# 7.3 Photosynthesis · 7.4 Gas Exchange

F.2 Integrated Science · Chapter 7 · Lesson 2

How green plants make food, how to prove it, and how plants and animals exchange gases with the air.

## Figure

A green plant respires all the time; in light photosynthesis is faster, so the net gas flows reverse.

## Key points

- **Photosynthesis ／ 光合作用**: $\text{carbon dioxide}+\text{water}\xrightarrow[\text{chlorophyll}]{\text{light energy}}\text{glucose}+\text{oxygen}$; 6CO~2~ + 6H~2~O → C~6~H~12~O~6~ + 6O~2~.
- **Raw materials**: carbon dioxide (air), water (soil). **Energy**: light, absorbed by **chlorophyll ／ 葉綠素**. **Product**: glucose, used or stored as **starch**. **By-product**: oxygen.
- **Starch test on a leaf**: **destarch** first (keep the plant in the dark) → boiling water (breaks the cell membrane) → hot alcohol, Bunsen burner **off**, warmed in a hot-water bath (removes chlorophyll; alcohol is flammable) → hot water (softens the leaf) → iodine solution (**light brown → dark blue** with starch).
- **Oxygen test**: a water plant in sodium hydrogencarbonate solution (supplies carbon dioxide) gives off bubbles; the gas **relights a glowing splint**.
- **Light, chlorophyll, carbon dioxide and water** are all needed. Tests: hydrogencarbonate indicator goes **purple** in light, **yellow** in the dark; only the green parts of a **variegated leaf** go dark blue; **soda lime** absorbs carbon dioxide (no soda lime = **control**); water cannot be tested, removing it kills the plant.
- **Food chains**: **producers ／ 生產者** (green plants) → **consumers ／ 消費者**; the arrow means "eaten by". The Sun is the ultimate source of energy for every food chain.
- Plants exchange gases through **stomata ／ 氣孔** (singular stoma), tiny pores in the leaf epidermis; a pair of **guard cells** opens and closes each one.
- **Humans**: **breathing** moves air in and out of the lungs; **gas exchange** is in the **air sacs** (oxygen in, carbon dioxide out). Path: nose → trachea → bronchus → bronchioles → air sacs.

## Must know

| | Photosynthesis | Respiration |
|---|---|---|
| Where? | In green cells (they contain chlorophyll) | In every living cell, plant or animal |
| When? | Only in light | All the time, day and night |
| Raw materials | Carbon dioxide + water | Food (glucose) + oxygen |
| Products | Glucose (stored as starch) + oxygen | Carbon dioxide + water |
| Energy | Light energy is stored as chemical energy in food | Chemical energy in food is released |

Inhaled → exhaled air: oxygen about 21 % → 16 %, carbon dioxide about 0.04 % → 4 %, water vapour variable → saturated, and exhaled air is warmer.

## Common mistakes

- ✗ "In the daytime a green plant carries out photosynthesis only." ✓ It respires **all the time**. In light photosynthesis is faster, so there is a **net** intake of carbon dioxide and a **net** release of oxygen.
- ✗ "Breathing is gas exchange." ✓ **Breathing** moves air in and out of the lungs. **Gas exchange** is oxygen and carbon dioxide crossing the air-sac wall into and out of the blood.
- ✗ Heating the alcohol over the Bunsen flame, with a stopper on the tube. ✓ Turn the burner **off** and warm the alcohol in a **hot-water bath** (alcohol is flammable). No stopper: the air inside expands on heating.
- ✗ "Soda lime gives the plant carbon dioxide." ✓ Soda lime **absorbs** carbon dioxide. **Sodium hydrogencarbonate** solution **supplies** it to a water plant.
- ✗ "Hawk → small bird → grasshopper → grass" (arrow pointing at what is eaten). ✓ The arrow means **"eaten by"**, from food to eater: grass → grasshopper → small bird → hawk.

## Quick check

1. Write the word equation for photosynthesis and state what is written above and below the arrow.
2. In the starch test, why is the leaf put in hot alcohol, and how must the alcohol be warmed?
3. A boiling tube with a green leaf in hydrogencarbonate indicator is left in sunlight for three hours. What colour does the indicator become, and why?
4. Arrange hawk, grass, grasshopper and small bird into a food chain, and name the producer.
5. Trace the path of inhaled air from the nose to the place where gas exchange happens, and state which gas enters the blood there.

## Answers

1. carbon dioxide + water → glucose + oxygen, with light energy above and chlorophyll below the arrow.
2. Alcohol removes the chlorophyll so the iodine colour change is easy to see. Warm it in a hot-water bath with the Bunsen burner off, because alcohol is flammable.
3. Purple. In sunlight the leaf photosynthesises faster than it respires, so there is a net absorption of carbon dioxide.
4. Grass → grasshopper → small bird → hawk. The producer is grass.
5. Nose → trachea → bronchus → bronchioles → air sacs. Oxygen enters the blood in the capillaries (and carbon dioxide leaves it).

## Did you know?

Leaves look green because chlorophyll absorbs red and blue light and reflects green. In autumn the chlorophyll breaks down, and the yellow and orange pigments that were there all along show through.