1. Photosynthesis “Making the Fuel”
GTCHS Biology

2. 1. The Light-Dependent Reactions
Take place in the _______________________
This step of photosynthesis creates _________________ as a byproduct
When light strikes chlorophyll (or any other accessory pigment), it energizes electrons within the pigment molecule

3. 1. The Light-Dependent Reactions
Energized electrons leave chlorophyll to go participate in other reactions
This leaves chlorophyll ___________________
This loss must be replaced by Photolysis

4. 1. The Light-Dependent Reactions
Photolysis: Splitting up of water molecules into:
1. H+ ions
2. O2 gas
3. Electrons
What happens to each of these components?

5. 1. The Light-Dependent Reactions
Excited electrons that have absorbed light E are unstable…
Electron Transport Chain (ETC): A highly organized electron carrier molecules order these electrons and their flow down the ETC
Two sequential transport chains harvest the energy of these electrons

6. 1. The Light Dependent Reactions
The Electron Transport Chain (ETC)

7. 1. The Light-Dependent Reactions
Electron Transport Chain:
1. First, electrons pass down the ETC which captures their energy
This energy is used to pump H+ ions into the thylakoids…these ions form a _____________________ ___________________
There is a higher concentration of both:

8. 1. The Light-Dependent Reactions
Electron Transport Chain:
2. Second, light re-energizes electrons as they travel down the second ETC
This serves to bond H+ ions to NADP+ to form a more stable energy storage molecule, NADPH
NADPH is sometimes called ‘hot hydrogen’ as its energy and H+ ions will be used to help build _________________ in the next part of photosynthesis

9. 1. The Light-Dependent Reactions
Now NADPH and ATP molecules store the energy from excited electrons (which started out as light energy from the sun)
Chloroplasts, with their orderly arrangement pigments, enzymes, and ETCs, have successfully transformed light energy into chemical energy!

10. 1. The Light-Dependent Reactions
Sidenote: Before photosynthesis evolved, the Earth’s atmosphere was totally lacking in:

11. 2. The Calvin Cycle
Remember – all autotrophs store their E as _________________
In the Calvin Cycle, E is briefly stored by NADPH and ATP (both of which are needed to make _____________)
“Making food from thin air”

12. 2. The Calvin Cycle
So what products/reactants have we tracked so far??

13. 2. The Calvin Cycle
So what products/reactants have we tracked so far??

14. 2. The Calvin Cycle
So what products/reactants have we tracked so far??

15. 2. The Calvin Cycle H+ ions
So what products/reactants have we tracked so far??

16. 2. The Calvin Cycle Electrons (e-) H+ ions
So what products/reactants have we tracked so far??

17. 2. The Calvin Cycle O2 gas Electrons (e-) H+ ions
So what products/reactants have we tracked so far??

18. 2. The Calvin Cycle
So what products/reactants have we tracked so far??

19. 2. The Calvin Cycle
What has happened to the carbon dioxide???
So what products/reactants have we tracked so far??

20. 2. The Calvin Cycle
Up until now, CO2 has been lurking around but has not been used yet
Animals and most other heterotrophs cannot take in carbon directly…they must eat other organisms or absorb organic molecules in order to get carbon
Only autotrophs can build low-energy, inorganic CO2 into high-energy molecules like glucose

21. The Calvin Cycle

22. 2. The Calvin Cycle Carbon fixation:
The process of turning low-E, inorganic carbon molecules into high-energy organic molecules

23. 2. The Calvin Cycle
Plants have actually evolved three different pathways for carbon fixation:
1. C3 photosynthesis (‘standard’ photosynthesis)
2. C4 photosynthesis
3. CAM photosynthesis

25. 2. The Calvin Cycle 1. C3 Photosynthesis:
One molecule of C (from CO2) is combined with a 5-carbon sugar
This 5-carbon sugar is called Ribulose Biphosphate (RuBP)
The enzyme which catalyzes this reaction is called RuBisCo
Resulting 6-carbon molecule is unstable so it splits into two 3-carbon molecules

26. 2. The Calvin Cycle Alternative forms of carbon fixation:
Dry air, hot temps, and bright sunlight all slow down the C3 pathway for carbon fixation
This is because stomata must eventually close to prevent excess water loss
Closing the stomata will eventually cause a CO2 shortage
2. C4 Photosynthesis: Some plants use separate compartments to fix CO2

27. C4 Photosynthesis
In this type of photosynthesis, the CO2 combines with a 3-carbon molecule – resulting in a 4-carbon molecule
Because the 4-carbon molecule is the first ‘stable’ organic molecule formed, this is called ‘C4 Photosynthesis’
C4 plants have special cells that let them separate the light and dark reactions in space

28. Anatomy of C3 versus C4 plants
C4 Photosynthesis
Light Reactions: in the mesophyll cells
Carbon Fixation: in bundle sheath cells
Anatomy of C3 versus C4 plants

29. CAM Photosynthesis Cacti and other succulents avoid water loss
by fixing carbon only
at night
These plants close their stomata during the day and re-open them only during cooler, more humid nighttime conditions
Was named after the family of plants it was originally discovered in

30. CAM Photosynthesis

31. C4 versus CAM Photosynthesis

32. 2. The Calvin Cycle
The Calvin Cycle transfers energy in small, controlled steps – slowly pushing molecules ‘uphill’ in terms of energy content
During this process of carbon fixation, NADPH and ATP provide energy to build glucose
Where did this energy originate from???

33. Photosynthesis Wrap-Up