Module 3

• what is photosynthesis 


  turns solar energy into biochemical energy
• what is the stored biochemical energy used for

  it converts CO2 and water into sugars to synthesize organic molecules
• how many glucose molecules made in photosynthesis

  one 6-carbon glucose molecule
• what helps power ATP


  sugars and oxygen produced by photosynthesis help power aerobic respiration so aid in ATP
• photosynthesis and respiration have what type of relationship

  interdependent
• where do light dependent reactions occur

  thylakoid membrane of the chloroplast
• where do light independent reactions occur

  in the stroma of the chloroplast
• what happens during light dependent reactions

  • water molecules split apart (photolysis) and they release electrons, hydrogen ions, and oxygen
  • electrons go through electron transport chain
  • ATP produced (phosphorylation of ADP into ATP)and NADP reduced to NADPH (electron donor) to power light independent reactions in stroma
• visible light in light dependent reactions

  • 40% of solar light on earth is in form of visible light
  • leaves absorb 80% of visible light reaching them
  • light is absorbed by pigments and each pigment absorbs light at different wavelengths
• what are pigments

  • pigment colour determined by light reflected or not absorbed
  • Chlorophyll absorbs photons in the blue (430 nm) and red (680 nm) portions of the visible light spectrum (400-700 nm), green light is reflected
  • Chlorophyll a reflects blue-green
  • Chlorophyll b reflects yellow-green
  • Carotenoids(accessory pigments) absorbs blue-green light and reflect yellow or yellow-orange light
• chlorophyll structure 


  magnesium and nitrogen ring absorb light photon and lipid tail anchors pigment into thylakoid membrane
• photosystems

  • In thylakoid membranes, pigments are clustered in discrete units of organization called photosystems
  • Each photosystem consists of an assembly of 200-300 pigments and associated proteins
  • Photosystems occur repeatedly throughout thylakoid membranes
• what does a photosystem consist of

  • reaction centre:
  • consists of chlorophyll a and primary electron acceptor
  • less than 1 % of pigments in photosystem are chlorophyll a
  • antenna pigment molecules:
  • chlorophyll and accessory pigments gather and transfer light energy to reaction centre
  • accessory pigment play critical role dissipating and funneling light energy to chlorophyll a
• electron energy

  • when pigments absorb light photons, the energy level of electrons is raised
  • energy from an excited electron is released when it drops back to its "ground state"
  • energy can be in form of heat, fluorescence or photochemistry
• LPD: reaction centre

  • Upon reaching the reaction center, light energy causes an electron to be ejected from the chlorophyll a molecule.
  • The electron is then transferred to a primary electron acceptor.
  • Chlorophyll a molecules of reaction centres behave differently than other chlorophyll and absorb light at a slightly different wavelength (longer wavelength = lower energy)
• LPD: PSI & PSII

  • PSII:
  • antenna pigment molecules: chlorophyll a and chlorophyll b and beta carotene
  • reaction centre: chlorophyll a molecule P680 and primary electron acceptor (pheophytin)
  • PSI:
  • antenna pigment: chlorophyll a and chlorophyll b and cartenoids
  • reaction centre: chlorophyll a molecule P700 and primary electron acceptor (iron surfer protein)
• what's the z scheme


  protein based electrons carriers that create a path for electron flow/ movement, this constitutes LPD
• Z-scheme: Steps 1 & 2

  • step 1: Light is funnelled to P680, energizing electron in chlorophyll a molecule to higher energy level. Excited electron passes to primary electron acceptor (pheophytin), then to plastoquinone (PQ) in electron transport chain to PSI
  • step 2: Each ejected P680 electron is quickly replaced by an electron from H2 O, after enzymatic splitting of water molecule (photolysis) into: 2 electrons + 2 protons (H +) + 1/2 oxygen atom
• Z-scheme: Steps 3 & 4

  • step 3: Electrons passed on from the primary electron acceptor gradually lose energy as they move through protein complexes in electron transport chain: plastoquinone (PQ), cytochrome complexes and plastocyanin (PC)
  • step 4: The energy released by the flow of electrons is used to move protons (H+) across thylakoid membrane (chemiosmosis)
  • Buildup of H+ gradient indirectly used to power the synthesis of ATP (photophosphorylation)
• Z-scheme: Steps 5 & 6

  • step 5: Light is funnelled to P700, energizing electron in chlorophyll a molecule to higher energy level. Excited electron passes to iron- sulphur protein electron acceptor. Electrons ejected from P700 are replaced by electrons from PSII
  • step 6: Excited electron moves through second electron transport chain from ferredoxin to NADP+ reductase, NADP+ reduced to NADPH (electron donor)
• chemiosmosis

  • As electrons move through the electron transport chain between PSII and PSI they lose energy
  • This energy is used to pump protons from the stroma into the thylakoid lumen causing a concentration gradient of H+ across the thylakoid membrane. This process is known as chemiosmosis.
  • The charge difference is also a pH difference (like a battery store of energy)
  • Protons (H + ) move across thylakoid membrane through protein channel, ATPase, producing ATP
• How is ATP made with chemiosmosis

  Movement of H + ions (protons) from thylakoid space (lumen) back into chloroplast stroma releases energy; when protons flow through ATP synthase this energy is used to add inorganic phosphate to ADP, forming ATP (higher energy)
• what is noncyclic electron movement

  movement of electrons from water to PSII to PSI to NADPH, PSII and PSI operate simultaneously
• Light-Dependent Reaction: O2 , ATP & NADPH


  Light-dependent reactions use:

  • energized electrons from PSII to produce ATP

  • generate oxygen via photolysis

  • use re-energized electrons from PSI to produce NADPH
• In photosynthesis, the efficiency of conversion of light energyinto chemical energy is

  27%
• Solar panels convert light into electricity at a photosyntheticefficiency of


  15-23%
• cyclic electron flow

  • PSI can, and does, work independently of PSII
  • Energized electrons from P700 are recycled back via ferredoxin to plastoquinone instead of being passed on to NADP+ reductase
  • Electrons pass back down to PSI driving H+ transport across the thylakoid membrane, making more ATP
  • No water molecules are split, so no oxygen or NADPH produced
• primitive form of photosynthesis

  cyclic electron flow, when PSII came, it helped cyanobacteria become more advanced with PSII and oxygen release
• what is a dark reaction

  • Light-dependent reaction uses light energy and H2 O to yield chemical energy in the form of ATP and NADPH.
  • The light-independent reaction or Calvin Cycle takes place in the chloroplast stroma, uses chemical energy produced in light-dependent reactions to make simple sugar phosphates, often called dark reaction
  • Cellular stockpiles of ATP and NADPH only last a few seconds or minutes and must be replenished again
• LIR: Calvin cycle

  • Officially known as Calvin-Benson-Bassham Cycle
  • Named after Melvin Calvin, who with his graduate students Andrew Benson and James Bassham, determined the pathway by which plants convert carbon dioxide (CO2 ) into sugars (1953)
  • The first product of the Calvin cycle is a 3-carbon sugar compound [3-phosphoglyceric acid (3PGA)]
  • Also known as C3 pathway
  • Starting compound (RuBP) is regenerated at end of cycle
• Calvin cycle steps

  1. One ATP powers the addition of a phosphate to a 5-carbon sugar phosphate

  2. CO2 is added to the 5-carbon sugar phosphate [catalyzed by RUBISCO enzyme]

  3. The resulting 6-carbon molecule is unstable and short-lived, it breaks down into two 3-carbon molecules (PGA)

  4. Two molecules of ATP add phosphates to PGA

  5. Two PGA molecules are converted into two (BPG)

  6. BPG is reduced by NADPH into (G3P)

  7. One G3P per three turns of the cycle contributes to sugars

  8. Majority of G3P continues through the rest of the cycle to replenish RuBP

  9. ATP from light-dependent reaction is used to phosphorylate Ru5p to regenerate RuBP
• how many turns for one glucose molecule

  Three turns of the cycle generates one G3P molecule, thus need six turns of the cycle to make one 6-carbon sugar (glucose to sucrose)
• ATP & NADPH in Calvin cycle

  • The Calvin cycle requires more ATP than NADPH (3:2)
  • BUT equal ratios of ATP and NADPH molecules are generated by the light-dependent reactions
  • The cyclic electron flow that takes place in PSI provides this extra ATP requirement
• sugar storage

  Once 6-carbon sugar is produced it can remain in chloroplast and bemade into starch, or be transported to other areas of cell (mitochondria) or plant (roots) as sucrose in sap
• what is rubisco's full name

  Ribulose 1,5 biphosphate carboxylase/oxygenase
• what % of plant biomass utilize C3 fixation

  95%
• C3 fixation

  • Majority of vascular plants perform C3 fixation including all woodytrees and temperate crop species (rice, wheat, soybeans andbarley)
  • C 3 plant are so named due to the three carbon product (3-PGA) of carbon fixation (carboxylation) by RubisCO
• The overall conversion of light energy to biomass (organiccompounds) in C3 plants is

  4%
• why is rubisco such a big deal

  • All phototrophs perform carbon fixation using the enzyme RubisCO, as a result it is the most abundant protein on earth.
  • RubisCO is both a carboxylase and an oxygenase, with a higher affinity for oxygen: oxygen blocks carbon fixation
• when did rubisco evolve

  a billion years ago during an anaerobic environment