Photosynthesis and Plants
The Photosynthesis Equation
Photosynthesis converts light energy into chemical energy stored in glucose:
Word equation: carbon dioxide + water → glucose + oxygen Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The reaction requires light energy, absorbed by chlorophyll in chloroplasts. Chlorophyll appears green because it reflects green light and absorbs red and blue wavelengths.
Leaf Structure
The leaf is the main organ of photosynthesis. Its adaptations:
| Structure | Adaptation |
|---|---|
| Palisade mesophyll | Tightly packed, tall cells near upper surface; many chloroplasts; maximum light absorption |
| Spongy mesophyll | Air spaces for CO₂/O₂ diffusion |
| Stomata | Pores in epidermis (mainly lower surface) for gas exchange; opened/closed by guard cells |
| Guard cells | Control stomatal opening; become turgid (open pores) in light; flaccid (close) when dehydrated |
| Waxy cuticle | Reduces water loss by evaporation |
| Veins (vascular bundles) | Xylem (water/minerals up) and phloem (sugars up/down) |
Limiting Factors of Photosynthesis
Rate of photosynthesis is limited by whichever factor is in shortest supply:
- Light intensity: increasing light increases rate until another factor becomes limiting. Rate ∝ light intensity at low intensities.
- CO₂ concentration: increasing CO₂ increases rate until light or temperature limits.
- Temperature: enzymes involved in photosynthesis (e.g. RuBisCO) work faster as temperature rises (up to ~40°C); above optimum, enzymes denature → rate drops sharply.
Required Practical: investigate the effect of light intensity on photosynthesis rate using aquatic plants (e.g. Elodea) by counting oxygen bubbles per minute at different distances from a lamp. Rate ∝ 1/distance².
Plant Transport
Xylem: dead cells forming continuous hollow tubes; transport water and dissolved minerals from roots to leaves by transpiration pull. One-directional (upwards).
Phloem: living cells (sieve tubes + companion cells); transport dissolved sugars (sucrose) from leaves (source) to all other parts (sinks — roots, fruits, growing regions). Bidirectional. This process is called translocation.
Transpiration: water evaporates from leaves through stomata → creates tension that pulls water up xylem. Factors increasing transpiration: higher temperature, lower humidity, higher wind speed, more light (stomata open wider).
Osmosis in roots: water moves from soil (high water potential) into root hair cells (lower water potential) by osmosis; then moves cell-to-cell across root cortex into xylem.
Uses of Glucose
Glucose made in photosynthesis is used for:
- Respiration (releases energy)
- Making cellulose (cell walls)
- Making starch (storage in leaves/tubers)
- Making sucrose (transport)
- Making amino acids (with nitrates from soil) → proteins
- Making lipids (oils in seeds)
⚠Common mistakes
- Confusing transpiration (water movement) with translocation (sugar movement).
- Forgetting that above optimum temperature, the rate drops (enzymes denature), not just levels off.
- Stating chlorophyll is in the nucleus — it is in chloroplasts in the cytoplasm.
- Saying photosynthesis only happens in daylight — it is light-dependent, not time-dependent.
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