ATP_synthase.html: 09_14ATPSynthase.jpg
ATP synthase, a molecular mill. The ATP synthase protein complex functions as a mill, powered by the flow of hydrogen ions. Each of the four parts of ATP synthase consists of a number of polypeptide subunits.

NAD.html: 09_04NADElectronShuttle.jpg
Nicotinamide Adenine Dinucleotide Two electrons and one proton (H⁺) from an organic molecule such as glucose is transferred to NAD⁺ which is reduced the to NADH. The NADH stores energy that can be tapped to make ATP when the electrons complete their “fall” down an energy gradient to oxygen.

aerobic.html: 09_15Chemiosmosis.jpg
The 4th protein complex (cytochrome c oxidase) in the ECT requires oxygen. Poisons such as cyanide and carbon monoxide bind to this protein, disrupting oxidative phosphorylation.

anaerobic.html: 09_18PyruvateInCatabolism_L.jpg
Fermentation. In the absence of oxygen, some cells undergo fermentation instead of aerobic respiration. Pyruvate serves as an electron acceptor for oxidizing NADH back to NAD⁺, which is reused in glycolysis. Two end products formed from fermentation are ethanol with the release of CO₂ and lactic acid. Note that fermentation. itself yields no ATP molecules; it serves to recycle NADH back to NAD⁺.

catabolic.html: 09_19Catabolism.jpg
The catabolism of various molecules from food. Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Monomers of these molecules enter glycolysis or the citric acid cycle at various points.

citric_acid_cycle.html: 09_11CitAcidCycleOverview.jpg
In the citric acid cycle, carbon dioxide (CO₂) is released as pyruvate is degraded. Each pyruvate yields one additional ATP and three molecules of NADH and one FADH₂ per pyruvate.

controlled_energy.html: 09_05ElectronTransChains_L.jpg

The uncontrolled exergonic reaction of hydrogen with oxygen to form water releases a large amount of energy: an explosion.		In cellular respiration, an electron transport chain breaks the “fall” of electrons into a series of smaller steps and stores some of the energy to make ATP, the rest of the energy is released as heat.

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At the ecosystem level, energy from sunlight is stored in organic compounds by producers through photosynthesis. The organic compounds are used by consumers to release energy for work in a process called cellular respiration. Byproducts of respiration are used as raw material for photosynthesis. Heat is released into the environment in this cycle.

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Energy flows into an ecosystem as sunlight and ultimately leaves as heat, while the chemical elements essential to life are recycled.

energy_cycling.html: 09_02EcosystemRecycling.jpg
At the cell level, energy from sunlight is stored in organic compounds by photosynthesis in chloroplasts. A byproduct of this process is oxygen. The organic compounds and oxygen are used by mitochondria to produce ATP for work in a process called cellular respiration. Byproducts of respiration are water and carbon dioxide, which are used as raw material for photosynthesis. Heat is released into the environment in this cycle.

etc.html: 09_15Chemiosmosis.jpg
As high-energy electrons move along a series of protein complexes, they pump hydrogen ions (protons) across the membrane, producing a gradient of H+. The protons diffuse back (chemiosmosis) through the enzyme ATP synthase to form ATP. This process requires oxygen (the final electron acceptor) and releases water.

fermentation_alcohol.html: 09_17FermentationA.jpg
Ethanol fermentation releases CO₂ in addition to producing ethanol. Only 2 molecules of ATP from glycolysis are harvested, the NADH is recycled back to NAD⁺.

fermentation_lactate.html: 09_17FermentationB.jpg
Lactic acid fermentation produces lactic acid, which can be used in dairy products. Only 2 molecules of ATP from glycolysis are harvested, the NADH is recycled back to NAD⁺.

glycolysis.html: 09_08GlycolysisEnergy.jpg
Glycolysis ("splitting of sugar") occurs in the cytosol and breaks down one glucose molecule into two molecules of pyruvate, yielding a net gain of 2 ATP amd 2 NADH molecules.

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In the energy investment phase of glycolysis, two ATP molecules are consumed to phosphorylate glucose.

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In the energy payoff phase of glycolysis, four ATP molecules are recovered, for a net gain of two ATP molecules.

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Methane combustion releases energy because the electrons lose potential energy when they end up closer to electronegative atoms such as oxygen.

oxidative.html: 09_16ATPYieldPerGlucose.jpg
ATP yield per molecule of glucose at each stage of cellular respiration.
Since oxygen is required to complete the citric acid cycle and oxidative phosphorylation, these two processes are known as aerobic respiration.

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phosphorylation.html: 09_07Phosphorylation_L.jpg
Substrate-level phosphorylation. Some ATP is made by direct enzymatic transfer of a phosphate group from an organic substrate to ADP.

pyruvate.html: 09_10PyruvateToAcetylCoA_L.jpg
Several enzymes catalyze the conversion of 3-carbon pyruvate to 2-carbon acetyl CoA. The acetyl CoA enters the citric acid cycle, while CO₂ diffuses out of the cell.

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In a redox reaction, one substance loses electrons (oxidation) and energy while another gains electrons (reduction) and energy.

respiration.html: 09_06CellRespiration.jpg
An overview of cellular respiration. During glycolysis, a glucose molecule is broken into 2 molecules of pyruvate in the cytosol. The pyruvate enters the citric acid cycle in the mitochondrion; these 2 steps produce a few molecules of ATP by substrate-level phosphorylation NADH and FADH2 transfer electrons to the ETC, where oxidative phosphorylation produces much more ATP by chemiosmosis.