1. Chapter 8
Photosynthesis

2. Trophic Roles
Autotrophs  produce organic molecules from CO2 & inorganic raw materials  “producers”  plants
Heterotrophs  “consumers”

3. Chloroplast Structure
Found in mesophyll
1 mesophyll cell may have 30 chloroplasts
Stomata regulate passage of CO2, O2 and H2O

4. Chloroplast Structure

5. Chloroplast Structure (cont.)
Pigments  light receptors
Absorb light energy
Boost e-
Chlorophyll
Chlorophyll “a”  main pigment  blue-green
Chlorophyll “b”  accessory pigment  yellow-green
Accessory pigments  absorb different wavelengths of light
Carotenoids  yellow-orange

6. Photosystems
Pigment molecules absorb energy  boost electrons  unstable
Passes energy to reaction center of antenna complex (chlorophyll “a” molecule)  transfers energy to primary electron acceptor

7. Photosynthesis Light reactions Dark reactions Calvin Cycle
Cyclic photophosphorylation
Non-cyclic photophosphorylation
Photolysis (breaking water up with light)
Dark reactions
Calvin Cycle

8. Photosystems (cont.) Located in thylakoid membrane
Photosystem I  P700  absorbs light of 700 nm
Photosystem II  P680  absorbs light of 680 nm
Both are primarily chlorophyll “a”
Electron acceptor  NADP+  NADPH

9. Non-cyclic photophosphorylation

15. Cytochrome complex synthase reductase
LIGHT
REACTOR
NADP+
ADP
ATP
NADPH
CALVIN
CYCLE
[CH2O] (sugar)
STROMA
(Low H+ concentration)
Photosystem II
H2O
CO2
Cytochrome
complex O2 1
1⁄2 2
Photosystem I
Light
THYLAKOID SPACE
(High H+ concentration)
Thylakoid
membrane
synthase Pq Pc Fd
reductase
+ H+
NADP+ + 2H+ To
Calvin
cycle P 3 H+
2 H+
+2 H+

16. Cyclic Photophosphorylation
Primitive  used by bacteria  only generates energy--no glucose
Electron “boosted” out of PI  ETC  returned to PI
Electron drives proton pump  chemiosmosis  ATP

17. Calvin Cycle aka Dark Reactions
Occur in the dark or the light
Light independent reactions
3 “steps”
Carbon fixation
Reduction
Regeneration of RuBP

18. Step 1: Carbon Fixation
RuBP (ribulose bisphosphate)  5 C sugar  catalyzed by RuBP carboxylase (Rubisco)  unstable compound (splits)  PGA

19. Step 2: Reduction Phosphorylated by ATP Reduced by NADPH
Produces pyruvate
some pyruvate  glucose
most pyruvate  regenerate RuBP

20. Step 3: Regeneration of RuBP
Pyruvate  rearranged into RuBP
Requires input of 3 ATP
Takes 12 turns of cycle  1 glucose

22. Alternative mechanisms: Photorespiration
Competitive reaction between RuBP, CO2 and O2
Rubisco substitutes O2 for CO2  2 C compound (phosphoglycerate)
Eventually broken down  releases CO2
Non-productive
C3 plants  rice, wheat, soybeans (hot, bright days)
Photo (light) respiration( releases CO2/consumes O2)

24. Alternative mechanisms: C4 Plants
Sunny ecosystems
Carbon “fixed” outside cells (in bundle sheath cells  very efficient  requires extra ATP Balances out photorespiration
Corn is a C4 plant

25. Alternative mechanisms: CAM Plants
Crassulacean acid metabolism  Hot/dry climates
Orchids, cacti, etc.
Stomates open at night to reduce water loss  evaporation
CO2 is fixed, used later