The Light Independent Reactions Reexamined

We have seen how synthesis of ATP and NADPH is set in motion during the light-dependent reactions. Both of these substances play key roles in the synthesis of carbohydrate from carbon dioxide from the atmosphere, which during the light-independent reactions reaches the interior of chlorenchyma tissues via stomata. As indicated earlier, the light-independent reactions are really a whole series of reactions, each mediated by an enzyme in this major phase of photosynthesis.

The light-independent reactions take place in the stroma of the chloroplasts and, as long as the products of the light-dependent reactions are present, they do not directly need light in the same sense that the light-dependent reactions do. However, they normally take place only during daylight hours because some of the enzymes involved in the

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light-independent reactions require light for their activation or conversion to a form in which they can actively catalyze steps of the light-independent reactions.

The Calvin Cycle The heart of the light-independent reactions is the Calvin cycle, during which carbon dioxide is fixed and converted to carbohydrate. The carbohydrate produced during these reactions facilitates growth, including the development of leaves, stems, roots, flowers, and other plant structures. From an ecological standpoint, this process is essential because it is the main biosynthetic pathway through which carbon enters the web of life. As discussed next, the Calvin cycle, or photosynthetic carbon reduction (PCR) pathway (Fig. 10.10), runs in five main steps:

1. Six molecules of carbon dioxide (CO2) from the air combine with six molecules of ribulose 1,5-bisphos-phate (RuBP, the 5-carbon sugar continually being formed while photosynthesis is occurring), with the aid of the enzyme rubisco (RuBP carboxylase/oxygenase).5

2. The resulting six 6-carbon unstable complexes are immediately split into twelve 3-carbon molecules known as 3-phosphoglyceric acid (3PGA), the first stable compound formed in photosynthesis.

3. The NADPH (which has been temporarily holding the hydrogen and electrons released during the light-dependent reactions) and ATP (also from the light-dependent reactions) supply energy and electrons that chemically reduce the 3PGA to twelve molecules of glyceraldehyde 3-phos-phate (GA3P, 3-carbon sugar phosphate).

4. Ten of the twelve glyceraldehyde 3-phosphate molecules are restructured, using another six ATPs, and become six 5-carbon molecules of RuBP, the sugar with which the Calvin cycle was initiated.

5. Rubisco is a huge, complex enzyme that may constitute up to 30% of the protein present in a living leaf. It is the most abundant protein known.

Plant Metabolism

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Life on earth depends on photosynthesis to provide oxygen and absorb carbon dioxide. Photosynthesis is also the foundation through which carbon enters the web of life, directly or indirectly providing food and shelter for most living organisms. Photosynthetic efficiency varies with changes in carbon dioxide concentration, water availability, temperature, and light intensity. Human activities, including the use of fossil fuels, pesticides, and pollutants such as PCBs (polychlorinated biphenyls), disrupt the factors affecting photosynthesis. As a result of human impact, there have been, and continue to be, fluctuations in photosynthetic efficiency within nature, as well as changes in the way natural elements, water, and organic materials are recycled. While some of the activities mentioned may be temporarily beneficial to humans in the name of "progress," the long-term detrimental effects of industrialization, deforestation, and pollution are becoming apparent. The vulnerability of photosynthesis to changes in the environment signifies our need to do as much as possible to preserve the balance of nature on our planet.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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