Chloroplasts

Chloroplasts

1. Structure

   • inner membrane contains transport proteins
     • allows an exchange of substances between cytoplasm and stroma
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   • stacked thylakoid membranes increase surface area for photosynthetic pigments, electron carriers and ATP synthase enzymes
     • assists light dependant stage of photosynthesis
   • photosynthetic pigments arranged into photosystems
     • allows maximum absorption of light energy
   • grana are sorrounded by the stroma
     • products of light dependant can pass into stroma for light independant
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   • stroma contains enzymes to catalyse reactions of light independant
     • increases rate of light independant
   • proteins embedded in grana
     • hold photosystem in place
   • chloroplasts contain basic ribosomes and DNA
     • can make some of the protein needed for photosynthesis

2. Photosynthetic pigments

   • absorbs certain wavelengths of light and reflects others
     • appears as colour that it reflects
   • many different pigments act together
     • to capture as much light energy as possible
   • they're found in thylakoid membranes
     • arranged in photosystems (funnel shape)

3. Chlorophyll

   • a mixture of pigments of similar structure
     • one long phytol (hydrocarbon) chain and porphryn group (similar to haem group but with magnesium atom)
   • when the right wavelengh of light hits it, two electrons become excited
   • chlorophll A is yellow-green and can either be P680 or P700
     • each absorbs red light at a slightly different wavelength (absorption peak)
     • both found at the centre of photosystems (primary pigment reaction centre)
     • P680 is in photosytem 2 and its peak absoption is 680 nm
     • P700 is in photosystem 1 and its peak absoption is 700 nm
     • both also absorb blue light of wavelength 450 nm
   • chlorophll B absorbs light of wavelengths around 500 and 640 nm
     • appears blue-green

4. Accessory pigments

   • carotenoids reflect orange and yellow light and absorb blue
     • carotene (orange) and xanthophyll (yellow) are the main carotenoid pigments
   • don't contain a porphryin group
   • they're not directly involved in the light dependant reaction
   • they pass the energy associated with wavelengths that are not well absorbed by chlorophyll to the primary pigment