Photosynthesis
What is Photosynthesis?
Photosynthesis is the process by which plants (and some algae and cyanobacteria) use light energy to convert inorganic raw materials into organic molecules. It is the foundation of almost all food chains on Earth and the source of the oxygen in our atmosphere.
Word equation:
Carbon dioxide + Water → Glucose + Oxygen (using light energy, in the presence of chlorophyll)
Balanced symbol equation:
6CO2 + 6H2O → C6H12O6 + 6O2
(This equation is endothermic — it takes in energy from light.)
Where Does it Happen?
Photosynthesis occurs in chloroplasts — organelles found mainly in leaf cells (also in green stems). Chloroplasts contain chlorophyll, a green pigment that absorbs light energy.
Chlorophyll absorbs red and blue wavelengths most strongly. It reflects green light, which is why leaves appear green to our eyes.
Within the chloroplast, photosynthesis occurs in two main stages:
- Light-dependent reactions (in the thylakoid membranes): light energy splits water, releasing oxygen and producing ATP
- Light-independent reactions / Calvin cycle (in the stroma): CO2 is fixed using ATP to produce glucose
Inputs and Outputs
Inputs (reactants):
- Carbon dioxide — from the air; enters leaves through tiny pores called stomata
- Water — absorbed by root hair cells; transported to leaves via xylem vessels
- Light energy — from the Sun; absorbed by chlorophyll
Outputs (products):
- Glucose — used by the plant for energy (respiration), growth, and storage
- Oxygen — released through stomata as a by-product; vital for aerobic life on Earth
How Plants Use Glucose
Glucose produced by photosynthesis is used in several ways:
| Use | Process |
|---|---|
| Energy release | Aerobic respiration |
| Short-term storage | Converted to starch (insoluble, stored in leaves and roots) |
| Cell wall building | Converted to cellulose |
| Protein synthesis | Combined with nitrates to make amino acids |
| Fat storage | Converted to lipids (oils and fats) |
Limiting Factors
The rate of photosynthesis is controlled by the factor in shortest supply — the limiting factor.
1. Light intensity Increasing light intensity speeds up photosynthesis. Beyond a certain point, other factors become limiting and the rate plateaus.
2. Carbon dioxide concentration Increasing CO2 concentration increases the rate. Greenhouses sometimes enrich CO2 levels to boost plant growth.
3. Temperature Enzymes control photosynthesis. As temperature rises towards the optimum (~25–35°C), rate increases. Above the optimum, enzymes denature and the rate drops sharply.
4. Water availability Severe water shortage causes stomata to close (preventing CO2 entry) and slows photosynthesis.
Worked Example — Limiting Factors
A plant is grown in low light. Increasing the light intensity speeds up photosynthesis significantly. However, once the light is bright enough, adding more light has no effect. Why?
At that point, CO2 concentration has become the new limiting factor — the plant cannot fix more CO2 despite having enough light. Increasing CO2 would then increase the rate again.
Testing for Starch — Practical
To show that a leaf has photosynthesised:
- Kill the leaf by placing it in boiling water (stops enzyme activity)
- Remove chlorophyll by boiling the leaf in ethanol (decolourises the leaf)
- Rinse with water to soften the leaf
- Add iodine solution
Result: Starch is present → leaf turns blue-black
This test can be combined with a variegated leaf (green and white sections) or a leaf kept in the dark to show that:
- Photosynthesis requires chlorophyll (white sections show no starch)
- Photosynthesis requires light (dark-covered sections show no starch)
Adaptations of Leaves for Photosynthesis
Leaves are highly adapted to maximise photosynthesis:
- Large, flat surface — maximises light absorption
- Thin — short diffusion distance for CO2
- Transparent upper epidermis — allows light through to palisade cells
- Palisade mesophyll cells — packed with chloroplasts; positioned at top of leaf for maximum light
- Stomata and air spaces — allow CO2 to diffuse in and O2 to diffuse out
- Network of veins (xylem and phloem) — deliver water and remove glucose
Key Terms
- Photosynthesis — process converting light energy into chemical energy (glucose)
- Chlorophyll — green pigment in chloroplasts that absorbs light energy
- Chloroplast — organelle where photosynthesis takes place
- Stomata — tiny pores in leaves through which gas exchange occurs
- Limiting factor — the factor that restricts the rate of photosynthesis
- Starch — insoluble storage molecule made from glucose
Common Mistakes
- Confusing respiration (releases energy from glucose) with photosynthesis (stores energy in glucose) — they are opposite processes
- Saying plants do not respire — plants respire continuously; they photosynthesise only in light
- Thinking oxygen is a product of splitting CO2 — the oxygen comes from splitting water (H2O)
- Confusing the colour change for starch: iodine turns blue-black with starch, not brown
Tips and Tricks
- Word equation: CO2 + H2O → glucose + O2 (reactants are gases and liquid; products are solid sugar and gas)
- Limiting factor graphs all follow the same pattern: rate increases then plateaus — the plateau indicates a new limiting factor
- Remember chlorophyll absorbs red and blue but reflects green — plants look green because green is the reflected colour
- The iodine test for starch is one of the most common practical questions — learn the step-by-step method