The Calvin Cycle Light Independent Phase of Photosynthesis

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The Calvin Cycle

• Light Independent Phase
• of
• Photosynthesis

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Calvin Cycle carbon fixation

• Similar to Krebs Cycle
• Metabolic pathway
• Starting material is regenerated in one turn of
  cycle
• Different than Krebs Cycle
• Carbon enters in CO2 molecule leaves as G3P
• Cycle spends ATP as energy source
• Consumes NADPH as reducing power for adding high
  energy electrons to make G3P

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Calvin Cycle has three phases

• Phase 1 Carbon Fixation CO2 is captured
• Phase 2 Reduction ATP is used NADPH gives
  away electrons
• Phase 3 Regeneration of CO2 acceptor (RuBP)
• Net Synthesis of one G3P molecule
• G3P can be used to make organic compounds
• 9 ATP consumed 6 NADPH

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• CO2 is attached to a 5-carbon RuBP with the help
  of an
• enzyme, rubisco.
• note Rubisco is the most abundant protein
  on Earth!
• 6-carbon product is unstable, splitting to create
  2 molecules
• of 3-phosphoglycerate for each CO2

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• ATP gives up a phosphate to
• form 1,3 Biphosphoglycerate
• NADPH donates 2 electrons
• creating G3P

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G3P same 3-carbon sugar in glycolysis

• G3P is the sugar of choice because it stores more
  potential energy
• Every 3 molecules of CO2 produce 6 molecules of
  G3P.
• To pay back the cycle which started with 15 total
  carbons (3 molecules of RuBP) only one molecule
  of G3P exits the cycle to be used by plant cell

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Phase 3 regeneration of RuBP

• Complex reactions rearrange the carbon skeletons
  of 5 molecules of G3P to create 3 molecules of
  RuBP.
• 3 more ATP are consumed in this process
• RuBP is ready to fix more carbon dioxide
• NO metabolic pathway is free energy must be
  available

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Alternative Mechanisms of CO2 fixation

• Dehydration is a constant problem for plants that
  live on land - even more so in hot, arid
  climates.
• Photosynthesis requires that stomata are open to
  allow gas exchange some water is lost due to
  transpiration.
• Result plants will close stomata in heat which
  reduces photosynthetic yield

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Photorespiration evolutionary relic?

• Occurs in the light and consumes O2 but
  generates no ATP or FOOD
• C-3 plants create a 3-carbon sugar in carbon
  fixation
• As plant is starved of CO2 O2 overtake CO2
  in leaf rubisco adds O2 to Calvin Cycle not CO2
• Counter productive why still around?
• Rubiscos affinity to accept O2 wasnt a problem
  in ancient times very little O2 in early earth
  atmosphere.

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Photorespiration is problematic

• Rice, wheat soybeans are important C-3 plants
  in terms of food production
• If hot, bright dry days linger, then food
  production in these plants can be cut by 50 -
  BAD NEWS for heterotrophs.
• TWO important adaptations have evolved in some
  plant species C4 pathway CAM

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C4 Plants ex .Sugarcane Corn (grass family)

• Unique leaf anatomy 2 photosynthetic cells
• Bundle sheath tight pack around veins
• Mesophyll loose arrangement in leaf
• Creation of 4-carbon sugar before Calvin Cycle
  begins in mesophyll.
• PEP high affinity for CO2 unlike rubisco

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Mesophyll cells of a C4 plant pump CO2 into
bundle sheath keeping CO2 high and reducing
photorespiration.
C4 Net consumption 12 ATP 6 NADPH
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CAM Pathwaysucculents (water storing plants)

• Stomata open at night closed during day.
• Closed stomata prevent water loss but also
  prevents CO2 from entering.
• To reduce photorespiration these plants FIX
  carbon at night
• Process named after the plant families that have
  this adaptation
• Organic acids are stored in vacuoles until ATP
  NADPH from the light reactions are available to
  help make sugar.

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C4 plants have structural separation from
Calvin Cycle CAM plants have a time of day
separation from Calvin Cycle C3, C4 and CAM
all use Calvin Cycle to make G3P
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Fate of G3P in plants?

• Sugar becomes either energy or carbon skeleton
  for other organic molecules.
• 50 of organic matter made is consumed as fuel
  for c. respiration in plants cells.
• Sucrose is transported around plant for use as
  energy or building material
• Lots of glucose linked makes up cellulose
  necessary for plant growth ? cell walls
• Excess stored in STARCH

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Photosynthesis a review

• Light Reactions
• Carried out by molecules in the thylakoid
  membranes.
• Split H2O release water to atmosphere.
• Convert light energy to
• the chemical energy of
• ATP NADPH.

• Calvin Cycle Reactions
• Take place in the stroma
• Use ATP NADPH to convert CO2 to G3P
• Return ADP, P1 and NADP to the light reactions

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ComparisonATP created by chemisosmosis

• ETC in a membrane pumps protons (H) across
  membrane as e- are passed from one protein
  complex to the next
• ETC transforms redox energy to a proton motive
  force PE stored in form of H gradient
• ATP synthase is built in the same membrane
  couples diffusion of H with phosphorylation of
  ADP

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Mitochondrial chemiosmosis

• Oxidative phosphorylation Krebs cycle
• High energy e- extracted from food molecules
• Inner membrane of mito pumps protons from matrix
  (outside) out to intermembrane space

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Chloroplast chemiosmosis

• Photophosphorylation Calvin Cycle
• High energy e- extracted from excited
  chlororphyll molecules by photons of light
• Thylakoid membrane pumps protons from stroma
  (outside) into the thylakoid space
• ATP synthase on the stroma side