Photosynthesis Flashcards
define absorption spectrum
graph showing the relative absorbance of different wavelengths of light by a pigment
define action spectrum
graph showing the effectiveness of different wavelengths of light in stimulating the process of photosynthesis
what wavelengths do chlorophyll absorb the best?
red and blue-violet light
function of carotenoid
- act as accessory pigments by absorbing other wavelengths of light and transferring energy to chlorophyll a at reaction centre complex of photosystem
- protect chlorophyll from structural damage induced by excess light and oxidation
outline non-cyclic phosphorylation
- a photon of light excites an electron from a light harvesting complex pigment molecule, energy is relayed to other pigment molecules until it reaches the chlorophyll a of P680 in PSII
- this excites an electron in chlorophyll a in P680, photoexcited electron is emitted from chlorophyll a and is transferred to the primary electron acceptor. chlorophyll a in P680 now lacks an electron
- an enzyme catalyses photolysis of water to give 2 electrons, 2 protons and an O atom. the O atom immediately combines with another O atom to give oxygen, electrons are used to fill the electron gap in chlorophyll a in P680
- each photoexcited electron passes from primary electron acceptor of PS II to the chlorophyll a in P700 of PS I via ETC
- as electrons are transferred from one electron carrier to the next down the chain, electron carriers are of progressively lower energy level than the preceding one, energy is released as electrons move down the chain. electron carriers use energy released to pump protons across the thylakoid membrane to generate a proton pool in the thylakoid lumen as thylakoid membrane is impermeable to protons. this establishes a proton gradient across the thylakoid membrane and is subsequently used to synthesise ATP via chemiosmosis
- concurrently, light energy is also transferred via light harvesting complex pigments of PS I to excite an electron of chlorophyll a in P700 at the reaction centre. an excited electron of chlorophyll a from P700 is received by another primary electron acceptor, electron gap in chlorophyll a in P700 is filled by electron from PS II
- high energy electrons are passed in a series of redox reactions through another ETC (not coupled with ATP synthesis)
- enzyme NADP+ reductase catalyses formation of reduced NADP from protons and NADP+ (stroma) and electrons (ETC), NADP+ is the final proton and electron acceptor
state the function of non-cyclic phosphorylation
generate ATP and reduced NADP, which provide chemical energy and reducing power to the carbohydrate synthesising reactions of the Calvin cycle
outline cyclic phosphorylation
- generates only ATP
- excited electron that leaves chlorophyll a P700 of PS I is returned back to chlorophyll a P700 of PSI via the ETC
outline the Calvin cycle
- CO2 combines with ribulose biphosphate (RuBP) to form an unstable 6C intermediate, reaction catalysed by enzyme rubisco. the intermediate then breaks down into two 3-phosphoglycerate (PGA) molecules
- ATP and reduced NADP from light-dependent reactions are used to reduce PGA into glyceraldehyde-3-phosphate (GALP), regenerating NADP+ which returns to light-dependent stage to receive more protons and electrons and regenerating ADP for further phosphorylation into ATP
- some GALP molecules leave the cycle and are used to synthesise other organic molecules (glucose)
- other GALP molecules used to regenerate RuBP to receive more CO2 molecules (requires ATP)
define limiting factor
factor that is nearest its minimum value and determines the rate of reaction
explain how the limiting factor light intensity affects rate of photosynthesis
- at low light intensity, rate of photosynthesis increases linearly with increasing light intensity
- more photosystems are photoactivated, leading to a higher rate of photophosphorylation
- higher rate of photophosphorylation means more oxygen is produced due to photolysis and more ATP and reduced NADP are formed
- this leads to a higher rate of Calvin cycle, more CO2 is accepted by RuBP
- CO2 from respiration is utilised and less CO2 is given out
what does light compensation point mean?
at further light intensities, this is the point where CO2 is neither evolved or absorbed, CO2 taken in by PTS = CO2 given out by respiration
what does light saturation mean?
further increases in light intensity does not affect rate of reaction
explain how CO2 affects rate of reaction
as concentration of CO2 increases, there are more effective collisions between CO2 and Rubisco, more enzyme-substrate complexes formed, increased rate of Calvin cycle
explain the effect of increasing temperature on rate of photosynthesis
- increasing temperature increases kinetic energy of enzymes of Calvin cycle (Rubisco) and substrates, leading to more effective collisions between enzymes and substrates, resulting in more enzyme-substrate complexes formed, increased rate of GALP synthesis
- above the optimum temperature, enzymes become denatured and rate of ES complex formation decreases
differences between Calvin cycle and Krebs cycle (6)
- 2 CO2 produced (Krebs cycle) vs 1 CO2 used (Calvin cycle)
- hydrogen carrier is NAD+ and FAD (Krebs cycle) vs hydrogen carrier is NADP+ (Calvin cycle)
- 3 molecules of reduced NAD and 1 molecule of reduced FAD produced (Krebs cycle) vs 2 molecules of reduced NADP (Calvin cycle)
- 1 ATP produced (Krebs cycle) vs 2 ATP used (Calvin cycle)
- oxaloacetate regenerated (Krebs cycle) vs RuBP regenerated (Calvin cycle)
- acetyl CoA enters cycle and binds with oxaloacetate (Krebs cycle) vs CO2 enters cycle and binds with RuBP (Calvin cycle)
