BIOLOGY · Stage 9 · Chapter 9: Plants and Photosynthesis
Biology 9.2 Photosynthesis — Chloroplasts, Light Energy and Word Equations
5 min read 6 parts Lesson 2 of 9
Lesson summary
9.2 Photosynthesis — Chloroplasts, Light Energy and Word Equations — Alhussan International Academy — Biology (Stages 7–9) notes: Describe photosynthesis, write its word and symbol equations, and explain the role of chloroplasts and how plants use glucose.Photosynthesis = the pro…
- Gas movement
- Chlorophyll
- Aerobic respiration
- Starch store
- Building other molecules
- Net effect
Section 1 of 6What photosynthesis is
- Photosynthesis = the process by which green plants, algae and some bacteria make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll.
- Light energy is transferred to chemical energy stored in glucose.
- It makes the biomass of plants, from wood to cereal crops to flowers, and is the source of nearly all food and energy in ecosystems.
- Biomass
- animals must eat plants to get biomass, but plants make their own from sunlight, air and water.
- Oxygen
- it is a product of photosynthesis and is used for aerobic respiration in most living things.
Section 2 of 6Equations
- Reactants
- carbon dioxide from the air, which is also made in aerobic respiration, and water from the soil.
- Energy
- light, usually from the Sun, captured by chlorophyll.
- Word equation: carbon dioxide + water → glucose + oxygen (in light, using chlorophyll).
- Symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2.
- Gas movement
- carbon dioxide goes into the leaf and oxygen comes out through the stomata.
Section 3 of 6Where it happens: chloroplasts
- Location
- photosynthesis happens in chloroplasts, found mostly in palisade cells in leaves.
- Chlorophyll
- the green pigment in chloroplasts absorbs red and blue light and reflects green, so leaves look green.
- Reactions
- light energy is used to split water molecules and to build glucose from carbon dioxide, and most reactions take place inside chloroplasts.
- Evidence
- a cell without chloroplasts cannot photosynthesise, and the number of chloroplasts shows how much photosynthesis a cell carries out.
Section 4 of 6What happens to the glucose
- Aerobic respiration
- about half of the glucose is used to release energy for all the other reactions in the cells.
- Starch store
- small, soluble glucose cannot be stored, so molecules join to form large starch molecules.
- Starch is stored in leaves and lasts several days without light, so cells can respire in the dark.
- Starch is also stored in root tubers such as potatoes.
- Building other molecules
- glucose is the building block for other carbohydrates, proteins, lipids and chlorophyll.
- It makes cellulose for cell walls and sucrose for transport in the phloem.
- It combines with nitrate to make amino acids and then proteins.
- It is converted to lipids (oils) in seeds.
Section 5 of 6Leaf adaptations
| Feature | How it helps |
|---|---|
| Broad, flat shape | Large surface area to absorb light |
| Thin | Short diffusion distance for gases and light reaches all cells |
| Palisade cells near top, many chloroplasts | Absorb most light |
| Air spaces in spongy layer | Allow carbon dioxide to diffuse to cells |
| Stomata, with guard cells | Gas exchange; open in light, closed to save water |
| Veins (xylem and phloem) | Bring water, remove sugars |
| Transparent upper epidermis and waxy cuticle | Lets light through; reduces water loss |
Section 6 of 6Photosynthesis compared with respiration
- Photosynthesis
- it stores energy and takes in carbon dioxide.
- Respiration
- it releases energy and produces carbon dioxide, and goes on all the time.
- Net effect
- in daylight a healthy plant photosynthesises faster than it respires, so there is a net release of oxygen.
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