1. Photosynthesis

2. You must know…
How photosystems convert solar energy to chemical energy…
How linear electron flow in the light reactions results in the formation of ATP, NADPH, and 02
How chemiosmosis generates ATP in the light reactions
How the Calvin cycle uses energy molecules of the light reactions to produce G3P
The metabolic adaptions of C4

3. Context Autotrophs Heterotrophs Self Feeders Producers Consumers
Depend on autotrophs for food and oxygen

4. Context Chloroplasts The site of photosynthesis in plants
Mesophyll tissue in interior of leaf
Chlorophyll
Located in thylakoid membranes
Light absorbing pigment

5. Chloroplast

6. Plant cell structure
Stomata: allow CO2 to enter and O2 and water vapor to exit

7. Photosynthesis Reaction
6CO2 + 6H2O + Light Energy  C6H12O6 + 6O2
Reverse of respiration
Water is split for electrons and H+ ions
It is a two step process
Light Reactions
The Calvin Cycle

8. Light Reactions Occurs in thylakoid membranes
Solar energy converted to chemical energy
Electrons transferred from water to NADP+
Oxygen released
ATP generated via photophosphorylation
Net products are…
NADPH
ATP
Oxygen

9. Calvin Cycle Occurs in stroma
Sugar formed from CO2 aka carbon fixation
Powered by NADPH and ATP from light reactions

10. Big Picture

11. Light Reaction Details
Light is electromagnetic energy, photons
Pigments: substances that absorb light.

13. Light Reactions
Photons absorbed by pigments called photosystems with 2 parts
Light Harvesting complex
Photon excites an electron, moves to higher orbital
Reaction center
2 Chlorophyll a molecules accept energy
Energy transferred to primary electron acceptor
At this point, light energy has been converted to chemical energy

14. Photosystems Photosystem 2 (P680) and 1(P700)
Major concept is the linear electron flow
Flow of electrons through photosystems in the thylakoid membrane

15. Major steps of light reactions
PS2 absorbs light energy, reaction center (2 chlorophylls) donate e- to primary electron receptor. Chlorophyll is now oxidized.
An enzyme splits water into 2 H+ and O and electrons go to PS2
Electron travels down electron transport chain from PS2 to PS1
The energy from this electron sets up proton gradient used for chemiosmosis to power ATP production. ATP used later to make carbohydrates in Calvin cycle

16. Major steps of light reaction
5. Meanwhile light also activated PS1, electrons donated to primary acceptor. What replaces these electrons? What is the source of these electrons? 6. Electrons go down another transport chain to NADP+, NADP+ and H form NADPH for Calvin cycle.

18. An alternative
Cyclic electron flow: PS1 only. Cycles electrons, produces ATP but no NADPH

19. Chloroplasts VS Mitochondria
Both generate ATP by chemiosmosis
Both generate proton motive force
Differences
Spatial
Mitochondria transfer energy from food to ATP
Chloroplasts use light energy to transfer to ATP

21. Calvin Cycle Uses ATP and NADPH to convert CO2 to sugar
To make one G3P you must complete the cycle 3 times.
Don’t memorize all intermediate steps… understand concept of reducing CO2 to sugar.

22. Calvin Cycle
3 CO2’s attach to 3 molecules of 5-carbon ribulose bisphosphate (RuBP)
Catalyzed by rubisco
Intermediate is unstable, breaks into 3 carbon compounds
6 NADPH reduce the intermediate
One G3P molecule exits cycle
2 G3P molecules combine to form glucose
Last step, RuBP regenerated

23. Calvin Cycle
Nine molecules of ATP consumed along with 6 molecules of NADPH

25. Alternate mechanisms of carbon fixation
Evolved in hot and arid climates
The stomata is the problem
CO2 enters the same place that H2O exits
Plant has to keep stomata closed hence CO2 shortage for Calvin cycle
Photorespiration: Oxygen binds and reduces RuBP, blocks up CO2 binding.
This can drain 50% of carbon fixed by Calvin cycle.

26. Adaptations
2 Types C4
CAM Plants

27. C4 Plants C4 plants have bundle-sheath cells and mesophyll cells.
CO2 is added to PEP to form 4 C compound oxaloacetate.
PEP carboxylase can not bind oxygen so photorespiration is not an issue
Mesophyll cells export oxaloactate to bundle sheath cells releasing CO2 to the normal Calvin cycle.

29. CAM Plants Adaption to hot dry climates
Stomata closed during day, prevents water loss
What else will this prevent?
Open stomata at night, fix CO2 in acids
Where do you think this is stored?
CAM plants separate stages of PS temporally

30. Adaptive Values C3
more efficient than C4 and CAM plants under “normal” conditions
requires less machinery, enzymes and anatomy C4
PS faster than C3 plants under high light intensity and high temperatures.
Prevents photorespiration.
Has better water use
PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration)
CAM
Better water use efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are low.

31. References