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B8.4 Making the Most of Photosynthesis

GCSE Combined Science · AQA · Chapter B8: Photosynthesis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: controlling limiting factors in greenhouses and polytunnels, and deciding whether it pays (Higher tier). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how growers control temperature, light and carbon dioxide in greenhouses and polytunnels
  • Explain how removing limiting factors increases crop yield
  • Use data to decide whether adding heat, light or carbon dioxide is cost-effective (Higher tier)
  • Explain why raising a factor that is not limiting wastes money

Key words

  • Yield: The amount of useful crop produced, e.g. kg of tomatoes per m².
  • Greenhouse: A glass building that traps heat and lets growers control conditions.
  • Polytunnel: A cheaper plastic-covered tunnel used in the same way.
  • Cost-effective: The extra income from a bigger yield is more than the extra cost.
  • Profit: Income minus costs.

Controlling the conditions

Food production matters in every country. If a grower removes the limiting factors, plants photosynthesise faster, make more glucose and grow faster. That means bigger crops, sooner, and crops grown out of season. Greenhouses and polytunnels also protect plants from wind, frost and many pests.

  • Temperature; How it is controlled Glass or plastic traps heat; heaters on cold days and at night; vents and fans stop overheating.
  • Light; How it is controlled LED lamps give extra light in winter and extend the “day”; blinds in very strong sun.
  • Carbon dioxide; How it is controlled Gas or paraffin heaters release CO₂ as they burn; or CO₂ is piped in from tanks, often to about 0.1%.
  • Water and minerals; How it is controlled Automatic watering; nutrient solutions supply the mineral ions plants need.

Exam tip: a heater that burns fuel raises two factors: temperature and carbon dioxide concentration.

Only the limiting factor matters

Increasing a factor that is not limiting does not raise the rate, so it wastes money. The graph shows that, in normal air, extra light helps only up to about 600 units. Above that, carbon dioxide is limiting.

Worked example: reading the graph

A grower uses lamps at 700 units in normal air. Should she buy brighter lamps or add CO₂?
At 0.04% CO₂ the line has levelled off by 700 units, so light is not limiting; CO₂ is.
At 0.10% CO₂ and 700 units the uptake is about 7.8 instead of about 5.3 units.

Answer: Add CO₂, not brighter lamps

Does it pay? (Higher tier)

Heating, lighting and CO₂ all cost money (fuel, electricity, gas, equipment, staff). Growers must balance the conditions to get the highest rate of photosynthesis at the lowest cost, so that crops can still be sold at a profit.

Worked example: is the lighting worth it?

Extra LED lighting in a cucumber greenhouse for one month costs £1 860. It gives 2 800 extra cucumbers, sold at £0.55 each.
Extra income = 2 800 × 0.55 = £1 540.
Extra profit = 1 540 − 1 860 = −£320.

Answer: No: the lighting loses £320 that month

Worked example: is CO₂ enrichment worth it?

Adding CO₂ to a tomato greenhouse costs £640 a month and increases the yield by 520 kg. Tomatoes sell for £1.70 per kg.
Extra income = 520 × 1.70 = £884. Extra profit = 884 − 640 = £244.

Answer: Yes: £244 extra profit

Evaluating: also think about whether the factor is limiting, the price of crops out of season (often higher), the cost of equipment, and the environmental impact of burning fuel.

Common mistakes

  • Saying “more of everything” always helps. Only raising the limiting factor raises the rate.
  • Forgetting the cost. Extra yield is only worth it if extra income is more than extra cost.
  • Saying heaters only add warmth. Burning fuel also releases carbon dioxide.
  • Heating too much. Above the optimum, enzymes denature and the rate falls.

Revision questions

Try these, then check your answers below each one.

Question 1. A grower burns natural gas in a heater inside a greenhouse. Which two limiting factors does this increase? (Choose from: Light intensity and temperature, Temperature and carbon dioxide concentration, Carbon dioxide concentration and light intensity, Water and mineral ions.)

Answer: Temperature and carbon dioxide concentration

Working: Burning gas releases heat (raising temperature) and carbon dioxide (raising CO₂ concentration).

Question 2. What word means the amount of useful crop that is produced, for example kg of tomatoes per m²?

Answer: yield

Working: Yield is the amount of agricultural product. Growers control limiting factors to increase it.

Question 3. (Higher tier) A grower’s lamps give a light intensity of 700 units and the greenhouse has normal air. Use the graph. Which change would increase CO₂ uptake the most? (Choose from: Buy brighter lamps, Add carbon dioxide to raise it to 0.10%, Switch the lamps off at night, Lower the temperature.)

Answer: Add carbon dioxide to raise it to 0.10%

Working: In normal air the line is flat by 700 units, so light is not limiting; CO₂ is. At 0.10% CO₂ the uptake at 700 units is about 7.8 instead of about 5.3.

Related lessons

Continue in the full course

This lesson is section B8.4 (AGCSC0804) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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