# Nutrient and Gas Exchange

> How plants and animals get nutrients and swap gases, from leaves and stomata to lungs, gills and the digestive system.

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

### Story time: Breathing through your skin
The green and golden bell frog lives in ponds around Sydney. It has lungs, but it also breathes through its thin, moist skin. On a cold day it can stay underwater for a long time, taking in all the oxygen it needs straight through its skin.

There is a catch. Because its skin is so thin and wet, it also lets in harmful chemicals and fungi from polluted water. That is one reason frogs are among the first animals to disappear when a habitat is damaged. Every exchange surface is a door, and doors work both ways.

**In short:** Organisms take in nutrients and gases through exchange surfaces that are large, thin, moist and close to a blood or air supply. Leaves exchange gases through stomata, roots take up water and minerals, and in humans the alveoli of the lungs and the villi of the small intestine do the job.

## Starting point: what you already know

- Diffusion moves particles from high to low concentration (see The Cell Membrane and Transport).
- Photosynthesis uses $\mathrm{CO_2}$ and releases $\mathrm{O_2}$. Respiration does the opposite.
- The digestive system breaks food into small molecules.

## What makes a good exchange surface?

### Key idea: Four features of every good exchange surface
1. **Large surface area** (many folds, branches or flat shapes).
2. **Thin**, often one cell thick, so diffusion distance is short.
3. **Moist**, so gases can dissolve before diffusing.
4. **Good supply** of blood or air flow, to keep the concentration gradient steep.

## Gas exchange in plants

Leaves exchange gases through tiny pores called **stomata** (one is a stoma), mostly on the lower surface. Each stoma is surrounded by two **guard cells**.

- When guard cells take in water they swell and bend, and the stoma **opens**.
- When they lose water they go floppy, and the stoma **closes**.

During the day, $\mathrm{CO_2}$ diffuses in and $\mathrm{O_2}$ diffuses out. Water vapour also escapes (transpiration), so plants close their stomata in hot, dry conditions. Inside the leaf, the **spongy mesophyll** has air spaces so gases reach every cell.

Roots take in **water** by osmosis and **mineral ions** (like nitrate and magnesium) by active transport through their **root hairs**.

## Gas exchange in animals

| Animal | Surface | How it works |
|---|---|---|
| Earthworm, frog | Moist skin | Gases diffuse straight through skin into blood vessels |
| Insects | Tracheal system | Holes called **spiracles** lead to tubes that take air right to the cells |
| Fish | **Gills** | Thin filaments with huge surface area. Water flows one way over them |
| Mammals, birds, reptiles | **Lungs** | Air is breathed in and out. Gases cross the walls of the alveoli |

In human lungs, air passes down the **trachea**, into the **bronchi** and **bronchioles**, and ends in about 480 million tiny air sacs called **alveoli**. Their total surface area is about the size of a tennis court. Each alveolus is one cell thick, wrapped in **capillaries**.

- Oxygen diffuses from the air in the alveoli into the blood.
- Carbon dioxide diffuses from the blood into the alveoli and is breathed out.

## Getting nutrients: digestion

Animals are **heterotrophs**: they eat other organisms. Food has to be broken into small molecules before it can be absorbed.

| Part | What happens |
|---|---|
| Mouth | Teeth chew (**mechanical** digestion). Saliva amylase starts on starch (**chemical** digestion) |
| Oesophagus | Muscle waves (peristalsis) push food down |
| Stomach | Churning, acid and the protease pepsin start on protein |
| Small intestine | Enzymes from the pancreas finish digestion. Bile from the liver breaks fat into small drops. Nutrients are **absorbed** through **villi** |
| Large intestine | Water is absorbed. Waste forms faeces |

**Villi** are finger-like folds in the small intestine wall, covered in even smaller **microvilli**. They give a huge surface area, a thin wall and a rich blood supply: a perfect exchange surface.

### Exam tip: Plant food is made, animal food is eaten
Plants are **autotrophs**: they make their own glucose. They still need to take in water, $\mathrm{CO_2}$ and mineral ions. Animals are **heterotrophs**: they digest food made by other organisms.

**Mind map**

```text
Exchange
  Good surfaces
    Large area
    Thin
    Moist
    Blood or air supply
  Plants
    Stomata and guard cells
    Spongy mesophyll air spaces
    Root hairs: water and ions
  Animals: gases
    Skin
    Tracheae
    Gills
    Lungs and alveoli
  Animals: nutrients
    Mechanical and chemical digestion
    Enzymes
    Villi absorb
```

### Year 11: Comparing gills and lungs
Water holds far less oxygen than air, so gills must be very efficient. Fish use **counter-current flow**: blood in the gill filaments flows in the **opposite direction** to the water. All along the filament, the water has a little more oxygen than the blood next to it, so oxygen keeps diffusing in. Fish can take about 80% of the oxygen out of the water. Gills collapse and stick together in air, which is why a fish suffocates out of water.

Investigating stomata: paint clear nail polish on a leaf, peel it off with tape and look at the print under a microscope. Count the stomata in the field of view and compare the upper and lower surfaces.

### Year 12: Exchange surfaces and disease
Damage to exchange surfaces causes many Module 8 diseases. **Emphysema**, often from smoking, breaks down alveolar walls, so the surface area falls and patients become breathless. **Coeliac disease** damages villi, so nutrients are poorly absorbed. Many pathogens in Module 7 also enter through these thin, moist surfaces.

## Quick summary

- Good exchange surfaces are large, thin, moist and well supplied with blood or air.
- Leaves exchange gases through stomata, opened and closed by guard cells.
- Roots absorb water by osmosis and mineral ions by active transport.
- Animals exchange gases through skin, tracheae, gills or lungs. Human lungs have millions of alveoli.
- Food is digested mechanically and chemically, then absorbed through villi in the small intestine.

## Quiz

Try these questions from the chapter quiz.

1. Which is NOT a feature of a good exchange surface?
   - A. Large surface area
   - B. Thin walls
   - C. Thick, dry layer
   - D. Good blood supply
   - Answer: C. Thick, dry layer. Exchange surfaces must be thin and moist.

2. Gases enter and leave a leaf mainly through
   - A. the cuticle
   - B. stomata
   - C. xylem
   - D. root hairs
   - Answer: B. stomata. Stomata are pores in the leaf surface.

3. What controls whether a stoma is open?
   - A. Guard cells
   - B. Palisade cells
   - C. Phloem
   - D. Chloroplasts in the stem
   - Answer: A. Guard cells. Guard cells swell to open and go floppy to close.

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