AQA GCSE Science BIOLOGY 2 Unit B2.3 Photosynthesis

• Know that all green plants and algae use light energy to make their own food.

• Know that all green plants and algae obtain the raw materials they need to make this food from the air and the soil.

• Know that the conditions in which plants are grown can be changed to promote growth.

• You should be able to use your skills, knowledge and understanding to be able to:

  • interpret data showing how factors affect the rate of photosynthesis

  • evaluate the benefits of artificially manipulating the environment in which plants are grown.

AQA GCSE Science BIOLOGY Unit B2.3.1 Photosynthesis

• a) Know that photosynthesis is summarised by the equation:

  • carbon dioxide + water == light energy/chlorophyll  ==> glucose + oxygen

  • This is the process by which plants make food, for themselves, and for most animal life, including us too!

  • The chlorophyll is in structures called chloroplasts.

  • The carbon dioxide diffuses in through the stomata of the guard cells - effectively pores that can open and close ie CO₂ in, and oxygen O₂ out in the day and O₂ in at night.

• b) Know that during photosynthesis:

  • light energy is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and algae,

  • this energy is used by converting carbon dioxide (from the air) and water (from the soil) into sugar (glucose),

  • oxygen is released as a by-product.

• c) Know that the rate of photosynthesis may be limited by:

  • (i) shortage of light (usually sunlight) slows photosynthesis - since the greater the light intensity, the greater the rate of photosynthesis

  • (ii) low temperature, slows down the rate of photosynthesis - a general rule for all chemical reactions

    • A combination of both (i) and (ii) will cause very different rates between photosynthesis in winter (much less, slower) compared to summer (much more, faster).

  • (iii) shortage of carbon dioxide will also slow down the rate of photosynthesis,

  • So these are three factors affect the rate of photosynthesis - see the three graphs below.

• d) Know that light, temperature and the availability of carbon dioxide interact and in practice any one of them may be the factor that limits photosynthesis.

  • You should be able to relate the principle of limiting factors to the economics of enhancing the following conditions in greenhouses:

    • light limitation

    • light intensity - lots of glass window panes and site the greenhouse in a non-shaded area.

      • At night artificial light can be supplied.

      • However, the light level with have its limit (either sunlight or artificial light at night), so for maximum effect you need a warm temperature and a fresh supply of carbon dioxide.

    • temperature - warms up by trapping the heat radiation from the sun

      • temperature limits

      • However, too high a temperature is just as bad as too a low temperature. At temperatures over 40^oC enzymes involved in the process are destroyed, so photosynthesis slows down and eventually stops because the photosynthesis enzymes are destroyed.

      • A graph of rate of photosynthesis versus temperature rises at first (usual rate of chemical reaction factor), goes through a maximum and then falls as the enzymes are denatured and cease to function.

      • In cold weather, heaters might be employed in a greenhouse. If they are not electric and burn a fuel like paraffin, then lots of carbon dioxide is produced - quite handy, two factors catered for at the same time.

    • carbon dioxide limitation

    • carbon dioxide concentration - you do need some ventilation or the level of carbon dioxide gas will fall if the air is not replenished, but for maximum effect you need a warm temperature and plenty of light.

      • All of these factors can be manipulated to increase the rate of photosynthesis and hence increase plant growth rates. It enables market gardeners to produce more crops per year.

      • However, the extra costs of heating and artificial lighting must be off-set by selling the produce at a sustainable market price that the consumer is prepared to pay.

• e) Know that the glucose produced in photosynthesis may be converted into insoluble starch for storage in leaves, roots and stems.

  • The insoluble nature of starch makes it a very useful concentrated chemical store of energy - if it was soluble, it would dissolve and diffuse all over the place.

    • When needed, it is hydrolysed (broken down) into the useful sugar glucose, so the process of starch formation is reversed.

  • Know that plant cells use some of the glucose produced during photosynthesis for respiration - release of energy to power the cell functions.

  • Plants need energy from sugars to power their own life supporting systems just as we do.

  • The energy released enables the plant to convert glucose plus other elements/ions like nitrogen/nitrate into other essential useful chemical substances - some are listed below.

• f) Know that some glucose in plants and algae is used:

  • to produce fat or oil for storage - sources of energy via aerobic respiration, seeds contain food stores based on oils and fats (think of cooking oil from olives or sunflower seeds),

  • to produce cellulose, which makes up and strengthens the cell walls eg of the xylem and phloem,

  • to produce proteins after amino acids are first synthesised from glucose and nitrate ions.

• g) Know that to produce proteins, plants also use nitrate ions that are absorbed from the soil.

• Your practical work to develop your skills and understanding may have included the following:

  • investigating the need for chlorophyll for photosynthesis with variegated leaves

  • taking thin slices of potato and apple and adding iodine to observe under the microscope

  • investigating the effects of light, temperature and carbon dioxide levels (using Cabomba, algal balls or leaf discs from brassicas) on the rate of photosynthesis

  • computer simulations to model the rate of photosynthesis in different conditions

  • the use of sensors to investigate the effect of carbon dioxide and light levels on the rate of photosynthesis and the release of oxygen.

  • You may have done/seen experiments on the rate of photosynthesis in which the volume of oxygen formed is measured with a gas syringe connected to a flask of water and Canadian pondweed.

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