Active Transport, Endocytosis and Exocytosis

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.5 — Biological membranes (refer to the official OCR H420 specification document for exact wording). It develops active transport against a concentration gradient using ATP and carrier proteins (pumps), co-transport, and the bulk-transport processes of endocytosis (phagocytosis, pinocytosis, receptor-mediated) 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 develops the OCR H420 Module 2.1.5 content on active transport and bulk transport by endocytosis and exocytosis.

---

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.

---

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.

---

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:

1. The phagocyte detects a pathogen, often via receptor binding to foreign surface molecules or opsonin markers.
2. The plasma membrane extends pseudopodia around the particle.
3. The membrane fuses, enclosing the particle in a phagosome.
4. The phagosome fuses with a lysosome to form a phagolysosome.
5. Hydrolytic enzymes (lysozyme, proteases, nucleases) digest the pathogen.
6. Soluble products are absorbed into the cytoplasm; waste is exocytosed.

3.2 Pinocytosis

Pinocytosis ("cell drinking") is the uptake of fluid and small dissolved solutes by formation of small vesicles. Unlike phagocytosis it is non-specific and occurs continuously in most cells.

3.3 Receptor-Mediated Endocytosis

A more selective form, in which a specific ligand (such as a hormone, LDL, or growth factor) binds to a membrane receptor. Clusters of receptors gather in coated pits, which pinch off as coated vesicles. This is how cells take in cholesterol (bound to LDL particles) and iron (bound to transferrin).

---

4. Exocytosis

Key Definition — Exocytosis: The active process in which a cell releases substances to its surroundings by fusion of intracellular vesicles with the plasma membrane.

Exocytosis is used for:

• Secretion of proteins — digestive enzymes by pancreatic cells, hormones such as insulin, antibodies by plasma cells.
• Secretion of neurotransmitters into synaptic clefts.
• Addition of new membrane material — the vesicle membrane becomes part of the plasma membrane.
• Removal of waste products from phagolysosome digestion.

Steps (for a protein being secreted):

1. The protein is synthesised on ribosomes bound to the rough endoplasmic reticulum (RER).
2. It is packaged into transport vesicles that bud off the RER.
3. Vesicles fuse with the cis face of the Golgi apparatus, where the protein is modified (glycosylation, folding).
4. Secretory vesicles bud off the trans face of the Golgi.
5. Vesicles are transported along the cytoskeleton to the plasma membrane.
6. Vesicles fuse with the plasma membrane and release their contents outside.

ATP is required for vesicle movement and membrane fusion.

graph LR
  A[Ribosome on RER] --> B[Protein in RER lumen]
  B --> C[Transport vesicle]
  C --> D[Golgi modification]
  D --> E[Secretory vesicle]
  E --> F[Vesicle fuses with plasma membrane]
  F --> G[Contents released outside]

---

5. Summary Comparison — Transport Across Membranes

Process	Direction	Carrier/channel?	ATP?	Examples
Simple diffusion	Down gradient	No	No	O₂, CO₂
Facilitated diffusion	Down gradient	Yes	No	Glucose (GLUT), ions
Osmosis	Down ψ gradient	Aquaporins or bilayer	No	Water
Active transport	Against gradient	Carrier (pump)	Yes	Na⁺/K⁺ pump
Co-transport	Both (coupled)	Co-transporter	Indirect	Na⁺/glucose in ileum
Phagocytosis	In (bulk solid)	N/A	Yes	Macrophages engulfing bacteria
Pinocytosis	In (bulk fluid)	N/A	Yes	Cells sampling extracellular fluid
Exocytosis	Out (bulk)	N/A	Yes	Insulin secretion, neurotransmitter release

---

6. Common Exam Mistakes

• Saying facilitated diffusion requires ATP — it does not.
• Forgetting to say active transport is against the gradient.
• Claiming the Na⁺/K⁺ pump moves "one of each" — it is 3 Na⁺ out and 2 K⁺ in per ATP.
• Confusing endocytosis and exocytosis — "endo" = in, "exo" = out.
• Saying phagocytosis and pinocytosis are "the same as endocytosis". They are types of endocytosis.
• Claiming exocytosis "makes" proteins. Proteins are made at ribosomes; exocytosis only releases them.
• Writing that co-transport "does not use ATP". It uses it indirectly — via the gradient set up by the Na⁺/K⁺ pump.
• Forgetting that endocytosis and exocytosis require ATP.

---

7. Exam-Style Questions

1. Compare active transport with facilitated diffusion. (4)
2. Describe the role of the sodium-potassium pump in animal cells. (4)
3. Explain how respiratory inhibitors can be used to distinguish active transport from facilitated diffusion. (3)
4. Describe the process of exocytosis and give one example of its biological importance. (5)

Model answer for (1): "Both use carrier proteins in the cell membrane, are specific to particular molecules, and can saturate when all carriers are occupied. However, active transport moves substances against their concentration gradient and requires ATP, whereas facilitated diffusion moves substances down the gradient and does not require ATP. Active transport can therefore be inhibited by respiratory poisons such as cyanide; facilitated diffusion cannot."

---

Summary

• Active transport moves substances against their concentration gradient using ATP and specific carrier proteins (pumps).
• The Na⁺/K⁺ pump exports 3 Na⁺ and imports 2 K⁺ per ATP — essential for nerve impulses and co-transport.
• Co-transport uses a gradient set up by active transport to pull a second molecule uphill — e.g. Na⁺/glucose in the ileum.
• Endocytosis (phagocytosis, pinocytosis, receptor-mediated) brings materials into the cell by invagination.
• Exocytosis releases proteins and waste by vesicle fusion with the plasma membrane.
• All forms of bulk transport require ATP.
• Respiratory inhibitors stop active transport and bulk transport but not diffusion — a key experimental diagnostic.

---

8. The Na⁺/K⁺ ATPase — SVG Mechanism

---

9. Co-transport — Sodium-Glucose in the Ileum (SVG)