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8.4 Making the Most of Photosynthesis

GCSE Biology · AQA · Chapter 8: Photosynthesis

A free GCSE Biology lesson for AQA (8461) from OBA School: controlling limiting factors in greenhouses, 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 light, temperature and carbon dioxide in greenhouses and polytunnels
  • Explain how controlling limiting factors increases crop yield
  • Use data to decide whether changing a factor is cost-effective (Higher tier)
  • Evaluate the benefits and costs of hydroponics and artificial conditions

Key words

  • Yield: The amount of useful crop produced, e.g. kg of tomatoes per square metre.
  • Greenhouse: A glass or plastic building that traps heat and lets growers control conditions.
  • Polytunnel: A cheaper, plastic-covered tunnel used in the same way as a greenhouse.
  • Hydroponics: Growing plants without soil, in a solution with the exact mineral ions they need.
  • Cost-effective: The extra income from a bigger yield is more than the extra cost of producing it.

Controlling the conditions

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.

  • Temperature; How it is controlled Glass traps heat; heaters on cold days and at night; vents and fans stop overheating.
  • Light; How it is controlled LED or other lamps extend the “day” in winter and at night; white-washing or blinds in strong sun.
  • Carbon dioxide; How it is controlled Paraffin or gas heaters release CO₂ as they burn; or CO₂ is piped in from tanks. Levels are often raised to about 0.1%.
  • Water and minerals; How it is controlled Automatic watering; hydroponics supplies mineral ions in exact amounts.

Greenhouses also keep out many pests and diseases, and computers can monitor and adjust everything automatically.

Hydroponics

In hydroponics plants grow with their roots in a nutrient solution, often supported by an inert material such as rock wool.

  • Mineral ions supplied in the right amounts, so they are never limiting; Disadvantages Expensive equipment and running costs
  • No soil-borne pests or weeds; Disadvantages Plants may need extra support
  • Water can be recycled; crops can be grown where the soil is poor; Disadvantages Needs constant monitoring of the solution

Does it pay? (Higher tier)

Every change costs money: fuel for heaters, electricity for lamps, CO₂, staff and equipment. A grower must get the highest rate of photosynthesis at the lowest cost, so that extra income is more than extra cost.

Raising a factor that is not limiting wastes money: the rate will not increase. The graph shows that extra light only helps at high CO₂: in normal air the line levels off above about 600 units.

Worked example: is the heating worth it?

Heating a tomato greenhouse for a month costs £1 350. It increases the yield that month by 900 kg. Tomatoes sell for £1.80 per kg.
Extra income = 900 × 1.80 = £1 620.
Extra profit = 1 620 − 1 350 = £270.

Answer: Yes: the heating gives an extra £270 profit that month

Worked example: reading the graph

A lettuce grower uses lamps at 800 units in normal air (0.04% CO₂). Should he buy brighter lamps or CO₂ enrichment?
At 0.04% CO₂ the rate has levelled off by 800 units, so light is not limiting; CO₂ is.
At 0.10% CO₂ and 800 units, uptake rises from 5.0 to 7.6 units.

Answer: CO₂ enrichment, not brighter lamps

Common mistakes

  • Saying “more of everything” always helps. Only increasing the limiting factor raises the rate.
  • Forgetting the cost. A higher yield is not worth it if the extra cost is more than the extra income.
  • Saying heaters only add warmth. Burning paraffin or gas also releases carbon dioxide.
  • Heating too much. Above the optimum temperature, enzymes denature and the rate falls.

Revision questions

Try these, then check your answers below each one.

Question 1. How can a paraffin heater increase the rate of photosynthesis in a greenhouse? (Choose from: It raises the temperature and releases carbon dioxide, It removes water vapour, It adds nitrate ions to the air, It releases oxygen and light.)

Answer: It raises the temperature and releases carbon dioxide

Working: Burning paraffin warms the air and adds CO₂, two possible limiting factors.

Question 2. What is the name for growing plants without soil, in a solution of mineral ions?

Answer: hydroponics

Working: Hydroponics supplies mineral ions in exact amounts without soil.

Question 3. True or false? Increasing a factor that is not limiting will increase the rate of photosynthesis.

Answer: False

Working: Only raising the limiting factor increases the rate; other increases waste money.

Related lessons

Continue in the full course

This lesson is section 8.4 (AGCBH0804) of our free GCSE Biology 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 Biology course.

 

 

 

 

 

 

 

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