1. PHOTOSYNTHESIS
Chapter 10

2. BASIC VOCABULARY Autotrophs – producers; make their own “food”
Heterotrophs – consumers; cannot make own food

3. LEAF STRUCTURE
Stomata (stoma) – microscopic pores that allow water, carbon dioxide and oxygen to move into/out of leaf
Chloroplasts – organelle that performs photosynthesis
Found mainly in mesophyll – the tissue of the interior leaf
Contain chlorophyll (green pigment)
Stroma – dense fluid in chloroplast
Thylakoid membrane – inner membrane of chloroplast
Grana (granum) – stacks of thylakoid membrane

4. Figure 10.2 Focusing in on the location of photosynthesis in a plant

5. PHOTOSYNTHESIS SUMMARY
6CO2 + 6H20 + light energy C6H12O6 + 6O2
Oxygen comes from water, not CO2
Two parts:
Light Reactions
The Calvin Cycle (Dark Reactions)

6. Figure 10.3 Tracking atoms through photosynthesis

7. Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle

8. LIGHT Photons – discrete packets of light energy
Chlorophyll a – (blue-green)only pigment that is directly used in light reactions
Chlorophyll b – (yellow-green) accessory pigment
Carotenoids - (yellow-orange)
accessory pigment

9. Figure 10.6 Why leaves are green: interaction of light with chloroplasts

10. Figure Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for photosynthesis in an alga
Aerobic bacteria gather where there is more oxygen along this algal filament, which was exposed to different colors of light.

11. PHOTOEXCITAION
When photons hit chlorophyll and other pigments, electrons are excited to an orbital of higher energy
In solution when the excited electrons fall, they give off energy (a photon) and fluoresce

12. Figure 10.9 Location and structure of chlorophyll molecules in plants

13. LIGHT REACTIONS Photosystems:
Made of proteins and other molecules surrounding chlorophyll a
Contain a primary electron acceptor
Photosystem I – P700
Photosytem II – P680

14. Figure 10.11 How a photosystem harvests light

15. Noncyclic (predominant route) Cyclic
Require light to occur
Two pathways:
Noncyclic (predominant route)
Cyclic
Noncyclic animation
Another animation

16. NONCYCLIC ELECTRON FLOW
Photosystem II absorbs light
Two electrons excited and captured by primary electron acceptor
“Hole” in photosystem II is filled by 2 electrons that come from the splitting of water
H2O H+ + ½ O2 + 2e-

17. Oxygen is released
Excited electrons pass from primary electron acceptor down an electron transport chain to photosystem I (filling its “hole”)
ATP is made by photophosphorylation as electrons fall down ETC

18. Photons excite 2 electrons from Photosystem I and are captured by its primary electron acceptor
Electrons then move down another ETC to ferredoxin (Fd)
Fd gives electrons to NADP+ (nicotinamide dinucleotide phosphate) making NADPH
The enzyme that helps this transfer of e- is called NADP+ reductase

19. Figure How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)

20. Figure 10.13 A mechanical analogy for the light reactions

21. Fig. 10-17- Photosystems in Thylakoid Membrane
STROMA
(low H+ concentration)
Cytochrome
complex
Photosystem II
Photosystem I
4 H+
Light
NADP+
reductase
Light Fd 3
NADP+ + H+ Pq
NADPH e– Pc e– 2
H2O 1
1/2 O2
THYLAKOID SPACE
(high H+ concentration)
+2 H+
4 H+ To
Calvin
Cycle
Figure The light reactions and chemiosmosis: the organization of the thylakoid membrane
Thylakoid
membrane
ATP
synthase
STROMA
(low H+ concentration)
ADP +
ATP P i H+

22. Figure 10.14 Cyclic electron flow

23. CYCLIC ELECTRON FLOW Only Photosystem I is used
Fd passes electrons back to Photosystem I via ETC
ATP made
No NADPH made
No oxygen released
Used when cell needs more ATP than NADPH

24. Figure 10.17 The Calvin cycle (Layer 3)

25. CALVIN CYCLE
Also called Dark Reactions because light is not needed; however products from light reactions are needed.
Carbon Fixation – initial incorporation of carbon into organic molecules
CO2 attaches to a 5-carbon sugar called ribulose bisphosphate (RuBP)
The enzyme that catalyzes this is called rubisco
Calvin cycle animation

26. One G3P molecule leaves cycle to be used by plant
Immediately splits into two 3-carbon molecules called 3-phosphoglycerate
3-phosphoglycerate is phosphorylated by ATP (from light reactions) making ,3-bisphosphoglycerate
1,3-bisphosphoglycerate is reduced by taking electrons from NADPH making glyceraldehyde 3-phosphate (G3P)
One G3P molecule leaves cycle to be used by plant
The remaining G3P’s are converted into RUBP in several steps and by getting phosphorylated by ATP
3 phosphoglycerate and 1-3 bisphosphoglycerate from glycolysis

27. Figure 10.17 The Calvin cycle (Layer 3)

28. C3 PLANTS – have a problem
Examples : rice, wheat, and soy beans
Problem - produce less food when stomata are closed during hot days because low CO2 starves Calvin Cycle and rubisco can accept O2 instead of CO2
High oxygen levels = O2 passed to RUBP (not CO2) and Calvin cycle stops
When this oxygen made product splits, it makes a molecule that is broken down by releasing CO2

29. This process is called photorespiration.
Occurs during daylight (photo)
Uses O2 and makes CO2 (respiration)
NO ATP made (unlike respiration) and NO food made
Early earth had low O2 so this would not have mattered as much
Photorespiration drains away as much as 50% of carbon fixed by Calvin Cycle in many plants.

30. Alternative methods of Carbon Fixation- the solution to the previous problem
C4- Plants
2 photosynthetic cells- bundle-sheath & mesophyll
PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll
new 4C molecule releases CO2 in bundle sheath cell
CAM Plants
open stomata during night
close during day (crassulacean acid metabolism)
cacti, pineapples, etc.

31. Figure 10.18 C4 leaf anatomy and the C4 pathway
Malate and oxaloacetate from Krebs

32. Figure 10.19 C4 and CAM photosynthesis compared

33. Figure 10.20 A review of photosynthesis