AP Biology - Light Dependent Reactions

inputs of light-dependent reactions

sunlight, water, NADP+, ADP, and an unhooked phosphate group

outputs of light dependent reactions

ATP, NADPH, oxygen

electron acceptor

NADP+

energy carrying molecule

NADPH

area where light hits. Clusters of pigments

photosystems

1st system to receive light. Reaction center receives light from wavelengths at 680 nm

photosystem II

2nd photosystem to receive light. Reaction center accepts wavelengths at 700nm

photosystem I

680nm

photosystem II

700nm

photosystem I

first step of light reactions

photolysis occurs

the splitting of water to form oxygen, hydrogen ions, and electrons

photolysis

step 2 of the light reactions

electrons from photolysis are transferred to system II

step 3 of the light reactions

sunlight hits and is absorbed by the chlorophyll molecules in photosystem II (oxidizes chlorophyll) to excite the electrons in the photosystem

step 4 of the light reactions

excited electrons from the photosystem are transferred to a primary electron acceptor are then passed down an ETC

step 5

as the electrons are passed down the ETC, the electron loses energy a little at a time

step 6

Lost energy is used to pump H+ from the stroma across the thylakoid membrane into the interior of the thylakoid - building up the H+ concentration inside the thylakoid

step 7

unexcited electrons at the bottom of the ETC are transferred to PI700, where they are struck by sunlight and excited again

step 8

the excited e- travel down another, but shorter ETC

step 9

at the end of the short ETC, the e- still have a little bit of energy, so they are transferred to an energy carrying molecule of NADP+

step 10

NADP+ and electrons combine with H+ with the help of NADP+

step 11

NADPH transfers e-/energy to stroma where it will be used in the light-independent reactions

12.

photolysis occurs again to help the process start over

the electrochemical gradient from H+ ions in the thylakoids passes through the membrane via ATP synthase, which adds a phosphate group to ADP to make ATP. ATP is sent to the stroma and used in the Calvin Cycle

Chemiosmosis

what replaces electrons lost by photosystem II?

electrons from water

what replaces electrons lost by PI?

ETC

electrons in PSI sometimes get sent back to the first ETC again, allowing more ATP to be sent to the Calvin Cycle

cyclic electron flow