Chapter 22
Photosynthesis
1
SUMMARY
Section 22.1
In eukaryotes, photosynthesis takes place in chloroplasts. The light reactions
take place in the thylakoid membrane, a third membrane in chloroplasts in
addition to the inner and outer membrane.
The dark reactions of photosynthesis take place in the stroma, the space
Section 22.2
Photosynthesis consists of two processes. The light reactions are electron
transfer processes, in which water is oxidized to produce oxygen and NADP+ is
reduced to produce NADPH. The dark reactions are also electron transfer
processes, but here carbon dioxide is reduced to carbohydrates.
excited-state chlorophyll of photosystem I to the ultimate electron acceptor
NADP+, producing NADPH; again, energy is provided by absorption of a photon
of light.
Section 22.3
The mechanism of ATP in chloroplasts closely resembles the process that takes
place in mitochondria. The structure of the ATP synthase in chloroplasts is
similar to that in mitochondria.
Section 22.4
When photosynthesis first evolved, it was most likely to have been carried out by
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Section 22.5
In the dark reactions of photosynthesis, the fixation of carbon dioxide takes place
when the key intermediate ribulose-1,5-bisphosphate reacts with carbon dioxide
Section 22.6
In tropical plants, four-carbon compounds are frequently involved in the pathway
LECTURE NOTES
This chapter is likely to take two lectures to complete, one for the light reactions,
and one for the dark reactions. The light reactions of photosynthesis can be easily
LECTURE OUTLINE
I. Location of photosynthesis
A. Chloroplasts
1. grana
B. Chlorophylls
1. Structure and variants
2. Absorption spectra
C. Accessory pigments
II. Photosystems I & II
A. Light vs. dark reactions
B. Splitting of water at PS II
1. Oxygen-evolving complex
Photosynthesis 3
C. Reduction of NADP+ via PS I
1. Reaction center electron transfers
2. Ferredoxin
B. ATP synthase
IV. Evolutionary implications
V. The Dark Reactions
A. Rubisco
1. Ribulose-1,5-bisphosphate
2. 3-phosphoglycerate
B. Production of six-carbon sugars
ANSWERS TO PROBLEMS
22.1 Chloroplasts Are the Site of Photosynthesis
1. In the fall, the chlorophyll in leaves is lost, and the red and yellow colors of the
accessory pigments become visible, accounting for fall foliage colors.
5. Chlorophyll has a cyclopentanone ring fused to the tetrapyrrole ring, a feature
that does not exist in heme. Chlorophyll contains magnesium, whereas heme
contains iron. Chlorophyll has a long side chain based on isoprenoid units, which
is not found in heme.
22.2 Photosystems I and II and the Light Reactions of Photosynthesis
8. By and large, the synthesis of NADPH in chloroplasts is the reverse of NADH
oxidation in mitochondria. The net electron flow in chloroplasts is the reverse of
that in the mitochondria, although different carriers are involved.
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9. When light impinges on the reaction center of Rhodopseudomonas, the special
pair of chlorophylls there is raised to an excited energy level. An electron is
10. In photosystem I and in photosystem II, light energy is needed to raise the
reaction-center chlorophylls to a higher energy level. Energy is needed to
generate strong enough reducing agents to pass electrons to the next of the
series of components in the pathway.
13. Probably the electron transport chain in chloroplasts. Chloroplasts generate
molecular oxygen; mitochondria use it. The early atmosphere almost certainly
lacked molecular oxygen. Only when photosynthesis introduced oxygen into the
atmosphere would oxygen be needed.
14. Electron transport and ATP production are coupled to each other by the same
mechanism in mitochondria and chloroplasts. In both cases, the coupling
16. With very few exceptions, life directly or indirectly depends on photosynthesis.
The electric current is the flow of electrons from water to NADP+, a light-requiring
process. The “current” continues in the light-independent reactions, with
electrons flowing from NADPH to bisphosphoglycerate, which ultimately yields
glucose.
17. Photosystem II requires more energy than photosystem I. The shorter
wavelength of light means a higher frequency. Frequency, in turn, is directly
Photosynthesis 5
20. The electron transport chains of mitochondria and chloroplasts are similar. In
mitochondria, antimycin A inhibits electron transfer from cytochrome b to
22. It is well established that the path of electrons in photosynthesis goes from
photosystem II to photosystem I. The reason for the nomenclature is that
photosystem I is easier to isolate than photosystem II and was studied more
extensively at an earlier date.
23. It would take much work to establish the number of protons pumped across the
thylakoid membrane. This is partly the result of experience with mitochondria and
25. When the loosely bound cytochrome diffuses away, a charge separation is
induced. This separation of charge represents stored energy.
22.3 Photosynthesis and ATP Production
27. In cyclic photophosphorylation, the excited chlorophyll of photosystem I passes
electrons directly to the electron transport chain that normally links photosystem
II to photosystem I. This electron transport chain is coupled to ATP production
(see Figure 22.8).
28. Both depend on a proton gradient, resulting from the flow of electrons. In
chloroplasts, protons come from the splitting of water to produce oxygen. In
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22.4 Evolutionary Implications of Photosynthesis with and without Oxygen
32. Many electron donors other than water are possible in photosynthesis. This is
especially the case in bacteria, whose photosystems do not have strong enough
22.5 Dark Reactions of Photosynthesis Fix CO2
34. Rubisco is the principal protein in chloroplasts in all green plants. This wide
distribution makes it likely to be the most abundant protein in nature.
35. The amino acid sequence of the catalytic subunits of rubisco is encoded by
oxidation of glucose and divide by 6.
38. Glucose synthesized by photosynthesis is not uniformly labeled because only
one molecule of CO2 is incorporated into each molecule of ribulose-1,5-
bisphosphate, which then goes on to give rise to sugars.
39. If rubisco was one of the first protein enzymes to arise early in the evolution of
life, it may not have the efficiency of protein enzymes that evolved later, when
evolution was more dependent on modifying and adapting existing proteins.
42. Atmospheric oxygen is a consequence of photosynthesis. Rubisco evolved
before there was a significant amount of oxygen in the atmosphere.
43. The condensation of ribulose-1,5-bisphosphate with carbon dioxide to form two
molecules of 3-phosphoglycerate is the actual carbon dioxide fixation. The rest of
Photosynthesis 7
atoms are incorporated into sugars for every six carbon dioxide molecules that
enter the Calvin cycle.
22.6 CO2 Fixation in Tropical Plants
46. In tropical plants, the C4 pathway is operative in addition to the Calvin cycle.
47. In C4 plants, when CO2 enters the leaf through pores in the outer cells, it reacts
first with phosphoenolpyruvate to produce oxaloacetate and Pi in the mesophyll
48. Photorespiration is a pathway in which glycolate is a substrate oxidized by
rubisco acting as an oxygenase, rather than as a carboxylase. Photorespiration
is not completely understood.
49. Three reasons come to mind. (1) Light energy is usually not limiting. (2) The
50. Most plants would be more productive in the absence of photorespiration. There
is another side to this picture, however. The oxygenase activity appears to be an