5.6.3 the light-dependent stage
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where does the light-dependent stage occur
in the grana (thylakoids) of chloroplasts
4 steps within the light-dependent stage
• light harvesting at photosystems • photolysis of water • phosphorylation - production of ATP in presence of light • formation of reduced NADP
define electron carriers
• molecules that can accept 1+ electrons & then donate them to another carrier • proteins embedded in thylakoid membranes are electron carriers, & form electron transport chain
describe photosystem I (PSI)
• pigment at primary reaction centre in chlorophyll a • peak absorption of red light of wavelength 700nm (P700)
describe photosystem II (PSII)
• pigment at primary reaction centre is chlorophyll a • peak absorption of red light of wavelength 680nm (P680)
which photosystem does photolysis occur in
photosystem II
what is water split into (photolysis)
protons (hydrogen ions), electrons & oxygen
photolysis equation
2H2O —-> 4H+ + 4e- + O2
when will much of the oxygen by-product diffuse out of the leaves
whe the rate of photosynthesis is higher than that rate of respiration (period of high light intensity)
roles of water
• source of protons (hydrogen ions) which are used in phosphorylation • donates electrons to chlorophyll to replace those lost when light strikes chlorophyll • source of by-product, oxygen • keeps plant cells turgid
2 types of phosphorylation
• non-cyclic = involves PSI & PSII - produces ATP, oxygen & NADPH • cyclic = involves PSI - produces ATP (less)
what do the two types of phosphorylation involve
iron-containing proteins embedded in thylakoid membranes - accept/donate electrons & form ETC
steps of non-cyclic phosphorylation
• photon strikes PSII & energy channelled to primary pigment reaction centre • light energy excites pair of electrons inside chlorophyll • energised electrons escape chlorophyll & captured by electron carrier • electrons replaced by electrons from photolysis • iron ion combines with electron to become reduced (Fe2+) & then donates electrons to next electron carrier in chain, becoming reoxidised (Fe3+) • electrons passed along chain of electron carrier embedded in thylakoid membrane, and some energy associated with electrons released at each • energy used to pump protons (H+) across thylakoid membrane into thylakoid space • electrons captured eventually by another chlorophyll a molecule in PSI which replace those lost • protein-iron-sulfur complex (ferroxidin) accepts electrons from PSI & passes to NADP in stroma • protons accumulate in thylakoid space creating concentration gradient across membrane • protons diffuse down concentration gradient through ATP synthase, forming ATP from ADP & Pi • protons accepted (alongside electrons) by NADP, causing it to become reduced (catalysed by NADP reductase)
what’s an electron carrier
protein with iron at its centre, embeddd in thylakoid membrane
describe cyclic phosphorylation
• light strikes PSI & pair of electrons in chlorophyll become excited by gaining energy • electrons escape from chlorophyll & pass to electron carrier system • then pass back to PSI • only uses PSI • small amount of ATP generated when electrons pass along electron carrier • no photolysis = no protons or oxygen produced • NADPH not generated
where does cyclic phosphorylation occur
• chloroplasts in guard cells only contain PSI • produce only ATP to actively bring potassium ions into cells = lowers water potential for osmosis into cell • causes guard cells to swell & opens stoma