carbon cycle

Examples of carbon cycle in the following topics:

• The Carbon Cycle

• The Carbon Cycle

  • Carbon enters the atmosphere in the form of carbon dioxide via the carbon cycle and returns to organic carbon via photosynthesis.
  • The carbon cycle is most easily studied as two interconnected sub-cycles: one dealing with rapid carbon exchange among living organisms and the other dealing with the long-term cycling of carbon through geologic processes .
  • The movement of carbon through the land, water, and air is complex and, in many cases, it occurs much more slowly than the biological carbon cycle.
  • Photosynthesis converts carbon dioxide gas to organic carbon, while respiration cycles the organic carbon back into carbon dioxide gas.
  • Volcanic activity and human emissions bring this stored carbon back into the carbon cycle.

• The Chemical Basis for Life

  • On earth, carbon circulates through the land, ocean, and atmosphere, creating what is known as the Carbon Cycle.
  • This global carbon cycle can be divided further into two separate cycles: the geological carbon cycles takes place over millions of years, whereas the biological or physical carbon cycle takes place from days to thousands of years.
  • In a nonliving environment, carbon can exist as carbon dioxide (CO2), carbonate rocks, coal, petroleum, natural gas, and dead organic matter.
  • The fundamental component for all of these macromolecules is carbon.
  • All living things contain carbon in some form, and carbon is the primary component of macromolecules, including proteins, lipids, nucleic acids, and carbohydrates.

• The Role of Prokaryotes in Ecosystems

  • Prokaryotes play vital roles in the movement of carbon dioxide and nitrogen in the carbon and nitrogen cycles.
  • Prokaryotes play an important role in the carbon cycle .
  • Carbon is cycled through earth's major reservoirs: land, the atmosphere, aquatic environments, sediments and rocks, and biomass.
  • In aqueous environments and their anoxic sediments, there is another carbon cycle taking place.
  • In this case, the cycle is based on one-carbon compounds.

• Acetyl CoA to CO2

  • The acetyl carbons of acetyl CoA are released as carbon dioxide in the citric acid cycle.
  • In the citric acid cycle, the two carbons that were originally the acetyl group of acetyl CoA are released as carbon dioxide, one of the major products of cellular respiration, through a series of enzymatic reactions .
  • For each acetyl CoA that enters the citric acid cycle, two carbon dioxide molecules are released in reactions that are coupled with the production of NADH molecules from the reduction of NAD+ molecules.
  • For each molecule of acetyl CoA that enters the citric acid cycle, two carbon dioxide molecules are released, removing the carbons from the acetyl group.
  • Describe the fate of the acetyl CoA carbons in the citric acid cycle

• Citric Acid Cycle

  • The citric acid cycle is a series of reactions that produces two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2.
  • Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons of one glucose molecule.
  • Two carbon dioxide molecules are released on each turn of the cycle; however, these do not necessarily contain the most recently-added carbon atoms.
  • The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide.
  • In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule.

• The Calvin Cycle

  • Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA.
  • Because the G3P exported from the chloroplast has three carbon atoms, it takes three "turns" of the Calvin cycle to fix enough net carbon to export one G3P.
  • 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.

• Biogeochemical Cycles

  • The elemental components of organic matter are cycled through the biosphere in an interconnected process called the biogeochemical cycle.
  • Carbon, found in all organic macromolecules, is an important constituent of fossil fuels.
  • The cycling of all of these elements is interconnected.
  • Furthermore, the ocean itself is a major reservoir for carbon.
  • It is important for leaching certain components of organic matter into rivers, lakes, and oceans, and is a reservoir for carbon.

• Breakdown of Pyruvate

  • After glycolysis, pyruvate is converted into acetyl CoA in order to enter the citric acid cycle.
  • Acetyl CoA is a molecule that is further converted to oxaloacetate, which enters the citric acid cycle (Krebs cycle).
  • (Note: carbon dioxide is one carbon attached to two oxygen atoms and is one of the major end products of cellular respiration. ) The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase; the lost carbon dioxide is the first of the six carbons from the original glucose molecule to be removed.
  • This molecule of acetyl CoA is then further converted to be used in the next pathway of metabolism, the citric acid cycle.
  • Each pyruvate molecule loses a carboxylic group in the form of carbon dioxide.

• Connecting Proteins to Glucose Metabolism

  • Each amino acid must have its amino group removed (deamination) prior to the carbon chain's entry into these pathways.
  • The remaining atoms of the amino acid result in a keto acid: a carbon chain with one ketone and one carboxylic acid group.
  • In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule.
  • The keto acid can then enter the citric acid cycle.
  • The carbon skeletons of certain amino acids (indicated in boxes) are derived from proteins and can feed into pyruvate, acetyl CoA, and the citric acid cycle.