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Features of a Chloroplast


  • Disc-shaped 2-10um long
  • Intermembrane space 10-20nm wide
  • Outer membrane is permeable to many small ions
  • Inner one is less permeable and has tansport proteins embedded in it - it is folded into lamellae which are stacked


  • fluid filled
  • visible with light microscope
  • enzymes for the light-independent stage 
  • starch grains, oil droplets, DNA and ribosomes (prokeryotic type)


  • stacks of flattened Thylakoid compartments
  • light is absorbed and ATP is made in the light-dependent stage
  • Visible with an EM
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Adaptations of Chloroplasts

  • inner membrane controls entry and exit substances
  • Grana have upto 100 thylakoids - large SA
  • Photosystems allow maximum light absorption
  • Enzymes catalyse the reactions
  • Grana and stroma close to each other
  • Proteins needed are made in the chloroplasts by DNA and ribosomes
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Light Dependent Stage - Non Cyclic Phosphorylation

  • Photolysis - A light wave/photon hits a water molecule in the presence of an enzymes and causes it to split into oxygen, an H+ ion and an electron. The energy from the light excites the electron and it jumps up an energy level - it goes to replace the electron lost at PSII
  • PSII - A light wave/photon is absorbed by accessory pigments and the energy transfered to the primary pigment reaction centre which is where chlorophyll a p680 is located. An electron is excited 
  • ETC - the electron accepter in the transport chain accepts the excited electron and the electron carriers take it to PSI. As the electron moves, it losses energy which is used to pump H+ ions into the Thylakoid lumen
  • PSI - an in PSII, light is absorbed and the energy transferred to the reaction centre but in this system, it is where chlorophyll a p700 is located. Again an ETC transports the electron, evetually this electron is transport through proteins into the stroma
  • THe accumulation of H+ions in the Thylakoid lumen creates an electrochemcial gradient (becomes acidic) until the ions diffuse down the chemiosmosis concentration gradients and out through the stalked particle, ATP synthetase
  • The proton motive force of the H+ ions passing through causes the head of the ATP synthetase to rotate which forces ADP and Pi to join and form ATP
  • The H+ ions and electrons join to for Hydrogen which then reacts with NADP to form NADPH
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Photosynthetic Pigments

...absorb a certain wavelength of light energy to excite electrons and many pigments act together to capture as much light as possible - they are embedded in the thylakoid membrane and are in funnel shaped photosystems held in place by proteins, passing on energy to the primary pigment reaction centre


  • long phytol chain - the tail
  • Phorphyrin group - similar to haem in haemoglobin and light excites the mg electron - the head

Chlorphyll a 

  • Photosystem II has P680 - yellow in colour and absorbs red light of 680nm wavelength
  • Photosystem I has P700 - yellow/green in colour and absorbs red light of 700nm wavelength

Chlorophyll b:- appears blue-green and absorbs light of 500nm (blue) and 640nm (yellow) wavelength

Accessory Pigments:- Carotenoids appear yellow-orange and absord blue light

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Calvin Cycle

  • RuBP and CO2 in the presence of Rubisco combine to form 2GP molecules.
  • GP can be converted into Amino acid with the additon of nitrates (to then becomes proteins for mitosis), fatty acids to become lipids or use hydrogen provided by NADPH and Energy from ATP to become 2TP
  • Out of 12molecules of TP, 2 will become hexose sugars, the remainder regenerates with the phosphate provided from ATP as it is converted in ADP and Pi, into RuBP and continue the cycle
  • Hexose sugars:- Sucrose (non reducing, transported in the phloem), Fructose, Cellulose (beta glucose for the cell wall) or Starch (alpha glucose as an energy store)

RuBP - Ribulose Bisphosphate (5C)

GP - Glycerate - 3 - phosphate (3C)

TP - Triose Phosphate (3C)

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Light Dependent Stage - Cyclic Phosphorylation

  • Only uses PSI
  • No photolysis of water or NADPH
  • Some ATP
  • Mostly used in guard cells to bring in K+ ions which lowers the water potential and water moves in through osmosis to cause cells to become turgid and open the stomata

Where does the oxygen for Photosynthesis Come from?

If a plant were given water with a radioactive isotope of oxygen, the oxygen released was also radioactive. It was not so when the COs was radioactive

Therefore the oxygen is released from water

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Limiting Factors for the rate of photosyntheis

Light:- ATP and NADPH are produced in the Light Dependent stage of photosynthesis and without them, the calvin cycle can not occur as there is no ATP or NADPH to convert GP into TP or ATP to regenerate TP back into RuBP

CO2- Little carbon dioxide means that little carbon fixation occurs so RuBP is not converted into GP but GP and TP are being converted into RuBP

Temperature:- As temperature increase to 25dc, reactant molecules gain kinetic energy so move faster adn these is an increased rate of successful reactions forming enzyme substrate complexes. This means more GP is produced. Above 25dc, competitive inhibition occurs as oxygen outcompetes carbon dioxide for the active site of Rubisco, leading to photorespiration

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