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Photosynthesis and energy different organisms gain

Photosynthesis is about turning the inorganic carbon from the atmosphere into an organic substance via the addition of water  to make glucose (used as energy source) and (oxygen waste product )


  • Autotrophic - organisms make their own organic compounds from carbon doxide most of which is transformed to the chemical bonds of starch and glucose
  • Heterotrophic - organisms which can't make their own organic compound from carbon but generally eat  plants or other animals which have eaten plants. Therefore using the products of photosynthesis indirectly
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ATP energy source

  • in living orgnisms chemical bonds are continuously being made or broken, so in a cell energy has to be continiously available for this to happen
  • ATP or Adenosine triphosphate - is the universal energy form found in all living things (so there is no need to waste energy converting different forms of energy)
  • ATP consists of a nucleotide with 3 phosphate groups attached. Its the chemical energy stored in the phosphate bonds which when broken releases energy and made available to the cell

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Photosynthesis - Photolysis

  • involves the reduction of carbon dioxide to carbohydrates used to provide ATP
  • the hydrogen for this process comes from the splitting of water with light the waste product of oxygen being released into the atmosphere
  • splitting of water via light = photolysis. The energy for this step is first trapped by the light absorbing pigment chlorophyll

the overall process is achieved vs 2 link stages called :

  • light dependant reaction and light independant reaction

Light dependant reaction :

  • water is split and ATP and reduced NADP is made

Light independant reaction

  • in which the energy from the atp and reducing power of the hydrogen from reduced NADP are used to make sugar
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Light dependent reaction

  • Located in the thylakoid membrane of the chloroplast
  • 2 functions to produce ATP which supplies energy needed to synthesise carbohydrates, the other is to split water molecules in photochemical reaction providing hydrogen ions to reduce carbon dioxide and produce carbohydrates.
  • a pair of electrons from the chlorophyll is boosted to a higher energy level by the light energy it has trapped.
  • accepted by an electron acceptor and then passed down a chain of carriers as the electrons are passed doen the transport chain they reduce the acceptors and oxidate again when they pass them on forming a REDOX reaction.  This releases energy.
  • This energy is used to convert ADP and inorganic Pi into ATP in a process called photophosphorylation (light driven addition of phosphate)
  • the electron then enters another chlorophyll molecule

  • the electron eventually passes to NADP with the hydrogen from the water to form reduced NADP.

  • the ATP and reduced NADP go on to be used in the light independant reaction to make carbohydrate from carbon dioxide

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Cyclic Photophosphorylation

  • involves only PS1 and drives the production od ATP
  • when light hits a chlorophyll molecule in PS1, a light excited electron leaves the molecule.
  • taken up by an electron acceptor and passed directly along an electron transport chain to produce ATP.
  • an electron returns to the chlorophyll molecule in PS1 it can be excited in the same way again.


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Non-cyclic photophosphorylation

  • involves both PSI and PS11
  • it splits water molecules to provide reducing power to make carbohydrates at the same time results in the production od more ATP
  • extra hydrogen ions present in the chloroplast from dissociated water
  • in light condittions photons of light hit both PS1 nd PS11 exciting electrons
  • an excited electron from the PD1  is taken up by an electron acceptor e.g NADP which also takes hydrogen ion from the dissociated water forming reduced NADP
  • this reduced NADP is the used as a source of reducing power in the light independant reaction of photosynthesis to make glucose
  • at the same time an excited electron from PS11 is picked up by another electron acceptor and passes along and electron transport chain until it reaches the PS1
  • This drives the synthesis of and ATP molecule
  • and the PS1 recieves an electron to replace the one lost to the light - independant reaction
  • However now the chlorophyll molecule in PS11 is short an electron and unstable (the original electron can't be returned as its passed to the chlorophyll in PS1
  • so the electron is found from the splitting of water (photolysis) to restore the Chlorophyll in PS11 to its original state
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Non-cyclic photophosphorylation continued


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Light independant stage of photosynthesis

  • The ATP and reduced NADP from the light dependant stage is used in the calvin cycle of the light- independant stage
  • The Reduced NADP provides the Hydrogen or reducing power
  • ATP provides the energy needed for making carbon dioxide into carbohydrate
  • takes place in the stroma of the chloroplast
  • Each step in the calvin cycle is controlled by enzymes
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The calvin cycle

  • Products of the light dependant stage are used in a continious cycle to fix carbon dioxide resulting in new carbohydrates

Step 1

  • carbon dioxide from the air combines with the 5 carbon compound Rubulose bisphate (RuBP)- making it part of the photosynthetic reactions - carbon is said to be fixed
  • the enzyme Ribulose Bisphosphate carboxylase/oxygenase known as (RUBISCO) is needed for the reaction of RuBP and carbon dioxideis a rate limiting enzyme in the process of photosynthesis
  • the result of the reaction is a unstable 6 carbon compound, which immediatly splits to give two molecules of glycerate 3-phosphate (GP) and a 3 carbon compound
  • GP is then reduced (hydreogen is added from reduced NADP) to form Glyceraldehyde 3 phosphate (GALP) a 3 carbon sugar (energy needed for the reaction from ATP from light dependant stage)
  • much of the 3 carbon GALP passes through a series of steps to replace ribulose bisphosphate needed in the beggining of the cycle
  • some is synthesised into 6 carbon simple sugar glucose
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Calvin cycle

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Where photosynthesis takes place - the importance

  • chloroplasts relatively large organelles found in cells of green parts of plants
  • Membranes within a chloroplast are arranged in stacks called Grana 
  • The Grana is made up of stacks of membrane disks known as thylakoids (area of light dependant reaction). This is where the green pigment chlorophyll is found they are arranged to allow maxamum light absorbtion
  • the membrane stacks are surrounded by a matrix called the stroma (place of light independant readction), which contains all enzymes needed for photosynthesis - glucose production.


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function of Chlorophyll

  • light capturing photosynthetic pigment chlorophyll is a group of 5 closely related pigments
  • consisting of blue green Chlorophyll a, yellow green Chlorophyll b and orange carrotine caratinoids
  • each pigment absorbs and captures light from a different are of the spectrum - as a result far more energy from the light falling on the plant can be used

2 distinct Chlorophyll complexes:

  • Photosystem 1 (PS1) - found on the intergranial lamellae
  • Photosystem 11 (PS11) - found on grana themselves
  • each contains a different combination of chlorophyll and so absorbs light in  a slightly different area of the spectrum
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