Photosynthesis

• Photosynthesis: process that converts atmospheric CO2 and H2O to carbohydrates

• Solar energy is captured in chemical form as ATP and NADPH

• ATP and NADPH are used to convert CO2 to hexose phosphates

• Phototrophs: photosynthetic organisms (some bacteria, algae, higher plants)

Two Major Reaction Sets

• Net reaction of photosynthesis is:

CO2 + H2O (CH2O) + O2

• The oxidation of water is driven by solar energy

• Electrons from this oxidation pass through an electron-transport chain (which resembles the mitochondrial ETC)

• Light-dependent Reactions (light reactions)

•Carbon-assimilation Reactions (dark reactions)

The light reactions

Light reactions (light-dependent reactions)

• H+ derived from H2O is used in the chemiosmotic synthesis of ATP

• Hydride ion (H:-) from H2O reduces NADP+ to NADPH

• Release of O2 from splitting 2H2O molecules

The dark reactions Dark reactions (light-independent, carbon

assimilation or carbon-fixation reactions)

• Reduction of gaseous CO2 to carbohydrate

• Requires energy of NADPH and ATP

Sum of light and dark reactions • Both processes can occur simultaneously

In the presence of light:

H2O + ADP + Pi + NADPH O2 + ATP + NADPH + H+

Reactions which can occur in the dark:

CO2 + ATP + NADPH + H+ (CH2O) + ADP + Pi + NADP+

Sum: CO2 + H2O (CH2O) + O2

The Chloroplast • Chloroplasts: specialized organelles in algae and

plants where photosynthesis occurs

• Thylakoid membrane: highly folded continuous membrane network, site of the light-dependent reactions that produce NADPH and ATP

• Stroma: aqueous matrix of the chloroplast which surrounds the thylakoid membrane

• Lumen: aqueous space within the thylakoid

membrane

Chlorophyll and Other Pigments Capture Light

• Recall

• Energy of a photon

• h = Planck’s constant

• = frequency

• Photons absorbed – electrons go to excited state

• Can decay back to ground state– Extra energy given off as light, heat, or used to do

work (chemical energy)

– Direct transfer of energy to a neighboring molecule

Exiton Transfer

hE

Light-Capturing Pigments

• Chlorophylls - usually most abundant and most important pigments in light harvesting

• Contain tetrapyrrole ring (chlorin) similar to heme, but contains Mg2+

• Chlorophylls a (Chl a) and b (Chl b) in plants

• Bacteriochlorophylls a (BChl a) and b (BChl b) are major pigments in bacteria

Other pigments

• phycobilins – cyanobacteria & red algae

• Accessory pigments – absorb wavelengths that chlorophylls do not

• Carotenoids – yellow, red or purple

• Most important one – lutein (yellow)

• Pigments arranged with specific proteins to form the light-harvesting complexes (LHCs)

• Plants usually have twice as much Chl a as Chl b

• Absorbance spectra complement each other

Photosystems I (PSI) and II (PSII):

• Functional units of photosynthesis in plants

• Contain many proteins and pigments embedded in the thylakoid membrane

• These two electron-transfer complexes operate in series, connected by cytochrome bf complex

• Electrons are conducted from H2O to NADP+

• PSI and PSII each contain a reaction center (site of the photochemical reaction)

• Special pair: two chlorophylls in each reaction center that are energized by light (reaction center chlorophylls)

• In PSI special pair is: P700 (absorb light maximally at 700nm)

• In PSII the special pair is: P680 (absorb light maximally at 680nm)

• Light can be captured by antenna molecules (light-harvesting) and transferred among themselves until reaching the special-pair chlorophyll molecules in the reaction center

Diagram of photosynthesis membrane systems

Photosystems vary

- Photosynthetic bacteria have one-photosystem

- Plants & cyanobacteria have 2

Electron Transport in Photosynthesis• Distribution of photosynthetic components

Overview of InitiationLight capture, electron transport and proton

translocation in photosynthesis

• Light is captured by antenna molecules• Light energy drives the transport of electrons from

PSII through cytochrome bf complex to PSI and ferridoxin and then to NADPH

• The proton gradient generated is used to drive ATP production

• For 2 H2O oxidized to 1 O2, 2 NADP+ are reduced to 2 NADPH

Steps to initiate redox chain

Absorption of Light

Reaction-center chlorophylls contains “special pair”

Exiton transfer between antennae molecules

Exiton transfer to special pair

Energy used to donate an electron

Get an electron hole

Separation of charge

Redox chain can begin

The Z-scheme• Z-scheme: path of electron flow and reduction

potentials of the components in photosynthesis

• Absorption of light energy converts P680 and P700 (poor reducing agents) to excited molecules (good reducing agents)

• Light energy drives the electron flow uphill

• NADP+ is ultimately reduced to NADPH

2H2O + 2NADP+ + 8 photons → O2 + 2NADPH + 2H+

Electron Transport From PSII through Cytochrome bf

• Electrons for transport are obtained from the oxidation of water (remember the e- hole created)

• Catalyzed by the oxygen-evolving complex (water-splitting enzyme) of PSII

• Oxygenic photosynthesis

• Organisms with only one photosystem do not generate O2

2H2O O2 + 4 H+ + 4 e-

Reduction, excitation and oxidation of P680• P680 special-pair

pigment of PSII

• Light excites P680 to P680*, increasing its reducing power

• e- released to give P680+

• P680+ is reduced by e- derived from oxidation of H2O

• Reduction of plastoquinone to plastoquinol

• Analogous to Q in the mitochondrial ETC

• Mobile carrier that goes on to cytochrome bf complex

• Q-cycle occurs at cyt bf

Details:

• Dimeric protein subunits hold rest of parts together

• located in granal lamellae (stacked thylakoid membranes)

•Close to LHCII (light harvesting complex)

•Electrons pass through only 1 of the dimers

• e- path: P680 → Pheophytin → PQA (plastoquinone) → PQB

• Net reaction:

4P680 + 4H+ +2 PQB + 4 photons → 4P680+ + 2PQBH2

Splitting of Water

• Water is split by Oxygen-evolving complex

• To make 1 O2 need:

– 2 H2O

– Waters release 4 electrons

• Electrons fill holes in P680

• Passed one at a time

• Mn passes through several oxidation states

• H+ released to the lumen (contributes to pH gradient)

•Cytochrome bf links PSI and PSII

•Similar in function and components to mitochondrial complex III

•Contains: cytochromes (hemes) and Fe-S protein

•Has a Q-cycle

•Is a proton pump – 4H+ per PQH2

•Protons pumped: stroma → lumen

•Proton gradient used to drive ATP synthesis

•Electrons passed to mobile carrier Plastocyanin

PSI • Reduced P700 is excited to P700* (the strongest

reducing agent in the chain) by light absorbed by the PSI antenna molecules

• P700* donates an electron through a series of acceptors to ferredoxin (Fd)

• e- path: plastocyanin → PSI special pair → A0 (Chl) → phylloquinonoe (A1 or QK) → several Fe-S centers → NADP+

• Reduction of NADP+ (Eo’ = -0.32 V) by Fd (Eo’= -0.43 V) is catalyzed by ferredoxin-NADP+ oxidoreductase on the stromal membrane side

• PSI also dimeric – electrons can pass through both

• Last reaction step 2Fdred + 2H+ + NADP+ 2Fdox + NADPH + H+

• Arrangement of photosystems

PSI stromal lamellae

PSII granal lamellae

LHCII – holds granal lamellae together

varies with phosphorylation of a Tyr

adjusts by [PQH2]

ATP Synthesis by Photophosphorylation • Photophosphorylation: synthesis of ATP

which is dependant upon light energy

• Chloroplast ATP synthase very similar to mitochondrial counterpart

• Consists of two major particles: CFo and CF1

• CFo spans the membrane, forms a pore for H+

• CF1 protrudes into the stroma and catalyzes ATP synthesis from ADP and Pi

• About 12 H+ passed per O2 produced

– 4 from oxygen-evolving complex

– Up to 8 from cytochrome bf

• 4 electrons passed

• Difference in pH across membrane pH 3

• pH difference is major contributor to free energy stored in the gradient

• Experimentally shown that ~3 ATP made per O2

• Net equation (non-cyclic photophosphorylation)

2H2O + 2NADP+ + 8 photons + ~3ADP + ~3Pi →

O2 + 2NADPH + 2H+ + ~3 ATP

Cyclic photophosphorylation• Produces ATP without also making NADPH

• Used to adjust ATP/NADPH ratio

• Uses only PSI

• Instead of tranferring e- to NADP+, they are passed to plastocyanin

• Returns e- to reaction center of PSI

• Still generate pH gradient and ATP

• Net reaction: ADP + Pi + light ATP + H2O

Summary of ATP Synthases