Biochemistry · Topic
Membrane transport !
Molecules cross membranes by simple diffusion, through channels and carriers down a gradient, or with energy against it.
Updated 2026-11-01
Key facts
- Simple diffusion: gases and small lipid-soluble molecules, down a gradient, with no protein and no saturation.
- Facilitated diffusion: through channels (ions, water via aquaporins) or carriers (GLUT); passive but saturable, with Michaelis–Menten kinetics.
- Primary active transport spends ATP directly (Na⁺/K⁺-ATPase: 3 Na⁺ out, 2 K⁺ in; H⁺/K⁺-ATPase; ABC pumps such as CFTR and MDR).
- Secondary active transport uses the Na⁺ gradient made by the pump (SGLT1 in the gut, SGLT2 in the kidney: Na⁺ + glucose symport).
- Endocytosis (phagocytosis, pinocytosis, clathrin-mediated receptor uptake) and exocytosis move particles and macromolecules.
In detail
- 1. Uniport: one molecule (GLUT1–4). Symport: two in the same direction (SGLT). Antiport: opposite directions (Na⁺/Ca²⁺ exchanger, Cl⁻/HCO₃⁻).
- 2. GLUT2 (liver, pancreas, high Km) senses blood glucose; GLUT4 (muscle, fat) moves to the membrane in response to insulin.
- 3. The Na⁺/K⁺-ATPase is electrogenic, keeps the resting potential and cell volume, and uses a large share of resting ATP.
- 4. Channels open by voltage, ligand or mechanical stimulus; they are much faster than carriers.
- 5. Receptor-mediated endocytosis (LDL, transferrin) forms clathrin-coated vesicles that mature into endosomes and lysosomes.
- 6. Oral rehydration solution uses SGLT1: glucose pulls Na⁺ in, and water follows.
- Also called
- Diffusion, carriers, pumps
Clinical notes
- Cystic fibrosis is a defect of CFTR, an ABC-family chloride channel.
- Digoxin inhibits the Na⁺/K⁺-ATPase; SGLT2 inhibitors make the kidney lose glucose in urine.
- Cystinuria and Hartnup disease are defects of amino acid carriers in kidney and gut.
- P-glycoprotein (MDR1) pumps drugs out of cells and explains part of chemotherapy resistance.
Related conditions
Related
- Membrane structure
- Na⁺/K⁺-ATPase (sodium pump)
- H⁺/K⁺-ATPase (gastric proton pump)
- Glucose
- ATP (adenosine triphosphate)
- Cell organelles and their metabolism
Test yourself
Think of the answer, then tap the question to see it.
How does primary active transport differ from secondary?
Primary spends ATP on the protein itself; secondary uses a gradient (usually Na⁺) made earlier by a pump.
How many ions does the Na⁺/K⁺-ATPase move per ATP?
It moves 3 Na⁺ out and 2 K⁺ in, which makes it electrogenic.
Why does oral rehydration solution contain glucose?
Intestinal SGLT1 absorbs Na⁺ together with glucose, and water follows the sodium.
Which glucose transporter depends on insulin?
GLUT4, in muscle and fat: insulin brings it to the membrane.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Transport of small molecules (Reactome, 2026); SLC-mediated transmembrane transport (Reactome, 2026); Ion channel transport (Reactome, 2026); ABC-family protein mediated transport (Reactome, 2026); Aquaporin-mediated transport (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.