Photosynthesis

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Photosynthesis Takes Place in The Chloroplast of Plant Cells

·         Chloroplasts are small flattened organelles in a plant cell

·         They have a double membrane - chloroplast envelope 

·         Thylakoids are stacked up into structures called the grana, linked together by bits of thylakoid membrane called lamellae

·         Chloroplasts have photosynthetic pigments which are coloured pigments that absorb light energy needed for photosynthesis, these pigments are found in the thylakoid membranes, attached to proteins. The protein and pigment is called a photosystem.

·         There are two photosystems used by plants to capture energy:

Photosystem 1 (PSI) - absorbs light best at a wavelength of 700nm

Photosystem 2 (PSII) - absorb light best at a wavelength of 680nm

  • Contained in the inner membrane of the chloroplast and the surrounding thylakoids is a gel-like substance called the stroma, it contains enzymes sugars and organic acids. 
  • The carbohydrates produced by photosynthesis which aren't used straight away as stored as starch grains in the stroma. 

Photosynthesis Can Be Split Into Two Stages

The Light-Dependant Reaction

  • The reaction needs light energy
  • Takes place in the thylakoid membranes 
  • Light energy is absorbed by photosynthetic pigments in the photosystems and converted to chemical energy
  • Light energy is used to add a phosphate group to ADP to form ATP and reduce NADP to form reduced NADP. ATP transfers energy and reduced NADP tranfers hydrogen to the light-independant reaction
  • During the process water is oxidised to oxygen

The Light-Independant Reaction

  • Also known as the Calvin cycle 
  • Takes place in the stroma
  • The ATP and reduced NADP from the light dependant reaction supply the energy and hydrogen to make glucose from carbon dioxide.

In the Light Dependant Reaction ATP is made by Photophosphorylation

Light energy is absorbed by the photosystems for:

  • Making ATP from ADP and inorganic phosphate - photophosphorylation
  • Making reduced NADP from NADP 
  • Splitting water into protons (H+ ions) electrons and oxygen - photolysis

Non-cyclic Photosynthesis Produces ATP Reduced NADP and O2

1) Light energy excites electrons in chlorophyll

  • Light energy is absorbed by PSII
  • Light energy excites electrons in chlorophyll
  • The electrons move to a higher energy level  (have more energy)
  • The high energy electrons move along the electron transport chain

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