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[Biochemistry](https://durgical.com/en/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

- [Cystinuria](https://durgical.com/en/conditions/cystinuria/)
- [Hartnup disease](https://durgical.com/en/conditions/hartnup-disease/)
- [Primary carnitine deficiency](https://durgical.com/en/conditions/primary-carnitine-deficiency/)
- [Lactose intolerance](https://durgical.com/en/conditions/lactose-intolerance/)

## Related

- [Membrane structure](https://durgical.com/en/biochemistry/topics/membrane-structure/)
- [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/)
- [H⁺/K⁺-ATPase (gastric proton pump)](https://durgical.com/en/biochemistry/enzymes/h-k-atpase/)
- [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/)
- [ATP (adenosine triphosphate)](https://durgical.com/en/biochemistry/molecules/atp/)
- [Cell organelles and their metabolism](https://durgical.com/en/biochemistry/topics/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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Transport of small molecules](https://reactome.org/PathwayBrowser/#/R-HSA-382551) (Reactome, 2026); [SLC-mediated transmembrane transport](https://reactome.org/PathwayBrowser/#/R-HSA-425407) (Reactome, 2026); [Ion channel transport](https://reactome.org/PathwayBrowser/#/R-HSA-983712) (Reactome, 2026); [ABC-family protein mediated transport](https://reactome.org/PathwayBrowser/#/R-HSA-382556) (Reactome, 2026); [Aquaporin-mediated transport](https://reactome.org/PathwayBrowser/#/R-HSA-445717) (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.

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