A2 OCR Biology - Photosynthesis

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Autotrophs - organisms that use light or chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules

Herterotrophs - organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them

Oxygen first appeared in the atmosphere after being released from photosynthesis - photosynthesis appeared before respiration

Photosynthesis takes place in organelle called chloroplasts

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Chloroplast Structure

  • disc shape between 2-10 micrometers long
  • surrounded my double membrane - envelope
  • there is an intermembrane space, 10-20 nm wide, between inner and outer membrane
  • outer membrane = permeable to small ions
  • inner membrane less permeable and has transport proteins
  • inner membrane folded in lamellae which are stacked up into grana
  • between grana is the intergranal lamellae
  • stroma is the fluid filled matrix, containingstarch grains, oil droplets, DNA and ribosomes
  • grana = stacks of flattened membrane compartments called thylakoids
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How Chloroplasts are Suited to their Function

  • inner membrane (containing transport proteins) controls the entry and exit of substances
  • grana provide a large SA for photosynthetic pigments, electron carriers and ATP synthase
  • photosynthetic pigments arranged into photosystems allow maximum aborption of light energy
  • proteins embedded in grana hold photosystems in place
  • stroma contains enzymes needed to catalyse reactions in the light independent stage
  • grana are surrounded by the stroma so the products of the light dependent reaction can readily pass into stroma to take part in light independent reaction
  • chloroplasts can make some proteins needed for photosynthesis using the DNA and ribosomes
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Photosynthetic Pigments

  • can absorb certain wavelengths of light and reflect others
  • appear as the colour they are reflecting 


  • chlorophyll = mixture of pigments containing a long hydrocarbon chain and a porphyrin group (similar to haem)
  • light hitting cholorphyll causes a pair of electrons to become excited
  • two forms of chlorophyll a (primary pigment) = P680 and P700, both appear yellow-green
  • each absorbs red light at a slightly different wavelength
  • both found at centre of photosystems  - primary pigment reaction centre
  • P680 - found in PSII and peak absorption is 680nm
  • P700 - found in PSI and peak absorption is 700nm
  • chlorophyll a also absorbs blue light, wavelength 450nm
  • chloropyhll b (accessory pigment) absorbs wavelengths around 500 and 640nm - appear blue-green
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Accessory Pigments

  • carotenoids reflect yellow-green light and absorb blue light
  • do not contain porphyrin group and not directly involved in light dependent reaction
  • absorb light wavelengths that are not well absorbed by chlorophylls and pass energy to the chlorophyll a at the base of the photosystem
  • carotene (orange) and xanthophyll (yellow) are main carotenoid pigments.
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Light Dependent Stage of Photosynthesis

  • takes place in thylakoid membranes
  • PSII contains enzyme that takes part in photolysis (splitting of water)
  • Some oxygen used in respiration, most of it diffuses out through the stomata into the air
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