Physiology · Cell and membrane
Membrane transport: diffusion, channels and carriers !
- Mechanism
- Exam priority: high
Substances cross the cell membrane by dissolving in it, through channels, on carriers, or by spending energy.
Updated 2026-11-01
Also called: Facilitated diffusion, Passive and active transport
What it is
The membrane is a lipid bilayer that lets gases and lipid-soluble molecules through easily but holds back ions, glucose and bulk water. For those there are proteins: channels (selective pores, very fast) and carriers (they change shape, slower and saturable). Passive transport follows the electrochemical gradient; active transport goes against it and needs energy, either direct (ATP) or borrowed from another gradient.
Key facts
- Electrochemical gradient = chemical gradient (concentration) + electrical gradient (voltage). For ions both count.
- Symport: both solutes move the same way. Antiport: opposite ways.
- Vesicular transport (endocytosis and exocytosis) moves large molecules and also costs energy.
How it works, step by step
- The solute reaches the membrane; how it crosses depends on its lipid solubility, size and charge.
- If small and lipid-soluble, it dissolves in the bilayer and diffuses down its gradient.
- If it is an ion, it crosses through a specific channel while the gate is open.
- If polar and large (glucose), it binds a carrier that changes shape and releases it on the other side.
- If it must go against its gradient, the carrier takes energy from ATP or from Na⁺ moving down its own gradient.
Equations
- J
- flux
- Jmax
- maximum flux with every carrier occupied
- Km
- concentration giving half the maximum
Simple diffusion, by contrast, is J ∝ C (no ceiling).
Mechanisms compared
| Mechanism | Energy | Saturates? | Examples |
|---|---|---|---|
| Simple diffusion | None (gradient) | No | O₂, CO₂, steroids, general anaesthetics |
| Channel | None (electrochemical gradient) | Barely | Na⁺, K⁺, Ca²⁺ channels; aquaporins |
| Facilitated diffusion | None (gradient) | Yes | GLUT1–4, urea transporter |
| Primary active | Direct ATP | Yes | Na⁺/K⁺-ATPase, H⁺/K⁺-ATPase, Ca²⁺-ATPase |
| Secondary active | Na⁺ gradient (made by ATP) | Yes | SGLT1/2, NKCC2, Na⁺/Ca²⁺ exchanger, Na⁺/H⁺ exchanger |
Clinical notes
- Glycosuria: when blood glucose exceeds the maximum capacity of proximal tubule SGLTs (≈180–200 mg/dL), glucose appears in the urine.
- Insulin and GLUT4: insulin moves GLUT4 into muscle and fat membranes; lack of insulin or resistance to it slows glucose uptake.
- Ischaemia: without ATP the Na⁺/K⁺-ATPase stops, Na⁺ enters with water and the cell swells.
Test yourself
Think of the answer, then tap the question to see it.
As a substance's concentration rises a lot, its flux stops increasing. What kind of transport is this?
Carrier-mediated (facilitated or active): all carriers are occupied and Jmax is reached. Simple diffusion has no ceiling.
How can SGLT concentrate glucose inside the cell without using ATP directly?
It uses the energy of Na⁺ entering down its gradient; that gradient is kept by the Na⁺/K⁺-ATPase, which does use ATP: secondary active transport.
Why does O₂ need no carrier while glucose does?
O₂ is small and non-polar and dissolves in the bilayer. Glucose is large and polar; the bilayer holds it back, so it needs a GLUT or SGLT.
Which two gradients drive Na⁺ into a resting cell?
The chemical one (≈142 outside, ≈14 inside) and the electrical one (negative interior). Both point inwards.
Related topics
- Diffusion and Fick's law
- Primary and secondary active transport
- Ion channels and gating
- Osmosis, osmolarity and tonicity
- Glucose transport maximum and renal threshold
- Endocytosis, exocytosis and vesicles
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 3.1 The Cell Membrane (OpenStax (Rice University), 2022); 26.3 Electrolyte Balance (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 4). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.