# The Plasma Membrane

> The plasma membrane is a double layer of lipids with proteins floating in it, a design called the fluid mosaic model. It decides what enters and leaves the cell while letting the cell sense its surroundings.

Web page: https://cellnaut.com/topics/plasma-membrane

![Teaching illustration: The Plasma Membrane](https://cellnaut.com/topic-illustrations/plasma-membrane-teaching.jpg)

## Labelled parts

1. **Phospholipid heads**: Water-loving heads that face the watery outside and inside of the cell.
2. **Fatty acid tails**: Oily tails that form a barrier most water-soluble molecules cannot cross.
3. **Channel protein**: A pore that lets specific ions or small molecules cross down their concentration gradient.
4. **Carrier protein**: Changes shape to carry a molecule across, sometimes using energy.
5. **Glycoprotein**: A protein with a sugar chain, used in cell recognition.
6. **Cholesterol**: Tucks between the tails and keeps the membrane from becoming too stiff or too runny.

## Key points

- Phospholipids face their water-loving heads outward and hide their oily tails in the middle.
- Channel and carrier proteins let ions and larger molecules cross when they could not slip through the lipids.
- Sugar chains on the outer surface act as identity tags that other cells can recognise.

## Check yourself

**1. Which part of a phospholipid is water-loving?**

- A. The head
- B. The tails
- C. Both
- D. Neither

**Answer:** A. The head - The charged head attracts water; the fatty acid tails avoid it.

**2. Why does a charged ion need a channel to cross the membrane?**

- A. The oily middle blocks charged particles
- B. Ions are too big
- C. The membrane has no lipids
- D. Ions are alive

**Answer:** A. The oily middle blocks charged particles - Charged particles cannot dissolve in the oily core, so proteins offer a path.

## Related lessons

- [The Plasma Membrane and Transport](https://cellnaut.com/learn/plasma-membrane-and-transport)

## See it in 3D

- [Animal Cell](https://cellnaut.com/studio/animal)
- [Epithelial Cell](https://cellnaut.com/studio/epithelial)

Explanations rewritten in our own words from Biology 2e, Chapter 5: Structure and Function of Plasma Membranes (OpenStax), licensed CC BY-NC-SA 4.0. https://openstax.org/books/biology-2e/pages/5-introduction