Plants do both: photosynthesis and respiration

The Photosynthetic Reaction H2O + CO2 --> CH2O + O2

– For a long time, the O2 released was thought to come from CO2 (wrong)– Studies on sulfur bacteria showed:

H2S + CO2 --> CH2O + 2S

– So, van Niel postulated a generic scheme:

H2X + CO2 --> CH2O + 2X

– And it was later shown that indeed the O2 comes from H2O

6H2O + 6CO2 --> C6H12O6 + 6O2

weak reducer + weak oxidizer --> strong reducer + strong oxidizer

Chloroplast structure and function• Membranes

– Outer: permeable to many things• Porins, large central pore

– Inner: highly impermeable• Specific channels for certain molecules

• Membranes– Thylakoid membrane system

• Contained within the inner membrane system• Arranged in stacks: Grana• Enzymes for light capture are embedded

within this membrane– Photosystem II (PSII)– Cytochrome b6f (like ETC Comp. III) (move protons)– Photosystem I (PSI)– ATP synthase

Chloroplast structure and function

• Enclosed spaces– Intermembrane space: between outer and inner membranes

– Stroma: space enclosed by inner mem.• Contains the thylakoids• Contains the Calvin cycle enzymes

for CO2 fixation into sugar• Contains DNA, ribosomes

– Lumen: Space enclosed by thylakoids• Accumulates high [H+] for ATP synthesis by ATP synthase

Chloroplast structure and function

stroma lumen

• Light dependent reactions– Capture E of light into ATP and NADPH– Produce O2 from H2O

• Light independent reactions– Use ATP and NADPH to capture and reduce CO2 into sugar

• Plants also use aerobic respiration (mitochondria)

Light-dependent and independent reactions

• Photon is absorbed by a molecule• ‘pushes’ an electron from an inner (lower E) to an outer (higher E) orbital

e- + photon --> e* (excited state)

• # orbitals is finite and E levels are specific• Different molecules can only absorb photons of certain E (wavelength)

Absorption of light by photosynthetic pigments

• Chlorophyll• Beta-carotene

• Conjugated systems– Alternating single and double bonds– Delocalized electron cloud– Can absorb more varied wavelengths– Strong absorbers of visible light

Absorption of light by photosynthetic pigments

• Light Harvesting Complexes– 100s of chlorophyll molecules– Noncovalent link to thylakoid membrane– Group acts as an antenna for light– Photon is passed around

• each pass reduces E

– Only one is the reaction-center• P680, PSII• P700, PSI• Transfers e* to a carrier

Organization into photosynthetic units

• Photosystem II (PSII)– Boost e* halfway to

NADP+• Photosystem I (PSI)

– Boost e* above NADP+

H2O + NADP+ --> 1/2O2 + NADPH + H+

Eo’ = 1.14V

Cell uses 2 photons, in 2 steps

Organization into photosynthetic units

• Photosystem II– 20 subunits, embedded in thylakoid membrane– Associated with Light Harvesting Complex II (LHCII)

• Antenna pigments (chlorophyll) + protein subunits– Light absorbed into D1/2 complex, e* transfer to Pheophytin

P680* + Pheo --> P680+ + Pheo- (charge separation)

P680* + Pheo --> P680+ + Pheo- (charge separation)– P680+ = strong oxidizing agent (most powerful in biology)

• Will accept e- from H2O and yield O2 in process (photolysis)– Pheo- = strong reducing agent

• Will pass e- to Plastoquinone (PQ) --> PQH2

• Cytochrome b6f (structure-function similarity to Complex III of ETC)– Accepts 2e- from PQH2– Translocates 4H+ per pair of e-– Transfers e- to Plastocyanin protein (PC)– PC carries e- to PSI

• Photosystem I (PSI)– LHCI

• Contains light antenna– P700 rxn center

P700* + A0 --> P700+ + A0-

– P700+ receives e- from PC

– A0- txfr e- to ferredoxin (Fd)

– Fd donates e- to:

NADP+ + H:- --> NADPH

Fd NADP+ reductase (FNR)

2H2O + 2NADP+ + 2H+ + 8photons -->

O2 + 2NADPH

Also, 18H+ difference generated acrossthylakoid membrane

– Acidic inside lumen

ATP synthase can generate ~ 5 ATP

Summary: Light-dependent reactions

• Noncyclic: passage of e- from H2O to NADP+ yielding H2O and NADPH plus, the proton gradient for ATP synthase• Cyclic: Fd passes e- to cytochrome b6f instead of Fd NADP+ reductase,

still creates proton gradient, but no NADPH

– ATP synthesis can be uncoupled from NADPH synthesis

Noncyclic vs. Cyclic photophosphorylation

3D structures for the light reaction complexes

Light independent rxns: Calvin cycle• Key step: Ribulose bisphosphate carboxylase (RuBisCo)

– 5C + CO2 --> 2x 3C (3-phosphoglycerate from glycolysis)– Plants that fix CO2 this way are called C3 plants because of the 3C intermediate

6CO2 + 18ATP + 12NADPH --> Fructose + 18ADP + 12NADP+ + 18Pi

– Calvin cycle enzymes are in the stroma

Cytoplasm

Outer mem

Inner mem

Photorespiration (distinct from respiration)• RuBisCo is at the mercy of the [CO2]/[O2] ratio• Only modest preference of enzyme for CO2

glycolate

CO2 release

• Coordination of cellular organelles in photorespiration

• Hot dry climates are hard on C3 plants– Must shut stomata to prevent H2O loss during day– Also keeps CO2 out (and O2 builds up inside = photorespiration problem)– C4 plants use PEP carboxylase enzyme

• PEP (3C) + CO2 --> OA (4C)

PEP carboxylase works at much lower [CO2],open stomata less often!