PhotosynthesisCambridge IGCSE Biology: Revision notes
Section 1
What is photosynthesis and how is it represented?
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. It occurs in chloroplasts, which contain the green pigment chlorophyll.
The process can be represented in two ways:
Word equation: Carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
Balanced chemical equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This equation shows that six molecules of carbon dioxide and six of water are needed to produce one molecule of glucose and six molecules of oxygen. The process requires light energy and chlorophyll as essential conditions.
Examiners expect you to know both the word equation and the balanced equation. Always state that light and chlorophyll are required conditions when writing about photosynthesis.
Think of chlorophyll as a solar panel and the chloroplast as a factory: the chlorophyll captures light energy, and the chloroplast uses that energy to build glucose from simple starting materials.
Section 2
What is the role of chlorophyll and how are photosynthetic products used?
Chlorophyll is a green pigment that transfers energy from light into chemical energy stored in carbohydrates. Without chlorophyll, plants cannot absorb light energy and photosynthesis cannot occur.
Once glucose is synthesised, plants use and store it in several ways:
| Product | Function | Notes |
|---|---|---|
| Starch | Energy storage | Stored in leaves and roots for later use |
| Cellulose | Structural support | Used to build and strengthen cell walls |
| Glucose | Immediate energy | Used in respiration to release energy |
| Sucrose | Transport molecule | Moved through phloem to other parts of plant |
| Nectar | Attraction | Attracts insects for pollination and seed dispersal |
Plants also use glucose to synthesise other organic compounds needed for growth and function.
When asked about the fate of glucose, cover at least three uses: starch storage, cellulose for structure, and glucose for respiration. If space allows, mention sucrose transport.
Students often confuse starch and cellulose. Remember: starch is storage (temporary), cellulose is structure (permanent in cell walls).
Section 3
Why are mineral ions important for photosynthesis?
Although photosynthesis requires only carbon dioxide, water, and light, plants need mineral ions from the soil for optimal photosynthetic performance and general health.
Two mineral ions are especially critical:
Nitrate ions (NO₃⁻)
- Used to synthesise amino acids
- Amino acids are needed to build proteins required for growth and enzyme production
- Without sufficient nitrate, enzyme synthesis is reduced, limiting the rate of photosynthesis
Magnesium ions (Mg²⁺)
- Essential for the synthesis of chlorophyll molecules
- A magnesium ion sits at the centre of each chlorophyll molecule
- Without magnesium, chlorophyll cannot be made, so less light energy is absorbed
- Plants appear yellow-green when magnesium is deficient
Both mineral ions are therefore limiting factors of photosynthesis under certain conditions.
Link mineral ions to photosynthesis rate: nitrate → proteins/enzymes → faster reactions; magnesium → chlorophyll → more light absorption. Examiners reward this cause-and-effect reasoning.
If a plant shows yellow leaves but green veins, suspect magnesium deficiency (chlorophyll cannot be made). If growth is stunted and pale, suspect nitrate deficiency (proteins cannot be made for growth).
Section 4
How can we investigate the requirements and limiting factors of photosynthesis?
Investigating requirements: Testing that chlorophyll, light, and carbon dioxide are all necessary
Essential control design: Compare the test plant with identical plants, each missing ONE variable:
| Variable Investigated | Test Setup | Expected Result if Variable Required |
|---|---|---|
| Chlorophyll | Use a plant without chlorophyll (variegated or albino) | Non-green parts show no starch; only green parts photosynthesize |
| Light | Keep plant in dark; compare with illuminated plant | Dark plant produces no starch; light plant produces starch |
| Carbon dioxide | Seal plant in container with soda lime (absorbs CO₂); compare with normal air | CO₂-depleted plant produces no starch |
Test for starch production using iodine solution: blue-black colour indicates starch presence.
Investigating limiting factors: Testing effects of light intensity, carbon dioxide concentration, and temperature
- Vary light intensity: Measure oxygen production (using a gas collection tube) or starch production at different light distances. Rate increases then plateaus when another factor becomes limiting.
- Vary CO₂ concentration: Measure oxygen release or starch formation. Rate increases with CO₂ until another factor limits further increase.
- Vary temperature: Measure photosynthetic rate. Rate increases until 25–35°C (enzyme optimum), then decreases due to enzyme denaturation.
Gas exchange investigation: Use hydrogencarbonate indicator solution to detect gas exchange:
- Indicator is red in normal air (neutral pH)
- Turns yellow when CO₂ is added (acidic)
- Turns purple when CO₂ is removed (alkaline)
In light conditions: Plant photosynthesises faster than respiring, removing more CO₂ than it releases. Indicator turns purple. In dark conditions: Plant only respires, releasing CO₂. Indicator turns yellow.
When describing an investigation, always state: (1) what variable you change (independent), (2) what you measure (dependent), and (3) what you keep constant (controls). Examiners mark this rigorous experimental design.
To investigate if light is needed: Place two identical aquatic plants in hydrogencarbonate indicator—one in light, one in dark. After 1 hour, the light plant's indicator turns purple (CO₂ removed by photosynthesis) whilst the dark plant's turns yellow (CO₂ released by respiration). This proves light is required.
Students often forget to keep variables constant. If testing the effect of light intensity, you must keep temperature and CO₂ constant, or you cannot identify which factor caused the change.
Section 5
What determines the rate of photosynthesis in different environments?
The rate of photosynthesis is controlled by limiting factors. A limiting factor is an environmental variable that restricts photosynthesis rate when not present in sufficient quantity.
The three main limiting factors:
1. Light intensity
- At low light: increases in light intensity increase photosynthesis rate (photosynthesis is light-limited)
- At high light: further increases have little effect; another factor becomes limiting
- Relationship is roughly linear at low intensities
2. Carbon dioxide concentration
- At low CO₂: increasing concentration increases photosynthesis rate (CO₂-limited)
- At high CO₂: rate plateaus; another factor (e.g. light, temperature, enzyme availability) becomes limiting
- Most environments have 0.04% CO₂; this is often limiting
3. Temperature
- Photosynthesis is enzyme-catalysed, so rate increases with temperature (within limits)
- Optimal temperature is typically 25–35°C
- Below optimum: molecular collisions increase, enzyme activity increases
- Above optimum: enzymes denature, rate decreases rapidly
- Very low temperatures: enzyme activity too slow; photosynthesis negligible
Identifying limiting factors in different conditions:
| Environment | Primary Limiting Factor(s) | Why |
|---|---|---|
| Dense forest floor | Light | Canopy blocks sunlight |
| Winter (temperate regions) | Temperature and light | Low light intensity, cold temperatures |
| Sealed greenhouse | Often CO₂ | Photosynthesis uses CO₂ faster than it is replenished |
| Bright, warm day | None until threshold | Usually light, CO₂, and temperature all adequate |
Key principle: When one factor is limiting, increasing other factors has little effect. Growers must identify and remedy the limiting factor to maximise yield.
When asked to explain why a factor is limiting, connect it to enzyme activity or light absorption: 'Low temperature slows enzyme reactions' or 'Low light reduces energy availability for photosynthesis'.
A farmer grows tomatoes in a greenhouse. On a cold, cloudy winter day, photosynthesis is slow. Adding extra CO₂ has minimal effect. Why? Because temperature and light intensity are the limiting factors, not CO₂. Improving these would increase rate much more.
Think of photosynthesis like a car factory assembly line: if you lack steel (light), adding more workers (enzymes) won't help. You must fix the shortage of steel (the limiting factor) first.
Must Know
- Photosynthesis equation (word and balanced): Carbon dioxide + water → glucose + oxygen (in light and chlorophyll); 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Chlorophyll is the green pigment in chloroplasts that absorbs light energy and converts it to chemical energy stored in glucose
- Glucose fate: Starch (storage), cellulose (cell walls), respiration (energy), sucrose (transport), nectar (pollination)
- Mineral ions: Nitrate ions make amino acids and proteins (needed for growth and enzymes); magnesium ions are required to synthesise chlorophyll
- Three limiting factors of photosynthesis: Light intensity, CO₂ concentration, and temperature; whichever is in shortest supply limits the overall rate
- Investigations: Use iodine to test for starch, hydrogencarbonate indicator to detect gas exchange (purple = photosynthesis > respiration; yellow = respiration > photosynthesis); always use proper controls when testing requirements
That's the notes covered.
Carry on to the next subtopic.