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Dark Reaction:

PhotosynthesisWinfred Louie DC.

VillalunaII-AristotleScience II

Dark Reaction In Photosynthesis

Calvin Cycle

Advantages of C4 plants

Photorespiration

Dark Reaction in Photosynthesis:Dark reactions don't require light, but they aren't inhibited by it, either. For most

plants, the dark reactions take place during daytime. The dark reaction occurs in the stroma of the chloroplast. This reaction is called carbon fixation or the Calvin cycle. In this reaction, carbon dioxide is converted to sugar using ATP and NADPH. Carbon dioxide is combined with a 5-carbon sugar to form a 6-carbon sugar. The 6-carbon sugar is broken into two sugar molecules, glucose and fructose, which can be used to make sucrose. The reaction requires 72 photons of light.

The efficiency of photosynthesis is limited by environmental factors, including light, water, and carbon dioxide. In hot or dry weather, plants may close their stomata to conserve water. When the stomata are closed, the plants may start photorespiration. Plants called C4 plants maintain high levels of carbon dioxide inside cells that make glucose, to help avoid photorespiration. C4 plants produce carbohydrates more efficiently than normal C3 plants, provided the carbon dioxide is limiting and sufficient light is available to support the reaction. In moderate temperatures, too much of an energy burden is placed on the plants to make the C4 strategy worthwhile (named 3 and 4 because of the number of carbons in the intermediate reaction). C4 plants thrive in hot, dry climates.

PhotorespirationAs its name suggests, RUBISCO catalyzes two different reactions:

• adding CO2 to ribulose bisphosphate — the carboxylase activity

• adding O2 to ribulose bisphosphate — the oxygenase activity.

Which one predominates depends on the relative concentrations of O2 and CO2 with

• high CO2, low O2 favoring the carboxylase action,

• high O2, low CO2 favoring the oxygenase action.

The light reactions of photosynthesis liberate oxygen and more oxygen dissolves in the cytosol of the cell at higher temperatures. Therefore, high light intensities and high temperatures (above ~ 30°C)favor the second reaction.

Calvin CycleThe Calvin cycle or Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle

or C3 cycle or CBB cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is also known (erroneously) as the "dark reaction" or "dark stage.“

The cycle was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14. It is one of the light-independent (dark) reactions, used for carbon fixation.

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. The light-independent Calvin cycle uses the energy from short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds[2] that can be used by the organism (and by animals that feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following biochemical equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.

The enzymes in the Calvin cycle are functionally equivalent to many enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway, but they are to be found in the chloroplast stroma instead of the cell cytoplasm, separating the reactions. They are activated in the light (which is why the name "dark reaction" is misleading), and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity.

Calvin CycleThe series of reactions in the Calvin Cycle comprise the third major step in

photosynthesis. This step involves the storage of chemical energy in glucose. In this step, CO2 combines with hydrogen in NADPH, with ATP and NADPH supplying the needed energy. The end product is glucose.

It is important to remember that glucose has much more energy than the raw materials, CO2 and water. This is because energy from sunlight has been eventually stored in the glucose molecules. The reactions in the Calvin Cycle complete the series of changes that light energy undergoes in photosynthesis.

The Calvin Cycle

Advantages of C4 plantsOver 8000 species of angiosperms have developed adaptations which minimize the

losses to photorespiration.

They all use a supplementary method of CO2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called C4 plants. (Plants that have only the Calvin cycle are thus C3 plants.)

• Some C4 plants — called CAM plants — separate their C3 and C4 cycles by time. CAM plants are discussed below.

• Other C4 plants have structural changes in their leaf anatomy so that • their C4 and C3 pathways are separated in different parts of the leaf with • RUBISCO sequestered where the CO2 level is high; the O2 level low.

These adaptations are described now...

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