Step 7. Water is added to fumarate during step seven, and malate is produced. The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate. Another molecule of NADH is produced. 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. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules. Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic both catabolic and anabolic. Privacy Policy. Skip to main content.
Cellular Respiration. Search for:. Oxidation of Pyruvate and the Citric Acid Cycle. Breakdown of Pyruvate After glycolysis, pyruvate is converted into acetyl CoA in order to enter the citric acid cycle. Learning Objectives Explain why cells break down pyruvate. Key Takeaways Key Points In the conversion of pyruvate to acetyl CoA, each pyruvate molecule loses one carbon atom with the release of carbon dioxide. In the final step of the breakdown of pyruvate, an acetyl group is transferred to Coenzyme A to produce acetyl CoA.
Key Terms acetyl CoA : a molecule that conveys the carbon atoms from glycolysis pyruvate to the citric acid cycle to be oxidized for energy production. Learning Objectives Describe the fate of the acetyl CoA carbons in the citric acid cycle. Acetyl CoA transfers its acetyl group to oxaloacetate to form citrate and begin the citric acid cycle. Key Terms TCA cycle : an alternative name for the Krebs cycle or citric acid cycle Krebs cycle : a series of enzymatic reactions that occurs in all aerobic organisms; it involves the oxidative metabolism of acetyl units and serves as the main source of cellular energy oxaloacetate : a four carbon molecule that receives an acetyl group from acetyl CoA to form citrate, which enters the citric acid cycle.
Learning Objectives List the steps of the Krebs or citric acid cycle. Key Takeaways Key Points The four-carbon molecule, oxaloacetate, that began the cycle is regenerated after the eight steps of the citric acid cycle. The eight steps of the citric acid cycle are a series of redox, dehydration, hydration, and decarboxylation reactions.
Licenses and Attributions. Read more about the source of our oxygenic atmosphere in Dismukes GC et al. USA ]. The Krebs cycle is the first pathway of oxygenic respiration.
Later, natural selection fleshed out the aerobic Krebs cycle, electron transport and oxidative phosphorylation pathways we see today.
As a pathway for getting energy out of nutrients, respiration is much more efficient than glycolysis. Animals rely on it, but even plants and photosynthetic algae use the respiratory pathway when sunlight is not available! Here we focus on oxidative reactions in mitochondria, beginning with pyruvate oxidation and continuing to the redox reactions of the Krebs cycle.
The free energy released in these redox reactions is coupled to the synthesis of only one ATP per pyruvate oxidized i. These entry of pyruvate into the mitochondrion and its oxidation are summarized below. Pyruvate oxidation converts a 3C carbohydrate into acetate, a 2C molecule, releasing a molecule of CO2.
Intermediates of the Krebs cycle also function in amino acid metabolism and interconversions. The removal of an electron from a molecule, oxidizing it, results in a decrease in potential energy in the oxidized compound. The electron sometimes as part of a hydrogen atom does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound.
The shift of an electron from one compound to another removes some potential energy from the first compound the oxidized compound and increases the potential energy of the second compound the reduced compound. The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons.
The transfer of energy in the form of electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food.
You will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.
In living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy electrons between compounds in pathways.
The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced that is, they accept electrons or oxidized they lose electrons.
Nicotinamide adenine dinucleotide NAD Figure 4. When electrons are added to a compound, they are reduced. A compound that reduces another is called a reducing agent. When electrons are removed from compound, it is oxidized. A compound that oxidizes another is called an oxidizing agent.
Its reduced form is FADH 2. A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store the energy safely and release it for use only as needed.
Living cells accomplish this by using the compound adenosine triphosphate ATP. It functions similarly to a rechargeable battery. When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The cell uses the energy to do work, usually by the released phosphate binding to another molecule, activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins.
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