Applied Biology- Plants


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is the process where green plants use water, carbon dioxide and sunlight to produce carbohydrates

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 dioxide and water are converted into glucose and oxygen. The reaction requires light energy, which is absorbed by a green substance called chlorophyll.

Here is the equation for photosynthesis:

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

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 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

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Factors limiting photosynthesis

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.

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.

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

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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

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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 - it's leaves will turn yellow
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