The cell membrane and transport !
- Topic
The membrane is the cell's border: a double layer of fat with proteins that lets some substances through and blocks others.
Updated 2026-11-01
What it is
The plasma membrane is a phospholipid bilayer with cholesterol and proteins, coated on the outside by the glycocalyx. It is selectively permeable: gases and small nonpolar molecules cross on their own, while ions, glucose and large molecules need channels, carriers, pumps or vesicles.
Structure
- On light microscopy the bilayer cannot be seen; only the cell's edge is visible.
- On electron microscopy it is trilaminar: two dark lines (polar heads) separated by a pale band (hydrophobic tails).
- The glycocalyx (glycoproteins and glycolipids) is PAS positive; it is thick on the intestinal brush border.
Transport mechanisms
| Mechanism | Uses ATP? | How it works | Example |
|---|---|---|---|
| Simple diffusion | No | Down the gradient, through the bilayer | O2, CO2, lipid-soluble molecules |
| Facilitated diffusion | No | Down the gradient, through a channel or carrier | Glucose via GLUT; ion channels |
| Osmosis | No | Water moves toward the more concentrated side, through aquaporins | A red cell swelling in a hypotonic solution |
| Primary active transport | Yes, directly | A pump moves ions against the gradient | Na+/K+-ATPase: 3 Na+ out, 2 K+ in |
| Secondary active transport | Indirectly | Uses the Na+ gradient built by the pump (symport or antiport) | SGLT in the gut and proximal tubule |
| Endocytosis and exocytosis | Yes | Vesicles carry material in or out; receptor-mediated endocytosis uses clathrin | LDL uptake; hormone secretion |
Clinical notes
- Cystic fibrosis: the CFTR chloride channel fails, so secretions in lung, pancreas and gut become thick.
- Familial hypercholesterolaemia: a faulty LDL receptor blocks LDL endocytosis and blood cholesterol rises.
- Digitalis glycosides inhibit the Na+/K+-ATPase of the cardiac myocyte, which raises intracellular calcium and contractile force.
Test yourself
Think of the answer, then tap the question to see it.
What kind of transport does the sodium-glucose cotransporter (SGLT) use?
Secondary active transport: it uses the sodium gradient made by the Na+/K+-ATPase to bring glucose in against its gradient.
How many ions does the Na+/K+-ATPase move per ATP?
It pumps three sodium ions out and two potassium ions in.
How does the plasma membrane look on electron microscopy?
As a trilaminar structure: two dark lines separated by a pale band.
Which membrane protein fails in cystic fibrosis?
CFTR, a chloride channel of epithelial cells.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Organelles: nucleus, ER, Golgi, mitochondria, lysosomes, peroxisomes
- Cell junctions: tight, adherens, desmosomes, gap, hemidesmosomes
- Cilia, microvilli and stereocilia
- Kidney: cortex, medulla and the nephron
In Pharmacology
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 3.1 The Cell Membrane (OpenStax (Rice University), 2022); Stains 6 (Digital Histology, 2024).
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Representative model, for educational purposes only.