TOPIC L: PHOTOSYNTHESIS Flashcards
What are the adaptations of thylakoids?
Large surface area of thylakoid membranes
- allows photosystems and electron carriers to be embedded
- allows stalked particles containing ATP synthase to be embedded
Thylakoid membrane is impermeable to protons.
- allows electrochemical proton gradient to be set up between the thylakoid space and stroma.
What is chlorophyll and what is its structure?
There are two main types of chlorophyll:
Chlorophyll a and b, they absorb mainly red and blue-violent light, reflecting green light and therefore giving plants their characteristic green colour.
Their chemical structure consists of:
A head made up of poryphyrin ring with a magnesium ion in the centre
A long hydrocarbon tail which is joined to its head by an ester linkage. =
Different chlorophylls have different side-chains on the head and these modify their absorption specta.
What are carotenoids?
There are 2 main types of carotenoids carotenes and xanthophylls. They are yellow, orange, red or brown pigments that absorb strongly in the blue-violet light spectrum.
Carotenoids protect chlorophylls from excess light and oxidation by oxygen produced during photosynthesis.
What do photosynthetic units comprise of?
- Light-harvesting complexes
- Consists of accessory pigments (Chlorophylls and Carotenoids)
- Accessory pigments funnel the energy absorbed from light to the reaction centre in either PSI or PSII.
- The light-harvesting complexes surround the reaction centre. - A reaction centre
- A protein complex that includes two special a molecules (primary pigments) and a molecule called the primary electron acceptor.
- Two special chlorophyll a molecules each emit one electron and the two electrons are accepted by the primary electron acceptor.
Outline the process of non-cyclic phosphorylation?
- Light of particular wavelengths strikes an accessory pigment molecule in the light harvesting complex of PSII and PSI.
This energy is relayed to neighbouring accessory pigment molecules until it accumulates and reaches one of the two P680 chlorophyll a molecules in the reaction centre of PSII.
The same occurs for the P700 chlorophyll a molecules in the reaction centre of PSI. - This excites one of the P680 electrons and one of the P700 electrons to a higher energy state, which subsequently gets emitted and captured by the primary electron acceptor within each PS.
A positive ‘hole’ is left behind in each P680 and P700 chlorophyll a molecule in PSII and PSI. - Photolysis of water occurs when an enzyme catalyses the splitting of a water molecule into protons, electrons and molecular oxygen.
Electrons from the photolysis of water are used to fill up positive ‘holes’ in the reaction centre of PSII to return P680+ to ground state. - The photoexcited electron passes from the primary electron acceptor of PSII to P700+ in PSI, to fill the positive holr in P700+
This occurs via an electron transport chain made up of electron carriers, each with an energy level lower than the one preceding it. - Energy from the electron transfer down the chain of electron carriers is used to actively pump protons from the stroma into the thylakoid.
This generates an electrochemical proton gradient for the synthesis of ATP.
Protons diffuse through the stalked particle containing ATP synthase back into the stroma, down the electrochemical proton gradient. This provides enough energy for ATP synthase to catalyze the synthesis of ATP from ADP and Pi
This process by which protons diffuse through a stalked particle for the synthesis of ATP is known as chemiosmosis. - Electron are subsequently passed from the primary electron acceptor of PSI to the protein freedoxin
- The enzyme NADP reductase catalyses the transfer of electrons from ferredoxin to oxidised NADP (final electron and proton acceptor) to form reduced NADP.
How is an electrochemical proton gradient established across the thylakoid membrane?
- Higher concentrations of proton in thylakoid space.
(A) Water undergoes photolysis in the thylakoid space. generating protons in the process.
(B) As electrons are passed from one electron carrier to the next, energy is released to actively pump protons across the membrane into the thylakoid space. - Lower concentration of protons in stroma.
(C) Protons are removed from the stroma when they are taken up by oxidised NADP. - Impermeable nature of thylakoid membrane to protons allows the gradient to be established.
What is cyclic phosphorylation?
PSI is both a donor and acceptor of electrons. The excited electrons in the primary electron acceptor of PSI pass to ferredoxin and back to the cytochrome complex in the ETC. The electrons eventually return back to the PSI reaction centre.
It does not involve the photolysis of water.
The energy released during the cycle of electrons down the chain of electron carriers allows protons to be actively pumped from the stroma into the thylakoid space, generating an electrochemical proton gradient across the thylakoid membrane, just like in non-cyclic phosphorylation.
It allows for the synthesis of ATP by the stalked particles embeddded on thylakoid membrane.
Cyclic phosphorylation yields only ATP whereas non-cyclic photophosphorylation yields O2, ATP and reduced NADP.
What is RuBP and GP?
RuBP is ribulose bisphosphate, 5C sugar.
GP is glycerate-3-phosphate, previously known as phosphoglyceric acid.
RuBP + CO2 + H2O —Rubisco–> unstable 6C intermediate ——> 2GP.
Rubisco is RuBP carboxylase-oxygenase
What occurs in the CO2 fixation stage in the Calvin cycle?
RuBP carboxylase-oxygenase (Rubisco) is present in large amounts in the stroma of the chloroplast.
It catalyses the fixation of CO2 by a 5C sugar known as RuBP which gives an unstable 6C intermediate that immediately breaks down to 2 molecules of 3C GP.
Rubisco thus regulates the rate of photosynthesis,
What is the role of GP in plants?
GP can be converted to pyruvate which is used to synthesise fatty acids. The pyruvate that is formed can also be converted to acetyl coenzyme A which undergoes the Krebs cycle to form a-ketoglutarate (can undergo further rxns to form amino acids in plants).
What occurs in the carbon reduction stage of the Calvin cycle?
GALP = Glyceraldehyde-3-phosphate 3C sugar (Triose Phosphate)
The reducing power of reduced NADP and energy from the hydrolysis of ATP are used to convert GP to GALP. GALP contains more chemical energy than GP, and is the first carbohydrate made in photosynthesis. About 1/6 of the total amount of GALP is used to synthesise glucose, other carbohydrates and glycerol.
What occurs in the regeneration of RuBP phase of CC?
About 5/6 of the total amount of GALP has to be used to regenerate the RuBP consumed in ther first reaction. This process requires energy from the hydrolysis of ATP.
What properties does the thylakoids possess that allows the light-dependent reactions of photosynthesis to occur?
- Large surface area of thylakoid membrane. Allows photosystems, electron carriers and stalked particles containing ATP synthase to be embedded.
- Thylakoid membranes are impermeable to protons
Allows electrochemical proton gradient to be set up betweeen the thylakoid space and stroma.
How does light intensity act as a limiting factor in P/S?
Light intensity is an important limiting factor in the light-dependent stage to excite the special chlorophyll a molecules for photophosphorylation to occur. P/S results in uptake of CO2 and evolution of O2. At the same time, respiration uses oxygen and produces CO2. There will come a point when the light intensity causes P/S and respiration to exactly balance each other. This is called the light compensation point (light intensity at which net gas exchange is zero).
Without light, as photophosphorylation cannot occur, the light dependent reactions of P/S cannot occur to generate reduced NADP and ATP for the next steps in photosynthesis.
How does the wavelength of light act as a limiting factor in P/S?
Wavelength of light is also a limiting factor as demonstrated by comparing the action and absorption spectra for photosynthesis. The rate of P/S is highest at the red and blue-violet regions of the action spectrum and lowest at the green region.
