane deformation and vesicle movement along the cytoskeleton) and are therefore active processes.

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3. Endocytosis

Key Definition — Endocytosis: The active process in which a cell takes in substances from its surroundings by invagination of the plasma membrane to form a vesicle inside the cell.

Three forms are examinable:

3.1 Phagocytosis

Phagocytosis ("cell eating") is the uptake of large solid particles such as bacteria or cell debris. It is used by specialised white blood cells called phagocytes (neutrophils and macrophages) as part of the innate immune response.

Steps: