UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 21/05/2023 [SEARCH]

Photosynthesis: 2. What is the chemical process of photosynthesis? What does in need? What does it make?

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(2) What is the chemical process of photosynthesis?

You need four 'basic' things for photosynthesis:

(i) water - absorbed through the roots,

(ii) carbon dioxide - absorbed from air by diffusion,

(iii) chlorophyll - green pigment in palisade cells of stems and leaves,

(iv) visible sunlight energy - converted to chemical potential energy in glucose molecules.

Obviously other nutrients are needed as well, including the magnesium ion, Mg2+, which is an essential part of the chlorophyll molecule.

A simplified version of the biochemistry of photosynthesis

  • Plants absorb water through their roots and carbon dioxide through their leaves and covert these into carbohydrate molecules, initially in the form of glucose, the waste product is oxygen! handy for us!

  • The carbon dioxide in air diffuses into leaves through the stomata, water comes up from the roots via the xylem tubes, oxygen diffuses out and sugars are transported around the plant by the phloem tubes.

  • Carbon dioxide into leaves and oxygen out of the leaves is an example of gas exchange system on the surface pores (stomata) of the leaves.

  • The biochemical process of photosynthesis takes place in the chloroplasts of plant cells in the green leaves and stems with the help of green molecules called chlorophyll.

    • It is the green pigment chlorophyll that absorbs the light energy to power photosynthesis.

  • The chemistry of photosynthesis

  • Photosynthesis is summarised by the equation:

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

    •  6H2O(l) + 6CO2(g) == sunlight ==> C6H12O6(aq) + 6O2(g)

    • This is overall an endothermic chemical reaction, energy is taken in, i.e. sunlight energy is absorbed in the process of photosynthesis.

    • Photosynthesis is the process by which plants make food, initially in the form of glucose, for themselves, and for most animal life, including us too via food chains!

      • The plant will use some of the glucose immediately to fuel all the necessary life maintaining processes.

      • The plant converts some of the glucose to starch - a chemical potential energy food store for the plant and animals like us too!

    • Photosynthesis utilises sunlight energy to convert carbon dioxide and water into glucose (basis of food) and oxygen.

    • The green pigment chlorophyll is in the subcellular structures called chloroplasts, where photosynthesis takes place in green plant cells.

      • All of the photosynthetic chemistry facilitated by enzymes (biological catalysts).

      • The chemistry of photosynthesis is very complicated but it takes place in two main stages.

      • 1. Chlorophyll absorbs a photon of light energy. This sunlight energy (visible light photons) splits water (H2O) into hydrogen ions (H+) and oxygen (O2).

        • From the plants point of view, the oxygen gas is given out as a waste material.

      • 2. The hydrogen ions combine with carbon dioxide (CO2) to form glucose molecules (C6H12O6).

    • 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.

      • In daylight the rate of photosynthesis will exceed the rate of respiration.

      • At night the rate of respiration will exceed that of photosynthesis.

      • Both processes are need to keep the plant alive.

    • During photosynthesis light energy is absorbed by the green chlorophyll, which is found in chloroplasts in some plant cells and algae.

      • Chlorophyll looks green because it absorbs in the violet-blue and orange-red regions of visible light, so plants can absorb use the energy from visible electromagnetic radiation.


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