Photosynthesis - The Light Reactions



Photosynthesis - The Light Reactions

Remember, there are two types of reactions in photosynthesis.

1. The Light Reactions ("photo")

- produce ATP and NADPH so the "Dark Reactions" can

occur

- need light

- occur in the thylakoid membrane of chloroplast

2. The Dark Reactions ("synthesis")

- ATP and NADPH produced in Light Reactions are used to

produce carbohydrate from CO2

- occur in the stroma of chloroplast

For the Light Reactions to occur light must be absorbed

• Photons of light are absorbed by molecules of Chlorophyll

(a "family" of several pigments)

• There are of two sets of pigments

-Photosystem I (P-700) and Photosystem II (P-680)

• Photosystem I is most excited by light at 700 nm

• Photosystem II is most excited by light at 680 nm

• P-680 is more powerful than P-700 because P-680 can capture higher energy light

• P-680 contains chlorophyll a, chlorophyll b, chlorophyll c, chlorophyll d, carotenes, xanthophylls and anthocyanins

• P-700 only contains chlorophyll a

The Light Reactions can be carried out by two different

photosystems.

a. Non-cyclic Electron Pathway

- electrons from water are moved from P-680 to P-700 then

to NADP+

b. Cyclic Electron Pathway

- in the P-700 photosystem electrons are recycled after their

energy is used to form ATP molecules

Non-Cyclic Electron Pathway

-green plants, algae and cyanobacteria

1. P-680 becomes

excited by a photon of

light and electrons are

shed. This is an

unstable state. A

molecule of water is

oxidized and reduces

the P-680. (A coupled

reaction)

2. P-680 passes

electrons on to a series

of electron carriers in

the thylakoid

membrane.

3. As electrons pass from P-680 to P-700 the energy of electron

transport may be used to produce ATP.

4. Electrons are then passed along to P-700. When P-700 becomes

"excited" after absorption of light the electrons are passed to

ferredoxin. Ferredoxin passes the electrons on to reduce NADP+ to

NADPH.

ATP Production in the Light Reactions

1. Electron transport establishes a proton gradient

2. Electrons are donated by water molecules, leaving H+ ions

in the intrathylakoid space (2 H+ for every H2O)

3. Also, electron carrier Plastoquinone (PQ) transports

electrons between P-680 (PSII) and the cytochrome b6/f

-- as PQ is reduced and re-oxidized during non-cyclic

transport, H+ may be pumped from stroma into intrathylakoid

space

4. Large proton-motive force favors the return of protons to the

stroma via ATP synthase complex in the thylakoid

membrane and allows for production of ATP

Cyclic Electron Transport

-some photosynthetic bacteria

1. P-700 (PS I) may operate

alone, without interacting

with P-680 (PS-II)

2. NO access to external

source of electrons

ie: no H2O as a donor

-- instead electrons are

returned to P-700 via

cytochrome b6/f and

plastocyanin (PC)

3. Cyclic electron transport

may produce ATP, but does NOT produce NADPH

ATP Production in Cyclic Electron Transport

In cyclic electron transport, the proton gradient is established by pumping protons through the cytochrome complex from the stroma to the intrathylakoid space as the electron goes to

P-700

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download