light dependent reactions

location of light independent reactions

takes place in the stroma of chloroplast

location of light dependent reactions

occurs in thylakoid membrane, requires chlorophyll

photosystems

• photosystems are clusters of pigments embedded in the thylakoid membrane

• photosystem is made of 2 chlorophyll a molecules (reaction molecules) and surrounding accessory pigments (antenna complex)

• antenna complex harvests the light and passes it to the reaction center

types of photosystems

• photosystem I (P1) → P700, absorbs up to 700nm wavelengths

• photosystem II (P2) → P680, absorbs up to 680nm in wavelength

• reaction centers are identical but accessory pigments are different, so they absorb slightly different wavelengths

NADPH

• electron carrier used in photosynthesis and is involved in anabolic reactions

• NAPH has P group

light reactions

• when light strikes the thylakoid membrane, photo systems absorb a photon, that energy is passed from pigment to pigment in the antenna complex until it reaches the reaction center

• energy from the photo eventually excites the electrons in the chlorophyll a molecules in the reaction center

5 components of light reactions (in order)

• photosystem II

• cytochrome complex

• photosystem I

• NADP+ reductase

• ATP synthase

non-cyclic — step 1 (photoexcitation)

• electrons excited by photons (antenna complex), photosystem II

• energy is passed until it reaches P680

• P680 is excited and leaves PS2 to pheophytin (primary e acceptor of PS2)

• photoexcitation happens in PS1 as well, electron of P700 is excited and goes to ferredoxin (primary electron acceptor in PS1)

• split water into O, 2H+, 2e by Z protein, provides electrons (O is waste product)

non-cyclic — step 2 (ETC)

Q cycle: some energy in the electron is used by the cytochrome complex to pump H+ into the lumen to make an electrochemical gradient

non-cyclic — step 3 (photoexcitation)

• PS1 accepts electron, replaces electron that was previously there

• photon absorbed by PS1 and re-excites the electron that just arrived

non-cyclic — step 4 (ETC)

• electrons (P700) re-excited by photon

• ferredoxin brings the electron to another ETC (Calvin cycle) where it reaches NADP+ reductase

• NADP+ reduced to NADPH

• in the Calvin cycle, electron goes back to cytochrome

non-cyclic — step 5 (photophosphorylation)

• cytochrome complex pumps H+ into the thylakoid lumen

  • H+ gradient (H+ lumen > H+ stroma)

• H+ moves through ATP synthase into the stroma, making ATP

cyclic electron flow

• happens when there’s low O₂, low levels of NADP+, low ATP : NADPH ratio

• only PS1 used, P700 donates its electron to ferredoxin, go to Q cycle and BACK to P700

• makes a proton gradient for ATP synthesis but doesn’t release electrons to get NADPH