Photosynthetic organisms harvest the energy of sunlight and use it to power the synthesis of organic compounds from CO2. Various pigments including chlorophylls, bacteriochlorophylls, carotenoids, and phycobilins may be used to capture radiant energy. These pigments are arranged in complexes called photosystems. When reaction-center chlorophyll absorbs the energy of light, a high-energy electron is emitted. This is then passed along an electron transport chain to generate a proton motive force, which is harvested to synthesize ATP. Plants and cyanobacteria use water as a source of electrons for reducing power, generating oxygen. Purple and green bacteria obtain electrons from a reduced compound other than water, and therefore do not evolve oxygen.
■ What is the role of the antenna pigments?
■ What is the advantage of having tandem photosystems?
■ It requires energy to reverse the flow of the electron transport chain. Why would this be so?
Chemolithoautotrophs and photoautotrophs convert carbon dioxide into an organic form, the process of carbon fixation. In photosynthetic organisms, the process occurs in the light-independent reactions. Carbon fixation consumes a great deal of ATP and reducing power, which should not be surprising considering that the reverse process—oxidizing those same compounds to CO2—liberates a great deal of energy. The Calvin cycle is by far the most common pathway used to fix carbon, but some prokaryotes incorporate CO2 using other mechanisms. For exam-
6.9 Carbon Fixation 159
ple, the green sulfur bacteria and some members of the Archaea use a pathway that effectively reverses the steps of the TCA cycle.
The Calvin cycle, or Calvin-Benson cycle, named in honor of the scientists who described much of it, is a complex cycle that can be viewed as having three essential stages—incorporation of CO2 into an organic compound, reduction of the resulting molecule, and regeneration of the starting compound (figure 6.28). Because of the complexities of the cycle, it is easiest to consider the process as consisting of six turns of the cycle. Together, these six "turns" generate a net gain of two molecules of glyceraldehyde 3-phosphate, which can be converted into one molecule of fructose 6-phosphate. The Calvin cycle consists of three stages:
■ Stage 1: Carbon dioxide enters the cycle when the enzyme ribulose bisphosphate carboxylase, commonly called rubisco joins it to a 5-carbon compound, ribulose 1, 5-bisphosphate. The resulting compound spontaneously hydrolyzes to produce two molecules of a 3-carbon compound, 3-phosphoglycerate (3PG). Interestingly, although rubisco is unique to autotrophs, it is thought to be the most abundant enzyme on earth.
■ Stage 2: A sequential input of energy (ATP) and reducing power (NADPH) is used in steps that, together, convert 3PG to glyceraldehyde 3-phosphate (G3P). This compound is identical to the precursor metabolite formed as an intermediate in glycolysis. It can be converted to a number of different compounds used in biosynthesis, oxidized to make other precursor compounds, or converted to a 6-carbon sugar. A critical aspect of the pathway of CO2 fixation, however, stems from the fact that it operates as a cycle—ribulose 1, 5-bisphosphate must be regenerated from G3P for the process to continue. Consequently, in six cycles, a maximum of 2 G3P can be converted to a 6-carbon sugar; the rest is used to regenerate ribulose 1, 5-bisphosphate.
■ Stage 3: Many of the steps that are used to regenerate ribulose 1, 5-bisphosphate involve reactions of the pentose phosphate cycle.
One molecule of the 6-carbon sugar fructose can be generated for every six "turns'' of the cycle. These six "turns" consume 18 ATP and 12 NADPH + H+.
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