Photosynthesis Flashcards
Non-cyclic Light Dependent Reaction
1) Photoactivation
Light energy is absorbed by accessory pigments in the light harvesting complexes of Photosystems II and I.
The electrons in these pigment molecules become excited and when they return to ground state, pass on released energy to neighbouring pigment molecules.
This resonance transfer of energy continues until it reaches 1 of 2 special chlorophyll a molecules (P680 in PSII or P700 in PSI) in the reaction centre.
When special chlorophyll a molecule absorbs energy, an excited electron is emitted and captured by primary electron acceptor. The hole in P680 is replaced by electrons from photolysis of water catalysed by an enzyme. Oxygen is formed as a by-product.
2) Photophosphorylation
As excited electrons flow down electron transport chain of carriers with progressively lower energy levels, energy released is coupled to actively pump H+ from stroma across thylakoid membrane into thylakoid space, establishing a proton-motive force. NADP acts as the final electron acceptor.
Accumulated H+ flows down proton gradient through ATP synthase out to stroma, generating ATP by phosphorylation of ADP with inorganic phosphate via chemiosmosis.
Cyclic LDR
Occurs when NADP concentration is limiting.
Instead of getting passed to 2nd ETC, electrons are cycled back to fill space in P700. ATP continues to be generated via chemiosmosis by electrons flowing down 1st ETC.
Calvin cycle
In the stroma of the chloroplast,
1) Carbon fixation
Catalysed by RuBP carboxylase (RuBisCO), CO2 combines with ribulose biphosphate (RuBP) to form unstable 6C compound which immediately splits into two 3C glycerate phosphate (GP).
2) Reduction by NADPH
6NADPH (from LDR) is used as the reducing power to reduce GP to glyceraldehyde-3-phosphate (G3P/TP/PGAL), using 6ATP as energy source.
G3P is the first sugar produced in photosynthesis and the end-product of Calvin Cycle.
3) Regeneration of RuBP
5 molecules of G3P are used to regenerate 3 RuBP so that carbon dioxide fixation can continue. This requires 3ATP as energy source.
The net synthesis of 1 G3P requires 3 CO2.
2 molecules of G3P can be used to form hexose sugar glucose.
How are products of photophosphorylation used?
ATP is source of energy
NADPH is the reducing power for reduction of GP to G3P
ATP is also used in regeneration of RuBP
What factors affect?
Low CO2 conc/Low Temp: decreases FOCeff btw CO2, RuBP and Rubisco, decrease E-S complex formed per unit time
Function of thylakoid membrane in photophosphorylation
1) Provides large surface area to embed MANY photosynthetic pigments/chlorophyll molecules for light absorption
2) Maintains the sequential arrangement of the photosystems and electron carriers of electron transport chain for the flow of electrons
3) Maintains proton gradient for ATP synthesis since the hydrophobic core is impermeable to protons and this is essential for chemiosmosis
4) Allows many ATP synthase to be embedded (in the correct orientation) so (active site faces the stroma)/ for protons flow down their gradient to generate ATP via chemiosmosis.
Role of ATP synthase
Diffusion of protons from thylakoid into stroma through ATP synthase leads to formation of ATP by phosphorylation of ADP with inorganic phosphate, via chemiosmosis.
Role of NADP
NADP is a coenzyme which carries both protons and high-energy electrons.
NADP is the final electron acceptor of the electron transport chain in non-cyclic light dependent reaction in the thylakoid membrane.
Electrons carried in reduced NADP, NADPH are used in the Calvin cycle in the stroma to reduce glycerate phosphate to glyceraldehyde-3-phosphate.
When GP is reduced to G3P, NADP is regenerated to continue carrying out its role as electron carrier in LDRs.
Cyclic phosphorylation vs Non-cyclic phosphorylation
1) NCP produces ATP, O2 and NADPH while CP produces only ATP.
2) NCP involves both PSII and PSI while CP involves only PSI
3) NCP has final electron acceptor NADP while CP has final electron acceptor P700.
4) NCP has electron donor P680/water and involves splitting of water by an enzyme, while CP has electron donor P700 and does not involve splitting of water by an enzyme.
5) In NCP, electron flow is in one direction from water as electron donor through two ETCs before getting accepted by NADP, while in CP electron flow is cyclical from PSI as electron donor to first ETC and going back to PSI.
Light-dependent reaction vs Non light-dependent reaction
1) LDR occurs in thylakoid space while NLDR occurs in stroma.
2) LDR produces ATP and NADPH, NLDR produces G3P and NADP
3) LDR has O2 as by-product, NLDR has no by-products
4) LDR has reactants H2O, ADP, Pi, NADP, NLDR has reactants CO2, ATP and NADPH
4) LDR requires light for photoactivation of electrons in non-cyclic and cyclic photophosphorulation while NLDR does not require light, it requires RuBisCO for carbon fixation in Calvin Cycle. (lmao so P2P)
Oxidative phosphorylation vs photophosphorylation
Differences:
1) PP occurs in the thylakoid membrane of the chloroplasts while OP occurs in the inner mitochondrial membrane.
2) In PP, water is the electron donor in non-cyclic pathway and PSI is the electron donor in cyclic pathway while NADH and FADH2 are the electron donors to the first electron carrier of ETC.
3) In PP, NADP+ is the final electron acceptor in non-cyclic pathway and PSI in cyclic pathway while water is the final electron acceptor in OP.
4) In PP, splitting of water produces O2 as by-product in cyclic pathway while in OP oxygen combines proton and electron to produce water as by-product.
5) In PP, light energy is converted to chemical energy while in OP chemical energy is converted to chemical energy.
6) In PP, source of energy for synthesis of ATP is light while in OP source of energy is the oxidation of glucose
Similarities:
1) Energy lost from flow of electrons down ETC is used to actively pump H+ across a membrane to generate proton gradient for chemiosmosis later.
2) ADP is phosphorylated to ATP using energy from flow of protons down proton gradient through ATP synthase via chemiosmosis.
3) Both processes take place on membranes.
Light and dark
In the dark, light-dependent reaction does not occur, so no ATP and NADPH is generated.
As carbon fixation continues since it does not need ATP/NADPH, RuBP decreases as it is carboxylated and converted to GP.
RuBP regeneration is slowed due to lack of ATP and NADPH to reduce GP to G3P/TP, leading to lack of G3P for regeneration.
Limiting factor
At higher light intensity, all photosynthetic pigments in chloroplasts are saturated with light. Hence rate of photosynthesis is already optimal. Light is no longer a limiting factor, but other factors such as temperature/CO2.
Light compensation point:
The value of light intensity at which rate of respiration is equivalent to rate of photosynthesis.
Amt of CO2 given out by the plant is the same as amt of CO2 taken in
There is no net gain of dry mass and no growth since all products of photosynthesis are used up in respiration
Light plays an important role in LDR of photosynthesis as it excites electrons in photosynthetic pigments to higher energy state through photoactivation.
Different wavelengths of light stimulate photosynthesis differently. Rate of photosynthesis is highest at 700nm (red) and 470nm (blue).
Light intensity is rarely limiting during daylight hours.
Oxygenase function of RuBisCO means that it accepts O2 as a competitive inhibitor.
