GCSE Biology: Trophic Levels & Food Production

Key Takeaways for Trophic Levels & Food Production

For GCSE Biology Revision

1. Trophic Levels in Ecosystems

• Food Chains & Webs
  • Food Chain: Linear sequence (e.g., grass → zebra → lion).
  • Food Web: Interconnected food chains (e.g., zebras and antelopes both eaten by lions and hyenas).
  • Arrows show energy/biomass transfer, not “eaten by.”
• Trophic Levels
  1. Producer (plants/algae).
  2. Primary Consumer (herbivores).
  3. Secondary Consumer (carnivores).
  4. Tertiary/Quaternary Consumer (apex predators, e.g., golden eagles).
• Energy Transfer
  • Only 10% of energy is transferred between levels.
  • 90% lost via respiration, waste, and uneaten parts.
  • Equation:
    Energy at next level=0.1×Energy at current levelEnergy at next level=0.1×Energy at current level
  • Example: If grass stores 10,000 kJ, zebras get 1,000 kJ, lions get 100 kJ.
• Pyramids of Biomass
  • Always pyramid-shaped (producers > primary consumers > secondary consumers).
  • Biomass = Total dry mass of organisms.
  • Calculation:
    Total biomass=Number of organisms×Dry mass per organismTotal biomass=Number of organisms×Dry mass per organism
  • Example (Rock Pool Activity):
    • Seaweed (producer): 256×3 g=768 g256×3g=768g.
    • Limpets (primary consumer): 44×3 g=132 g44×3g=132g.

2. Food Production & Security

• Food Security
  • Definition: Reliable access to sufficient affordable food.
  • Threats: Population growth, climate change, pests, conflict.
  • Example: 2007–2008 grain price rise caused global riots.
• Farming Techniques
  • Intensive Farming:
    • Maximises yield using machinery, fertilisers, and pesticides.
    • Pros: High productivity.
    • Cons: Biodiversity loss, antibiotic resistance, pollution.
  • Organic Farming:
    • Uses natural fertilisers (manure) and pesticides (pyrethrin).
    • Pros: Eco-friendly. Cons: Higher costs, lower yields.
• Sustainable Fisheries
  • Quotas: Limits on fish catches to prevent overfishing.
  • Larger net holes: Allow juvenile fish to escape and reproduce.
  • Example: North Sea cod quotas helped stabilise populations.
• Biotechnology
  • Mycoprotein (Quorn):
    • Grown in fermenters with glucose, oxygen, and ammonia.
    • Advantages: Low land use, high protein, no cholesterol.
  • GM Insulin:
    • Human insulin gene inserted into E. coli bacteria.
    • Steps: Gene extraction → plasmid insertion → bacterial growth.

3. Key Rules & Tips

• Energy Calculations:
  • Always apply the 10% rule.
  • Example: If a cow consumes 72,056 kJ, only 72,056×0.1=7,205.6 kJ72,056×0.1=7,205.6kJ becomes biomass.
• Drawing Pyramids:
  • Label trophic levels and scale axes accurately.
  • Use data from tables (e.g., total biomass per level).
• Common Mistakes:
  • Confusing food chains (linear) with webs (complex).
  • Misinterpreting arrows in food chains (energy flow, not consumption direction).

4. Practice Questions

1. Calculate energy transfer:
   • Producers: 90,000 kJ → Primary consumers: ?
     90,000×0.1=9,000 kJ90,000×0.1=9,000kJ
2. Explain why pyramids of biomass are always shaped:
   • Biomass decreases at each level due to energy loss (respiration, waste).

50 GCSE Biology Questions

Section 1: Trophic Levels in Ecosystems

1. Define the term trophic level.
2. What is an apex predator? Give an example.
3. Explain why food chains rarely have more than six trophic levels.
4. Draw a food chain with four trophic levels. Label each level.
5. Calculate the energy transferred to the tertiary consumer if the producer stores 50,000 kJ.
6. Why are pyramids of biomass always pyramid-shaped?
7. Describe how biomass is measured.
8. Calculate the total biomass of flat periwinkles in the rock pool (Table 21.1).
9. What do arrows in a food chain represent?
10. Explain why there are more prey than predators in an ecosystem.
11. Name the trophic level of zooplankton in the marine food chain (Figure 21.3).
12. Define biomass.
13. Why are organisms dried before measuring biomass?
14. If a cow consumes 72,056 kJ of energy, how much becomes new tissue?
15. Describe two ways energy is lost between trophic levels.
16. What percentage of sunlight energy do producers use?
17. Explain why marine food chains are more energy-efficient than terrestrial ones.
18. Calculate the percentage of energy transferred from producers (90,000 kJ) to tertiary consumers (56 kJ).
19. What is a food web? How does it differ from a food chain?
20. Define primary consumer.
21. Why is a pyramid of biomass more accurate than a pyramid of numbers?
22. If sardines have a total biomass of 1,500 g, calculate the biomass of tuna (Table 21.3).
23. Name the apex predator in Figure 21.3.
24. Explain why plankton biomass estimates are often inaccurate.
25. What happens to the remaining 90% of biomass not transferred between trophic levels?

Section 2: Food Production

26. Define food security.
27. List three factors that threaten food security.
28. Explain how the 2007–2008 grain price rise affected global communities.
29. What caused the 1983–1985 Ethiopian famine?
30. Define monoculture. Give an example.
31. Compare intensive farming with organic farming.
32. Why are hedgerows removed in intensive farming?
33. Describe two environmental impacts of using pesticides.
34. What is biological pest control?
35. Explain the ethical concerns around factory farming.
36. Define overfishing.
37. How do fishing quotas protect fish stocks?
38. Why are nets with larger holes used in sustainable fisheries?
39. Name a fish species overfished in the North Sea.
40. What is mycoprotein? How is it produced?
41. Calculate the land saved by producing mycoprotein instead of beef (15% land use).
42. Describe how bacteria are genetically modified to produce insulin.
43. Why is GM insulin better than pig insulin?
44. What is added to fermenters for mycoprotein production?
45. Explain the role of plasmids in genetic modification.
46. Define quota in fisheries.
47. Why might fishing communities oppose quotas?
48. Name a natural fertiliser used in organic farming.
49. How does crop rotation improve soil health?
50. Explain why organic food is often more expensive.

Detailed Answers

Section 1: Trophic Levels

1. Trophic level: A stage in a food chain/web representing an organism’s position (e.g., producer, primary consumer).
2. Apex predator: The top predator in a food chain (e.g., lion, golden eagle).
3. Energy loss: 90% is lost at each level via respiration/waste, leaving insufficient energy for higher levels.
4. Example: Grass → Grasshopper → Frog → Snake.
5. Energy calculation:
   50,000×0.1=5,000 kJ (primary)50,000×0.1=5,000kJ (primary)
   5,000×0.1=500 kJ (secondary)5,000×0.1=500kJ (secondary)
   500×0.1=50 kJ (tertiary)500×0.1=50kJ (tertiary)
6. Pyramid shape: Biomass decreases at each level due to energy loss; producers have the greatest biomass.
7. Measuring biomass: Dry organisms in an oven to remove water, then weigh.
8. Flat periwinkle biomass:
   46×3 g=138 g46×3g=138g
9. Arrows: Show energy/biomass transfer to the next organism.
10. More prey: Energy loss means fewer organisms can be supported at higher trophic levels.
11. Zooplankton: Primary consumer (eats phytoplankton).
12. Biomass: Total dry mass of living/recently dead organisms.
13. Drying organisms: Removes water to standardise measurements.
14. Cow tissue:
    72,056 kJ×0.05=3,602.8 kJ72,056kJ×0.05=3,602.8kJ
15. Energy loss: Respiration, faeces, uneaten parts.
16. Sunlight energy: Producers use ~1% of sunlight energy.
17. Marine efficiency: Less energy lost to movement/support (buoyancy reduces effort).
18. Energy transfer:
    (5690,000)×100=0.062%(90,00056​)×100=0.062%
19. Food web: Interconnected food chains showing complex feeding relationships.
20. Primary consumer: Herbivores that eat producers (e.g., zebra).
21. Biomass pyramid: Accounts for organism size; numbers pyramids can invert (e.g., one tree → many insects).
22. Tuna biomass:
    2×650 g=1,300 g2×650g=1,300g
23. Apex predator: Great white shark.
24. Plankton inaccuracy: Sampling difficulty due to tiny size and vast distribution.
25. 90% loss: Used for respiration, excreted as waste, or uneaten.

Section 2: Food Production

26. Food security: Reliable access to sufficient, affordable, nutritious food.
27. Threats: Population growth, climate change, conflict, pests.
28. 2007–2008 crisis: Rising grain prices caused riots in Africa/Asia due to unaffordable staples.
29. Ethiopian famine: Drought, civil war, and reduced healthcare spending.
30. Monoculture: Growing one crop repeatedly (e.g., soya beans).
31. Intensive vs. organic:

• Intensive: High yield, chemicals, machines.
• Organic: Natural inputs, lower yield.

32. Hedgerow removal: Allows machinery use but reduces biodiversity.
33. Pesticide impacts: Kills pollinators, contaminates water.
34. Biological control: Using predators (e.g., ladybirds) to control pests.
35. Ethical concerns: Animal welfare issues (e.g., battery cages).
36. Overfishing: Harvesting fish faster than they reproduce.
37. Quotas: Limit catch sizes to prevent population collapse.
38. Larger nets: Allow juveniles to escape and breed.
39. Overfished species: North Sea cod.
40. Mycoprotein: Fungal protein (Quorn) grown in fermenters with glucose/ammonia.
41. Land saved:
    Beef land×0.15=Mycoprotein landBeef land×0.15=Mycoprotein land
42. GM insulin steps:

• Human insulin gene cut from DNA.
• Inserted into bacterial plasmid.
• Bacteria grown in fermenters to produce insulin.

43. GM insulin benefits: Fewer allergies, no animal slaughter.
44. Fermenter inputs: Glucose (food), ammonia (nitrogen), oxygen (respiration).
45. Plasmids: Carry genes between bacteria during genetic modification.
46. Quota: Legal limit on fish catch to sustain populations.
47. Opposition: Quotas reduce income for fishing communities.
48. Natural fertiliser: Manure/bone meal.
49. Crop rotation: Prevents soil depletion and pest buildup.
50. Organic cost: Lower yields and manual labour increase production costs.