1. 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

2. 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)

3. Photosynthesis Chloroplast structure and function Membranes
Outer: permeable to many things
Porins, large central pore
Inner: highly impermeable
Specific channels for certain molecules

4. 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

5. 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

6. Photosynthesis H2X + CO2 --> CH2O + 2X 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

7. 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)

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

9. 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

10. 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

11. Photosynthetic units Photosystem II (PSII) Photosystem I (PSI)
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

12. 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 --> P Pheo- (charge separation)

13. P680+ = strong oxidizing agent (most powerful in biology)
P680* + Pheo --> P 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

14. 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

15. Photosystem I (PSI) P700* + A0 --> P700+ + A0- LHCI
Contains light antenna
P700 rxn center
P700* + A0 --> P A0-
P700+ receives e- from PC
A0- txfr e- to ferredoxin (Fd)
Fd donates e- to:
NADP+ + H:- --> NADPH
Fd NADP+ reductase (FNR)

16. 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

17. 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

18. The light reactions: structures

19. 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

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

21. 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)

23. 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

24. Coordination of cellular organelles in photorespiration

25. 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!