Chap 13: Photosynthesis

• Thylakoid membrane

  • Involved in the light-dependent reaction or photophosphorylation
  • Thylakoid membrane contains pigments such as chlorophyll a, chlorophyll B, carotene, xanthophyll, photosystems, and has a large surface area to absorb more light
• Stroma:

  • Used for the light-independent reaction or Calvin cycle
  • Contains enzymes like ribulose bisphosphate RuBP
  • Colorless or contains water to allow light to reach the thylakoid
• DNA/Ribosomes:

  • Responsible for making chloroplast proteins and proteins for photosynthesis
• Starch grains / lipid droplets:

  • Store chemical energy and products of photosynthesis
• Envelope:

  • Provides compartmentalization for the chloroplast
• Explain how grana is adapted to its function.

  • Stack of thylakoids
  • Membrane/thylakoids form large surface area
  • For (named pigments) / photosystems / light-harvesting clusters
  • For absorption of light energy
  • So large number of enzymes/ETC/ATP synthase 
  • For light dependent stage / photophosphorylation
• State the pigments involved in the light-harvesting complex

  • Chlorophyll a
  • Chlorophyll b
  • Carotene
  • Xanthophyll
• Role of accessory pigments

  • Passes energy to chlorophyll a / reaction centre
  • May absorb light wavelengths that chlorophyll a does not absorb (maximise the amount of light absorb)
  • Forms part of light-harvesting cluster of pigments
• Method used to separate and identify pigments

  1. Chromatography
  2. Place sample on base line of paper
  3. Dry and repeat
  4. Place paper in solvent
  5. Measure distance travelled by solvent and pigment
  6. Calculate Rf value (Rf = distance travelled by pigment divided by the distance travelled by solvent)
  7. Compare Rf values against published values to identify pigment
• What is meant by absorption spectrum and action spectrum

  Absorption spectrum: Absorbance of different wavelengths.

  Action spectrum: Rate of photosynthesis at different wavelengths.
• Similarities between non cyclic and cyclic photophosphorylation

  • Photoactivation of chlorophyll
  • ETC involved in both
  • ATP produced in both
• Difference between non cyclic and cyclic photophosphorylation

  Cyclic 

  • Only PSI
  • No reduced NADP/oxygen produced
  • No photolysis // oxygen-evolving complex involved
  • e- emitted from PSI returned to PSI / PSI is source of electron

  Non-cyclic

  • PSI and PSII both involved
  • Reduced NADP / oxygen produced
  • Photolysis // oxygen-evolving complex involved
  • Electrons emitted from PSII are replaced by water / water is source of electrons
• Describe the process of non-cyclic photophosphorylation

  1. Light energy is absorbed by photosynthetic pigment at PSII
  2. Light energy is passed onto the reaction centre and electron is photoactivated
  3. E- taken up by electron carrier and pass along ETC, releasing energy to pump H+ ions into lumen of thylakoid
  4. This creates a proton gradient, H+ diffuses through the ATP synthase
  5. ATP synthase generate ATP
  6. E- is photoactivated again at PSI, NADP is the final electron acceptor
  7. NADP + 2e- + 2H+ → reduced NADP
• Describe the photolysis of water

  1. Break down of water in presence of light
  2. Oxygen diffuses out of chloroplast and into the air
  3. E- in water replace the e- that have left the primary pigment
  4. Combination of water splitting and the proton pumping caused build up of proton gradient
  5. Proton diffuse down the gradient via the ATP synthase
  6. ATP synthase enzyme produce ATP 
• Describe cyclic photophosphorylation

  1. Electrons in PSI is photoactivated and accepted by electron carrier
  2. Pass ETC, releasing energy
  3. Energy used to synthesise ATP by chemiosmosis 
  4. ATP passed onto light independent reaction
  5. Electron return to PSI
• Describe the Calvin cycle

  1. Rubisco catalyses the fixation of COs: combination of ribulose bisphosphate (RuBP) + unstable 5C compound → 2 x glycerate-3-phosphate (GP) 3C
  2. GP is reduced to triose phosphate (TP) involving reduced NADP and ATP
  3. RuBP is regenerated from TP using ATP
• Outline the uses of triose phosphate in the mesophyll cells of the leaf.

  1. Regeneration of RuBP - requires ATP
  2. Triose phosphate used in the production of glucose/sucrose, amino acids, ribose/deoxyribose
  3. Triose phosphate used to produce useful organic molecules:
  4. Condense to become glucose → to produce starch, sucrose, cellulose
  5. Convert to glycerol to fatty acids → to form lipids for cell membrane
  6. Used in production of amino acids for protein synthesis
• Describe the role of reduced NADP 

  • Reduced/donates hydrogen 
  • Glycerate-3-phosphate (GP) to triose phosphate (TP) in Calvin cycle
• What is meant by limiting factor

  • When a process is affected by more than 1 factor
  • The factor that prevents any further increase in the rate of the process is in shortest supply
• Why can light energy be a limiting factor in photosynthesis?

  • Light energy/protons
  • Used for light dependent stage / photophosphorylation
  • To make ATP or reduced NADP 
  • To open stomata for Co2 to enter
• Describe the role of photosystem II in the absorption of light

  • Light harvesting complex
  • Accessory pigments present - chlorophyll b, carotent, xanthophyll
  • Passes light to reaction centre (chlorophyll a)
  • Non-cyclic photophosphorylation
  • More different wavelengths are absorbed