Follow a meal from ingestion to absorbtion, focusing on what happens to the 4 major macromolecules. Learn about the sources of sugar in our diets, their chemical similarities and differences, and their effects on our cells and our bodies. Hormones help us balance periodic food intake with varying energy needs.

See how hormones regulate metabolic pathways during fasting, feeding, exercise, starvation, and more. Travel inside the body to see how regulation goes wrong in people with diabetes, and how various treatments work to correct the problem.

Once nutrients arrive in the blood stream, the body finds a way to use them. Molecules from our food can be burned for energy, stored for later, or used to build and maintain the body. These pages look at the chemistry of our food.

The nutrients from our food can be turned into cellular structures, burned for energy, or stored away for later. View metabolism by cell type, where you can compare the metabolic activities of muscle, fat, liver, and other cells.

Or view by nutrient, where you can see how our cells process carbohydrates, fats, and proteins. But what does this all mean to your body and the physiological processes it carries out each day?

You need to absorb a range of nutrients so that your cells have the building blocks for metabolic processes that release the energy for the cells to carry out their daily jobs, to manufacture new proteins, cells, and body parts, and to recycle materials in the cell.

This chapter will take you through some of the chemical reactions essential to life, the sum of which is referred to as metabolism. The focus of these discussions will be anabolic reactions and catabolic reactions.

Metabolism varies, depending on age, gender, activity level, fuel consumption, and lean body mass. Your own metabolic rate fluctuates throughout life.

By modifying your diet and exercise regimen, you can increase both lean body mass and metabolic rate. Factors affecting metabolism also play important roles in controlling muscle mass. The TCA cycle is also known as the Krebs cycle, named after its discoverer, Sir Hans Kreb. Krebs based his conception of this cycle on four main observations made in the s.

The first was the discovery in of the sequence of reactions from succinate to fumarate to malate to oxaloacetate by Albert Szent-Gyorgyi, who showed that these dicarboxylic acids present in animal tissues stimulate O 2 consumption. The second was the finding of the sequence from citrate to α-ketoglutarate to succinate, in , by Carl Martius and Franz Knoop.

Next was the observation by Krebs himself, working on muscle slice cultures, that the addition of tricarboxylic acids even in very low concentrations promoted the oxidation of a much higher amount of pyruvate, suggesting a catalytic effect of these compounds.

And the fourth was Krebs's observation that malonate, an inhibitor of succinate dehydrogenase, completely stopped the oxidation of pyruvate by the addition of tricarboxylic acids and that the addition of oxaloacetate in the medium in this condition generated citrate, which accumulated, thus elegantly showing the cyclic nature of the pathway.

When 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate, substrate-level phosphorylation occurs and ATP is produced from ADP. Then, 3-phosphoglycerate undergoes two reactions to yield phosphoenolpyruvate.

Next, phosphoenolpyruvate is converted to pyruvate, which is the final product of glycolysis. During this reaction, substrate-level phosphorylation occurs and a phosphate is transferred to ADP to form ATP. Interestingly, during the initial phase, energy is consumed because two ATP molecules are used up to activate glucose and fructosephosphate.

Part of the energy derived from the breakdown of the phosphoanhydride bond of ATP is conserved in the formation of phosphate-ester bonds in glucosephosphate and fructose-1,6-biphosphate Figure 4.

In the second part of glycolysis, the majority of the free energy obtained from the oxidation of the aldehyde group of glyceraldehyde 3-phosphate G3P is conserved in the acyl-phosphate group of 1,3- bisphosphoglycerate 1,3-BPG , which contains high free energy.

Then, part of the potential energy of 1,3BPG, released during its conversion to 3-phosphoglycerate, is coupled to the phosphorylation of ADP to ATP. The second reaction where ATP synthesis occurs is the conversion of phosphoenolpyruvate PEP to pyruvate.

PEP is a high-energy compound due to its phosphate-ester bond, and therefore the conversion reaction of PEP to pyruvate is coupled with ADP phosphorylation. This mechanism of ATP synthesis is called substrate-level phosphorylation.

For complete oxidation, pyruvate molecules generated in glycolysis are transported to the mitochondrial matrix to be converted into acetyl-CoA in a reaction catalyzed by the multienzyme complex pyruvate dehydrogenase Figure 5.

When Krebs proposed the TCA cycle in , he thought that citrate was synthesized from oxaloacetate and pyruvate or a derivative of it. Only after Lipmann's discovery of coenzyme A in and the subsequent work of R. Stern, S. Ochoa, and F. Lynen did it become clear that the molecule acetyl-CoA donated its acetyl group to oxaloacetate.

Until this time, the TCA cycle was seen as a pathway to carbohydrate oxidation only. Most high school textbooks reflect this period of biochemistry knowledge and do not emphasize how the lipid and amino acid degradation pathways converge on the TCA cycle.

The cell is depicted as a large blue oval. A smaller dark blue oval contained inside the cell represents the mitochondrion. The mitochondrion has an outer mitochondrial membrane and within this membrane is a folded inner mitochondrial membrane that surrounds the mitochondrial matrix.

The entry point for glucose is glycolysis, which occurs in the cytoplasm. Glycolysis converts glucose to pyruvate and synthesizes ATP. Pyruvate is transported from the cytoplasm into the mitochondrial matrix.

Pyruvate is converted to acetyl-CoA, which enters the tricarboxylic acid TCA cycle. In the TCA cycle, acetyl-CoA reacts with oxaloacetate and is converted to citrate, which is then converted to isocitrate.

Isocitrate is then converted to alpha-ketoglutarate with the release of CO 2. Then, alpha-ketoglutarate is converted to succinyl-CoA with the release of CO 2. Succinyl-CoA is converted to succinate, which is converted to fumarate, and then to malate.

Malate is converted to oxaloacetate. Then, the oxaloacetate can react with another acetyl-CoA molecule and begin the TCA cycle again. In the TCA cycle, electrons are transferred to NADH and FADH 2 and transported to the electron transport chain ETC.

The ETC is represented by a yellow rectangle along the inner mitochondrial membrane. The ETC results in the synthesis of ATP from ADP and inorganic phosphate P i. Fatty acids are transported from the cytoplasm to the mitochondrial matrix, where they are converted to acyl-CoA. Acyl-CoA is then converted to acetyl-CoA in beta-oxidation reactions that release electrons that are carried by NADH and FADH 2.

These electrons are transported to the electron transport chain ETC where ATP is synthesized. Amino acids are transported from the cytoplasm to the mitochondrial matrix. Then, the amino acids are broken down in transamination and deamination reactions. The products of these reactions include: pyruvate, acetyl-CoA, oxaloacetate, fumarate, alpha-ketoglutarate, and succinyl-CoA, which enter at specific points during the TCA cycle.

This pathway is known as β-oxidation because the β-carbon atom is oxidized prior to when the bond between carbons β and α is cleaved Figure 6. The four steps of β-oxidation are continuously repeated until the acyl-CoA is entirely oxidized to acetyl-CoA, which then enters the TCA cycle.

In the s, a series of experiments verified that the carbon atoms of fatty acids were the same ones that appeared in the acids of TCA cycle. Holmes, F. Lavoisier and the Chemistry of Life. Madison: University of Wisconsin Press, Krebs, H. Nobel Prize Lecture org, Kresge, N. ATP synthesis and the binding change mechanism: The work of Paul D.

Journal of Biological Chemistry , e18 Lusk, G. The Elements of the Science of Nutrition , 4th ed. Philadelphia: W. Saunders, Luz, M. Glucose as the sole metabolic fuel: A study on the possible influence of teachers' knowledge on the establishment of a misconception among Brazilian high school stucents.

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