Chapter 10 - Photosynthesis Flashcards
who is performing photosynthesis?
Plants, algae (protists), some prokaryotes - all photoautotrophic organisms perform photosynthesis
what is the largest biosynthetic activity on the planet?
photosynthesis - it creates 160 billion tons of carbohydrate per year - lots of biomass being produced
what produces almost all of the organic material required by consumers?
photosynthesis
where does photosynthesis occur (in land plants)?
in the chloroplasts of green plant tissues
how many membranes does a chloroplast have?
2
how many chloroplasts (on average) are in a green plant cell?
usually 30-40
what are the important parts of chloroplasts?
inner and outer membranes, intermembrane space, thylakoid, thylakoid space, granum
what are chloroplasts similar too?
mitochondria
what is photosynthesis biochemically?
water + carbon dioxide + light -> sugar + oxygen
the correct formula for photosynthesis
6 H20 + 6 CO2 + light energy -> C6H12O6 + 6 O2
the three steps of photosynthesis
- photochemistry (light reaction) 2. electron transfer and production of NADPH and ATP (light reaction) 3. incorporation of CO2 into carbohydrate and production of O2 (dark reaction/Calvin cycle)
what is light?
electromagnetic radiation, characterized by its energy (wavelength is lambda)
relationship between wavelength and energy
High wavelength = low energy, low energy = high wavelength
what happens when light meets matter (3 options)
reflected, transmitted (goes through the matter and nothing happens), or absorbed
the three classes of light-absorbing compounds (pigments) found in plants
chlorophylls (chl a and chl b), carotenoids, phycobilins (only found in red algae and cyanobacteria)
what do pigments do?
absorb certain wavelengths of the visible light (and reflect or transmit the rest) - each pigment has its own absorption spectrum
why is the world green?
chlorophylls absorb about 70% of red and blue wavelength, while the green light is reflected or transmitted and bounces back to the eye - so everything appears green (note that chloroplasts do NOT absorb green light)
difference between chl a and chl b
CH3 in chl a and CHO (aldehyde group) in chl b
how is chlorophyll capturing the light?
there are a whole bunch of double bonds that are organized in an alternating pattern with single bonds, which make the uptake of photons really easy, because the energy is dissipated along the whole plane of molecules
if a photon of the right energy level (colour) hits the chlorophyl, an electron gets excited and moves orbitals (doesn’t leave the molecule, just gets excited and moves to a new orbital) then releases that extra energy (as heat or a photon) it gained very quickly, and falls back down into its normal energy state
the emitted photon is of a different colour (because some of the energy was also released as heat) so the new light is a different energy, and thus different wavelength (colour)
how is excitation energy exploited in the chloroplasts?
the pigments are linked together in ‘light harvesting complexes’ (chl a, chl b, and carotenoids transfer energy (not electrons)) which are connected to ‘reaction centres’ - the transfer of electrons to primary electron acceptor
describe photosystem II and the steps
- light excited pigment in PSII -> as excited electron falls to ground state, another electron is excited, which continues until P680 is excited
- transfer of electron from chloropyll a pair (P680) to primary electron acceptor
- electron hole in P680+ must be filled -> pulls it from water
describe electron transport chain and photosystem II and the steps
in the thylakoid membrane, two photosystems (PS I with P700 and PS II with P680)
- PS I gets excited by light
- P700 looses an electron
- electron “hole” is filled with an electron coming from PS II
what is the solution to the fact that dark reactions require more ATP than NADPH, but in photosystem I and II ATP and NADH have been produced in a 1:1 ratio?
electron recycling: cyclic electron flow within PS I
what gradient is used to produce ATP?
conversion of light energy to H+ gradient -> H+ (pH) gradient used for ATP synthesis
what happens in plants when the lights go out?
all light reactions stop (photochemistry, electron transport, splitting of water/O2 production, H+ gradient generation)
however, CO2 absorption and sugar production continues (as long as substrates are available)
what are the dark reactions?
the Calvin cycle (incorporation of CO2 into carbohydrates) (because it can occur in the dark) -> involves proteins that transfer chemical energy
three steps of the Calvin cycle (occur in the stroma and do not require light)
- carbon fixation
- reduction
- regeneration
how many turns of the Calvin cycle are needed to produce one molecule of glucose?
6
