ZIMSEC O Level · Biology · Form 3
Plants are autotrophs: they make their own food by photosynthesis.
carbon dioxide + water → (light, chlorophyll) → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The leaf
- Large surface area and thin, for absorbing light and quick diffusion of gases.
- Palisade mesophyll: many chloroplasts. Spongy mesophyll: air spaces for gas exchange.
- Stomata (mainly on the lower surface) open and close using guard cells.
- Veins: xylem brings water; phloem takes sugars away.
Limiting factors
The rate is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature.
Uses of glucose
Respiration; stored as starch; made into cellulose, proteins (with nitrates) and oils.
Mineral nutrition
- Nitrates: for proteins. Lack causes stunted growth and yellow leaves.
- Magnesium: for chlorophyll. Lack causes yellowing between the veins.
Exam tip: To show a factor is needed, destarch the plant first (keep it in the dark for 48 hours), then test leaves with iodine.
Check yourself
- Give the balanced equation for photosynthesis.
- Why is a plant kept in the dark before a starch experiment?
- State two symptoms of nitrate deficiency.
Show answers
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- To destarch it, so any starch found was made during the experiment.
- Stunted growth and yellow (older) leaves.
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