Lecture 14

chloroplasts produce their own nicotinamide adenine dinucleotide phosphate (NADPH) and ATP during photosynthesis

NADPH and ATP remain in it and used to produce sugars in chloroplast

sugars are exported to cytosol for ATP & insoluble polys

steps:

photosynthetic electron-transfer reactions (light reactions)

carbon fixation reactions (the dark reactions)

light energizes electrons in chlorophyll

electrons move along photosynthetic electron transport chain embedded in thylakoid membrane

chlorophyll obtains electrons from water & generate O2 via water-splitting enzyme

electron flow pumps H+ across thylakoid membrane resulting in an electrochemical proton gradient used to produce ATP in stroma

high-energy ekectrons reduce NADP+ to NADPH

ATP and NADPH from light reactions are used as energy source to convert CO2 to carbon hydrate (carbon fixation)

fixation begins in stroma with formation of glyceraldehyde 3-phosphate, transported to cytosol

can be used as starting block

blue & red light is absorbed primarily by chlorophylls

its electrons in network altering double & single bonds are raised to high energy level

it's oxidized only in reaction centers

chlorophyll function: as part of large multi-protein complexes (phtosystems= atenna and photochemical reaction center)

antenna complex: a cluster of chlorophyll & accessory molecules that absorb light & pass photons to photochem rxn center via FRET

photochem rxn cener energy is irreversibly trapped by electrons of a special pair of chlorophyll molecules that are immediately passed to close electron acceptors and ultimately into electron transport chain.

two photosystems and two photons required to produce ATP & NADPH

ATP is synthesized after the first photon is absorbed by PS II

NADPH synthesized after second photon is absorbed by PS I

electron removed from chlorophyll molecule in rxn center of PS II travels all the way through ETC in thylakoid until from NADP+ to NADPH

electron is replaced by low energy electron from water returning system to unexcited state

between PS II and PS I a proton electrochemical gradient generated & used to drive ATP synthesis

all oxygen in EArth atmosphere has been generated/ is being generated from splitting of water at PS II

non-cyclic photosynthesis

ATP & NADPH from light reaction are used to fix carbon (calvin cycle)

initial reaction: carbon dioxide (atmospheric, inorganic carbon) combine w/ 5-Carbon sugar (ribulose 1, 5-biphosphate) & H2O to make 2 molecules of 3-phosphoglycerate

catalyzed in the chloroplast stroma by enzyme called rebulose biphosphate carboxylase (RuBisco)

Rubisco is a very slow acting enzyme, thus there is a large number of rubisco complexes in stroma

for every 3 molecules of CO2 one new molecule of glyceraldehyde 3-phosphate produced

its  the sugar that provides starting material for synthesis of many other sugars & organic molecules

ATP & NADPH required to make ribulose 1, 5-biphosphate

most glyceraldehyde to cytosol, some enter glycolysis & converted to pyruvate

it is converted into sucrose & transported throughout the plant

glyceraldehyde 3-phosphate that remains in chloroplast stroma is converted into starch