photosynthesis

• The Calvin cycle


  1. Carbon dioxide

  2. Unstable 6C compound

  3. RUBISCO

  4. 2 x glycerate-3-phosphate (GP)

  5. Reduced NADP

  6. NADP

  7. ATP

  8. 2 x Triose phosphate

  9. Hexose sugar

  10. ATP

  11. Ribulose bisphoshate (5C)
• The Calvin cycle


  • In the stroma, the 5C compound ribulose bisphosphate takes up carbon dioxide to form an unstable 6C compound
  • The carbon dioxide can be described as 'fixed' as it has been converted from a gas into a solid carbohydrate compound
  • The unstable 6C compound immediately breaks down into 2x 3C compounds called glycerate-3-phosphate
  • Glycerate-3-phosphate is reduced by reduced NADP and using energy from ATP (both received from the light dependent cycle) the 2x 3C glycerate-3-phosphate are converted into 2x 3C carbohydrates called triose phosphate
  • 1C from the triose phosphates will be used to create a hexose sugar, the other 5C form ribulose phosphate which is regenerated to ribulose bisphosphate using ATP, and the cycle begins again
• Products of the Calvin cycle

  • Glucose (fructose bisphosphate)
  • Lipids from triose phosphate
  • Amino acids using nitrogen from nitrates
• Essential minerals


  • Nitrogen - Synthesis of proteins, nucleic acids and chlorophylls
  • Magnesium - Chlorophyll production
• Deficiency of essential minerals

  • Nitrogen - Reduced growth of all organs and yellowing of leaves (chlorosis)
  • Magnesium - Yellowing of the leaves (chlorosis)
• Limiting factors


  • Light intensity
  • Carbon dioxide
  • Temperature
• Light intensity is limiting the rate


  When the light intensity increases, the rate also increases
• Carbon dioxide can be described as a limiting factor


  When in short supply the process of photosynthesis is limited
• Temperature


  • Increase in temperature leading to an increase in kinetic energy of molecules will lead to increased successful collisions and an increased rate of reaction
  • Low temperatures will limit the reaction
  • Increases in temperature above the optimum can cause denaturation of enzymes that similarly limits the rate
• Chloroplasts


  Transducers that convert light energy into chemical energy
• Use the 'Unit 2 – Adaptations for gaseous exchange in plants' to study the adaptations of a leaf for photosynthesis
• Chloroplasts


  • Mainly located in the palisade mesophyll
  • Able to move and rotate in that layer to maximise light absorption
  • Have a large surface area for maximum light absorption
• Photosystems


  Light capturing complexes located in the thylakoid membranes containing different pigments that absorb different wavelengths of light
• Chromatography


  1. Mixture of pigments extracted from leaves and applied to the origin of the chromatogram

  2. Chromatogram placed into a solvent and left to run

  3. Pigments travel up the chromatography paper different distances according to their solubilities

  4. Distance moved by the solvent and the Rf values calculated and compared to known data to identify the different pigments
• Rf = distance travelled by pigment / Distance travelled by solvent front
• Absorption and action spectra


  Give evidence that the light absorbing pigments are responsible for photosynthesis
• Absorbance spectra show peak absorbances of different wavelengths of light for each pigment
• Action spectra show the rate of photosynthesis at different wavelengths of light
• Both absorbance and action spectra show peaks in the red and blue regions and a dip in the green region, supporting the theory that red and blue light is absorbed and used in photosynthesis whereas green light is reflected and does not contribute
• Light-dependent reaction


  1. Photon of light absorbed by pigment in antennae complex of PSII

  2. Energy passed to reaction centre where electron from chlorophyll a excited

  3. Excited electron reduces electron acceptor, oxidising chlorophyll a

  4. Electrons passed along electron transport chain powering proton pumps to increase proton concentration in thylakoid space

  5. Protons flow down gradient through ATP synthetase driving ATP production
• Photolysis of water


  1. Light splits water molecules in thylakoid spaces into hydrogen ions, electrons and oxygen

  2. Electrons replace those lost by chlorophyll a of PSII

  3. Protons picked up by reduced NADP

  4. Oxygen is a by-product and removed
• Photosystem I


  1. Photon of light hits PSI

  2. Same process as in PSII, but electron acceptor passes electron to NADP, forming reduced NADP
• Antennae complex


  • Contains chlorophyll a, chlorophyll b, and the carotenoids xanthophyll and beta carotene
  • Light energy absorbed and passed to the reaction centre
• Reaction centre


  • Contains 2 molecules of chlorophyll a
  • Electrons in these molecules excited and raised to a higher energy level
• ATP and Reduced NADP will now be used in the stroma for the light independent stage of photosynthesis to create hexose sugars