Light dependent reactions

Biology Unit 4

Light dependent reactions

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Biology Unit 4
Revision Notes
Topic 5: On the wild side
4. Describe the light-dependent reactions of
photosynthesis including how light energy is
trapped by exciting electrons in chlorophyll and
the role of these electrons in generating ATP,
and reducing NADP in photophosphorylation and
producing oxygen through photolysis of water.
The light-dependent reactions only occur in light and take place on the thylakoid membranes of the
chloroplasts. There are 2 main functions:
Production of ATP, which supplies the energy needed for the synthesis of carbohydrates.
To split water molecules in a photochemical reaction, providing hydrogen ions to reduce CO2
and produce carbohydrates.
There are 2 parts to the light-dependent reactions:
Cyclic photophosphorylation
o This only produces ATP (which is later used in the LIR as a source of energy)
o This only uses PhotoSystem I (PSI)
o Light excites electrons in PSI to a higher energy level
o These are then passed through a chain of carriers (ETC), dropping in energy as it
o The energy given out is used (along with ATP synthase) to turn ADP into ATP.
Non-cyclic photophosphorylation
o This produces ATP, NADPH, H and O ( All needed to supply energy in LIR except O
which is a waste gas)
o PSI and PSII are used.
o Light excites electrons in PSII to a higher energy level.
o These are passed through a chain of carriers, dropping in energy as they go, to PSI.
Some of this energy makes ATP from ADP.
o More light excites these electrons (now in PSI) to a higher energy level than before.
o The electrons are so excited that they are passed to a different electron acceptor
and don't return to the chlorophyll.
o The chlorophyll (PSII) needs its electrons replaced and so it splits water:
HO 2H + ½O + 2e¯
Text Book: p.14-16

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