IGCSE Biology • Topic 5

Food Production

Use of Biological Resources — Interactive Lesson

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Sections

Glasshouses & Polythene Tunnels

How controlled environments increase crop yield

Retrieval Practice: Before We Begin

Think back to what you already know: What conditions do plants need to grow effectively? Write down 3–4 factors before continuing.

What are Glasshouses and Polythene Tunnels?

Glasshouses (greenhouses) and polythene tunnels are protective structures that allow farmers to control growing conditions for crop plants.

How They Work

Key Features:

  • Transparent materials — Glass or clear plastic allows light to enter
  • Enclosed space — Traps heat and maintains temperature
  • Protection — Shields crops from harsh weather, pests, and diseases
  • Controlled environment — Enables optimization of growing conditions

How Do They Increase Crop Yield?

Crop yield refers to the amount of crop harvested per unit area. Glasshouses increase yield by:

Factor How It's Controlled Effect on Yield
Temperature Heating systems, ventilation, thermal mass Optimum temperature increases enzyme activity and photosynthesis rate
Light Transparent materials, artificial lighting Extended growing season, increased photosynthesis
Carbon dioxide CO₂ enrichment systems, burners Enhanced photosynthesis rate (CO₂ is a raw material)
Water Irrigation systems, drip feeds Prevents water stress, maintains turgor
Protection Physical barrier Reduces crop loss from weather, pests, and disease
Exam Focus: Be able to describe specific ways that glasshouses increase yield, not just list features. Link each feature to its effect on plant growth processes.
Multiple Choice
Which of the following BEST explains why glasshouses can increase crop yield?
A They make plants grow bigger by providing more space
B They prevent all pests from reaching the crops
C They allow control of environmental conditions to optimize photosynthesis and growth
D They increase the amount of soil nutrients available

🔄 Spaced Recall Practice

Without looking back, list THREE ways that glasshouses can be used to increase crop yield:

Effects of Increased CO₂ & Temperature

Understanding limiting factors in photosynthesis

🔗 Connecting to Prior Knowledge: This section builds on your understanding of photosynthesis from earlier topics. Try to make connections as you learn!

The Effect of Increased Carbon Dioxide

Carbon dioxide is a raw material for photosynthesis. The equation reminds us:

Photosynthesis equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(carbon dioxide + water → glucose + oxygen)

Effects of increased CO₂ concentration:

  • ✓ Increases rate of photosynthesis — More raw material available for the reaction
  • ✓ Increases glucose production — More product formed per unit time
  • ✓ Increases crop yield — More glucose → more growth → more biomass
  • ⚠ Only works up to a point — Other factors become limiting (limiting factor concept)
Typical CO₂ Levels
  • Atmospheric CO₂: ~0.04% (400 ppm)
  • Optimum in glasshouse: 0.1% – 0.15% (1000–1500 ppm)
  • Method: Paraffin burners or CO₂ cylinders

The Effect of Increased Temperature

Temperature affects enzyme-controlled reactions, including those in photosynthesis and respiration.

Temperature and Enzyme Activity

Key Points:

  • Below optimum (0–25°C): Increasing temperature increases enzyme activity → faster photosynthesis
  • Optimum temperature (~25–30°C for most crops): Maximum enzyme activity → maximum rate of photosynthesis
  • Above optimum (>35°C): Enzymes denature → reduced photosynthesis

Effects of controlled temperature increase:

  • ✓ Enzymes work faster → increased metabolic rate
  • ✓ Faster photosynthesis (if temperature is optimized)
  • ✓ Extended growing season (can grow year-round)
  • ✓ Faster growth rate → quicker harvest → more crops per year
  • ⚠ Higher respiration rate (uses glucose) — net gain must be positive
  • ⚠ Risk of overheating → need ventilation control
Common Misconception: Higher temperature is NOT always better! The key is maintaining the OPTIMUM temperature for the specific crop. Too hot = enzyme denaturation = crop damage.
Multiple Choice
A farmer increases the CO₂ concentration in their glasshouse from 0.04% to 0.1%. What is the MOST LIKELY immediate effect?
A The temperature inside the glasshouse will increase
B The rate of photosynthesis will increase
C The plants will need less water
D The plants will produce less oxygen
Multiple Choice
Why must temperature in a glasshouse be carefully controlled rather than simply maximized?
A High temperatures cost more money in heating bills
B Temperatures above optimum denature enzymes and reduce photosynthesis
C Plants cannot tolerate any temperature above 25°C
D High temperatures prevent CO₂ from entering the leaves

Use of Fertilizers

Mineral ions, soil nutrition, and environmental impact

Retrieval Practice: Recall from Earlier Topics

What are the three main mineral ions that plants need? What does each one do? Try to recall before reading on.

What Are Fertilizers?

Fertilizers are substances added to soil to provide essential mineral ions that plants need for growth.

Essential Mineral Ions
Mineral Ion Use in Plants Deficiency Symptoms
Nitrates (NO₃⁻) Make amino acids → proteins (for growth) Stunted growth, yellow older leaves
Phosphates (PO₄³⁻) Make DNA and cell membranes, respiration Poor root growth, purple leaves
Potassium (K⁺) Enzyme activation, photosynthesis Yellow leaves with dead spots
Magnesium (Mg²⁺) Make chlorophyll for photosynthesis Yellow leaves (chlorosis)

How Fertilizers Increase Crop Yield

The mechanism:

  1. Fertilizers replace mineral ions removed from soil by previous crops
  2. Increased mineral availability → increased protein synthesis
  3. More proteins → more enzymes → faster metabolism
  4. More proteins → more growth (cell division and expansion)
  5. Healthier plants → increased biomass → higher yield
NPK Fertilizers: The most common type containing Nitrogen (N), Phosphorus (P), and Potassium (K). The ratio varies depending on crop needs. For example, 15-15-15 means 15% of each element.

Types of Fertilizers

Type Advantages Disadvantages
Organic
(manure, compost, bone meal)		 • Improves soil structure
• Releases nutrients slowly
• Increases soil organisms
• Reduces waste 		• Bulky to transport
• Slow-acting
• Nutrient content variable
• May contain weed seeds/pathogens
Inorganic / Artificial
(ammonium nitrate, NPK)		 • Fast-acting
• Precise nutrient content
• Easy to apply and transport
• Immediate results 		• Can leach into waterways → eutrophication
• Doesn't improve soil structure
• More expensive
• Requires fossil fuels to produce
Environmental Issue — Eutrophication:
  1. Excess fertilizer washes into water bodies (leaching)
  2. Algae grow rapidly (algal bloom) blocking light
  3. Plants below die due to lack of light for photosynthesis
  4. Bacteria decompose dead plants, using oxygen
  5. Oxygen depletion → fish and other organisms die
Multiple Choice
A farmer notices that their crop plants have stunted growth and yellowing older leaves. Which mineral deficiency is MOST LIKELY responsible?
A Nitrate deficiency
B Phosphate deficiency
C Potassium deficiency
D Magnesium deficiency

🔄 Application Question

Explain why using too much inorganic fertilizer can lead to problems in nearby rivers and lakes.

Pest Control

Chemical and biological methods of protecting crops

Why Control Pests?

Pests are organisms that damage crops, reducing yield and quality.

Types of Crop Pests
  • Insects — Eat leaves, stems, roots, or fruits (e.g., aphids, caterpillars, beetles)
  • Fungi — Cause diseases and rot (e.g., potato blight, wheat rust)
  • Weeds — Compete for light, water, minerals, and space
  • Mammals — Eat crops or damage plants (e.g., rabbits, deer, rodents)

Economic reasons for pest control:

  • Prevent crop damage → maintain yield
  • Maintain crop quality → higher market value
  • Reduce economic losses for farmers
  • Ensure food security and supply

Method 1: Chemical Control (Pesticides)

Pesticides are chemicals that kill pests. Different types include insecticides, herbicides, fungicides, and molluscicides.

Advantages of Pesticides Disadvantages of Pesticides
✓ Act very quickly
✓ Very effective at killing pests
✓ Can treat large areas easily
✓ Relatively cheap to produce
✓ Prevent significant crop losses 		✗ Kill non-target species (including beneficial insects)
✗ May harm organisms throughout food chain
✗ Some pesticides are persistent (don't break down)
✗ Can bioaccumulate in food chains
✗ Pests can develop resistance
✗ Pesticide residues may remain on food
✗ Can pollute water sources
Key Concept — Bioaccumulation: Pesticides can accumulate at increasing concentrations along a food chain. For example: pesticide in water → eaten by plankton → eaten by small fish → eaten by large fish → eaten by birds. Birds at the top of the chain have highest concentrations.

Method 2: Biological Control

Biological control uses natural predators, parasites, or diseases to control pest populations instead of chemicals.

Examples of Biological Control
  • Ladybirds → eat aphids on crops
  • Parasitic wasps → lay eggs in pest caterpillars
  • Bacteria (Bt) → produce toxins that kill specific insects
  • Nematodes → parasitize slugs and vine weevils
  • Myxomatosis virus → controls rabbit populations
Advantages of Biological Control Disadvantages of Biological Control
✓ Highly specific to target pest
✓ No chemical pollution
✓ Doesn't harm other organisms
✓ No bioaccumulation
✓ Pests don't develop resistance
✓ Long-lasting (predators reproduce)
✓ No pesticide residues on food 		✗ Slow to take effect (needs time to establish)
✗ Doesn't eliminate pests completely
✗ Control agent may become a pest itself
✗ May disrupt local ecosystems
✗ More expensive initially
✗ Requires expertise to implement
✗ Difficult to control once introduced
🔗 Connecting Concepts: Notice how this links to ecology and food chains (Topic 4). Pesticide bioaccumulation affects energy transfer and pyramid of biomass. Biological control involves predator-prey relationships.
Multiple Choice
Which of the following is an advantage of biological control over using pesticides?
A It works much faster to reduce pest numbers
B It completely eliminates the pest population
C It is specific to the target pest and doesn't harm other organisms
D It is always cheaper than using chemicals
Multiple Choice
A farmer wants rapid control of an aphid infestation that is currently destroying their crop. Which method would be MOST appropriate?
A Introduce ladybirds to eat the aphids
B Apply an insecticide pesticide
C Wait for natural predators to control the population
D Use organic fertilizers to make plants stronger

🔄 Comparison Task

Create a comparison between pesticides and biological control. Include at least 2 advantages and 2 disadvantages of each method.

Practice Questions

Apply your knowledge across all four sections

Extended Response
A commercial tomato grower wants to maximize yield in their glasshouse. Suggest THREE changes they could make and explain how each would increase yield.
Extended Response
Explain why a farmer might choose to use both pesticides AND biological control methods rather than relying on just one approach.
Evaluate
A student claims: "Increasing temperature in a glasshouse will always increase crop yield." Evaluate this statement.

Quick Fire Quiz

Short recall questions — test yourself without looking back.

1. What are the three main mineral ions in NPK fertilizer?
2. Give one example of biological control.
3. What is the normal atmospheric CO₂ concentration?

End of Topic Assessment

Test your understanding of the entire topic. Answer all questions, then submit.

Question 1 of 5
Which of these is NOT a way that glasshouses increase crop yield?
A Controlling temperature to optimize enzyme activity
B Increasing CO₂ concentration to enhance photosynthesis
C Providing additional minerals through the glass structure
D Protecting crops from adverse weather conditions
Question 2 of 5
A farmer notices their crop has yellow leaves. What should they add to the soil?
A Phosphate only
B Nitrate or magnesium depending on which leaves are affected
C Potassium only
D Water only
Question 3 of 5
What is bioaccumulation?
A The process of biological control increasing in effectiveness
B The buildup of substances at increasing concentrations along a food chain
C The accumulation of fertilizers in the soil
D The growth of algae in water bodies
Question 4 of 5
Why is biological control considered more environmentally friendly than pesticides?
A It is cheaper to implement
B It works faster than chemical control
C It doesn't cause pollution or harm non-target species
D It completely eliminates pest populations
Question 5 — Extended
A commercial lettuce grower uses a glasshouse with CO₂ enrichment and temperature control. Explain how these two factors work together to increase crop yield. Include reference to photosynthesis in your answer.

Topic Summary

Key points for revision and exam preparation

🌱 Glasshouses and Polythene Tunnels

  • Increase yield by controlling temperature, light, CO₂, water, and protection from pests/weather
  • Allow year-round growing and optimization of growing conditions
  • Enable control of limiting factors for photosynthesis

🔬 Effects of CO₂ and Temperature

  • Increased CO₂: Raw material for photosynthesis → increases rate of photosynthesis → more glucose → higher yield
  • Optimum temperature: Increases enzyme activity → faster photosynthesis → faster growth
  • Warning: Too high temperature denatures enzymes and reduces photosynthesis
  • Both factors work as limiting factors — must be optimized together

🌾 Fertilizers

  • Provide essential mineral ions: Nitrates, Phosphates, Potassium, Magnesium
  • Increase yield by enabling protein synthesis → more enzymes → faster metabolism and growth
  • Organic fertilizers: Slow-release, improve soil structure, but variable content
  • Inorganic fertilizers: Fast-acting, precise content, but can cause eutrophication
  • Eutrophication: Excess fertilizer → algal bloom → oxygen depletion → death of aquatic organisms

🐛 Pest Control

  • Why control pests: Prevent damage, maintain yield and quality, economic security
  • Pesticides: Fast, effective, but harm non-target species, bioaccumulate, pests develop resistance
  • Biological control: Specific, no pollution, long-lasting, but slow and may have unintended consequences
  • Best approach: Often integrated pest management using both methods strategically

🎯 Key Exam Skills

  • Explain mechanisms — don't just list facts
  • Compare and contrast different methods (pesticides vs biological control)
  • Apply knowledge to novel scenarios (e.g., new pest control situations)
  • Evaluate approaches considering advantages AND disadvantages
  • Link to other topics (photosynthesis, food chains, ecology)