Photosynthesis and Cellular Respiration

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.

Photosynthesis

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

$$6C{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}\stackrel{\text{light, chlorophyll}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$$

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:

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\to 6C{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}+\text{energy (about 36 ATP)}$$

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).

Quick summary

• Photosynthesis: $6C{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$, 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 $\mathrm{C}{\mathrm{O}}_2$.
• Plants do both photosynthesis and respiration.
• Cells need glucose, oxygen, water and minerals, and must remove $\mathrm{C}{\mathrm{O}}_2$ and nitrogen wastes.

Quiz