Photosynthesis• Heterotrophs: depend on external C sources (e.g. animals)

• Autotrophs: can survive on CO2 as sole C source (e.g. plants, etc)– Requires large E input

• Chemoautotrophs: use E in inorganic chemical compounds (e.g. NH3, etc)

• Photoautotrophs: use E in light E = hc / – Shorter wavelength = higher energy photon

• Evolution of modern metabolic pathways– Initial CO2 atmosphere

1. Heterotrophs (anaerobic)2. Chemoautotrophs (anaerobic)3. Photoautotrophs: CO2 + H2O --> CH2O + O2

– O2 atmosphere, aerobic metabolism evolved• Symbiotic bacterium --> --> --> modern mitochondrion• Symbiotic cyanobacterium --> --> --> modern chloroplast

Photosynthesis• The photosynthetic redox reaction:

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

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

• E = hc / @ = 680nm, E = -42 kcal/mol photons ( ~ 6 ATP)

Photosynthesis• Chloroplast structure and function

– Membranes• Outer: permeable to many things

– Porins, large central pore• Inner: highly impermeable

– Specific channels for certain molecules

• Chloroplast structure and function– 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

Stroma

stroma

lumen

Photosynthesis• 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

• 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)

• Absorption of light– 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)

• 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

• Photosynthetic units– 100s of chlorophyll molecules– Noncovalent link to thylakoid membrane (Light Harvesting Complexes)

– Group acts as an antenna for light– Photon is passed around

• each pass reduces E (wavelength longer)– Only one is the reaction-center

• P680, PSII• P700, PSI• Transfers e* to a carrier

• 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

In cell, need ~ 2V

Cell uses 2 photons, in 2 steps

• 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)– 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)

• Light reactions summary

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

O2 + 2NADPH

Also, 18H+ difference generated across

thylakoid membrane– Acidic inside lumen

ATP synthase can generate ~ 5 ATP

• Noncyclic versus cyclic photophosphorylation– 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

creates proton gradient, but no NADPH

• ATP synthesis can be uncoupled from NADPH synthesis

The light reactions: structures

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

Light independent rxns: Calvin cycle 6CO2 + 18ATP + 12NADPH --> Fructose + 18ADP +

12NADP+ + 18Pi

Light independent rxns: Calvin cycle• Many steps are actually light sensitive

• Redox control: in absence of light, enzymes become inactivated– Transfer e- from Fd to Thioredoxin– Reduce disulfide bridges in proteins for light-dependent regulation of activity:

R-S-S-R(inactive) + 2e- + 2H+ -->

R-SH(active) + R-SH(active)

Photorespiration• RuBisCo is at the mercy of the [CO2]/[O2] ratio

• Rise in global [CO2] likely linked to increased crop yields– 270ppm(1870) --> 380ppm(now)

• 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!