8.2 – Photosynthesis

8.2.1 - Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs

  • cell wall
  • double membrane
  • starch grain
  • grana
  • thylakoid
  • internal membrane - location of the light dependent reaction
  • stroma - surrounds the thylakoids - location of the light independent reaction, including the Calvin cycle. Often contain large starch grains and oil droplets, products of photosynthesis


8.2.2 - State that photosynthesis consists of light-dependent and light-independent reactions


Light Dependent Reaction


  • sun's energy is trapped by chlorophyll
  • light energy is used to split water - photolysis
  • hydrogen is retained by the hydrogen acceptor, NADP+
  • ATP is generated from ADP and phosphate, using light energy
  • called photophosphorylation
  • oxygen is given off as a waste product
  • occurs in the grana, in the thylakoid membranes


Light Independent Reaction


  • chemical energy is used - ATP and the reduced hydrogen acceptor NADPH + H+
  • sugars are built up using CO2
  • occurs in the stroma
  • can occur during dark periods if products of the light dependent reactions are available


8.2.3 - Explain the light-dependent reactions

In the light dependent reactions, light energy from the sun is converted into chemical energy. This is trapped in the chlorophyll, which are grouped into structures called photosystems. The photosystems are found on the thylakoid membranes of the grana.

There are multiple types of chlorophyll found in each photosystem, each of which absorb a different wavelength of light. Chlorophyll A is in the centre of the photosystem. When light hits the chlorophyll, electrons are excited and lost in oxidation.

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Cyclic Photophosphorylation

ATP is produced in a cyclic process when the ratio of NADPH + H+ : NADP+ is high. Photosystem 1 does not generate any NADPH + H+, but acts to send electrons to the proton pump.

Photosystem 1 is oxidised, releasing an excited an electron to reduce the membrane proton pump. Protons, in the forms of H+ are pumped into the thylakoid space. This creates a concentration gradient necessary for the later production of ATP. The electrons are the cycled back to the first photosystem to reduce it.


Non-cyclic Photophosphorylation


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