Photosynthesis

• Light absorption occurs in the thylakoid membranes of chloroplasts.
• Chlorophyll a is the most abundant photosynthetic pigment.
• chlorophyll b and carotenoids are accessory pigments that channel absorbed light energy to chlorophyll a.
• photosystems are embedded in the thylakoid membrane, made of:

  • reaction centre (has 2 special chlorophyll a molecules and a primary electron acceptor)
  • light harvesting complexes (photosynthetic pigments)
• Photosystem I contains P700
• Photosystem II contains P680
• Non-cyclic light dependent reaction

  1. Photoactivation at PSII
  2. photolysis of water
  3. electron transport from PSII to PSI
  4. photoactivation at PSI
  5. electron transport from PSI to NADP+
• Photoactivation: light is absorbed by photosynthetic pigments in the light harvesting complex, absorbed energy is relayed from molecule to molecule until it reaches the special chlorophyll a molecules in the reaction centre, one of the electrons are excited and emitted, electron captured by primary electron acceptor
• photolysis of water occurs to replace electrons lost from P680 molecules in psii (non-cyclic)

  H2O —> 2 e + 2 H+ + 1/2 O2
• electrons transported from PSII to PSI via an electron transport chain, electron carriers are of progressively lower energy levels, energy lost when electrons are transported down is coupled to formation of ATP (photophosphorylation)
• electrons that travel down the electron transport chain from PSII and reach PSI replaces the electrons lost from special chlorophyll a in PSI
• photo-excited electrons passed from primary electron acceptor in PSI down the 2nd electron transport chain, electrons transferred to NADP+ (final electron acceptor)

  NADP+ + 2e + 1H+ —> NADPH

  reaction catalysed by NADP+ reductase
• Cyclic light dependent reaction:

  • photoexcited electron from one special chlorophyll a molecule captured by PSI primary electron acceptor
  • electron goes to the middle of the 1st electron transport chain instead of moving on to the 2nd one
  • travels down the first electron transport chain
  • repeats upon reaching PSI again
• Photophosphorylation: process of synthesising ATP by means of a proton-motive force generated across the thylakoid membrane using light energy captured during light dependent reactions
• Photophosphorylation:

  • electrons travel down electron transport chain, releases energy that pumps H+ across the thylakoid membrane from stroma into thylakoid space
  • H+ accumulates in thylakoid space (steep proton gradient)
  • H+ diffuses down conc gradient via ATP synthase across thylakoid membrane, chemiosmosis occurs
  • ATP synthase catalyses conversion of ADP to ATP
• Light independent stage (Calvin cycle) takes place in the stroma of the chloroplast (contains enzymes)
• Calvin cycle phase 1: carbon fixation

  • RuBP accepts carbon dioxide (reaction catalysed by rubisco)
  • forms a 6C intermediate that immediately splits to form 2 glycerate phosphate (GP)
• Calvin cycle phase 2: reduction of GP to G3P

  • Glycerate phosphate (GP) reduced to form glyceradehyde-3-phosphate (G3P)
  • NADPH provides reducing power, ATP provides energy
  • G3P is end product, used to build other carbohydrates
  • for 1 G3P, 3 carbon dioxide have to be fixed
• Calvin cycle phase 3: regeneration of RuBP

  • 5 molecules of G3P (3C compound) is used to regenerate 3 RuBP (5C compound)
  • 3 ATP used in the reaction
• during Calvin cycle,

  • 1 G3P = 3 CO2, 6 NADPH, 9 ATP
  • 1 glucose = 6 CO2, 12 NADPH, 18 ATP
• light compensation point: point of light intensity where the rate of photosynthesis is equal to the rate of respiration, no net exchange of gases, light intensity above this point will result in growth of plant
• advantage of having accessory pigments:

  • accessory pigments channel absorbed light energy to chlorophyll a
  • increases range of wavelengths at which light can be absorbed
  • increase photosynthesis rate
• how light energy is converted to chemical energy

  • photoactivation, photon of light absorbed by photosynthetic pigment, electron excited to higher energy state
  • electron travels down ETC, electron carriers of progressively lower energy levels
  • releases energy, pumps protons from stroma into thylakoid space, sets up steep proton conc gradient
  • protons diffuse down conc gradient from thylakoid space into stroma via ATP synthase, phosphorylates ADP to ATP via chemiosmosis
• why rate of photosynthesis low at certain wavelengths:

  • wavelengths correspond to green light, not efficiently absorbed
  • will not cause effective photo-activation of photosystems during light dependent reactions