photosynthesis Flashcards
what is photosynthesis?
process by which photosynthetic organisms convert light energy to chemical energy to fix CO2 and produce sugar
light dependant and independent reactions
light dependant reaction
converts light energy into chemical energy
require light in visible range of spectrum
produces ATP and NADPH
produce oxygen as byproduct
occurs on thylakoid membrane
light independent reaction
carbon fixation
must be preceded by light dependant
uses ATP and NADPH from dependant
carbon is fixed
sugar/carbohydrates produced
occurs in stroma
chloroplast structure
thylakoid discs stacked into granum
each surround by a thylakoid membrane
membrane can extend between grana creating a thylakoid lamellae
surrounded by storm containing metabolites and enzymes
2 membranes
envelope surrounding stroma and
inner membrane impermeable to most molecules
has many transporter proteins
structure of chlorophyll
tetraphytol ring
- nitrogen atoms surrounding magnesium atom
- required for chlorophyll to capture light
phytol tail
- hydrocarbon tail
- allows molecule to embed itself into thylakoid membrane
different chlorophylls have different functional groups
form complexes called antennae
one chlorophyll acts as a reaction centre
antennae embedded in large protein complexes in thylakoid membrane
create complexes called photosystems
chlorophyll and light
absorb light at the extremes of the visible spectrum
chla and b exhibit two absorption maxima at the extremes
maximal are where most light is absorbed
- at blue and red part of spectrum
photosystem 1
reaction centre = chla
P700
primary electron acceptor = Fe-S protein
final electron acceptor = ferredoxin
responsible for photo reduction
photosystem 2
reaction centre = chla and chlb
P680
primary electron acceptor = pheophytin
final electron acceptor = plastocyanin
responsbile for photo-oxidation (photolysis) and photophosphorylation
what is redox potential?
measure of the tendency (how easy) of a molecule to loose or acquire electrons
- measured in volts
drives electrons in electron transport chain
more positive = greater the affinity for electrons
accepts electrons
therefore becomes reduced
more negative = lower affinity
easily donate electrons and become oxidised
donates to molecule with more positive potential
electrons move from molecules with more negative to more positive potential
reductants vs oxidants
reductants
- easily donate electrons to another with more positive potential
become oxidised
electron acceptor becomes reduced
oxidants
- easily accepts electrons
becomes reduced
acceptor becomes oxidised
lose electrons = oxidised
gain electrons = reduced
redox potential in the electron transport chain
electrons flow from more negative to more positive electrode potential
allows flow of electrons along chain
redox gets increasingly positive
eventually ends at reaction centre of P700
becomes excited and gives electrons to primary electrons acceptor at photosystem 1
overview of light dependant reaction
excitation of reaction centre initiates an etc
pigments in antennae harvest light
energy passed onto reaction centre
becomes excited and loses an electron
electrons passed to primary electron accepts
= etc along thylakoid membrane
LDR step 1 - photo-oxidation and photolysis
P680 loses electrons
passed to pheophytin
- primary electron acceptor of P680
P680 becomes strong oxidant
pulls electrons from H2O
water split to give electrons, hydrogen and oxygen
H2O oxidised to oxygen in photooxidation
LDR step 2
pheophytin passes electron to plastoquinone
2 protons pumped from stroma
plastoquinone reduced to plastoquinol
plastoquinol dissociates from P680
LDR step 3
plastoquinol carries the 2 electrons to cytb6f
releases 2 protons into lumen
at same time, more protons pumped into lumen
LDR step 4 - end of 1st chain
cytb6f passes electrons to plastocyanin
plastocyanin passes electrons to the reaction centre of photosystem 1 - P700
LDR step 5 - P700 photo reduction
P700 reduced
electrons passed to Fx
- creates new etc
Fx passes electrons to ferredoxin
then to ferredoxin enzyme - NADP+ reductase
enzyme carries electrons and becomes strong reductant
reduces NADP in photo reduction
how is proton gradient created in LDR?
all this time protons have been building up in the lumen
protons pumped from stroma to lumen
- released into lumen from splitting of water
- plastoquinone picked uo protons from stroma, reduced to plastoquinol
- releases protons into lumen
creates proton gradients from stroma to lumen
LDR overview
starts at P680
harvests light and becomes excited
electrons given away, etc initiated
electrons passed from P680 to cytb6f
to plastocyanin
to P700
gives electrons to Fx
then to ferredoxin
to ferredoxin enzyme in stroma
enzyme becomes reductant and reduces NADP = NADPH
process of photolysis
P680 loses electrons to pheophytin
becomes strong oxidant
pulls electrons from H20
water split
= electrons, hydrogen and oxygen
photophosphorylation
excess protons in the lumen return to stroma through ATP synthase
ATP synthase couples ATP production with movement of protons back to the stroma
at least 2 ATP produced for every 4 electrons that go through etc
- chemiosmotic synthesis of ATP
what is needed from the LDR for the LIR
ATP
NADPH
light independent reactions
carbon fixation
Calvin cycle
- carbon fixation
CO2 reacted with a 5C molecule - reduction phase
series of reactions that produce 2x 3 carbon sugar phosphates - regeneration stage
initial 5 carbon molecule is regenerated so that a new cycle can start again
carbon fixation - LIR 1
one CO2 reacts with one ribulose 1-5 bisphosphate
- carbon fixation
= 2 molecules of 3-phosphoglycerate
catalysed by ribulose 1-5 bisphosphate carboxylate oxygenate - rubisco
3 stages of carbon fixation by rubisco
enolisation - rearrangment
- double bond in chain
- produces an intermediate (unstable)
carboxylation
- immediately reacts with CO2 = 6C molecule
- unstable molecule
cleavage
= 2 molecules of 3-phosphoglycerate
reduction and sugar production - LIR 2
2 reactions
convert 3-phosphoglycerate into glyceraldehyde 3 phosphate
- requires ATP and NADPH
first
ATP used
catalysed by phosphoglycerate kinase
3-phosphoglycerate converted to 2 molecules of 1,3-bisphosphoglycerate
second
uses NADPH from light dependant
catalysed by NADPH-specific glyceraldehyde-3-phosphate dehydrogenase
reduction of 3-GPA to glyceraldehyde 3 phosphate
uses of glyceraldehyde 3 phosphate
removed from cycle
used to synthesise sugars
- eg glucose and sucrose
also used to recycle G3P
regeneration of RuBP - LIR 3
requires ATP from LDR
regenerated from G3P
