Science

Plants

Green plants absorb light energy using chlorophyll in their leaves. They use it to react carbon dioxide with water to make a sugar called glucose. The glucose is used in respiration, or converted into starch and stored. Oxygen is produced as a by-product.

This process is called photosynthesis. Temperature, carbon dioxide concentration and light intensity are factors that can limit the rate of photosynthesis.

Plants also need mineral ions, including nitrate and magnesium, for healthy growth. They suffer from poor growth in conditions where mineral ions are deficient.

Photosynthesis

Photosynthesis [photosynthesis: The chemical change that occurs in the leaves of green plants. It uses light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product of photosynthesis. ] is the chemical change which happens in the leaves of green plants. It is the first step towards making food - not just for plants but ultimately every animal on the planet.

During this reaction, carbon dioxidecarbon dioxide: A gaseous compound of carbon and oxygen, which is a by-product of respiration, and which is needed by plants for photosynthesis. and water are converted into glucose and oxygenoxygen: Gaseous element making up about 20 per cent of the air, which is needed by living organisms for respiration. The reaction requires light energylight energy: Visible electromagnetic radiation., which is absorbed by a green substance called chlorophyll.

Photosynthesis takes place in leaf cells. These contain chloroplasts, which are tiny objects containing chlorophyll.

the cell includes: a waxt outer cuticle, the upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis, stoma, and guard cells with chloroplasts

Cross-section through a leaf cell

Here is the equation for photosynthesis:

carbon dioxide + water (+ light energy)    →    glucose + oxygen

'Light energy' is shown in brackets because it is not a substance. You will also see the equation written like this:

Plants absorb water through their roots, and carbon dioxide through their leaves. Some glucose is used for respiration, while some is converted into insoluble starch [starch: A type of carbohydrate. Plants can turn the glucose produced in photosynthesis into starch for storage, and turn it back into glucose when it is needed for respiration. ] for storage. The stored starch can later be turned back into glucose and used in respiration. Oxygen is released as a by-product of photosynthesis.

Factors limiting photosynthesis

Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.

  • rate of photosynthesis plotted against light intensity. the rate begins to slow as the light intensity continues to increase

    Light intensity

    Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.

  • rate of photosynthesis plotted against carbon dioxide concentration. the rate begins to slow as the carbon dioxide concentration continues to increase

    Carbon dioxide concentration

    Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.

  • rate of photosynthesis plotted against temperature. the rate begins to slow as the temperature continues to increase

    Temperature

    If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.

If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.

In practice, any one of these factors could limit the rate of photosynthesis.

Maximising growth

Farmers can use their knowledge of these limiting factors to increase crop growth in greenhouses. They may use artificial light so that photosynthesis can continue beyond daylight hours, or in a higher-than-normal light intensity. The use of paraffin lamps inside a greenhouse increases the rate of photosynthesis because the burning paraffin produces carbon dioxide, and heat too.

Plants and minerals

Plants need to take in a number of elements to stay alive. The most important are:

  • carbon
  • hydrogen
  • oxygen

Plants get hydrogen and oxygen from water in the soil, and carbon and oxygen from carbon dioxide and oxygen in the atmosphere. Water and carbon dioxide are used to synthesise food during photosynthesis. Oxygen is used to release energy from food during respiration.

In addition to these three elements, plants need a number of minerals for healthy growth. These are absorbed through the roots as mineral ions dissolved in the soil water. Two important mineral ions needed by plants are:

  • nitrate - for making amino acids, which are needed to make proteins
  • magnesium - for making chlorophyll

If a plant does not get enough minerals, its growth will be poor. It will suffer from deficiency symptoms:

  • deficient in nitrate - it will suffer from stunted growth
  • deficient in magnesium - its leaves will turn yellow
A tomato plant stunted as a result of mineral deficiency

The tomato plant on the left is healthy, the one on the right is growing in conditions where mineral ions are deficient

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