Jamie Pope, Steven Nizielski, and Alison McCook NUTRITION for a Changing World FIRST EDITION Metabolism Supplement © 2016 by W. H. Freeman and Company & Scientific American

What Is Metabolism? Metabolism is all of the life-sustaining chemical reactions that occur in living organisms that convert one molecule to another molecule.

Many metabolic processes fall into the categories of catabolism and anabolism. Catabolism is the breakdown of large molecules into smaller ones and is generally accompanied by the release of energy. Anabolism is the synthesis of larger molecules from smaller ones, requiring an input of energy that is obtained from catabolic reactions.

Enzymes are biological molecules that speed up a chemical reaction.

Enzymes have organic partners called coenzymes that assist in chemical reactions. Coenzymes are organic molecules that must be present for an enzyme to catalyze a chemical reaction.

In the cell, energy transformation depends on oxidation-reduction reactions. Oxidation and reduction reactions always occur together. Every electron lost by an atom through oxidation is gained by another atom through reduction.

What is energy metabolism? Energy metabolism is the chemical reactions that are involved in storing fuels or breaking them down to produce ATP.

Cells use the energy in ATP molecules to power activity, synthesize compounds, and fuel active transport. ATP contains adenosine (adenine combined with ribose) and three phosphates held together with high-energy bonds. When the endmost phosphate bond breaks, energy is released.

What is cellular respiration? Cellular respiration is the process by which the energy stored in fuels is transferred to ATP by a series of enzyme-catalyzed reactions.

Overview of the Metabolic Pathways in Cellular Respiration The metabolic pathways by which energy is released and used to produce ATP are glycolysis, the citric acid cycle, and the electron transport chain (ETC).

The reactions of glycolysis breaks down glucose to pyruvate. Glycolysis is a critical component of cellular respiration as it is the first phase in the breakdown of glucose. This metabolic pathway ultimately splits glucose into 2 three-carbon pyruvate molecules and yields ATP and reduced coenzymes (NADH). Glycolysis occurs in the cytosol of the cell and does not require oxygen.

The stages of glycolysis include an energy investment stage, followed by an energy payoff phase. 1. Energy Investment Phase. The first phase of glycolysis requires an input of energy from ATP, using enzymes to add two phosphate molecules from ATP to glucose. Energy expenditure: Two phosphates from two molecules of ATP are added to six-carbon glucose, producing a six-carbon molecule with two phosphates. 2. The phosphorylated (phosphate-containing) sugar, now fructose, is cleaved into two, three-carbon molecules, each with one phosphate.

Glycolysis: The Energy Payoff Phase 4. Enzymes add a phosphate to each three-carbon molecule. 5. The three-carbon molecules are then oxidized while two coenzymes are reduced. 6. When 02 is available, the reduced coenzymes transfer the hydrogen atoms with their high-energy electrons into the mitochondria. 7. In two separate enzymatic reactions, all four phosphates are transferred to ADP to form four molecules of ATP.

Glycolysis Summary Each molecule of glucose produces two molecules of pyruvate, two reduced coenzymes, and four ATP. Since two ATP were used during the energy investment phase, glycolysis results in the net production of two ATP.

Cellular Respiration Overview: Glycolysis 1. In the cytosol, glycolysis breaks down glucose to pyruvate. 2. Pyruvate enters the mitochondria, 1. Glycolysis glucose is broken down to two molecules of pyruvate, which produces two ATP and transfers two electrons to coenzymes. 2. Pyruvate is transported into the mitochondria.

Cellular Respiration Overview: Glycolysis 1. In the cytosol, glycolysis breaks down glucose to pyruvate. 2. Pyruvate enters the mitochondria. 1. Glycolysis glucose is broken down to two molecules of pyruvate, which produces two ATP and transfers two electrons to coenzymes. 2. Pyruvate is transported into the mitochondria.

Cellular Respiration: Pyruvate and fatty acids are oxidized to acetyl-CoA. 3. In the mitochondria, pyruvate and fatty acids are broken down to acetyl-CoA. One electron is released for the breakdown of pyruvate. Two electrons are released for every acetyl-CoA formed during breakdown of fatty acids. During this transition step, one CO2 is removed and NAD+ is reduced to NADH (not shown). The remaining two-carbon molecule is transferred to a coenzyme forming acetyl-CoA.

Cellular Respiration: Citric Acid Cycle 4. The two-carbon acetyl portion of acetyl-CoA reacts with a four-carbon molecule that is part of the citric acid cycle, forming the six-carbon molecule citric acid. Coenzyme A is released in the process. Acetyl-CoA enters the citric acid cycle yielding ATP, energized electrons, and two CO2 molecules. 5. Through a series of reactions, the citric acid cycle removes two carbons from citric acid to produce two molecules of CO2 and one ATP, and transfers four electrons to coenzymes.

Cellular Respiration: Electron Transport Chain 6. Coenzymes transfer high-energy electrons to the electron transport chain (ETC). 7. As electrons move down the ETC they lose energy, which is captured and then used to synthesize the vast majority of the ATP that is produce during cellular respiration 8. At the end of the ETC, electrons combine with oxygen and hydrogen to produce water.

Overview of Energy Metabolism

Aerobic and Anaerobic Energy Systems