Photosynthesis

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Overview

  • The overall equation of photosynthesis: 6CO2 + 6H2O -light energy-> C6H12O6 + O2

  •  Light-dependent reactions (left on the diagram) involves light to chemical energy and the formation of red. NADP and ATP.
  • Light-independent reactions (the Calvin cycle etc.) involves the production of energy-rich glucose.
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Englemann's Experiment

  • Experiment to show the most effective wavelengths of light for carrying out photosynethesis.
  • Using green alga, containing spirical chloroplasts he distributed bacteria; eventually over time the bacteria clustered around the red and blue ends of light.
  • This is because there is more O2 produced from increased photosynthesis.
     
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Structure Of A Chloroplast

  • Grana - Stacks of up to 100 thylakoid discs, this is where the ligh-dependent stage occurs.
  • Stroma - Fluid-filled interior where light-independent stage takes place.
  • Starch grain - Where the starch and other polysaccharides are stored. 
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Absorption and Action Spectra

  • The absorption and the action spectra line up, indicating that the wavelengths of light absorbed are used in photosynthesis; shown by the rate of photosynthesis at those points.
  • The accessorry pigments are important as they absorb wavelengths of light not absorbed by primary pigments such as chlorophyll; meaning it can be more efficient.
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Light Harvesting

  • The chlorophyll and accessory pigments group in groups of hundreds, associating with special funnel proteins that guide light energy to the reaction centre - chlorophyll a.
  • Photons are transfered down the antenna complex until reaching the reaction centre, of which there are two types:
    -PSI, which has an absorption peak of 700nm.
    -PSII, which has an absorption peak of 680nm. 
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Light-dependent Stage

  • Occurs in the THYLAKOID MEMBRANES, involving:
    -Photolysis: splitting of H20 by light to form H+ and e-
    -The synthesis of ATP from ADP and Pi, this is photophosphorylation.
    -Combination of H+ with NADP to form to reduced NADP.
  • When light hits the reaction centre, an electron is raised to a higher energy level (Z scheme).
  • The following photophosphorlyation occurs in two ways:
    -Non-cyclic.
    -Cyclic. 
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Non-cyclic photophosphorylation

  • This involves both PSI and PSII.
    -Photons are absorbed by PSII and passes to the reaction centre.
    -This causes the displacement of two e- which are picked up by electron acceptors, to PSI.
    -The energy lost is used to generate ATP from ADP and Pi.
    -Photons of light are passed to PSI and reach the reaction centre.
    -This causes two more e- to be picked up by more electron acceptors.
    -PSII is now unstable as it has lost e-, replaced by those formed from photolysis.
    -The second acceptor passes these e- to reduce NADP with H+
    -Since nothing is recycled then its non-cyclic.

     

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Diagram of non-cyclic photophosphorylation

(http://images.tutorvista.com/content/photosynthesis/non-cyclic-photophosphorylation.jpeg)

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Cyclic Photophosphorlyation

  • Involves only PSI:
    -Light is absorbed by PSI and passed to chlorophyll in the reaction centre.
    -Then e- are passed to higher energy levels and then recieved by the electron acceptor.
    -Those electrons which are not taken up in the production of red. NADP then pass along a chain before returning to PSI.
    -As electrons are passed along they generate energy to form ATP.
    -No reduced NADP is formed though.
  • All these reactions in both forms of photophosphorylation take place in the thylakoid membranes, electrons provide energy to pump H+ into thylakoid space.
  • This sets up an electrochemical gradient, and so H+ flow into the membrane through protein channels and through ATP synthetase; forming ATP. 
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Light-independent Stage

  • Occurs in the STROMA.
  • This uses the products of the light-dependent stage, ATP and red. NADP.
  • Reduced NADP provides reducing power to reduce CO2 and synthesise hexose sugars.
  • The stage occurs as thus:
    -5C acceptor molecule called RuBP combines with CO2 to form unstable 6C compound.
    -The 6C immediately splits into two 3C molecules, GP.
    -GP is phosphorylated by ATP and reduced by red. NADP to TP.
    -Some of this can be built into glucose phosphate and starch.
    -As the cycle continues TP can reform RuBP using ATP.
    -The NADP is reformed and goes back to the light-dependent reaction to reduce.
  • This is called the Calvin cycle. 
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Calvin Cycle

  • This shows how the events take place.
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Product Synthesis

  • Following the metabalism of carbs, lipids and amino acids and absorption of minerals the plant can manufacture everything it needs for life.
  • Macronutrients such as K, Na, Mg etc. are used for growth.
  • N is taken up as nitrates and used to form amino acids, and without it the plant will not grow.
  • Mg2+ is absorbed also and used to produce chlorophyll, and without it pronounced chlorosis occurs in older plant leaves. 
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