8.3 Capturing Sunlight Into Chemical Forms Flashcards
visible light
the portion of the electromagnetic spectrum apparent to our eyes
each point along the electromagnetic spectrum has:
a different energy level and a corresponding wavelength
pigments are molecules that absorb:
some wavelengths of visible light
pigments look coloured because:
they reflect light enriched in the wavelengths that they do not absorb
chlorophyll
the major photosynthetic pigment contained in the thylakoid membrane; it plays a key role in the chloroplast’s ability to capture energy from sunlight. chlorophyll appears green because it is poor at absorbing green wavelengths
describe a molecule of chlorophyll
large, light absorbing “head” containing a magnesium atom at its centre and a long hydrocarbon “tail”
chlorophyll molecules are bound by their tail regions to :
integral membrane proteins in the thylakoid membrane
photosystem
a protein-pigement complex that absorbs light energy to drive redox reactions and thereby sets the photosynthetic electron transport chain in motion
accessory pigment
a light-absorbing pigment other than chlorophyll in the photosynthetic membrane; carotenoids are important accessory pigments
accessory pigments allow photosynthetic cells to absorb:
a broader range of visible light than would be possible with chlorophyll alone
when visible light is absorbed by a chlorophyll molecule, what happens?
one of its electrons is elevated to a higher energy state
in labs, chlorophyll molecules which have absorbed light energy, rapidly release the energy as:
heat or light (fluorescence)
absorption of visible light by an isolated chlorophyll molecule results in:
the release of heat and fluorescence when the electron returns to its ground energy state
absorption of visible light by an antenna chlorophyll results in:
the transfer of energy to an electron in a neighbouring chlorophyll molecule (little energy is lost as heat)
most chlorophyll molecules in the thylakoid membrane function as an antenna, meaning:
energy is transferred between chlorophyll molecules until it is finally transferred to a specially configured pair of chlorophyll molecules (reaction centre)
reaction centre
specially configured chlorophyll molecules where light energy is converted into electron transport
the reaction centre is where light energy is converted into:
chemical energy as a result of the excited electron’s transfer to an adjacent molecule
the reaction centre chlorophylls have a distinct configuration from antenna chlorophylls which results in:
when excited, the reaction centre transfer an electron to an adjacent molecule (oxidized photosystem) that acts as an electron acceptor, converts light energy into chemical, electron transfer initiates chain of redox reactions that ultimately lead to formation of NADPH
once the reaction centre has lost an electron:
it can no longer absorb light or contribute additional electrons, another electron must replace it and reduce the photosystem
replacement electrons for reduced photosystems ultimately come from:
water
why is water a challenging electron donor?
it takes a lot of energy to pull electrons from water
how is there enough energy to extract an electron from water AND reduce NADP+?
two photosystems are arranged in a series, first photosystem pulls electrons from water, second photosystem allows electrons to be transferred to NADP+, both use energy separately
why do electrons move in one “direction” through the series of redox reactions that make up the photosynthetic electron transport chain?
because these reactions are exergonic, running the reactions in the opposite way would require an input of energy
Z scheme
another name for the photosynthetic electron transport chain, so called because the overall energy trajectory resembles a “Z”
absorption of light energy at PSII allows:
electrons pulled from water to enter the photosynthetic electron transport hain
a second input of light energy by PSI produces:
electron donor molecules capable of reducing NADP+
the use of water as an electron donor requires:
input of light energy at two places in the photosynthetic electron transport chain
photosystem II
the photosystem that supplies electrons to the beginning of the electron transport chain. when photosystem II loses an electron, it can pull electrons from water
photosystem I
the photosystem that energizes electrons with a second input of light energy so they have enough energy to reduce NADP+
associate photosystem II with strong….
OXIDATION and photosystem I with strong REDUCTION
cytochrome f complex (cut)
part of the photosynthetic electron transport chain, through which electrons pass between photosystem II and photosystem I
what are some compounds that convey electrons between protein complex (PSII, Cyt, PSI)
plastoquinone (Pq)-lipid soluble, similar to coenzyme Q, carries electrons from PSII to Cyt
plastocyanin (Pc)-water soluble carries electrons from Cyt to PSI through thylakoid lumen
during photosynthetic electron transport, protons accumulate in the:
thylakoid lumen, protons flow back into the stroma through the ATP synthase
where does water splitting occur?
on the lumen side of photosystem II (inside)
NADPH is formed when electrons are passed from PSI to a membrane-associated protein called:
ferredoxin (Fd) and its associated enzyme ferredoxin-NADP+ reductase which catalyzes formation of NADPH
in the chloroplasts, just like the mitochondria, ATP is synthesized by:
ATP synthase, a transmembrane protein powered by a proton gradient
in chloroplasts, the ATP synthase results in the movement of protons from:
the thylakoid lumen to the stroma
what are two features that lead to accumulation of protons in the thylakoid lumen?
oxidation of water releases protons and oxygen into the lumen, Cyt complex functions as a proton pump
in photosynthesis, the proton pump involves:
- the transport of two electrons and two protons, by the diffusion of Pq from the stroma side of PSII to the lumen side of the Cyt complex
- the transfer of electrons within the Cyt complex to a different molecule of Pq which results in additional protons being picked up from the stroma and released into the lumen
the accumulation of protons on one side of the thylakoid membrane can then be used to power:
the synthesis of ATP by oxidative phosphorylation
an additional pathway for electrons is needed to:
increase the production of ATP (the Calvin cycle requires more ATP than NADPH)
cyclic electron transport
an alternative pathway for electrons during the Calvin cycle that increases the production of ATP
describe the process of cyclic electron transport:
electrons from PSI are redirected from ferredoxin back to the electron transport chain. electrons reenter by plastoquinone where they eventually return to PSI. as they pass through Cyt complex, more protons are pumped=more energy for ATP synthesis
