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While diffusion and osmosis are passive processes that move substances down their concentration gradients, cells also need to transport substances against their concentration gradients — from low to high concentration. This requires metabolic energy in the form of ATP. The Edexcel A-Level Biology specification (9BI0) requires you to understand active transport, co-transport, endocytosis and exocytosis.
Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient (from a region of lower concentration to a region of higher concentration). It requires metabolic energy in the form of ATP and involves specific carrier proteins (also called pumps) in the membrane.
| Feature | Description |
|---|---|
| Direction | Against the concentration gradient (low → high concentration) |
| Energy source | ATP (from cellular respiration) |
| Proteins involved | Specific carrier proteins (pumps) |
| Specificity | Each carrier protein transports a specific molecule or ion |
| Examples | Na+/K+-ATPase, H+/K+-ATPase, mineral ion uptake by root hairs |
Exam Tip: To distinguish active transport from facilitated diffusion in exam answers, always emphasise two key points: (1) active transport moves substances against the concentration gradient, and (2) it requires ATP. Facilitated diffusion is passive and moves substances down the gradient.
The sodium-potassium pump is one of the most important examples of active transport and is found in virtually all animal cells. It is essential for maintaining the resting membrane potential of neurons, muscle contraction and other vital processes.
Co-transport is a type of secondary active transport in which the movement of one substance down its concentration gradient (established by active transport) drives the movement of another substance against its gradient. No ATP is directly used by the co-transporter itself, but ATP was used to establish the gradient in the first place.
| Type | Description | Example |
|---|---|---|
| Symport (co-transport) | Both substances move in the same direction | Na+-glucose co-transporter in the small intestine (SGLT1) |
| Antiport (counter-transport) | Substances move in opposite directions | Na+/K+-ATPase (Na+ out, K+ in); Na+/H+ exchanger |
The absorption of glucose from the lumen of the small intestine into the epithelial cells of the villi involves both active transport and co-transport:
Exam Tip: The absorption of glucose in the ileum is a very popular exam question. You must explain the role of the Na+/K+-ATPase in creating the sodium gradient, the role of SGLT1 in co-transporting glucose and sodium, and the role of GLUT2 in the exit of glucose into the blood. Draw and annotate a diagram if possible.
Several lines of evidence confirm that active transport is an energy-requiring process:
| Evidence | Explanation |
|---|---|
| Metabolic inhibitors (e.g. cyanide, DNP) | Substances that inhibit ATP production (by blocking the electron transport chain or uncoupling oxidative phosphorylation) also stop active transport, confirming that ATP is required |
| Oxygen concentration | Cells deprived of oxygen cannot carry out aerobic respiration, produce less ATP, and show reduced rates of active transport |
| Temperature | Active transport rates are affected by temperature (as it affects enzyme activity of the carrier protein/ATPase). This would not be the case if the process were purely passive |
| Respiratory inhibitors | Treating cells with respiratory inhibitors reduces active transport but does not affect passive diffusion, confirming the dependence on cellular respiration |
Some molecules are too large to pass through the membrane via transport proteins. These are moved into or out of the cell by bulk transport mechanisms that involve the formation or fusion of vesicles. Bulk transport requires ATP.
Endocytosis is the process by which cells take in large molecules, particles or even other cells by engulfing them with the cell membrane to form a vesicle inside the cell.
There are two main types:
| Type | What is engulfed | Specificity | Example |
|---|---|---|---|
| Phagocytosis | Large solid particles | Low | Neutrophil engulfing a bacterium |
| Pinocytosis | Fluid droplets | Low | Uptake of nutrients by intestinal cells |
| Receptor-mediated endocytosis | Specific molecules bound to receptors | High | LDL uptake by liver cells |
Exocytosis is the process by which cells release large molecules to the outside of the cell. It is essentially the reverse of endocytosis.
| Example | Substance released | Cell type |
|---|---|---|
| Neurotransmitter release | Acetylcholine, dopamine, etc. | Neurons (at the presynaptic terminal) |
| Hormone secretion | Insulin | Beta cells of the pancreas |
| Enzyme secretion | Digestive enzymes (e.g. pancreatic amylase) | Pancreatic acinar cells |
| Mucus secretion | Mucus (glycoproteins) | Goblet cells |
| Antibody secretion | Immunoglobulins | Plasma cells (B lymphocytes) |
Exam Tip: Exocytosis involves the fusion of the vesicle membrane with the cell surface membrane, which means the cell surface membrane is constantly being added to. This is balanced by endocytosis, which removes membrane from the cell surface. This recycling of membrane material is important for maintaining the surface area of the cell.
| Feature | Simple diffusion | Facilitated diffusion | Active transport | Endocytosis / Exocytosis |
|---|---|---|---|---|
| Direction | Down gradient | Down gradient | Against gradient | N/A (bulk) |
| ATP required? | No | No | Yes | Yes |
| Proteins? | No | Yes (channels/carriers) | Yes (carrier pumps) | No (vesicle-mediated) |
| Specificity | Low | High | High | Variable |
| Saturation | No | Yes | Yes | N/A |
| Examples | O2, CO2, ethanol | Glucose, ions | Na+/K+ pump, mineral ions | Phagocytosis, insulin secretion |
The Edexcel 9BI0 specification places active and bulk transport within Topic 2 (Cells, Viruses and Reproduction), building on the previous lesson's passive transport. Candidates must: define active transport as net movement of solutes against their concentration gradient via carrier proteins that hydrolyse ATP; distinguish primary AT (direct ATP hydrolysis, e.g. Na+/K+-ATPase) from secondary AT (gradient-driven co-transport, e.g. SGLT1 Na+–glucose symport); describe bulk transport as vesicle-mediated movement, comprising endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis; and recognise that all these processes require an intact membrane and metabolically supplied ATP. Synoptic links: Topic 5 (respiration supplies ATP for active transport), Topic 6 (phagocytosis by neutrophils/macrophages), Topic 7 (Na+/glucose co-transport in ileum microvilli) and Topic 8 (Na+/glucose co-transport in the proximal convoluted tubule) — refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
The Na+/K+-ATPase pump is found in the cell-surface membrane of all animal cells.
(a) Describe the stoichiometry of the Na+/K+-ATPase and explain why it is described as electrogenic. (3)
(b) Explain how the Na+/K+-ATPase establishes the gradient that drives Na+–glucose co-transport (SGLT1) in the ileum. (3)
(c) Predict, with reasoning, the effect of a respiratory inhibitor (e.g. cyanide) on glucose absorption from the ileum. (2)
Solution with mark scheme:
(a) Step 1 — recall the stoichiometry. Per ATP hydrolysed, the Na+/K+-ATPase exports 3 Na+ out of the cell and imports 2 K+ into the cell.
M1 (AO1.1) — correct stoichiometry stated (3 Na+ out / 2 K+ in / 1 ATP).
Step 2 — explain electrogenicity. Because three positive charges leave the cell while only two enter, there is a net export of one positive charge per cycle, so the pump itself contributes directly to the negative resting membrane potential.
A1 (AO2.1) — credited language: "the pump is electrogenic because the charge transferred per cycle is not balanced — net +1 charge is exported, contributing to the negative interior". Many candidates lose this mark by writing "the pump is electroneutral" or by stating only that "the inside becomes negative" without linking to the unequal ion stoichiometry.
A1 (AO1.2) — links the electrogenic action to the maintenance of the resting membrane potential (typically ∼−70mV in neurons), distinguishing the pump's direct electrogenic contribution from the dominant K+-leak diffusion potential.
(b) M1 (AO1.2) — the Na+/K+-ATPase on the basolateral membrane continuously exports Na+ from the epithelial cell, lowering intracellular [Na+] (typically ∼12mM vs ∼140mM in lumen).
M1 (AO2.1) — this generates a steep electrochemical Na+ gradient directed inward across the apical membrane (the ileum lumen has high Na+, the cytoplasm is low Na+ and electrically negative).
A1 (AO3.2a) — the apical SGLT1 carrier exploits this gradient: Na+ binding triggers a conformational change that simultaneously translocates glucose against its concentration gradient into the cell. The energy is therefore secondary — provided by the Na+ gradient that the pump originally created with ATP. A common pitfall is to state "SGLT1 hydrolyses ATP" — it does not; it is secondary active transport (gradient-driven), and only the Na+/K+-ATPase consumes ATP directly.
(c) M1 (AO3.1a) — cyanide blocks complex IV of the electron transport chain, halting oxidative phosphorylation and collapsing the cellular ATP supply.
A1 (AO3.2a) — without ATP, the Na+/K+-ATPase cannot function; intracellular [Na+] rises and the inward Na+ gradient collapses; SGLT1 can no longer co-transport glucose against its gradient; glucose absorption falls dramatically even though SGLT1 itself is not directly inhibited. This illustrates the chain: mitochondrial ATP → primary AT → secondary AT.
Total: 8 marks.
Question (6 marks): Compare and contrast primary active transport, secondary active transport and endocytosis as mechanisms by which substances enter cells.
Mark scheme decomposition by AO:
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