1. Chapter 10: Photosynthesis

2. Photosynthesis??:
Captures light energy from the sun and converts it to chemical energy.
What types of organisms??

3. Photosynthesis in nature
Autotrophs: self feeders/ producers
photoautotrophs/ chemoautotrophs
obtains organic food without eating other organisms
Heterotrophs: consumers
obtains organic food by eating other organisms or their by-products (includes decomposers)

4. The chloroplast Double membrane organelle Conducts photosynthesis
Plant and algal cells
Plastid

9. Photosynthesis: an overview
Redox process
Compared to cellular respiration, it reverses the flow of electrons
Photons split water, excite electrons
Electrons are then transferred to CO2
Reducing it to a sugar
Electrons increase in potential energy

10. Two stage process: 1. Light reactions
Converts solar energy to potential energy
Produces ATP (photophosphorylation)
Reduces NADP+ to NADPH

11. Two Stage Process: 2. Calvin Cycle
Incorporates CO2 from the air to organic molecules
Requires ATP
Occurs during the day time because needs light to reduce NADP+
Light independent reaction(dark reaction)

13. Stage One: Light Reactions
Converts solar energy to chemical energy (ATP and NADPH)
Uses photosynthetic pigments
Chlorophyll a and b
Carotenoids

16. Photosystems Light harvesting units of the thylakoid membrane
Composed mainly of protein and pigment antenna complexes
Antenna pigment molecules are struck by photons
Energy is passed to reaction centers (redox location)
Excited e- from chlorophyll is trapped by a primary e- acceptor

17. Noncyclic electron flow
Photosystem II (P680):
photons excite electrons of chlorophyll to an acceptor
electrons travel to Photosystem I down an electron transport chain (PqcytochromesPc)
as electrons fall, ADP  ATP (noncyclic photophosphorylation)

18. Photosystem I (P700): Fallen electrons are excited by a photon
Electrons enter second electron transport chain
Ferrodoxin (Fp)
Electrons from Fp reduce NADP+ to NADPH

21. Cyclic electron flow Alternative cycle when ATP is deficient
Photosystem I used but not II; produces ATP but no NADPH
Why? The Calvin cycle consumes more ATP than NADPH…….
Cyclic photophosphorylation

23. http://dendro.cnre.vt.edu/forestbiology/photo synthesis.swf

24. The Calvin Cycle:
Uses ATP and NADPH from light reactions to convert CO2 to sugar
Anabolic
Endergonic

25. The Calvin Cycle: Glucose is not produced directly
3 carbon sugar molecule (G3P)
Glyceraldehyde 3- phosphate
3 molecules of CO2 enter the cycle one at a time
Produce 6 molecules of G3P- one will exit the cycle to become a sugar

26. Three phases of the Calvin Cycle:
1. Carbon Fixation:
Each molecule of CO2 is fixed to ribose bisphosphate(RuBP)
5 carbon sugar
Catalyzed by RuBisCo

27. RuBisCo Ribulose- 1,5- Bisphosphate carboxylase/oxygenase
Most abundant enzyme in the chloroplast
Most abundant protein on the planet
Very large protein- 560,000 Da
Very important for carbon fixation

28. Three phases of the Calvin Cycle:
Product of Carbon Fixation:
6 carbon intermediate molecule
Short lived/ unstable
Breaks down into two -3 carbon molecules
For each molecule of CO2

29. Three phases of the Calvin Cycle:
2. Reduction:
Each 3- carbon molecule is phosphorylated by ATP
Produces 1,3 bisphophoglycerate
Each 1,3 bisphosphoglycerate gets reduced to G3P by NADPH
1 of the 6 G3P produced will go on to produce glucose

30. Three phases of the Calvin Cycle:
3. Regeneration of RuBP:
The other 5 G3P molecules will be rearranged into 3 molecules of RuBP
Requires an additional 3 ATP’s

31. Input vs. Output 3 CO2 molecules enter one at a time
3 short lived 6 carbon molecules are produced
6, 3-phosphoglycerate molecules result

32. Input vs. Output
6, 3 –phosphoglycerate are phosphorylated into 6, 1,3 biphosphoglycerate
6 ATP

33. Input vs. Output
6, 1,3 biphosphoglycerate are reduced to 6 G3P by NADPH
1 of the 6 G3P will go onto form glucose
The other 5 of the G3P will go onto regenerate RuBP
Phosphorylated by 3 ATP

34. Input vs. Output
9 ATP consumed
6 NADPH oxidized

36. How do plants fix carbon in hot/arid climates?
Plants close their stomata to prevent dehydration during hot and dry days.
How does this affect photosynthesis?

37. Alternative carbon fixation methods:
Photorespiration:
Plant adds O2 to the Calvin cycle when CO2 is unavailable
No benefit to the plant
No ATP produced
No sugar produced

38. C4 Pathway C4 plants: alternative mode of carbon fixation
2 photosynthetic cells, bundle-sheath & mesophyll
PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll
new 4C molecule releases CO2 to Calvin cycle (grasses, corn, sugar cane)

40. Alternative carbon fixation methods, II
CAM plants: (crassulacean acid metabolism)
open stomata during night, close during day
Cacti and other succulents(Jade plant) pineapples
CO2 is taken in at night and fixed into different organic acids
During the day, when stomata are close but light reaction is active the organic acids deliver CO2 to the Calvin cycle

42. A review of photosynthesis