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Bioc 460 Spring 1999
Lecture 31 - Chapter 26
- Overview
of photosynthesis
- Photosystems I and II
- The Calvin cycle
Overview
of photosynthesis
The present biosystem on this planet exists because of the light activated biochemical process called photosynthesis. Light excitation of Photosystems I and II results in oxygen evolution from the splitting H2O, and the generation of chemical energy in the form of ATP and NADPH. Plants use this chemical energy to convert CO2 into sugars via the Calvin cycle (carbon fixation). Taken together, the net reaction of photosynthesis and carbon fixation is:
H2O + CO2 -----(light)-----> (CH2O) + O2
Figure 26.10
Figure 26.37
I t was discovered over 200 yrs ago that live plants and respiring animals could co-exist in a closed system for a limited period of time as long as H2O and light were provided. It was later shown that chloroplast contain light gathering proteins called chlorophyll that convert light energy into chemical energy in the form of reduction potential. The stroma of chloroplasts functions in the same way as the matrix of mitochondria.
Figure 26.3, Figure 26.2 (read more about closed systems at Biosphere2)
Chlorophyll molecules function as antenna by absorbing light at fixed wavelengths and then pass this energy to reaction centers which are special chlorophyll proteins that convert the light energy to chemical energy via Photosystems I and II.
Figure 26.4, Figure 26.5
Figure 26.7, Figure 26.8
Biochemical experiments using radioactive oxygen (18O) in the form of radioactive H20, showed that the oxygen evolved by photosynthesis comes from the dehydrogenation of H20:
H2O + CO2 -----(light)-----> (CH2O) + O2
Photosystems I and
II
Light energy captured by chlorophyll molecules actually involves the interplay of two reaction centers that are linked together by redox reactions.
Figure 26.15
- Photosystem II (PSII) contains chlorophylls a and b and absorbs light at 680 nm.
Figure 26.11, Figure 26.12
- Photosystem I (PSI) contains mostly chlorophyll a and absorbs light at 700 nm.
Figure 26.17, Figure 26.18
PSII and PSI and linked through a series of redox reactions characterized by proton pumping at the cytochrome bf complex. Note that PSII and PSI work together to provide energy to generate O2 and NADPH. The net reaction of PSII and PSI functioning together is:
2 H2O + 2 NADP ----(light)-----> O2 + 2 NADPH + 2 H
Importantly, in addition to O2 and NADPH production, protons are pumped across the thylakoid membrane. This proton gradient drives ATP synthesis via the ATP synthase reaction.
Figure 26.23
The Calvin cycle
Plants store light energy in the form of carbohydrate, primarily starch and sucrose. The energy for this process comes from the ATP and NADPH made during photosynthesis.
Read a report from Columbia University's Biosphere2 project about CO2 in the rain forests.
The conversion of CO2 to carbohydrate is called the Calvin cycle and is named after Melvin Calvin who discovered it. This cycle requires the enzyme ribulose 1,5-bisphosphate carboxylase commonly called RUBISCO. The carbohydrate hexoses made by the Calvin cycle reactions are used at night as a source of ATP production in mitochondria. The Calvin cycle is sometimes called the Dark Reactions or Dark Cycle. but do not be fooled by this name - the Calvin Cycle is the most active during the daylight hours!
Rubisco catalyzes the formation of two C3 molecules (3-phosphoglycerated; 3-PG) from ribulose (C5) + CO2. Other reactions in the Calvin CYCLE are devoted to regenerating ribulose for another round of carbon fixation. The 3-PG is converted to fructose 6-phosphate by gluconeogenesis inside the chloroplasts.
The net reaction of the Calvin cycle is:
6 CO2 + 18 ATP + 12 NADPH + 12 H2O ----> C6H12O6 + 18 ADP + 18 Pi + 12 NADP + 6 H
Figure 26.31
Figure 26.36
Rubisco is activated by light via thioredoxin which reduces Rubisco (oxidized) to the active form (reduced).
Figure 26.39
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