Which catalyzes the fixation of carbon dioxide in photosynthesis

At this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed.

Three more molecules of ATP are used in these regeneration reactions. Learning Objectives Describe the Calvin Cycle. Key Points The Calvin cycle refers to the light-independent reactions in photosynthesis that take place in three key steps. Although the Calvin Cycle is not directly dependent on light, it is indirectly dependent on light since the necessary energy carriers ATP and NADPH are products of light-dependent reactions.

In fixation, the first stage of the Calvin cycle, light-independent reactions are initiated; CO 2 is fixed from an inorganic to an organic molecule. Key Terms light-independent reaction : chemical reactions during photosynthesis that convert carbon dioxide and other compounds into glucose, taking place in the stroma rubisco : ribulose bisphosphate carboxylase a plant enzyme which catalyzes the fixing of atmospheric carbon dioxide during photosynthesis by catalyzing the reaction between carbon dioxide and RuBP ribulose bisphosphate : an organic substance that is involved in photosynthesis, reacts with carbon dioxide to form 3-PGA.

In one form, a more efficient use of CO 2 allows plants to photosynthesize even when CO 2 is in short supply, as when the stomata are closed on hot days. The other adaptation performs preliminary reactions of the Calvin cycle at night, because opening the stomata at this time conserves water due to cooler temperatures.

In addition, this adaptation has allowed plants to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods. The two parts of photosynthesis—the light-dependent reactions and the Calvin cycle—have been described, as they take place in chloroplasts.

However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles. Prokaryotic photosynthetic autotrophic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis Figure 5. It is here that organisms like cyanobacteria can carry out photosynthesis. Living things access energy by breaking down carbohydrate molecules.

However, if plants make carbohydrate molecules, why would they need to break them down? Carbohydrates are storage molecules for energy in all living things.

Although energy can be stored in molecules like ATP, carbohydrates are much more stable and efficient reservoirs for chemical energy. Photosynthetic organisms also carry out the reactions of respiration to harvest the energy that they have stored in carbohydrates, for example, plants have mitochondria in addition to chloroplasts. Photosynthesis produces oxygen as a byproduct, and respiration produces carbon dioxide as a byproduct.

In nature, there is no such thing as waste. Every single atom of matter is conserved, recycling indefinitely. Substances change form or move from one type of molecule to another, but never disappear Figure 5.

CO 2 is no more a form of waste produced by respiration than oxygen is a waste product of photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to break down carbohydrates. Both organelles use electron transport chains to generate the energy necessary to drive other reactions. Sixteen thousand species of plants use CAM.

Vascular bundles shown. CAM concentrates it temporally, providing CO 2 during the day and not at night, when respiration is the dominant reaction. Cross section of a C4 plant, specifically of a maize leaf.

C 4 plants can produce more sugar than C 3 plants in conditions of high light and temperature. Many important crop plants are C 4 plants, including maize, sorghum, sugarcane, and millet.

Plants that do not use PEP-carboxylase in carbon fixation are called C3 plants because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. In plants, carbon dioxide CO 2 enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO 2 diffuses into the stroma of the chloroplast, the site of light-independent reactions of photosynthesis.

These reactions actually have several names associated with them. Other names for light-independent reactions include the Calvin cycle, the Calvin-Benson cycle, and dark reactions. The most outdated name is dark reactions, which can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it. These energy-carrying molecules are made in the stroma where the Calvin cycle takes place.

The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO 2 ,two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase RuBisCO and three molecules of ribulose bisphosphate RuBP.

RuBP has five atoms of carbon, flanked by two phosphates. The Calvin Cycle : The Calvin cycle has three stages. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule. This is a reduction reaction because it involves the gain of electrons by 3-PGA.

Recall that a reduction is the gain of an electron by an atom or molecule. Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized. At this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed.

Three more molecules of ATP are used in these regeneration reactions.