1. Photosynthesis
The process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. Occurs in chloroplasts containing chlorophyll.
Glucose produced is used for: respiration (energy), building cellulose (cell walls), making starch (storage), making amino acids and proteins (with nitrate ions), making lipids.
1. Boil leaf in water (kills cells, stops reactions). 2. Boil in ethanol (decolourises leaf, removes chlorophyll). 3. Rinse with water (softens leaf). 4. Add iodine solution. Blue-black = starch present.
2. Limiting Factors
A limiting factor is the factor present in the shortest supply that limits the rate of photosynthesis. Even if other conditions are ideal, the limiting factor determines the maximum rate.
| Limiting factor | Effect of increasing it | How to investigate |
|---|---|---|
| Light intensity | Rate increases until another factor limits | Move lamp closer; measure O₂ bubbles per minute |
| CO₂ concentration | Rate increases until another factor limits | Add sodium hydrogencarbonate to water |
| Temperature | Rate increases up to enzyme optimum (~25–30°C), then falls (enzyme denaturation) | Use water bath |
If a graph shows rate plateauing despite increasing light intensity, the limiting factor has changed to something else (CO₂ or temperature). You must identify WHICH factor is now limiting, not just say "another factor".
3. Leaf Structure and Adaptations
| Structure | Adaptation | Function |
|---|---|---|
| Broad, flat lamina | Large surface area | Maximises light absorption |
| Thin leaf | Short diffusion distance | CO₂ reaches mesophyll cells quickly |
| Transparent upper epidermis | No chloroplasts; allows light through | Light reaches palisade layer |
| Palisade layer | Many chloroplasts; columnar cells | Maximum photosynthesis |
| Spongy mesophyll | Air spaces between cells | CO₂ and O₂ diffusion |
| Stomata (lower epidermis) | Open and close via guard cells | Gas exchange; controls water loss |
| Network of veins | Xylem and phloem | Water and dissolved substances in; sugars out |
4. Mineral Requirements
| Mineral ion | Needed for | Deficiency symptom |
|---|---|---|
| Nitrate (NO₃⁻) | Making amino acids and proteins | Stunted growth; yellow older leaves |
| Magnesium (Mg²⁺) | Making chlorophyll | Yellow leaves (chlorosis) — especially younger leaves |
| Phosphate (PO₄³⁻) | DNA, ATP and cell membranes | Poor root growth; purple discolouration |
- Three limiting factors: light intensity, CO2 concentration, temperature
- Each factor limits independently - the lowest factor sets the rate
- Nitrate deficiency: stunted growth (no amino acids, no protein). Magnesium deficiency: yellow leaves (no chlorophyll)
- Stomata: open in light (CO2 in, O2 out); close in darkness to reduce water loss
- Leaf adaptations for photosynthesis: large flat surface, thin, many stomata, palisade layer near top
5. Common Exam Traps
"Chlorophyll produces light energy" is wrong. Chlorophyll absorbs light energy and uses it to drive photosynthesis. The source of energy is sunlight; chlorophyll is the absorber.
Plants photosynthesise only in light, but they respire 24 hours a day. In bright light, photosynthesis rate exceeds respiration rate so net gas exchange shows CO₂ in and O₂ out. In darkness, only respiration occurs: CO₂ out and O₂ in.
Most stomata are on the lower (abaxial) surface of a leaf — not the upper. This reduces water loss since the lower surface is shaded from direct sunlight and thus cooler.
Key Terms — Flashcard Review
Tap each card to reveal the definition.
🎯 Practice Quiz — Test Yourself
8 O Level-style questions on this topic. Select an answer to see instant feedback.
Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.