Biology - General Certificate of Secondary Education, 2003

AQA

Life Processes and Living Things

Green Plants as Organisms

10.13 Plant Nutrition KS3 Green plants photosynthesise when it is light.
    Photosynthesis is summarised by the equation:
carbon dioxide + water [+ light energy] -> glucose + oxygen.
  FT and HT During photosynthesis:
  • light energy is absorbed by a green substance called chlorophyll
  • which is found in chloroplasts in some plant cells;
  • this energy is used by converting carbon dioxide and water into a
  • sugar (glucose);
  • oxygen is released as a by-product.
The rate of photosynthesis may be limited by:
  • low temperature;
  • shortage of carbon dioxide;
  • shortage of light.
The glucose produced in photosynthesis may be converted into insoluble starch for storage.

Plant cells use some of the glucose produced during photosynthesis for respiration.
Plant roots absorb mineral salts including nitrate needed for healthy growth.

Light, temperature and availability of carbon dioxide interact and in practice any one of them may be the factor that limits photosynthesis.
  HT The energy released by plants during respiration is used to build up smaller molecules into larger molecules:
  • sugars into starch;
  • sugars into cellulose for cell walls;
  • sugars, nitrates and other nutrients into amino acids which are then built up into proteins;
  • sugars into lipids (fats or oils) for storage in seeds.
For healthy growth plants also need mineral ions including:
  • nitrate — for the synthesis of proteins;
  • phosphate — which has an important role in the reactions involved in photosynthesis and respiration;
  • potassium - which helps enzymes involved in photosynthesis and respiration to work.
The symptoms shown by plants growing in conditions where mineral ions are deficient include:
  • stunted growth and yellow older leaves if nitrate ions are deficient;
  • poor root growth and purple younger leaves if phosphate ions are deficient;
  • yellow leaves with dead spots if potassium ions are deficient.
10.14 Plant Hormones  FT and HT Plants are sensitive to light, moisture and gravity:
  • their shoots grow towards light and against the force of gravity;
  • their roots grow towards moisture and in the direction of the force of gravity.
Plants produce hormones to co-ordinate and control growth.

The responses of plant roots and shoots to light, gravity and moisture are the result of unequal distribution of hormones, causing unequal growth rates.

The hormones which control the processes of growth and reproduction in plants can be used by humans to:
  • produce large numbers of plants quickly by stimulating the growth of roots from cuttings;
  • regulate the ripening of fruits on the plant and during transport to consumers;
  • kill weeds by disrupting their normal growth patterns.
10.15 Transport and Water Relations  KS3 Most of the water and minerals which enter a plant root are absorbed by root hair cells.
  FT and HT Plants lose water vapour from the surface of their leaves. This loss of water vapour is called transpiration. Transpiration is more rapid in hot, dry and windy conditions. Most plants have a waxy layer (cuticle) on their leaves which stops them losing too much water. Plants living in dry conditions have a thicker cuticle.

Most of the transpiration is through tiny holes called stomata. Plants need stomata to obtain carbon dioxide from the atmosphere.

The size of the stomata is controlled by guard cells which surround them. If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.

The water inside plant cells gives support for young plants. This is the main method of support and the plant wilts if the cells are short of water.

Flowering plants have separate transport systems for water and nutrients:
  • xylem tissue transports water from the roots to the stem and
  • leaves;
  • phloem tissue carries nutrients such as sugars from the leaves tothe rest of the plant including the growing regions and the storage organs.
  HT When water moves into plant cells by osmosis it increases the pressureinside the cell. The cell walls are sufficiently strong to withstand the pressure. It is this pressure which keeps the cells rigid (maintains their turgor) and hence provides support.
Next section       Previous section