Photosynthesis
Green plants make glucose from CO₂ and water using light, in the chloroplast. Light reactions in grana make ATP and NADPH₂; dark reactions in stroma fix CO₂.
Site, chloroplast and pigments
- Site: mainly the mesophyll cells of leaves, inside chloroplasts.
- Chloroplast: an organelle with a double membrane. Inside are stacks of disc-like thylakoids called grana, joined by stroma lamellae (fret channels), lying in a fluid matrix, the stroma.
- Grana hold the pigments and carry out the light reaction. The stroma holds enzymes and carries out the dark reaction.
- Pigments: chlorophyll a (the main pigment, bluish green), chlorophyll b (yellowish green) and carotenoids: carotenes (orange) and xanthophylls (yellow).
- Chlorophyll absorbs mainly blue and red light and reflects green light, so leaves look green.
Light reaction vs dark reaction
The dark reaction also runs in daytime. It is called 'dark' only because it does not need light directly.
Dark reaction: the Calvin cycle
- back to RuBP (5-carbon)
Six turns of the cycle fix 6 CO₂ to make one glucose. The cycle is named after Melvin Calvin. Your textbook names only the Calvin cycle; the steps are shown here to help you understand it.
Factors affecting photosynthesis
| Factor | Effect |
|---|---|
| Light (external) | More light gives more photosynthesis, up to a limit; red and blue light work best |
| Carbon dioxide (external) | Air has very little CO₂, so it often limits the rate; more CO₂ speeds photosynthesis up to a limit |
| Temperature (external) | Works best at moderate warmth; very high or very low temperatures slow it, because enzymes work only in a narrow range |
| Water (external) | Shortage of water closes the stomata and slows the rate |
| Mineral elements (external) | Magnesium and nitrogen are needed to make chlorophyll; their lack turns leaves yellow |
| Pigments, leaf age, sugar build-up, hormones (internal) | More chlorophyll helps; old leaves work less; sugar piling up in the leaf slows the process |
How to draw a chloroplast
- 1Draw an oval with a double outline. Label outer membrane and inner membrane.
- 2Inside, draw 4 to 6 stacks that look like piles of coins. Label one stack granum and one disc thylakoid.
- 3Join the stacks with thin lines. Label stroma lamellae.
- 4Label the fluid filling the rest of the oval stroma. Add a few dots for ribosomes and a small loop for DNA.
Every grain of rice and every banana began as glucose made in a leaf, and the oxygen we breathe comes from the splitting of water in the grana. Farmers add magnesium and nitrogen fertilisers to keep leaves green and productive.
1 marks: “Where does the oxygen released in photosynthesis come from?” From water, by photolysis in the light reaction.
4 marks: “Distinguish between the light and dark reactions of photosynthesis.” Grana vs stroma; needs light vs not directly; photolysis and O₂ release vs CO₂ fixation; ATP and NADPH₂ made vs used to make glucose; Calvin cycle.
7 marks: “Explain the process of photosynthesis with a labelled diagram of a chloroplast.” Definition and equation; labelled chloroplast; pigments; light reaction in the grana (photolysis, ATP, NADPH₂, O₂); dark reaction in the stroma (Calvin cycle: CO₂ fixed into glucose using ATP and NADPH₂); factors.
Light in Grana, Dark in Stroma: 'Lovely Garden, Dusty Stones'. The light reaction makes the power (ATP, NADPH₂); the dark reaction spends it to fix CO₂.