Lesson 1 of 4 · Grade 9 Integrated Science
Photosynthesis and respiration
18 min
Photosynthesis
Green plants make their own food (glucose) using light.
Carbon dioxide + Water → Glucose + Oxygen (in the presence of light and chlorophyll)
Requirements: light, carbon dioxide, water and chlorophyll. It takes place mainly in the leaves, in chloroplasts.
Test for starch: boil a leaf in water, then in alcohol (to remove chlorophyll), rinse, and add iodine. A blue-black colour shows starch is present.
Respiration
All living things release energy from food by respiration.
- Aerobic respiration (with oxygen): Glucose + Oxygen → Carbon dioxide + Water + Energy
- Anaerobic respiration (without oxygen): in yeast it produces alcohol and carbon dioxide (used in baking and brewing); in muscles during hard exercise it produces lactic acid, causing cramps.
Comparison
Photosynthesis stores energy and happens only in green plants in light; respiration releases energy and happens in all living cells all the time.
Key points to remember
- Photosynthesis needs light, CO₂, water and chlorophyll
- Iodine turns blue-black with starch
- Aerobic respiration releases energy using oxygen
- Yeast anaerobic respiration makes CO₂ and alcohol
Test yourself
