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Energy in Living Systems
Cellular respiration is the process of transforming chemical energy into forms usable by the cell or organism.
Electrons and Energy
The transfer of electrons between molecules via oxidation and reduction allows the cell to transfer and use energy for cellular functions.
ATP in Living Systems
ATP, produced by glucose catabolized during cellular respiration, serves as the universal energy currency for all living organisms.
Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism.
First Half of Glycolysis (Energy-Requiring Steps)
In the first half of glycolysis, energy in the form of two ATP molecules is required to transform glucose into two three-carbon molecules.
Second Half of Glycolysis (Energy-Releasing Steps)
In the second half of glycolysis, energy is released in the form of 4 ATP molecules and 2 NADH molecules.
Outcomes of Glycolysis
One glucose molecule produces four ATP, two NADH, and two pyruvate molecules during glycolysis.
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.
Acetyl CoA to CO2
The acetyl carbons of acetyl CoA are released as carbon dioxide 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.
Electron Transport Chain
The electron transport chain utilizes the reduction of molecular oxygen by moving protons across the mitochondrial membrane to produce ATP.
Chemiosmosis uses the energy of the electrochemical proton gradient across the mitochondrial membrane to perform oxidative phosphorylation.
The amount of energy (as ATP) gained from glucose catabolism varies across species and depends on other related cellular processes.
Metabolism without Oxygen
Anaerobic Cellular Respiration
Some prokaryotes use anaerobic respiration in which they can create energy for use in the absence of oxygen.
Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
Connections of Other Sugars to Glucose Metabolism
Sugars, such as galactose, fructose, and glycogen, are catabolized into new products in order to enter the glycolytic pathway.
Connections of Proteins to Glucose Metabolism
Excess amino acids are converted into molecules that can enter the pathways of glucose catabolism.
Connections of Lipid and Glucose Metabolisms
Lipids can be both made and broken down through parts of the glucose catabolism pathways.
Regulation of Cellular Respiration
Cellular respiration can be controlled at each stage of glucose metabolism through various regulatory mechanisms.
Control of Catabolic Pathways
Catabolic pathways are controlled by enzymes, proteins, electron carriers, and pumps that ensure that the remaining reactions can proceed.