1. The Chemistry of Living Systems
CHAPTER 18
The Chemistry of Living Systems
18.2 Photosynthesis and Respiration

2. Glucose Glucose is the most important energy-yielding molecule
Photosynthesis
Cellular respiration
6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)

3. Photosynthesis Glucose is the most important energy-yielding molecule
“light”
This reaction is an endothermic reaction that is driven by sunlight.
It occurs exclusively in plants and algae.
Photosynthesis
6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)

4. Why do leaves have a green color?
Sunlight
(all wavelengths)
Why do leaves have a green color?

5. Chlorophyll
Sunlight
(all wavelengths)
Chlorophyll absorbs blue, violet and red wavelengths
Plants appear green because of a large molecule called chlorophyll.
Chlorophyll emits green wavelengths

6. Chlorophyll
Chlorophyll absorbs sunlight, and transfers the energy to electrons. These high-energy electrons are used in nonspontaneous reactions.

7. Photosynthesis
Plant cells contain structures called chloroplasts. This is where photosynthesis takes place.
Chlorophyll molecules are found inside chloroplasts.

8. 6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)
Photosynthesis
light
6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)
Step 1: light dependent reactions
Step 2: “dark” reactions

9. Photosynthesis Step 1: light dependent reactions
sunlight
Chlorophyll absorbs sunlight, and transfers the energy to electrons
Step 1: light dependent reactions
high-energy electrons help break down water molecules
2H2O(l) → 4H+ + O2(g) + 4e–

10. Photosynthesis Step 1: light dependent reactions
We need special “carrier” molecules to transport these high-energy electrons to the next step
2H2O(l) → 4H+ + O2(g) + 4e–

11. Photosynthesis NADP+ + 2e– + H+ → NADPH Energy from the light reaction
Energy stored to be transported to the next step
NADPH: a molecule that carries two high-energy electrons and stores sunlight as chemical energy.

12. Photosynthesis ATP ADP + Phosphate group
Energy is stored in the bonds between phosphate groups
ATP ADP + Phosphate group
energy
ATP: adenosine triphosphate; a molecule that carries chemical energy from spontaneous chemical reactions to nonspontaneous reactions.

13. Energy is transported to the next step of photosynthesis
Step 1: light dependent reactions
Step 2: “dark” reactions

14. Photosynthesis During the Calvin cycle, no light is needed.
Energy comes from ATP and NADPH molecules.
CO2 is added, glucose is formed
6 CO2 ~ 1 glucose molecule, C6H12O6
Step 2: “dark” reactions
for energy
(starch)
for plant structure
(cellulose)

15. NADP+ and ADP are recycled
Photosynthesis
NADP+ and ADP are recycled
Step 1: light dependent reactions
Step 2: “dark” reactions

16. Photosynthesis Photosynthesis takes place in chloroplasts.
Chlorophyll molecules are found inside chloroplasts.
Light reactions
Dark reactions

17. Glucose Glucose is the most important energy-yielding molecule
Photosynthesis
Cellular respiration
6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)

18. Cellular respiration
Unlike plants, we cannot produce glucose from sunlight, but must obtain it from food.
Glucose is broken down for energy through a process called cellular respiration.
As the name indicates, this process takes place inside the cell.

19. Energy stored for transport
Like NADPH in photosynthesis, NADH is a molecule that chemically stores energy.
NAD e– + H+ → NADH
Energy
Energy stored for transport
NADH: nicotinamide adenine dinucleotide; a molecule capable of carrying high-energy electrons and transferring them to another pathway.

20. Energy carrying molecules
NAD e– + H+ → NADH
Energy
Energy stored for transport
NADH and ATP help to store and transport energy
ATP ADP + Phosphate group
energy

21. Cellular respiration Step 1: Glycolysis Step 2: Krebs cycle
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)
Step 1: Glycolysis
Step 2: Krebs cycle
Step 3: Electron transport chain

22. The first step occurs in the cytoplasm (cellular fluid).
Cellular respiration
Glyco-lysis
for “glucose”
“breakdown”
The first step occurs in the cytoplasm (cellular fluid).
Step 1: Glycolysis

23. Glycolysis
Some energy in
glucose
(6 carbons)
At the beginning of glycolysis, some energy is needed to drive the breakdown of glucose

24. 1 molecule of glucose yields 2 molecules of pyruvic acid
Glycolysis
Some energy in
More energy released
glucose
(6 carbons)
pyruvic acid
(3 carbons)
More energy released
More energy is obtained at the end of the process
1 molecule of glucose yields 2 molecules of pyruvic acid

25. Glycolysis
This process is also called “anaerobic respiration” because it requires no oxygen
glucose
(6 carbons)
pyruvic acid
(3 carbons)
Sprint runners can get some energy from glycolysis, but cannot do this for a long time.
They also have to breathe heavily afterward to compensate for this oxygen debt.

26. Glycolysis More energy can be extracted from glucose!
to step 2 of respiration
glucose
(6 carbons)
pyruvic acid
(3 carbons)
to step 3 of respiration

27. Cellular respiration Step 1: Glycolysis Step 2: Krebs cycle
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)
Step 1: Glycolysis
Step 2: Krebs cycle
Step 3: Electron transport chain

28. Krebs cycle Pyruvic acid → → → CO2 + ATP + e–
many steps
Pyruvic acid → → → CO2 + ATP + e–
Step 2: Krebs cycle
Steps 2 and 3 of cellular respiration take place in the mitochondrion (an organelle in animal cells)

29. Electron transport chain
The last step receives electrons from NADH molecules to produce more ATP
Step 3: Electron transport chain

30. Electron transport chain
The last step receives electrons from NADH molecules to produce more ATP
Very large and specific proteins, called cytochromes, help to pass electrons around. Oxygen is the final electron acceptor.
4H e– + O2(g) → 2H2O(l)
Step 3: Electron transport chain

31. Cellular respiration
100%
90%
Energy left that can be used
62%
Only about 38% of the energy in glucose is extracted chemically by the body. The rest is lost as heat.

32. 6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)
Photosynthesis
light
6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)
Light reactions
Dark reactions

33. Cellular respiration Step 1: Glycolysis Step 2: Krebs cycle
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)
Step 1: Glycolysis
Step 2: Krebs cycle
Step 3: Electron transport chain