Once in the bloodstream, different cells can metabolize these nutrients. We have long known that these three classes of molecules are fuel sources for human metabolism , yet it is a common misconception especially among undergraduates that human cells use only glucose as a source of energy. This misinformation may arise from the way most textbooks explain energy metabolism, emphasizing glycolysis the metabolic pathway for glucose degradation and omitting fatty acid or amino acid oxidation.
Here we discuss how the three nutrients carbohydrates, proteins, and lipids are metabolized in human cells in a way that may help avoid this oversimplified view of the metabolism. Figure 1 During the eighteenth century, the initial studies, developed by Joseph Black, Joseph Priestley, Carl Wilhelm Scheele, and Antoine Lavoisier, played a special role in identifying two gases, oxygen and carbon dioxide, that are central to energy metabolism.
Lavoisier, the French nobleman who owns the title of "father of modern chemistry," characterized the composition of the air we breathe and conducted the first experiments on energy conservation and transformation in the organism. One of Lavoisier's main questions at this time was: How does oxygen's role in combustion relate to the process of respiration in living organisms?
Using a calorimeter to make quantitative measurements with guinea pigs and later on with himself and his assistant, he demonstrated that respiration is a slow form of combustion Figure 1. Based on the concept that oxygen burned the carbon in food, Lavoisier showed that the exhaled air contained carbon dioxide, which was formed from the reaction between oxygen present in the air and organic molecules inside the organism. Lavoisier also observed that heat is continually produced by the body during respiration.
It was then, in the middle of the nineteenth century, that Justus Liebig conducted animal studies and recognized that proteins, carbohydrates, and fats were oxidized in the body. Voit demonstrated that oxygen consumption is the result of cellular metabolism, while Rubner measured the major energy value of certain foods in order to calculate the caloric values that are still used today.
Rubner's observations proved that, for a resting animal, heat production was equivalent to heat elimination, confirming that the law of conservation of energy, implied in Lavoisier's early experiments, was applicable to living organisms as well. Therefore, what makes life possible is the transformation of the potential chemical energy of fuel molecules through a series of reactions within a cell, enabled by oxygen, into other forms of chemical energy, motion energy, kinetic energy, and thermal energy.
Energy metabolism is the general process by which living cells acquire and use the energy needed to stay alive, to grow, and to reproduce. How is the energy released while breaking the chemical bonds of nutrient molecules captured for other uses by the cells? The answer lies in the coupling between the oxidation of nutrients and the synthesis of high-energy compounds, particularly ATP , which works as the main chemical energy carrier in all cells.
There are two mechanisms of ATP synthesis: 1. The latter occurs in both the mitochondrion, during the tricarboxylic acid TCA cycle, and in the cytoplasm , during glycolysis. In the next section, we focus on oxidative phosphorylation, the main mechanism of ATP synthesis in most of human cells.
Later we comment on the metabolic pathways in which the three classes of nutrient molecules are degraded. B Scheme of the protein complexes that form the ETS, showing the mitochondrial membranes in blue and red; NADH dehydrogenase in light green; succinate dehydrogenase in dark green; the complex formed by acyl-CoA dehydrogenase, electron transfer flavoprotein ETFP , and ETFP-ubiquinone oxidoreductase in yellow and orange; ubiquinone in green labeled with a Q; cytochrome c reductase in light blue; cytochrome c in dark blue labeled with cytC; cytochrome c oxidase in pink; and the ATP synthase complex in lilac.
On the left is an electron micrograph showing three oval-shaped mitochondria. Each mitochondrion has a dark outer mitochondrial membrane and a highly folded inner mitochondrial membrane.
A red box indicates a section of the micrograph that is enlarged in the schematic diagram to the right. The schematic diagram illustrates the electron transport chain. Two horizontal, mitochondrial membranes are depicted.
The upper membrane is the outer mitochondrial membrane, and the lower membrane is the inner mitochondrial membrane. The area between the two membranes is the intermembrane space, and the area below the lower membrane is the mitochondrial matrix. Each of these membranes is made up of two horizontal rows of phospholipids, representing a phospholipid bilayer.
Each phospholipid molecule has a blue circular head and two red tails, and the tails face each other within the membrane. A series of protein complexes are positioned along the inner mitochondrial membrane, represented by colored shapes. The proteins that make up the electron transport chain start on the left and continue to the right.
At the far left, NADH dehydrogenase is represented by a light green rectangular structure that spans the membrane. Next, succinate dehydrogenase is represented by a dark green bi-lobed shape embedded in the half of the inner membrane and facing the matrix. Next, acyl-CoA dehydrogenase, electron transfer flavoprotein ETFP , and ETFP-ubiquinone oxidoreductase form a complex, and are represented by three yellow and orange ovals on the matrix-facing side of the inner membrane.
Next, ubiquinone is represented by a lime green circle labeled with a Q located in the side of the inner membrane facing the intermembrane space. Next, cytochrome c reductase is represented by a light blue oval-shaped structure that spans the membrane. Next, cytochrome c oxidase is represented by a pink oval-shaped structure that spans the inner membrane.
Next, the ATP synthase complex is represented by an upside-down lollipop-shaped structure that traverses the inner membrane and contains a channel through the membrane; the round, purple head enters the mitochondrial matrix, and the lilac-colored stem spans the membrane. These electrons are transferred to ubiquinone. Succinate dehydrogenase converts succinate to fumarate and transfers additional electrons to ubiquinone via flavin adenine dinucleotide FAD.
During this reaction, additional electrons are transferred to ubiquinone by the FAD domain in this protein complex. Next, the electrons are transferred by ubiquinone to cytochrome c reductase, which pumps protons into the intermembrane space.
The electrons are then carried to cytochrome c. Next, cytochrome c transfers the electrons to cytochrome c oxidase, which reduces oxygen O 2 with the electrons to form water H 2 O. During this reaction, additional protons are transferred to the intermembrane space. The electrons are "transported" through a number of protein complexes located in the inner mitochondrial membrane, which contains attached chemical groups flavins, iron-sulfur groups, heme, and cooper ions capable of accepting or donating one or more electrons Figure 2.
These protein complexes, known as the electron transfer system ETS , allow distribution of the free energy between the reduced coenzymes and the O 2 and more efficient energy conservation. Electron transport between the complexes occurs through other mobile electron carriers, ubiquinone and cytochrome c. FAD is linked to the enzyme succinate dehydrogenase of the TCA cycle and another enzyme, acyl-CoA dehydrogenase of the fatty acid oxidation pathway. These observations led Peter Mitchell, in , to propose his revolutionary chemiosmotic hypothesis.
The reaction catalyzed by succinyl-CoA synthetase in which GTP synthesis occurs is an example of substrate-level phosphorylation. Acetyl-CoA enters the tricarboxylic acid cycle at the top of the diagram and reacts with oxaloacetate and water H 2 O to form a molecule of citrate and CoA-SH in a reaction catalyzed by citrate synthase.
Next, the enzyme aconitase catalyzes the isomerization of citrate to isocitrate. Fumarate combines with H 2 O in a reaction catalyzed by fumerase to form malate. Then, oxaloacetate can react with a new molecule of acetyl-CoA and begin the tricarboxylic acid cycle again.
The diagram shows the molecular structures for citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate. The enzymes that act at each of the eight steps in the cycle are shown in yellow rectangles.
In aerobic respiration or aerobiosis, all products of nutrients' degradation converge to a central pathway in the metabolism, the TCA cycle. In this pathway, the acetyl group of acetyl-CoA resulting from the catabolism of glucose, fatty acids, and some amino acids is completely oxidized to CO 2 with concomitant reduction of electron transporting coenzymes NADH and FADH 2.
Consisting of eight reactions, the cycle starts with condensing acetyl-CoA and oxaloacetate to generate citrate Figure 3. In this case, the hydrolysis of the thioester bond of succinyl-CoA with concomitant enzyme phosphorylation is coupled to the transfer of an enzyme-bound phosphate group to GDP or ADP. Also noteworthy is that TCA cycle intermediates may also be used as the precursors of different biosynthetic processes.
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. 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. The excess sugars in the body are converted into glycogen and stored in the liver and muscles for later use.
Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Excess glycogen can be converted to fats, which are stored in the lower layer of the skin of mammals for insulation and energy storage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility.
Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function.
The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must be excreted as it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories.
Fats are required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones. While the animal body can synthesize many of the molecules required for function from the organic precursors, there are some nutrients that need to be consumed from food. These nutrients are termed essential nutrients , meaning they must be eaten, and the body cannot produce them.
The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids. Vitamins are another class of essential organic molecules that are required in small quantities for many enzymes to function and, for this reason, are considered to be co-enzymes.
Absence or low levels of vitamins can have a dramatic effect on health, as outlined in Table Both fat-soluble and water-soluble vitamins must be obtained from food. Minerals , listed in Table Among their many functions, minerals help in structure and regulation and are considered co-factors. Certain amino acids also must be procured from food and cannot be synthesized by the body.
The human body can synthesize only 11 of the 20 required amino acids; the rest must be obtained from food. The essential amino acids are listed in Table Animals need food to obtain energy and maintain homeostasis.
Homeostasis is the ability of a system to maintain a stable internal environment even in the face of external changes to the environment. Humans maintain this temperature even when the external temperature is hot or cold.
It takes energy to maintain this body temperature, and animals obtain this energy from food. The primary source of energy for animals is carbohydrates, mainly glucose. ATP is produced by the oxidative reactions in the cytoplasm and mitochondrion of the cell, where carbohydrates, proteins, and fats undergo a series of metabolic reactions collectively called cellular respiration. For example, glycolysis is a series of reactions in which glucose is converted to pyruvic acid and some of its chemical potential energy is transferred to NADH and ATP.
ATP is required for all cellular functions. It is used to build the organic molecules that are required for cells and tissues; it provides energy for muscle contraction and for the transmission of electrical signals in the nervous system. Glycogen is a polymeric form of glucose and is stored in the liver and skeletal muscle cells. When blood sugar drops, the liver releases glucose from stores of glycogen. Skeletal muscle converts glycogen to glucose during intense exercise. The process of converting glucose and excess ATP to glycogen and the storage of excess energy is an evolutionarily important step in helping animals deal with mobility, food shortages, and famine.
Obesity is a major health concern in the United States, and there is a growing focus on reducing obesity and the diseases it may lead to, such as type-2 diabetes, cancers of the colon and breast, and cardiovascular disease. How does the food consumed contribute to obesity? Fatty foods are calorie-dense, meaning that they have more calories per unit mass than carbohydrates or proteins.
One gram of carbohydrates has four calories, one gram of protein has four calories, and one gram of fat has nine calories. Animals tend to seek lipid-rich food for their higher energy content. Foods that are rich in fatty acids tend to promote satiety more than foods that are rich only in carbohydrates.
Well, one of them you may be familiar with — the aerobic system. The aerobic energy system is also known as the oxygen energy system and it uses both carbohydrates and fat in a slow energy burn. The aerobic energy system is the most complex of the three using oxygen to create something called glycolysis and, ultimately, produce that all-important ATP. As a result, the aerobic energy system is the slowest to act of the three.
It is incredibly important for tissue repair, digestion, temperature control and hair growth as well. Think of all those short, sharp bursts of energy such as a metre sprint or fast and furious bench press set. So the aerobic energy system deals with our longer endurance energy needs and the ATP-PC system covers all the super short bursts. The lactic acid energy system deals with everything in-between — a few minutes of intense activity. Also called the anaerobic glycolysis system, the lactic acid system uses stored glucose muscle glycogen to create energy.
Generally though too much lactic acid will lead to that burning feeling in the muscles and, ultimately, fatigue. By submitting this form, you acknowledge that you have read, understood and accept our Privacy Policy and Terms and Conditions.
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