Key Takeaways for Photosynthesis

1. Photosynthesis Basics

  • Definition: Process where plants/algae convert light energy into chemical energy (glucose) using CO₂ and H₂O.
  • Word Equation:
    carbon dioxide + water→light, chlorophyllglucose + oxygencarbon dioxide + waterlight, chlorophyll​glucose + oxygen
  • Balanced Symbol Equation:
    6CO2+6H2O→lightC6H12O6+6O26CO2​+6H2​Olight​C6​H12​O6​+6O2​
  • Key Structures:
    • Chloroplasts: Organelles containing chlorophyll (green pigment) where photosynthesis occurs.
    • Palisade mesophyll cells: Leaf cells packed with chloroplasts for maximum light absorption.

Example: Algae contribute ~70% of Earth’s oxygen via photosynthesis.

2. Factors Affecting Photosynthesis (Limiting Factors)

Four main factors limit the rate:

  1. Light Intensity
    • Light provides energy for the reaction.
    • Graph Tip: Rate increases with light until another factor (e.g., CO₂) becomes limiting.
  2. Carbon Dioxide Concentration
    • CO₂ is a reactant; low levels slow the reaction.
  3. Temperature
    • Affects enzyme activity. Optimum: ~25°C. Below 10°C, enzymes work slower; above 45°C, enzymes denature.
  4. Chlorophyll Levels
    • Magnesium deficiency (chlorosis) causes yellow leaves, reducing chlorophyll.

Experiment Tip: Use pondweed to measure oxygen bubbles under varying light (inverse square law: Light Intensity∝1distance2Light Intensity∝distance21​).

3. Uses of Glucose

Plants use glucose for:

  1. Respiration: Releases energy.
  2. Starch Storage: Insoluble, so doesn’t affect osmosis (e.g., potatoes).
  3. Cellulose: Strengthens cell walls.
  4. Proteins: Combined with nitrates absorbed by roots.
  5. Fats/Oils: Long-term energy storage.

Food Chains: Only ~10% of energy transfers to the next trophic level.

4. Key Experiments

  • Testing for Starch:
    • Boil leaf → soak in ethanol → rinse → add iodine. Blue-black = starch present.
  • Light Intensity Practical:
    • Control variables: Temperature, CO₂ concentration.
    • Measure oxygen bubbles at different distances (e.g., 10 cm, 20 cm).

Exam Trick: If bubbles slow, light is no longer the limiting factor (check CO₂ or temperature).

5. Greenhouses & Farming

  • Optimising Conditions:
    • Heaters → maintain temperature.
    • Artificial light → extend daylight.
    • CO₂ enrichment → boost photosynthesis.
  • Cost-Benefit: Farmers balance expense of artificial conditions against crop yield.

Graph Analysis: In Figure 9.6, CO₂ above 1000 ppm has no effect (plateau) → other factors limit.

6. Common Exam Mistakes

  • Confusing Tissues:
    • Xylem: Transports water (roots → leaves).
    • Phloem: Transports glucose (leaves → plant).
  • Endothermic vs. Exothermic:
    • Photosynthesis absorbs energy (endothermic). Respiration releases energy (exothermic).
  • Starch vs. Glucose:
    • Plants store starch (insoluble), not glucose (soluble).

7. Practice Questions

  1. Word Equation: Fill in the blanks:
    carbon dioxide + X → glucose + Ycarbon dioxide + X → glucose + Y
    Answer: X = water, Y = oxygen.
  2. Limiting Factors: Why might a plant in a dark room grow slowly?
    Answer: Low light intensity limits photosynthesis.

Revision Tip: Draw labelled diagrams of chloroplasts and photosynthesis graphs. Memorise equations using flashcards! 🌱🔬

50 GCSE Photosynthesis Questions & Detailed Answers

Section A: Photosynthesis Basics

  1. What are the reactants in photosynthesis?
    • Answer: Carbon dioxide (CO2CO2​) and water (H2OH2​O).
  2. Give the word equation for photosynthesis.
    • Answer:
      carbon dioxide + water→light, chlorophyllglucose + oxygencarbon dioxide + waterlight, chlorophyll​glucose + oxygen
  3. Write the balanced chemical equation for photosynthesis.
    • Answer:
      6CO2+6H2O→lightC6H12O6+6O26CO2​+6H2​Olight​C6​H12​O6​+6O2​
  4. Where does photosynthesis occur in plant cells?
    • Answer: In chloroplasts, which contain chlorophyll.
  5. Which leaf cells are specialised for photosynthesis?
    • Answer: Palisade mesophyll cells (packed with chloroplasts).
  6. Why is photosynthesis an endothermic reaction?
    • Answer: It absorbs light energy to convert CO2CO2​ and H2OH2​O into glucose.
  7. How do plants absorb carbon dioxide?
    • Answer: Through stomata in the leaves, which open to allow gas exchange.
  8. What is the role of chlorophyll?
    • Answer: It absorbs light energy and transfers it for use in photosynthesis.
  9. Why are root hair cells adapted for water absorption?
    • Answer: They have a large surface area and thin walls for efficient osmosis.
  10. Name the process by which water moves from roots to leaves.
    • Answer: Transpiration, via xylem vessels.

Section B: Limiting Factors

  1. Define a limiting factor.
    • Answer: A factor that restricts the rate of a process (e.g., photosynthesis) when in short supply.
  2. List the four limiting factors of photosynthesis.
    • Answer:
      1. Light intensity
      2. Carbon dioxide concentration
      3. Temperature
      4. Chlorophyll availability
  3. Explain why low light intensity reduces photosynthesis.
    • Answer: Light provides energy for the reaction; insufficient light slows glucose production.
  4. How does temperature affect enzyme activity in photosynthesis?
    • Answer: Enzymes work best at 25°C. Low temperatures reduce kinetic energy; high temperatures denature enzymes.
  5. Why does magnesium deficiency cause yellow leaves?
    • Answer: Magnesium is needed to make chlorophyll; its absence leads to chlorosis.
  6. Interpret Figure 9.3a: Why does the rate plateau at high light intensity?
    • Answer: Another factor (e.g., CO2CO2​ or temperature) becomes limiting.
  7. Calculate light intensity at 40 cm if it is 100 units at 20 cm (use the inverse square law).
    • Answer:
      Intensity=100(40/20)2=1004=25 unitsIntensity=(40/20)2100​=4100​=25 units
  8. Why do farmers add carbon dioxide to greenhouses?
    • Answer: To prevent CO2CO2​ from becoming a limiting factor, boosting photosynthesis.
  9. What happens to photosynthesis if chlorophyll levels are reduced?
    • Answer: Less light energy is absorbed, slowing glucose production.
  10. Why might tropical plants photosynthesise faster than Arctic plants?
    • Answer: Higher temperatures and light intensity in tropical regions.

Section C: Experiments & Data Analysis

  1. Describe the steps to test a leaf for starch.
    • Answer:
      1. Boil the leaf to stop metabolic processes.
      2. Soak in ethanol to remove chlorophyll.
      3. Rinse and add iodine solution. Blue-black = starch present.
  2. Why is a water bath used in the starch test?
    • Answer: To safely heat ethanol, which is flammable.
  3. In the pondweed experiment, why count bubbles for 1 minute?
    • Answer: To calculate the rate of oxygen production (bubbles per minute).
  4. Why use a beaker of water between the lamp and pondweed?
    • Answer: To absorb heat, preventing temperature changes from affecting results.
  5. What does a plateau in a light intensity vs. rate graph indicate?
    • Answer: Another factor (e.g., CO2CO2​) is now limiting.
  6. How does sodium hydrogen carbonate help in the leaf disc experiment?
    • Answer: It releases CO2CO2​, ensuring it is not a limiting factor.
  7. Identify the independent variable in Figure 9.9.
    • Answer: Carbon dioxide concentration (%).
  8. From Figure 9.9, what temperature gives the highest rate of photosynthesis?
    • Answer: 25°C (see graph axis).
  9. Why repeat bubble counts three times in the pondweed experiment?
    • Answer: To calculate a mean, improving reliability.
  10. Why might a plant in a dark cupboard not produce starch?
    • Answer: No light → no photosynthesis → no glucose/starch.

Section D: Uses of Glucose

  1. List five uses of glucose in plants.
    • Answer:
      1. Respiration (energy release)
      2. Stored as starch
      3. Converted to cellulose for cell walls
      4. Combined with nitrates to make proteins
      5. Stored as fats/oils
  2. Why is starch better than glucose for storage?
    • Answer: Starch is insoluble, so it does not affect cell osmosis.
  3. How do plants obtain nitrates?
    • Answer: Absorbed from soil via active transport in root hairs.
  4. What percentage of energy transfers between trophic levels?
    • Answer: ~10% (90% lost as heat/waste).
  5. Why is cellulose vital for plant structure?
    • Answer: It strengthens cell walls, supporting stems and leaves.

Section E: Advanced Concepts

  1. Why do algae produce 70% of Earth’s oxygen?
    • Answer: They are abundant in oceans and photosynthesise efficiently.
  2. Name two pigments in algae other than chlorophyll.
    • Answer: Fucoxanthin (brown), phycoerythrin (red).
  3. How does light intensity affect oxygen levels over 24 hours?
    • Answer: Oxygen peaks during daylight (photosynthesis) and drops at night (respiration).
  4. Why is glucose converted to sucrose for transport?
    • Answer: Sucrose is less reactive and soluble, making it safer for translocation.
  5. What happens to oxygen produced in photosynthesis?
    • Answer: Released via stomata or used in plant respiration.

Section F: Exam-Style Questions

  1. Complete: Carbon dioxide + ______ → glucose + ______.
    • Answer: Water; oxygen.
  2. Explain why farmers use polytunnels.
    • Answer: To control temperature, light, and CO2CO2​, increasing yield.
  3. Compare xylem and phloem.
    • Answer:
      • Xylem: Dead cells, transports water/minerals upwards, no energy required.
      • Phloem: Living cells, transports glucose bidirectionally (translocation), requires energy.
  4. Calculate light intensity at 30 cm if it is 90 units at 10 cm.
    • Answer:
      Intensity=90(30/10)2=909=10 unitsIntensity=(30/10)290​=990​=10 units
  5. Why might a greenhouse in winter have low CO2CO2​ levels?
    • Answer: Closed ventilation reduces air exchange, and plants use CO2CO2​ during photosynthesis.
  6. What is the chemical formula for glucose?
    • Answer: C6H12O6C6​H12​O6​
  7. Why does photosynthesis stop at night?
    • Answer: No light energy to drive the reaction.
  8. Describe how magnesium deficiency affects crop yield.
    • Answer: Reduces chlorophyll → less photosynthesis → lower glucose → stunted growth.
  9. Why might a plant in a cold environment grow slowly?
    • Answer: Low temperature limits enzyme activity in photosynthesis.
  10. Explain the term ‘yield’ in agriculture.
    • Answer: The amount of useful crop produced (e.g., fruits, grains).