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# 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

## Key points

- Simple diffusion: O₂, CO₂, N₂, alcohol, steroid hormones; does not saturate and rises linearly with the gradient.
- Facilitated diffusion (GLUT): passive but saturable, with a maximum rate (Tmax), specificity and competition.
- Primary active: uses ATP directly (Na⁺/K⁺-ATPase). Secondary active: uses the Na⁺ gradient (SGLT, Na⁺/Ca²⁺ exchanger).
- Channels are thousands of times faster than carriers, and gates control their opening.
- Water crosses through aquaporins; ADH sets how many sit in the collecting duct.

## 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

**Carrier-mediated (saturable) transport**J = Jmax × C / (Km + C)

**J**
flux

**J max**
maximum flux with every carrier occupied

**K m**
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.

## Memory aid

Dissolve, slip, carry or push: bilayer, channel, passive carrier and active pump.

## 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.

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## Related topics

- [Diffusion and Fick's law](https://durgical.com/en/physiology/diffusion-and-fick-s-law/)
- [Primary and secondary active transport](https://durgical.com/en/physiology/primary-and-secondary-active-transport/)
- [Ion channels and gating](https://durgical.com/en/physiology/ion-channels-and-gating/)
- [Osmosis, osmolarity and tonicity](https://durgical.com/en/physiology/osmosis-osmolarity-and-tonicity/)
- [Glucose transport maximum and renal threshold](https://durgical.com/en/physiology/glucose-transport-maximum-and-renal-threshold/)
- [Endocytosis, exocytosis and vesicles](https://durgical.com/en/physiology/endocytosis-exocytosis-and-vesicles/)

## In Biochemistry

- [Membrane transport](https://durgical.com/en/biochemistry/topics/membrane-transport/)
- [Membrane structure](https://durgical.com/en/biochemistry/topics/membrane-structure/)
- [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/)

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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](https://openstax.org/books/anatomy-and-physiology-2e/pages/3-1-the-cell-membrane) (OpenStax (Rice University), 2022); [26.3 Electrolyte Balance](https://openstax.org/books/anatomy-and-physiology-2e/pages/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.

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