Photosynthesis

• 6CO2 + 6H2O ----> C6H12O6 + 6O2
• Adaptations for plants for photosynthesis;

  • Stomata (allow gas exchange)
  • Large SA so a lot of light hits leaves
  • Densely packed palisade layer containing chloroplasts, air spaces allow CO2 to diffuse to photosynthesising cells
• Chloroplast

  1. 70s Ribosomes
  2. Starch Grain
  3. Grana
  4. Double Plasma Membrane
  5. Stroma
  6. Plasmid DNA
  7. Thylakoid Membrane
  8. Thylakoid

  
• Primary Pigment:

  • Chlorophyll a - blue and red wavelengths

  Accessory Pigments:

  • Chlorophyll b - blue and red wavelengths
  • Xanthophyll - violet and blue wavelengths
  • Beta-Carotene - violet and blue wavelengths
• The purpose of accessory pigments is to increase the range of wavelengths that can be absorbed by a plant, increasing the rate of photosynthesis.
• Magnesium is needed for the production of Chlorophyll a
• Chromatography to separate photosynthetic pigments:

  1. Tear up leaves and grind with acetone (organic solvent) to form a dark green pigment solution
  2. Use a capillary tube to spot the pigment onto chromatography paper
  3. Place the paper into a solvent (such as acetone/petroleum), this dissolves the solute and carries the pigment up the paper
  4. Remove the paper once once the solvent has travelled to the top of the paper, measure the distances travelled by each pigment
  5. Calculate Rf value for each pigment
  6. Compare to known values to identify pigments
• Absorption spectrum - graph that shows the amount of light absorbed by each photosynthetic pigment at each wavelength of light.

  
• Action spectrum - graph that shows the rate of photosynthesis at different wavelengths of light

  
• Photosystems:

  • Found in thylakoid membrane
  • Made of a cluster of many accessory pigments
  • Accessory pigments absorb light energy and pass it down to the reaction centre, containing chlorophyll a
  • The energy is then used to excite electron to a high energy level
• Two types of photosystems involved in photosynthesis:

  Photosystem II (P680) - best absorbs light at 680nm

  Photosystem I (P700) - best absorbs light at 700nm
• 1. The low energy electrons are absorbed by PS2 and are excited to a higher energy level at the reaction centre by light energy
  2. Electrons are passed to an electron carrier and are transported down the electron transport chain, losing energy as it travels
  3. some goes to the proton pump, and pump protons, creating electrochemical gradient
  4. Protons flow down the proton channel in the stalked particle down electrochemical gradient, activating ATP synthase
  5. causing synthesis of ATP from ADP + Pi
  6. Electrons go to PS1 and are excited again
  7. Electrons bond with H+ ions and NADP to form NADPH
• When energy is absorbed by the proton pump, a change in it's tertiary structure occurs, causing it to pump protons from the stroma into the thylakoid space
• Electrons for non-cyclic photophosphorylation are provided by photolysis of water by light:

  H2O --> O2 + 4H+ + 4e-
• NADP is the final electron acceptor in photosynthesis
• Cyclic Photophosphorylation:

  • Takes place in primitive plants and bacteria
  • Higher plants do use when CO2 is in short supply
  • Only PS1 is used
• Cyclic Photophosphorylation:

  1. Light energy is absorbed by PS1
  2. Electrons are excited to a higher energy level at the reaction centre and then released
  3. Electrons are passed down the electron transport chain and protons are pumped into the thylakoid space
  4. Protons diffuse through proton pump in stalked particle and ATP is synthesised
  5. Electrons return to PS1
  6. Cycle repeats

  No NADPH is formed
• Light independent stage (calvin cycle):

  • Does not require light
  • Uses products of LDR (ATP and NADH)
  • Takes place in stroma of chloroplast
• Calvin Cycle

  1. Unstable 6C compound
  2. Glycerate-3-phosphate
  3. ATP
  4. NADP
  5. Triose Phosphate
  6. Ribulose phosphate
  7. ADP
  8. Ribulose bisphosphate
  9. Rubisco
  10. CO2

  
• Role of inorganic nutrients in plant nutrients:

  Nitrogen

  • synthesis of proteins
  • synthesis of nucleic acids/DNA/RNA/nucleotides
  • cause chlorosis if not in high enough numbers
• Magnesium:

  • For chlorophyll production
  • Activation of ATP hydrolase
  • Low conc leads to chlorosis between veins of older leaves as Mg2+ is moved to newer leaves
• Phosphate:

  • Form phospholipids
  • Form nucleotides
  • Stunt plant growth if not high enough conc
• Chlorosis - yellowing leaves due to inadequate chlorophyll production so plant no longer able to absorb light energy