Photosynthesis

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Photosynthesis is the process by which plants and other things make food.[1] It is a chemical process that uses sunlight to turn carbon dioxide into sugars the cell can use as energy. As well as plants, many kinds of algae, protists and bacteria use it to get food. Photosynthesis is very important for life on Earth. Most plants either directly or indirectly depend on it. The exception are certain organisms that directly get their energy from chemical reactions; these organisms are called chemoautotrophs.

Photosynthesis can happen in different ways, but there are some parts that are common.

6 CO2(g) + 6 H2O(l) + photonsC6H12O6(aq) + 6 O2(g)
carbon dioxide + water + light energy → glucose + oxygen

Reactions[change | edit source]

Diagram of a chloroplast

Photosynthesis has two main sets of reactions. Light-dependent reactions need light to work; and light-independent reactions, which do not need light to work.

Light-dependent reactions[change | edit source]

Light energy from the sun is used to split water (photolysis) which has been sucked in by plants by transpiration. The sunlight hits chloroplasts in the plant, causing an enzyme to break apart the water. Water, when broken, makes oxygen, hydrogen, and electrons.

The hydrogen converts to NADPH which is then used in the light-independent reactions. Oxygen diffuses out of the plant as a waste product of photosynthesis and ATP is synthesized from ADP and inorganic phosphate. This all happens in the grana of chloroplasts.

Light-independent reactions[change | edit source]

During this reaction, sugars are built up using carbon dioxide and the products of the light-dependent reactions (ATP and NADPH) and various other chemicals found in the plant in the Calvin Cycle. Therefore, the light-independent reaction cannot happen without the light-dependent reaction. Carbon dioxide diffuses into the plant and along with chemicals in the stroma of the chloroplast and ATP and NADPH, glucose is made and finally, transported around the plant by translocation.

Factors affecting photosynthesis[change | edit source]

There are three main factors affecting photosynthesis:

Light intensity[change | edit source]

If there is little light shining on a plant, the light-dependent reactions will not work efficiently. This means that photolysis will not happen quickly, and therefore little NADPH and ATP will be made. This shortage of NADPH and ATP will lead to the light-independent reactions not working as NADPH and ATP are needed for the light-independent reactions to work.

Carbon dioxide levels[change | edit source]

Carbon dioxide is used in the light-independent reactions. It combines with NADPH and ATP and various other chemicals (such as Ribulose Biphosphate) to form glucose. Therefore, if there is not enough carbon dioxide, then there will be a build up of NADPH and ATP and not enough glucose will be formed.

Temperature[change | edit source]

There are many enzymes working in photosynthetic reactions - such as the enzyme in photolysis. These enzymes will not work as well, or stop working at all at high or low temperatures and therefore, so will the light-dependent and light-independent reactions.

Other pages[change | edit source]

References[change | edit source]

  1. Fullick, Ann (2011). Edexcel IGCSE Biology Revision Guide. Pearson Education. p. 40. ISBN 9780435046767.