photosynthesis

• adaptations:

  • palisade mesophyll has 5x more chloroplast than spongey mesophyll is closer to the upper epidermis where more sunlight is received
  • thin leaf allows CO2 to quickly diffuse through intracellular airspaces of SMLayer to palisade cells
  • long axis of PMC arranged perpendicular to leaf surface, minimizing amt of light scattered and absorbed by cell walls before it reaches chloroplast
  • chloroplast in peripheral (outer) cytoplasm can move via cytoplasmic streaming for optimal capture of light
• special chl a: P680 (found in PS II) & P700 ((found in PS I)

  • found in RC where it acts as PEA
• chl b: accessory pigment

  • absorbs light and channels light energy to RC
• carotenoids: a class of accessory pigments: have photoprotective roles

  • absorb and dissipate excess light E that could damage chlorophyll
  • helps bind to reactive O2 species, that can cause oxidative damage to cell
  • add colour to fruits & flowers (helping in dispersal & pollination)
• abs spectrum: shows wavelength of light abs by each pigment

  action spectrum: shows effectiveness of different wavelengths of light, in stimulating photosynthesis (shows R of P at each WL)
• action & abs spectrum similar but not the same: indicates chl a,b and carotenoids are important pigs involved in absorbing light for p. however, other accessory pigs that abs light efficiently in 540-600m WL, broadening spectrum of WL which P can occur
• location of p: thylakoid membrane
• photosystems: a reaction centre (2SCAM + PEA) surrounded by a number of light-harvesting complexes
• photosynthesis process:

  1. photoactivation
  2. light dependent cycle (PHOTOphosphorylation)
  3. light independent cycle (calvin cycle)
• photoactivation:

  • when a photon of light is absorbed by an accessory pigment in the light harvesting complex, an e- is excited to a higher energy level
  • when excited e- is dropped to ground state, E released is passed on to next pigment mol, this resonance transfer of energy continues till P680/P700 is reached
• Non-cyclic photophosphorylation is the predominant route in photosynthesis
• When P680 absorbs energy from the accessory pigments in the light-harvesting complex, it loses an electron, leaving an electron hole in Photosystem II (PS II)
• The displaced electron is accepted by a Primary Electron Acceptor (PEA) in the Reaction Center (RC)
• An electron hole in PS II is filled by an electron released from the splitting of water, in an enzyme-catalyzed reaction in the thylakoid space
• During the splitting of water, H+ ions are released contributing to a high concentration of H+ ions in the thylakoid space, while the oxygen atoms combine to form O2 as a byproduct
• The electron from the PEA is then passed down a series of increasingly electronegative electron carriers
• The energy released is coupled to pumping H+ ions from the stroma to the thylakoid space, generating a proton gradient across the membrane
• Chemiosmosis occurs when H+ ions diffuse down the proton gradient back into the stroma via ATP synthase, converting ADP to ATP
• Meanwhile, Photosystem I (PS I) loses an electron in a similar manner to PS II
• When P700 absorbs energy from the Accessory Pigments (AP) in the Light-Harvesting Complex, it loses an electron, leaving an electron hole in PS I
• The displaced electron is accepted by a PEA in the RC, and the electron hole is filled by the displaced electron from PS II when it reaches the end of the first Electron Transport Chain (ETC)
• The electron from the PEA is then passed down a series of electron carriers in the second ETC (Energy is not released in the second ETC)
• The electron is accepted by NADP, which is converted to NADPH by NADP reductase
• ATP and NADPH produced will be used for the Calvin Cycle
• non-cyclic photophosphorylation: (predominant route)

  • when p680 absorbs the energy from the AP in LHC, it loses an electron, leaving an e- hole in PS II. displaced e- is accepted by a PEA in RC
  • e- hole in PS II filled by an e- released from splitting of h2o, in an enzyme catalysed rxn in thylakoid space. during splitting of water, H+ released contri to high [H+] in TS, while O atom combines with another O atom forming O2 as by product
  • e- from PEA passed down a series of increasingly electroneg e- carries. E released coupled to pumping of H+ from stroma to TS, gen proton gradient across memb
• non-cyclic photophosphorylation: (predominant route)

  • chemiosmosis occurs when H+ diffuses down proton gradient, back into stroma via ATP synthase, ADP phos to ATP
  • meanwhile, PS I loses an e- in similar manner to PS II. when P700 abs E from AP in LHC, it loses an e- leaving e- hole in PS I. displaced e- is accepted by a PEA in RC. e- hole is filled by the displaced e- from PS II when it reaches end of first ETC
  • e- from PEA then passed down a series of e- carries in 2nd ETC (E not released in 2nd ETC!). e- accepted by NADP, NADP -> NADPH by NADP reductase
  • ATP & NADPH produced will be used for CC
• cyclic: only involves PSI

  • e- displaced from p700 accepted by pea, e- hole created in p700, e- transferred to middle of 1st etc and transp down 1st ETC, finally returned to PS I

  (same H+ pumping as non-cyclic)
• cyclic: why e- from PS1 not passed on to 2nd etc instead?

  happens when NADP limting
• location of calvin cycle: stroma
• calvin cycle requires: ATP, NADPH, CO2
• calvin cycle:

  1. C fixation: CO2 combines w RuBp - ribulose bisphosphate (5C) to form unstable 6C intermediate (cat by RuBisCo - ribulose bisphosphate carboxylase oxygenase), which breaks down into 2 3C compounds (GC/PGA) (glycerate phosphate/ phosphoglyceric acid)
  2. reduction: GP reduced to G3P (3C) (ATP & NADPH req for this rxn, NADPH provides reducing power)
  3. regen of RuBP: 5 G3P mols (5x3) used to regen 3 RuBp mols (3x5) (3 ATP req)
• O2 is a limiting factor:

  • RuBisCo accepts O2 as competitive inhbitor when CO2:O2 ratio is low
  • on hot days, stomata closes to retstrict water loss: CO2 entering leaf decrease, [CO2] decrease, [O2] build up due to photosynthesis
  • RuBisCo splits RuBp (5C) -> G3P (3C) + glycolate (2C). glycolate exported to peroxisomes and mitochondria, where it is broken down into CO2 (ie. photorespiration which generates no ATP)