Ch. 22 Light Reactions

What is the goal of photosynthesis?

light energy —> electrons —> ATP —> glucose

Light reactions

reactions powered by sunlight

Where does photosynthesis occur?

inside the chloroplast, in the thylakoid membranes of the stroma

pigment / photoreceptor molecule

absorbs the light

chlorophyll a

principle photoreceptor in chloroplasts of green plants

What happens when a photon is absorbed by a pigment?

the electron jumps to a higher energy state and can be released as heat or light or it can be transferred to a nearby molecule with a lower energy state

photoinduced charge separation

charged electron transfers to a lower energy molecule, leaving original molecule negatively charged and the new one positively charged

energy must be transferred from the accessory pigments to

reaction centers

reaction center

photochemical reaction converts the energy of a photon into a separation of charge, initiating electron flow

What do the photosystems generate?

a proton gradient and NADPH

Reaction center for photosystem I

P700

Flow of electrons in PSI

P700 —> ETC —> ferredoxin

reaction center in PSII

P680

Photosystem II

P680 —> pheophytin (Ph) —> Qa —> Qb —> QH2

Reaction center of PSII steps

P680 —> P680* —> reacts with 2 H2O in Mn⁴⁺ center —> O₂

Cytochrome bf reaction

electrons from QH2 transfer to Pc, protons are released from QH2 and cytc bf pumps in 2 more protons

What is the cofactor for cytochrome bf?

Cu²⁺

Water-oxidizing complex

site of photolysis of water in PSII

Photolysis of water

P680+ removes electrons from water

the electrons from the WOC do what?

reduce 2 Q —> 2 QH2

What is ferredoxin made of?

iron-sulfur protein

Flow of electrons in PSI

Pc—> P700 —> Ao —> A1 —> 4 Fe- 4 S —→ Fd —> FNR —> NADPH

What ion is needed for Fd-NADP+ - Reductase?

Fe

What happens if NADPH needs are met?

cyclic electron flow generates ATP without forming NADPH

Cyclic electron flow

PS I electrons —> Fd —> cytochrome bf —> plastocyanin —> return to P700

8 photons produce

2 NADPH and 3 ATP

In cyclic photophosphorylation, 2 photons make

no NADPH and 1 ATP

2 H2O + 2 NADP⁺ + 10 H⁺_stroma —>

O2 + 2 NADPH + 12⁺ _lumen

3 ADP^3- + 3 Pi^2- + 3 H⁺ + 12 H⁺ _lumen —→

3 ATP^4- + 3 H2O + 12 H⁺_stroma