# The Cell Membrane and Transport

> The fluid mosaic model, diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis, and why cells stay small.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/membranes-and-transport/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The slug and the salt
Jai sprinkles salt on a garden slug, and it shrivels. His grandmother soaks wilted lettuce in cold water, and it goes crisp again. A nurse in hospital never puts pure water into a patient's vein, only a salty drip.

All three come down to one idea. Water moves across cell membranes from where it is more plentiful to where it is less plentiful. Salt on a slug pulls water out of its cells. Fresh water fills up lettuce cells. And pure water in blood would rush into red blood cells until they burst.

**In short:** The cell membrane is a phospholipid bilayer with proteins in it (the fluid mosaic model), and it controls what enters and leaves the cell. Diffusion and osmosis move substances from high to low concentration without energy, while active transport uses ATP to move them the other way.

## Starting point: what you already know

- Particles are always moving, and they spread out from where they are crowded (Year 9 and 10 chemistry).
- A **solution** is a solute (like salt) dissolved in a solvent (like water).
- Cells need to take in oxygen and glucose and get rid of carbon dioxide and wastes.

## The fluid mosaic model

The cell membrane is a thin double layer of **phospholipids**. Each phospholipid has a **head** that likes water (hydrophilic) and two **tails** that avoid water (hydrophobic). In water, they line up tail to tail.

*Diagram: A phospholipid bilayer with heads facing outward and tails facing inward, with a protein channel passing through it*
*The phospholipid bilayer. Proteins (orange) float in it like tiles in a moving mosaic.*

It is called the **fluid mosaic model** because the phospholipids and proteins move about (fluid) and the proteins are scattered through it like tiles (mosaic). The membrane also has **cholesterol** (keeps it stable) and **glycoproteins** (sugar chains that act as ID tags).

The membrane is **selectively permeable**: small or fat-soluble things like oxygen and carbon dioxide pass through easily, but large or charged things like glucose and ions need help from proteins.

## Passive transport (no energy needed)

**Diffusion** is the net movement of particles from a **high** concentration to a **low** concentration, down a **concentration gradient**. Oxygen enters cells and carbon dioxide leaves by diffusion.

**Facilitated diffusion** is diffusion through a **channel** or **carrier protein**. It is still downhill and needs no energy. Glucose enters many cells this way.

**Osmosis** is the diffusion of **water** across a selectively permeable membrane, from where water is more concentrated (a dilute solution) to where it is less concentrated (a concentrated solution).

### Key idea: Osmosis in one line
Water moves toward the **saltier** (more concentrated) side.

| Solution outside the cell | Name | Animal cell | Plant cell |
|---|---|---|---|
| More dilute than the cell | **Hypotonic** | Water in, swells, may burst (lysis) | Water in, firm (turgid). The wall stops it bursting |
| Same as the cell | **Isotonic** | No net change | Soft (flaccid) |
| More concentrated than the cell | **Hypertonic** | Water out, shrivels (crenation) | Water out, membrane pulls away from the wall (plasmolysis) |

## Active transport (needs energy)

**Active transport** moves substances **against** their concentration gradient, from low to high. It needs a **carrier protein** (a pump) and energy from **ATP**. Root hair cells use it to take in mineral ions from soil that has fewer ions than the cell.

**Bulk transport** moves very large things in vesicles and also uses energy:

- **Endocytosis**: the membrane folds in and wraps something to bring it **in**. A white blood cell swallowing a bacterium is phagocytosis ("cell eating").
- **Exocytosis**: a vesicle joins the membrane and releases its contents **out**, such as hormones or enzymes.

## Why cells are small

As a cell grows, its volume grows faster than its surface area. A big cell would not have enough membrane to supply its insides.

### Worked example: Surface area to volume ratio
| Cube side | Surface area $6s^2$ | Volume $s^3$ | SA : V |
|---|---|---|---|
| 1 µm | 6 | 1 | 6 : 1 |
| 2 µm | 24 | 8 | 3 : 1 |
| 4 µm | 96 | 64 | 1.5 : 1 |

Doubling the side **halves** the ratio. Small cells have a large SA : V, so materials move in and out fast enough.

**Mind map**

```text
Membrane transport
  Membrane
    Phospholipid bilayer
    Fluid mosaic
    Selectively permeable
  Passive, no ATP
    Diffusion: high to low
    Facilitated: through proteins
    Osmosis: water to the saltier side
  Active, needs ATP
    Pumps: low to high
    Endocytosis: in
    Exocytosis: out
  Size
    Small cells, big SA:V
```

### Year 11: Factors that affect the rate of diffusion
Diffusion is faster when there is

- a **steeper concentration gradient** (a bigger difference),
- a **larger surface area** (think of folded villi in the gut and millions of alveoli in the lungs),
- a **shorter distance** (thin walls, one cell thick), and
- a **higher temperature** (particles move faster).

This is why blood flow and breathing keep removing oxygen from the lungs: it keeps the gradient steep. A classic Year 11 practical uses agar cubes stained with an indicator to show that colour reaches the centre of small cubes first.

### Year 12: Membranes in disease and medicine
In **cystic fibrosis** (Module 8 and Module 5), a faulty channel protein called CFTR cannot move chloride ions out of cells. Water does not follow by osmosis, so thick sticky mucus builds up in the lungs. Doctors use **isotonic** saline drips so that red blood cells neither swell nor shrink, and they use hypertonic saline to draw water into sticky mucus and thin it.

## Quick summary

- The membrane is a phospholipid bilayer with proteins: the fluid mosaic model. It is selectively permeable.
- Diffusion and osmosis are passive: high to low concentration, no energy.
- Osmosis moves water toward the more concentrated solution.
- Active transport moves substances from low to high using protein pumps and ATP.
- Endocytosis brings large things in. Exocytosis sends them out.
- Small cells have a larger surface area to volume ratio.

## Quiz

Try these questions from the chapter quiz.

1. The cell membrane is mostly made of
   - A. cellulose
   - B. phospholipids and proteins
   - C. starch
   - D. DNA
   - Answer: B. phospholipids and proteins. A phospholipid bilayer with proteins scattered through it.

2. Diffusion is the movement of particles from
   - A. high concentration to low concentration
   - B. low concentration to high concentration
   - C. inside to outside only
   - D. the nucleus to the membrane
   - Answer: A. high concentration to low concentration. Particles spread down the gradient.

3. Osmosis is the diffusion of
   - A. salt
   - B. glucose
   - C. water
   - D. oxygen
   - Answer: C. water. Osmosis is water moving across a selectively permeable membrane.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/membranes-and-transport/#quiz