BIOLOGY - Photosynthesis

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  • Created by: Priscilla
  • Created on: 15-09-11 19:17

Light Dependent Reactions

Photosynthesis consists of light dependent and light independent reactions.

Light dependent reactions takes place in the CHLOROPLASTS,in the Granium and the Thylakoid membrane.

PRODUCTION OF ATP BY CHEMIOSMOSIS:

  • excited electrons are passed along a chain of carriers in the THYLAKOID membrane
  • electrons causes protons tobe pumped from the stroma into the thylakoid space - a PROTON GRADIENT is established.
  • ATP synthase located in the membrane lets protons across the membrane and uses the energy to make ATP.

PRODUCTION OF NADPH AND H+

  • electron excited from the Photosystem II pass along electron carriers and NADP+ picks up 2e- and 2H+ (from the stroma) to form reduced NADPH.
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NADP+  +  2H+  +  2e-  ->  NADPH  +  H+ PRODUCTION OF OXYGEN:

  • The photosystem II needs to replace 2e-'s 
  • With the help of light enzymes, WATER is split

H2O  ->  2H+  +  2e-  +  1/2O2

  • This process is called PHOTOLYSIS (using light to split water)

Note:

  • PHOTOPHOSPHORYLATION is using light to make ATP
  • PHOTOEXCITATION is using energy from light to excite electrons
  • If there is too much light, the NADP+ comesin shortage so the chloroplasts switch to CYCLIC phosphorylation.
  • This is an example of NON-CYCLIC photophosphorylation.
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Light Independent Reactions

This takes place in the STROMA.

  • Ribulose Bisphosphate a 5 carbon molecule
  • Through the the enzyme Rybisco, and CO2 is lost and glycerate 3-phosphate is formed
  • ATP from the light dependent reactions are reduced to 2ADP +P
  • 2NADPH from the light dependent reactions are reduced to 2NADP+
  • These go back to the LDR to pick up some more hydrogen.
  • So 2x triose phosphate is formed
  • 1/6 of triose phosphate is used to produce glucose phosphate - this is usedfor respiration, making cellulose and starch.
  • 5/6 of triose sulphate is used to regenerated RuBP, were an ATP is also reduced to form ADP + P
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The Chloroplast

HOW IS THE CHLOROPLAST ADAPTED TO ITS JOB?

  • The THYLAKOID membrane has a large surface area for maximum light absorptions.
  • A small space in the Thylakoid membrane (Thylakoid space) for the accumilation of Hydrogen (H+)
  • The stroma is fluid, for the enzymes of the Calvin Cycle.
  • The chloroplast contains thylakoids which have photosynthetic pigments embedded in the thylakoid membrane in clusters that absorb light and convert it into chemical energy. 
  • The thylakoids are pushed tightly together, meaning that light striking the chloroplasts will cause several light-dependent reactions to occur simultaneously, increasing energy and ultimately, sugars produced.


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THE RELATIONSHIP BETWEEN THE ACTION SPECTRUM AND THE ABSORPTION SPECTRUM OF PHOTOSYNTHETIC PIGMENTS IN GREEN PLANTS:

  • The visible light spectrum (400-700 nanometers) is used by plants in photosynthesis. 
  • If a plant is green, then it reflects green wavelengths of light and absorbs red, orange, some yellow, blue, indigo, and violet wavelengths of light. 
  • This means that the absorption spectrum of photosynthetic pigments in green plants is high for low and high wave lengths of light, but not for middle wave lengths of light (green), which are reflected by the photosynthetic pigments and are not absorbed.
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Limiting Factors In Photosynthesis

  • As light intensity, temperature, and concentration of carbon dioxide increase, so does the rate of photosynthesis.
  • Photosynthesis increases as light increases
  • Photosynthesis reaches a plateau after a specific light intensity
  • Photosynthesis increases as temperature increases
  • There is an optimal temperature
  • Too high of temperature denatures/melts the chloroplasts and slows photosynthesis
  • Photosynthesis increase as CO2 increases
  • There is a maximum rate of photosynthesis in response to CO2
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