Biology 1202B Lecture Notes - Lecture 5: Sucrose, Starch, Exergonic Reaction

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30 Apr 2012
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Lecture 5
Noncyclic electron flow:
o 1) 1 H2O splits and creates 1 ATP and 1 NADPH
o 2) 1 photon to drive 1 electron through 1 photosystem
o When H2O splits it creates 2 electrons
o 1 electron through 2 photosystems requires 2 photons
o 2 electrons through 2 photosystems requires 4 photons
o To produce 1 O2 would require 8 photons
Cyclic electron flow under certain conditions photoexcited electrons from photosystem I
can take this alternative pathway
o Excited electrons cycle from their reaction centre to a primary acceptor, along an
electron transport chain and returns to the oxidized P700 chlorophyll
o As electrons flow along the electron transport chain, they generate ATP by cyclic
photophosphorylation
Noncyclic produces ATP and NADPH in roughly equal quantities
o But, the Calvin cycle consumes more ATP than NADPH
Cyclic allows the chloroplast to generate enough surplus ATP to satisfy the higher demand of
ATP in the Calvin cycle
Chemiosmosis the mechanism by which chloroplasts and mitochondria generate ATP
o Oxidative phosphorylation mitochondria
o Photophosphorylation chloroplast
Electrons pass down the ETC
Cytochrome complex pumps H+ from stroma to thylakoid lumen
H+ is high concentration in lumen and low concentration in stroma
Also H+ comes from the splitting of water molecules
This causes an electrical gradient
Proton-motive force: provides energy for ATP synthase
Photosynthesis: carbon dioxide + water + light glucose and oxygen
Calvin cycle:
o Reactions occur in the chloroplast stroma and the enzyme is Rubisco
o Light independent reaction
o Rubisco attaches the carbon from CO2 to ribulose bisphosphate
Rubisco is the most common and complex protein on the Earth
o The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
o The starting material is regenerated at the end of the cycle, so it can continue
o The sugar product is not glucose, it is a three-carbon sugar (glyceraldehyde-3-
phosphate or G3P)
o Each turn of the cycle fixes one carbon
o For the net synthesis of one G3P molecule, the cycle must occur 3 times (fixing 3
molecules of CO2)
o To make one glucose molecule would take 6 cycles and the fixation of 6 CO2
molecules
o The carbon fixation phase each CO2 molecule is attached to a five-carbon sugar
(RuBP)
This is catalyzed by rubisco and then this molecule splits in half to form two
3-Phosphoglycerate molecules per CO2
o Reduction phase each molecule that was created from the split receives another
phosphate group from ATP to form 1,3 bisphosphoglycerate
A pair of electrons from NADPH reduces each 1,3 bisphosphoglycerate to
G3P (reduced from carboxyl to carbonyl group)
o Regeneration phase the CO2 acceptor (RuBP) is regenerated by the rearrangement
of 5 G3P molecules, which forms 3 RuBP molecules
For this to occur, the cycle must spend 3 more molecules of ATP (1 per
RuBP) to complete the cycle and prepare for the next
o For the net synthesis of one G3P molecule:
The Calvin cycle consumes 9 ATP and 6 NADPH (3 and 2 per CO2 molecule)
o G3P is transported from the chloroplast to the cytosol of the cell
Very little glucose is created, rather most fixed carbon is converted to
sucrose
Sucrose is the major transport sugar in plants
Or it is converted to starch the major storage carbohydrate
Starch grains appear in the chloroplast (stroma) in day and are converted to
sucrose at night
Summary of Calvin cycle: for the input of 3 CO2
o 9 ATP consumed (9 ADP produced)
o 6 NADPH oxidized (6 NADP+ produced)
o ½ glucose (C3) produced (3 CO2 consumed)
Takes 2 turns of the Calvin cycle to create 1 glucose
Carbon fixation is a highly endergonic reaction (lots of energy consumed)
Photosynthesis is an anabolic endergonic reaction
Respiration is a catabolic exergonic reaction
Energy is not stored in ATP and NADPH because they are too unstable
o Plants “fix” CO2 into carbohydrates
o Carbohydrates are the starting point for synthesis of lipids, amino acids, proteins
and nucleic acids
Uses of products:
o Starch production storage (usually in roots)
o Export within the plant to needed locations (Ex: flowers)