Lecture 19 and 20 - Photosynthesis Flashcards
What organisms contribute what % of global photosynthesis?
40% plants
60% algae/bacteria
Overall photosynthesis equation
3CO₂ + 9ATP + 6NADPH + 6H⁺ ⇌ 3-phosphoglycerate + 9ADP + 6NADP + 9Pᵢ + 3H₂O
Photosynthesis process
Light hits photosystem 2, knocking off some electrons which move down the ETC and eventually gives them to photosystem 1 (electron balance is restored by the hydrolysis of water)
Light hits photosystem 1 and as electrons are passed even more down an ETC, even more reducing power is generated which is used to synthesise ATP (electron balance is restored by photosystem 2)
Plant cell structure: C, C, F, G, L, M, N, P, P, P, R, R, S, and V
Cell wall - protects the cell
Chloroplast - Convert sunlight into glucose
Filamentous cytoskeleton - made of MTs, actin, and IF, forming the cytoskeleton and cell shape
Golgi apparatus - protein sorting/processing
Leukoplast - Starch formation and storage
Mitochondria - generate ATP/energy from glucose
Nucleus - contains DNA
Peroxisome - H₂O₂ for detoxification/metabolism
Plasma membrane - controls what enters/exits
Plasmodesmata - small channels between cells
rER - synthesis of proteins for secretion
Ribosomes - protein synthesis
sER - storage/steroids storage/creation
Vacuole - Water storage that also allows turgidity
Chloroplast structure
Nucleoid - DNA rings
Stroma - Liquid space
Thylakoids - Used to synthesise ATP
Grana - Stacks of thylakoids
Plastoglobulus - Lipid synthesis, thylakoid storage
Thylakoid membrane
PSII - Starts the photosystem chain
PSI- Passes electrons through the ETC
Cytochrome bf - links both photosystems
Photosystem 2
Enormous transmembrane protein (>20 subunits) which responds to wavelengths shorter than 680nm and has several antenna pigments which capture and transfer light until a special pair of chlorophyll molecules (which can be ionised - energy trap) at the reaction centre of the photosystem is reached
Requires light with shorter wavelengths (higher energy) than PSI
Also known as P680 as the maximum frequency (?) it can absorb is 680nm
Chlorophyll a and b
Planar structure for easy ionisation, allows for easy electron delocalisation
A - better at absorbing lower wavelengths of light
B - better at absorbing higher wavelengths of light
Transfer of electrons in PSII
P680 get excited -> pheophytin -> plastoquinone at fixed site (Qa) -> plastoquinone at mobile site (Qb)
Excited P680 extracts electrons from water bound at a manganese centre, forming O₂
Qb plastoquinone is reduced from plastoquinone (Q) into plastoquinol (QH₂)
Oxygen evolution centre of PSII
Composed of 4 manganese atoms, 1 calcium atom, and oxygen atoms
Oxidised by one electron at a time
Attaches water molecules to the manganese and calcium atoms and combines them to form O₂
Plastoquinone electron transfer
Plastoquinone (PQ) is reduced into plastoquinol (PQH₂) using hydrolysis of water
PQH₂ can diffuse through the membrane (into the middle) while carrying its two electrons
Once next to cytochrome bf, it gets oxidised and causes 2 protons to be released into the lumen
Cytochrome bf electron transfer: what does it do, where does it pass its electrons to, and what is the overall equation?
Once it oxidises plastoquinol, the Q cycle causes two more protons to be released into the lumen
The electrons are then passed to plastocyanin (PC), the final electron carrier, along with the 4 protons (2 from plastoquinol and two from the stroma) being passed into the lumen
PQH₂ + 2PC(ox) + 2H⁺ → PQ + 2PC(red) + 2H⁺ + 2H⁺
Plastocyanin: what is its structure, how does reduction work, and what is the equation?
Blue copper protein that is soluble and resides in the thylakoid lumen
Reduction occurs one electron at a time at the copper atom
PQH₂ + 2PC-Cu2⁺ → PQ + 2PC-Cu⁺
PSI: what is its structure, how does it capture light, and what type of light does it capture?
Large transmembrane assembly (>100 cofactors with 14 polypeptide chains) with a core made of two subunits psaA and psaB (~82kDa each)
Antenna complex contains several chlorophylls
and carotenoids on two proteins, capture light
at different wavelengths, (<700 nm)
Special pair of chlorophyll a molecules in the reaction centre of P700, absorb light at 700 nm
PSI electron transfer
P700* -> chlorophyll A₀ -> phylloquinone -> 3 iron-sulphur clusters -> ferredoxin
P700* then is restored to normal by receiving electrons from plastocyanin
Fd-NADP⁺ reductase: what does it do and what is the mechanism behind the process?
Reduces NADP⁺ to NADPH
(H⁺ + Fdᵣₑ -> Fdₒₓ) occurs in two separate steps, the first step occurs and causes FAD to be turned into the semiquinone intermediate (FADH⁺) which is then turned into FADH₂ after the reaction occurs again
The Z-scheme: what is it, what is the process behind it, and why is this mechanism needed?
The transfer of electrons from light hitting PSII all the way to the final electron carrier
Electron flow from H₂O to NADPH is an
endergonic process (ΔG > 0) that is made possible by the absorption of 2 photons of light at PSII (P680) and PSI (P700)
Absorption of light energy converts special
pairs of chlorophylls P680 and P700 (poor
reducing agents) to excited molecules (P680* and
P700* good reducing agents) which are also powerful oxidising agents (particularly P680⁺) after losing their electrons
Chloroplast ATP synthase
Very similar to ATP synthase (e.g. β subunits from human complex V and corn chloroplasts are >60% identical)
Structure with knob (CF1) and stalk (CF0)
- CF0: 12 subunits/rotations that span the membrane, forming a proton channel for H⁺ travel from the thylakoid lumen to the stroma
- CF1: protrudes into the stroma, containing the catalytic subunits for ATP synthesis from ADP + Pᵢ
Overall photosynthesis stoichiometry: PSII, PSI, overall photosystem, ATP synthesis, and overall ATP synthesis
Photosystem II:
ET = PQ/PQH2 –> Cyt bf –> PC
4 photons, 4e⁻ produces one O₂
8 protons from stroma -> thylakoid lumen
4 protons from H₂O -> thylakoid lumen
12 protons transferred into the thylakoid lumen
Photosystem I:
ET = Fd –> NADP-reductase
4 more photons, 4e⁻ to reduce 2NADP⁺ along with 2 protons from stroma to make NADPH
Overall photosystem reaction:
2H₂O + 2NADP⁺ + 10H⁺ stroma + 4H⁺ water→ O₂ + 2NADPH + 12H⁺ lumen
CF0-CF1 ATP synthase:
12 protons flow back to the stroma in one rotation
3 ATP molecules are generated per full rotation
Overall reaction including ATP:
2NADP+ + 3ADP + 3Pᵢ+ 4H⁺ → O₂ + 2NADPH + 3ATP + H₂O
- 8 photons required to yield 3 ATP (~2.7 photons per ATP)
What happens if there is no CO₂ or NADP⁺ available or ferredoxin cannot give an electron?
Cyclic phosphorylation
Plastocyanin is used to restore PSI to normal
(not sure of the process)
Net result: pumping of 8 protons to lumen per 4
photons at PSI, ATP is synthesized, but no O₂ or NADPH is produced
Yield ~ 2 photons per ATP
Photosynthesis - light-independent reactions
Energy from sunlight is transformed into ATP
and reducing power (NADPH)
Electrons are obtained from H₂O oxidation and
used to reduce NADP⁺ to NADPH
Proton gradient is created using the free energy of electron transport. The gradient is later used to synthesize ATP
3Pᵢ + 3ADP → 3ATP + 3H₂O
2H₂O + 2NADP⁺ + 8 photons → 2NADPH + 2H⁺ + O₂
Calvin cycle: where does it take place, what does it use, and what does it do>
Reactions taking place in the stroma of chloroplasts powered by ATP and NADPH
Converts CO₂ (fully oxidized carbon atoms) into
carbohydrates (more reduced state) using NADPH
Calvin cycle: the three steps to it
1 - Fixation of atmospheric CO₂ by RuBisCO by using ribulose 1,5-bisphosphate (5C) to form two molecules of 3-phosphoglycerate (2×3C)
2 - Reduction of 3-phosphoglycerate using NADPH, forming glyceraldehyde 3-phosphate (3C) which can be converted to hexoses (6C) or used in step 3
3 - Regeneration of ribulose 1,5-bisphosphate
(5C) from 2G3P (2×3C) for more CO₂ fixation
RuBisCO: what does it stand for, what does it do, and what are its key characteristics?
Ribulose bisphosphate carboxylase oxygenase
Fixes CO₂ from the atmosphere and uses it with ATP and 1,5-ribulose bisphosphate to form 3-phosphoglycerate
- Very inefficient (slow) enzyme fixes ~3CO2 per second
- Bad selectivity - occasionally it binds O₂ instead of CO₂ and carries out an oxygenase reaction instead of the correct carboxylase reaction
- RuBisCo is the most abundant protein in plants and likely on the Planet too
The conversion of 3-phosphoglycerate into hexoses
3-phosphoglycerate (3PG) is an intermediate in glycolysis and gluconeogenesis
3PG is phosphorylated to 1,3- bisphosphoglycerate (1,3BPG) with consumption of ATP and it is then reduced to glyceraldehyde 3-phosphate (G3P) with oxidation of NADPH and G3P can be used to either regenerate ribulose bisphosphate or form hexoses
The requirements for the formation of one glucose molecule
Making one molecule of glucose requires
6CO2 (atmosphere),18ATP/12NADPH (light reactions)
What can plants convert glucose/fructose into?
1- Sucrose (in the cytosol)
2 - Starch (in the chloroplast)
3 - Cellulose (in the cell wall)
