Lecture 24: Photosynthesis Flashcards
Photosynthesis
Synthesis of carbohydrates and oxygen in plants
Light dependent and light independent reactions
E + 6CO2 + 6H2O = C6H12O6 + 6O2
Light reactions
Use light energy to oxidize water to oxygen and synthesize NADPH and ATP
Take place in thylakoid membrane: Photosystems 1 and 2
Excitation of reaction centres:
1. Oxidation of water to oxygen
2. Reduction of NADP to NADPH
Generation of transmembrane proton gradient that powers ATP synthesis
Light-independent reactions
Convert CO2 to carbohydrates using NADPH and ATP
Occur within the stroma
Photoreceptors
Variety to light absorbing groups: pigments
Cause different colour of a plant
Chlorophyll, beta-Carotene, Phycocyanin
Absorb light of different wavelengths
Absorbing a photon increases the potential energy of a pigment, energy can be released in different ways when the molecule returns to ground state
Chlorophyll A
Absorbs red and blue light
We see plants as green
Chlorophyll B
Absorbs red and blue light
We see plants as green
Planck’s Law
Energy of light is inversely related to its wavelength
E=hc/wavelength
Light-harvesting complexes
Membrane proteins that contain pigments
Protein environment influences the wavelength of the light that is absorbed
Energy from absorbed photons is transferred between neighbouring chlorophyll to a chlorophyll that acts as a reaction centre
Part of two different super complexes: photosystems 1 and 2
Reaction centres
Primary reactions of photosynthesis occur at specific chlorophyll molecules called reaction centres
Antennae
Other pigments that transfer absorbed light to reaction centre
Photosystem II
Begins light reactions
In chloroplasts grant with little contact with stroma
Contains several light-absorbing pigments and redox-active cofactors as prothetic groups
P680
Chlorophyll dimer Reaction centre of PSII Large absorption peak at wavelength 680 Oxidized form: P680+ Can only be reoxidized after is absorbs light: reduction potential changes dramatically when it absorbs a photon
P680+
Within photosystem II
Strongest biological oxidizing agent known
Captures electrons from water, resulting in O2 and P680
Requires manganese containing cofactor
2H2O = O2 + 4H +4e
P680*
Activated P680 by photon
Very negative reduction potential
Very easily loses an electron and is oxidized
Oxidized by PQ to P680+
Plastoquinone
Accepts two protons and electron from P680* to make plastoquinol
For every H2O, two plastoquinone molecules are reduced
Electrons in plastoquinol are transferred to cytochrome b6f
Cytochrome b6f
Resembles mitochondria complex III
Transfer of 4e to 4 plastocyanin (Cu2+) to give 4 plastocyanin (Cu1+) - reduction
Regeneration of PQ
Pumping of protons into thylakoid lumen
Photosystem I
Super complex containing light harvesting complexes located in individual (unstacked) storm lamella
Contains chlorophyll and carotenoids
Chlorophyll dimers form reactions centres: P700
P700
Absorption of wavelength 700nm
Generates high-energy electrons in response to light energy, which are transferred to ferredoxin (reduced)
Reduced to original state by plastocyanin from cytochrome b6f complex
Final outcome: recution of NADP+ to NADPH
Ferredoxin
Accepts electrons from excited P700 reaction centre
Transfers electrons to ferredoxin-NADP+ reductase
Ferredoxin-NADP+ reductase
Accepts electrons from ferredoxin
Reduces NADP+ to NADPH
NADPH is final acceptor of electrons removed from H20 in PSII
P700+
Reduced to original state by accepting electrons from plastocyanin
Photophosphorylation
Chloroplasts use the same mechanism as mitochondria to synthesize ATP
ATP produces in stroma
Cyclic electron flow
Between PSI and cytochrome b6f leads to proton pumping without generation of NADPH or oxygen
Does not require PSII
Light energy harvested by PSI fuels the proton pumping activity of cytochrome b6f
Dark reactions
Calvin cycle, light-independent reactions
Utilize the products of light reactions
Occur in chloroplast storm and fix atmospheric CO2 into carbohydrates
Can occur in light but do not light themselves
Require ATP and NADPH
CO2 fixation
Synthesis of carbohydrates from CO2
Catalyzed by enzyme Rubisco
Rate limiting step
Rubisco
Ribulose bisphosphate carboxylase/oxygenase
Carboxylates 5-carbon sugar and cleaves it into two 3C sugars
Glyceraldehyde-3-phosphate dehydrogenase
Reduction of C3 glycerates
To two glyceraldehyde-3-phosphates
Synthesis of hexoses
Phosphoglycerate kinase
Phosphorylates 3-phosphoglycerate (produce of rubisco) to 1,3 bisphosphoglycerate
2 ATP required
Photorespiration
Rubisco acts as an oxygenate
Uses oxygen instead of CO to generate 2C molecule phosphoglycolate and 3-PG instead of 2 molecules of 3-PG
Some phosphoglycolate can eventually be converted back to RuBP: costs ATP and NADPH and produces CO2
Occurs at low CO2 levels and high temperatures
