Photosynthesis involves two processes. The first is the light dependant stage, this is where the plant requires light waves to allow the production of reduced NADP and also ATP. These products are made when the water molecules splits up during photolysis. the oxygen is produced as a waste product and may be used in respiration but the electron is sent into the electron transport chain. The electron is accepted by photosystem 2 which excites the electron due to the light energy and passes it along the chain to photosystem 1. At this point the electron is excited again and passed along another electron transport chain where it is recieved by the final electron acceptor which is the NADP. The NADP is reduced and gains the electrons.

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The products of the light dependant reaction are required for the calvin cycle. ATP and NADPH are used to reduce carbon dioxide. The light independant reaction takes place in the stroma of the chloroplasts.

  • In the stroma the carbon dioxide combines with the 5 carbon compound RuBP using the enzyme rubisco.
  • The combination of RuBP and carbon dioxide produces two molecules of GP one has 3 carbons.
  • ATP and NADPH are used to reduce the activated TP/GALP.
  • The NADP is reformed and goes back to the light dependant stage to be reduced again by accepting more hydrogen.
  • GALP/TP molecules are converted into organic molecules if the cycle turns 6 times.
  • Most TP/GALP are used to regenerate the RuBP using ATP from the dependant stage.
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The light dependant stage of photosynthesis takes place in the grana. This stage is dependant on the capture of light, of which the energy is used for two purposes.

1. To add inorganic phosphate tp make ATP    2. For photolysis.

Oxidation results in energy being given out in the form of an electron and reduction is the gaining of energy / uptake, when an electron is taken in.

  • During photolysis oxygen is produced as a waste product and may be used in respiration.  
  • NADP - final electron acceptor. The NADP is reduced and gains the electrons.
  • When a photosystem absorbs light energy it boosts the energy of the pair of electrons EXCITES!
  • The electrons then pass through the electron carrier.
  • They pass through the carriers in a series of REDOX reactions.
  • The trasfer chain is located in the thylakoid membrane.
  • The electrons loose energy at each stage because the photosystems have slightly lower energy levels.
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There are specific optimum conditions for photosynthesis. These include:

High light intensity - to provide energy for lightdep.

Temp. around 25 degrees - photosyn. involves enzymes ATP synthase and Rubisco. If temp. falls below 10 or above 45 the enzymes will become denatured and therefore inactive. Also at high temps. the stomata close the avoid losing water = less CO2 can enter so slow photosynthesis.

Carbon dioxide - 0.04% in atmosphere increasing this to 0.4% gives a higher rate of photosynthesis.

All of these factors can limit photosyn. Because of this, the growers make sure they maintain the optimum conditions in greenhouses for agriculture.

E.G --> light = lamps sun through glass / temp. = trap heat from sunlight heaters + cooling system / CO2 generators burning propane.

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