# Photosynthesis and Cellular Respiration

> How plants make glucose from light, how all cells release energy as ATP, aerobic and anaerobic respiration, and what cells need and get rid of.

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

### Story time: The tree made of air
Around 1640, Jan van Helmont planted a 2 kg willow in 90 kg of dry soil. He gave it only rainwater for five years. The tree grew to 77 kg. He weighed the soil again: it had lost only about 60 grams.

Where did 75 kg of tree come from? Van Helmont guessed water. He was partly right. But most of a tree's mass comes from something you cannot see: carbon dioxide from the air. A tree is built mostly out of air, using the energy of sunlight.

**In short:** Photosynthesis uses light energy in chloroplasts to turn carbon dioxide and water into glucose and oxygen. Cellular respiration does the reverse in mitochondria, releasing energy from glucose as ATP: about 36 ATP with oxygen, and only 2 without it.

## Starting point: what you already know

- Plants make food by photosynthesis. Animals eat food to get energy.
- Chloroplasts and mitochondria are organelles (see Cell Organelles).
- In a chemical equation, atoms are rearranged, not created or destroyed.

## What cells need and what they get rid of

Every cell needs **energy**, **matter** (raw materials to build with) and a way to remove **wastes**.

| Cells need | Cells remove |
|---|---|
| Glucose (energy and carbon) | Carbon dioxide |
| Oxygen (for aerobic respiration) | Water (extra) |
| Water | Nitrogen wastes such as urea |
| Mineral ions, amino acids, fatty acids, vitamins | Heat |

## Photosynthesis

Plants, algae and some bacteria capture light energy and store it in glucose.

$$\mathrm{6CO_2} + \mathrm{6H_2O} \xrightarrow{\text{light, chlorophyll}} \mathrm{C_6H_{12}O_6} + \mathrm{6O_2}$$

In words: carbon dioxide + water → glucose + oxygen.

It happens in **chloroplasts**, which contain the green pigment **chlorophyll**. Chlorophyll absorbs red and blue light and reflects green, which is why leaves look green.

The oxygen that plants release comes from **splitting water**, not from carbon dioxide.

The glucose is used for respiration, turned into **starch** for storage, into **cellulose** for cell walls, or combined with nitrogen from the soil to make **proteins**.

## Cellular respiration

Every living cell, plant or animal, breaks down glucose to release energy. The energy is stored in small molecules called **ATP** (adenosine triphosphate), which the cell spends like coins.

**Aerobic respiration** (with oxygen) happens mostly in **mitochondria**:

$$\mathrm{C_6H_{12}O_6} + \mathrm{6O_2} \rightarrow \mathrm{6CO_2} + \mathrm{6H_2O} + \text{energy (about 36 ATP)}$$

Notice it is photosynthesis backwards. Plants make glucose and oxygen. Every living thing, plants included, uses them to release energy.

**Anaerobic respiration** (without oxygen) happens in the cytoplasm and releases much less energy (2 ATP per glucose).

| | Product in animals | Product in yeast and plants |
|---|---|---|
| Anaerobic respiration | Lactic acid | Ethanol + carbon dioxide (this is **fermentation**) |
| Real-life example | Sore, tired muscles during a sprint | Bread rising, making wine and beer |

### Worked example: Back to the willow tree
The carbon atoms in the willow's wood came from $\mathrm{CO_2}$ in the air, joined into glucose by photosynthesis, then built into cellulose. The water mostly supplied hydrogen. The soil gave only a few grams of mineral ions. So van Helmont's tree really was built mostly from air.

**Mind map**

```text
Energy in cells
  Photosynthesis
    In chloroplasts
    CO₂ + water → glucose + O₂
    Needs light and chlorophyll
  Aerobic respiration
    In mitochondria
    Glucose + O₂ → CO₂ + water
    About 36 ATP
  Anaerobic respiration
    In cytoplasm
    Animals: lactic acid
    Yeast: ethanol + CO₂
    Only 2 ATP
  Cell needs
    Glucose, O₂, water, minerals
    Remove CO₂, urea, heat
```

### Year 11: The stages
**Photosynthesis** has two stages.

1. **Light-dependent reactions** on the **thylakoid membranes** (grana). Light energy splits water (**photolysis**), releasing $\mathrm{O_2}$ and making ATP and NADPH.
2. **Light-independent reactions** (the **Calvin cycle**) in the **stroma**. ATP and NADPH are used to fix $\mathrm{CO_2}$ into glucose.

**Aerobic respiration** has three stages.

| Stage | Where | Net ATP |
|---|---|---|
| Glycolysis: glucose split into 2 pyruvate | Cytoplasm | 2 |
| Krebs cycle: pyruvate broken down, $\mathrm{CO_2}$ released | Mitochondrial matrix | 2 |
| Electron transport chain: oxygen accepts electrons, water forms | Inner membrane (cristae) | about 32 to 34 |

**Limiting factors.** The rate of photosynthesis is held back by whichever of light intensity, $\mathrm{CO_2}$ concentration or temperature is in shortest supply. Greenhouse growers add $\mathrm{CO_2}$ and light to boost crops.

### Year 12: Energy flow links to ecosystems
In Module 4, the glucose made by photosynthesis is the start of almost every food chain. Each time energy passes from one organism to the next, much of it is released as heat by respiration. This is why only about 10% of energy passes up each trophic level. In Module 8, diabetes is a problem with getting glucose into cells for respiration.

## Quick summary

- Photosynthesis: $\mathrm{6CO_2} + \mathrm{6H_2O} \rightarrow \mathrm{C_6H_{12}O_6} + \mathrm{6O_2}$, in chloroplasts, using light.
- Aerobic respiration is the reverse, in mitochondria, and releases about 36 ATP per glucose.
- Anaerobic respiration releases 2 ATP. Animals make lactic acid. Yeast makes ethanol and $\mathrm{CO_2}$.
- Plants do both photosynthesis and respiration.
- Cells need glucose, oxygen, water and minerals, and must remove $\mathrm{CO_2}$ and nitrogen wastes.

## Quiz

Try these questions from the chapter quiz.

1. Where does photosynthesis happen?
   - A. Mitochondria
   - B. Chloroplasts
   - C. Nucleus
   - D. Ribosomes
   - Answer: B. Chloroplasts. Chloroplasts contain chlorophyll.

2. What are the reactants of photosynthesis?
   - A. Carbon dioxide and water
   - B. Glucose and oxygen
   - C. Oxygen and water
   - D. Glucose and carbon dioxide
   - Answer: A. Carbon dioxide and water. $\mathrm{CO_2}$ and $\mathrm{H_2O}$ are used to make glucose.

3. What gas do plants release during photosynthesis?
   - A. Carbon dioxide
   - B. Nitrogen
   - C. Oxygen
   - D. Hydrogen
   - Answer: C. Oxygen. Oxygen is a product of photosynthesis.

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