[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Histology](https://durgical.com/en/histology/) › [The cell](https://durgical.com/en/histology/#a-cell) › The cell membrane and transport

[Histology](https://durgical.com/en/histology/) · [The cell](https://durgical.com/en/histology/#a-cell)

# 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

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/) · [Ileum](https://durgical.com/en/anatomy/ileum/)

## Key points

- Phospholipid bilayer + cholesterol (controls fluidity) + integral and peripheral proteins.
- Na+/K+-ATPase: 3 Na+ out, 2 K+ in per ATP; builds the gradient for secondary transport.
- SGLT = secondary active (symport with Na+); GLUT = facilitated diffusion.
- Receptor-mediated (clathrin) endocytosis: LDL; its failure causes familial hypercholesterolaemia.

## 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.

## Memory aid

Three out, two in: the sodium-potassium pump trades unevenly and leaves the inside negative.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/histology/the-cell-membrane-and-transport/)

## See it in 3D

The structures of Durgical's 3D anatomy where it is found.

- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Ileum](https://durgical.com/en/anatomy/ileum/)

## Related

- [Organelles: nucleus, ER, Golgi, mitochondria, lysosomes, peroxisomes](https://durgical.com/en/histology/organelles-nucleus-er-golgi-mitochondria-lysosomes-peroxisomes/)
- [Cell junctions: tight, adherens, desmosomes, gap, hemidesmosomes](https://durgical.com/en/histology/cell-junctions-tight-adherens-desmosomes-gap-hemidesmosomes/)
- [Cilia, microvilli and stereocilia](https://durgical.com/en/histology/cilia-microvilli-and-stereocilia/)
- [Kidney: cortex, medulla and the nephron](https://durgical.com/en/histology/kidney-cortex-medulla-and-the-nephron/)

## In Pharmacology

- [Digoxin](https://durgical.com/en/pharmacology/digoxin/)

[← All of histology](https://durgical.com/en/histology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/3-1-the-cell-membrane) (OpenStax (Rice University), 2022); [Stains 6](https://digitalhistology.org/histology-basics/stains/stains-6/) (Digital Histology, 2024).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/histology/the-cell-membrane-and-transport/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/histology/the-cell-membrane-and-transport/