Photosynthesis
What Is Photosynthesis?
Photosynthesis is the process by which plants (and some protoctists) convert light energy into chemical energy stored as glucose. It is the primary source of organic molecules and energy for almost all life on Earth.
Equations
Word equation:
carbon dioxide + water → glucose + oxygen
Balanced symbol equation:
- Occurs in chloroplasts (contains chlorophyll which absorbs light)
- Requires light energy
- Produces glucose (used for energy and as a building block) and oxygen (released as a by-product)
Factors Affecting the Rate of Photosynthesis (Limiting Factors)
The rate of photosynthesis is controlled by whichever factor is in shortest supply: the limiting factor.
| Factor | Effect of increasing | Reason |
|---|---|---|
| Light intensity | Increases rate (up to a limit) | More energy available to power photosynthesis |
| CO₂ concentration | Increases rate (up to a limit) | More raw material available |
| Temperature | Increases rate up to optimum; falls above | Enzyme-controlled reactions; denaturation at high temperatures |
Graph interpretation:
- If increasing light intensity increases rate but increasing CO₂ does not, then CO₂ is the limiting factor.
- The curve levels off when another factor becomes limiting.
Leaf Adaptations for Photosynthesis
| Adaptation | How it helps |
|---|---|
| Large flat surface area | Maximises light absorption |
| Thin | Short diffusion distance for CO₂ and O₂ |
| Palisade cells packed with chloroplasts near the top | Maximum light absorption near the surface |
| Spongy mesophyll with air spaces | CO₂ and O₂ can diffuse freely inside the leaf |
| Stomata (pores in lower epidermis) | Allow CO₂ in and O₂ out by diffusion |
| Guard cells control stomatal opening | Regulate gas exchange and water loss |
| Vascular bundles (veins) | Xylem brings water; phloem takes away glucose (as sucrose) |
| Transparent upper epidermis | Allows light to reach palisade cells |
Mineral Ions for Plant Growth (Spec 2.22)
Plants need mineral ions absorbed through roots. Two key ones:
| Mineral ion | Needed for | Deficiency symptom |
|---|---|---|
| Nitrate (NO₃⁻) | Making amino acids → proteins; growth | Stunted growth, yellow leaves |
| Magnesium (Mg²⁺) | Making chlorophyll | Yellow leaves (chlorosis) |
Exam Questions
Q: Write the balanced symbol equation for photosynthesis.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (requires light energy and chlorophyll)
Q: Explain how a leaf is adapted for photosynthesis.
The leaf has a large, flat surface area to absorb light. It is thin, reducing the diffusion distance for CO₂. Palisade cells near the top contain many chloroplasts to absorb light efficiently. Air spaces in the spongy mesophyll allow CO₂ to diffuse to cells. Stomata allow CO₂ to enter and O₂ to leave.
Q: A plant is given plenty of light but its rate of photosynthesis does not increase. Suggest what the limiting factor might be.
The limiting factor is likely to be CO₂ concentration (or temperature). If CO₂ is in short supply, adding more light cannot increase the rate further.
Q: A plant shows yellowing of leaves. Suggest what mineral ion is deficient and explain why.
Magnesium is deficient. Magnesium is needed to make chlorophyll. Without sufficient magnesium, the plant cannot produce enough chlorophyll, so the leaves turn yellow (chlorosis).
Exam Tip
The equation for photosynthesis is the reverse of the equation for aerobic respiration. Both involve CO₂, H₂O, glucose and O₂, but in opposite directions. Light is only needed for photosynthesis (not respiration).