What happens during the carbon fixation step of the calvin cycle?

For every three turns of the Calvin cycle, three atoms of carbon are fixed from three molecules of carbon dioxide.

Explanation:

In the carbon fixation reactions that occur in the stroma, NADPH and ATP, produced in the energy capture reactions, are used to reduce a three-carbon compound, glyceraldehyde phosphate. This route in which carbon is fixed by means of glyceraldehyde phosphate is called the three-carbon route or C3. In this case, carbon fixation is carried out through the Calvin cycle, in which the ribulose bisphosphate (RuBP) carboxylase enzyme combines a carbon dioxide molecule with the starting material, a five-carbon sugar called ribulose bisphosphate.

In each complete cycle, enter a molecule of carbon dioxide. The number required to make two glyceraldehyde phosphate molecules, equivalent to a six-carbon sugar, is six turns. Six molecules of RuBP, a compound of five carbons, are combined with six molecules of carbon dioxide, producing six molecules of an unstable intermediate that is soon cleaved into twelve molecules of phosphoglycerate, a compound of three carbons. The latter are reduced to twelve molecules of glyceraldehyde phosphate. Ten of these three-carbon molecules combine and regenerate to form six five-carbon RuBP molecules. The two "extra" molecules of glyceraldehyde phosphate represent the net gain of the Calvin cycle. These molecules are the starting point of numerous reactions that can involve, for example, the synthesis of carbohydrates, amino acids and fatty acids.

The energy that drives the Calvin cycle is the ATP and NADPH produced by the energy capture reactions in the first stage of photosynthesis.

Gliceraldehyde phosphate can also be used as a starting material for other organic compounds necessary for the cell. Other plants that live in dry and warm environments have mechanisms that allow them to initially fix CO2 in one of two ways, and thus minimize water loss. These pathways are known as the four-carbon pathway, or C4 and the CAM plant pathway, and precede the Calvin cycle.

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according to the u.s. department of agriculture, the label of "certified organic" comes with a set of highly regulated standards for environmental stewardship and resource conservation. some of these include maintenance and improvement of soil quality, prevention of ground and surface water contamination, greenhouse gas regulation, encouragement of biodiversity and conservation of water and energy (see references 1). organic practices integrate natural methods such as mulching, plant diversity, native and adapted plants, crop rotation and the introduction of beneficial insects to control pests without synthetic pesticides.

environmental implications of pesticides

synthetic pesticides ensure high crop yields. while effective in achieving this goal, pesticides contaminate soil, air, ground and surface water, as well as the plants and animals that come into contact with these contaminated sources. freshwater and marine organisms are particularly susceptible to pesticides through agricultural runoff. pesticides also reduce soil fertility (necessitating the use of synthetic fertilizers) and can travel far from the original source, contaminating water and air hundreds of miles from the location of application (see references 2).

health implications of organics

although a subject of particular debate, many recent findings show that organic food is healthier than conventionally grown food. according to the usda, an extensive european union-funded study found that organically grown foods contain more beneficial compounds, such as the vitamins and antioxidants needed to combat many diseases (see references 5). organic milk, meat and eggs are regulated to ensure they are free of hormones administered to conventional livestock and poultry.

health implications of pesticides

pesticides affect human health in three ways: through skin contact, inhalation and ingestion. acute and long-term exposure to pesticides can cause serious health problems for farmers who apply them. the majority of other people are affected by drinking contaminated water and eating contaminated marine life (see references 3). pesticides have been linked to neurological disorders, endocrine and reproductive disorders, immune system deficiencies and cancer. according to the world health organization, acute pesticide poisoning (app) is responsible for significant annual mortality, especially in developing nations (see references 4).

other important considerations

not every small farm or business owner can afford the costly certification process required to be considered "certified organic," even if the farm is not using chemicals. "transitional" farms include those that once used chemicals and are now transitioning to organic-only production. in addition, though organic farming methods offer obvious environmental benefit, an organic apple shipped from around the world creates a different set of environmental problems. locally grown food (particularly locally grown, organic food) is key in sustainable food production.