Photosynthesis

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Site of Photosynthesis- structure of the leaf

  • Large surface area- collects as much light as possible.
  • Arrangement of leaves that minimises overlapping.
  • Thin, most light is absorbed in the first few milimetres also keeps diffusion distance low.
  • A transparent cuticle and epidermis allows light through.
  • Lots of stomata for gasous exchange.
  • stomata open and close depandant on light intensity.
  • network of xylems bring water to leaf cells and phloems bring sugars.
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outline for photosynthesis

1) Capturing of light energy- by chloroplast pigment eg chlorophyll

2) The light independant reaction-convert light energy into chemical energy. Photolysis occurs (splitting of water molecule by light to produce 2e, 2H+, O)

3) Light independant reaction- protons from light dependant reaction are used to reduce C02 into sugars and other oganic molecules.

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The making of ATP- Light dependant reaction

  • light hits the chlorophyll molecule and boosts the energy of a pair of electrons in the chlorophyll.
  • Rasising them to a higher energy level and the electrons are said to be Exited.
  • This causes the electrons to leave the chlorophyll.
  • These electrons are taken up by electron carriers. (the chlorophyll has lost electrons so is oxidised the electron carriers have gained electrons so is reduced)
  • electrons are passed along the chain in a series of oxidation reduction reactions.
  • Each electron carrier has less energy than the one previous the exess energy is used to form ATP from ADP+Pi
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Photolysis of Water- Ligh dependant reaction

  • The loss of electron in the chlorophyll when the light hits it leaves it short of electrons and the elctrons are replaced
  • This replacement comes from water molecules which are split using light.

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Photolysis of Water 2- Light dependant reaction

  • The hydrogen ions (protons) are taken up by electron carrier NADP and becomes reduced to NADPH, it then goes and enters the light independant reaction
  • This NADPH has futer chemical potential.
  • oxygen combines and diffuses out of the leaf.

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Site of the light dependant reaction

  • The light dependant reaction takes place in the thylakiods of the chloroplast.
  • The thylakiod membranes provide a large surface area for electron carriers and enzymes to carry out the light dependant reation.
  • Proteins in the grana hold the chlorophyll in a very precise manner that allows maximum absorbtion of light.
  • Chloroplast contain DNA and ribsomes to synthesis protein needed for photosynthesis quickly
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The light independant reaction- Calvin Cycle

  • CO2 diffuses into the leaf through the stomata and dissolves in water to allow it to enter the stroma of the chloroplast
  • CO2 comibes with Ribulose-Bisphosphate (5 carbon) (RuBP) to give 2 molecules of Glycerate-3-phosphate (G3P) (3 carbon)
  • ATP and NADPH are used to reduce G3P into Triose Phosphate (TP)
  • NADP is reformed and enters the site of the light dependat stage 
  • Some TP is used to make Gluecose
  • But most is used to reform Ribulose-Bisphosphate
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Calvin Cycle

(http://legacy.owensboro.kctcs.edu/gcaplan/bio/notes/07_10bCalvinCycle_3_L.jpg)

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Site of the light independant reaction

  • The stroma contains all the enzymes needed for the light independant reaction to occur.
  • Contains DNA and ribosomes for the production of any proteins that are needed for the light independant reaction.  
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Factors affecting photosynthesis

  • Darkness- no photosynthesis would occur if there is no light
  • Increased light intensity- rate of photosynthesis will increase upto a point where it has no futher effect on rate of photosynthesis.
  • At this point other factors such as Carbon Dioxide is the limiting factor so that will increase photosynthsis rate.
  • At this point CO2 will have no futher effect and so a different factor will limit photosynthesis eg Tempreture
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