Metabolism is the sum of all energy-requiring and energy-consuming processes of the body. Many factors contribute to overall metabolism, including lean muscle mass, the amount and quality of food consumed, and the physical demands placed on the human body. Eating is essential to life. Many of us look to eating as not only a necessity, but also a pleasure. You may have been told since childhood to start the day with a good breakfast to give you the energy to get through most of the day. You most likely have heard about the importance of a balanced diet, with plenty of fruits and vegetables. 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.
Metabolism of Carbohydrates
Carbohydrates are one of the major forms of energy for animals and plants. Plants build carbohydrates using light energy from the sun (during the process of photosynthesis), while animals eat plants or other animals to obtain carbohydrates. Plants store carbohydrates in long polysaccharides chains called starch, while animals store carbohydrates as the molecule glycogen. These large polysaccharides contain many chemical bonds and therefore store a lot of chemical energy. When these molecules are broken down during metabolism, the energy in the chemical bonds is released and can be harnessed for cellular processes.
“All living things use carbohydrates as a form of energy.: Plants, like oak tree and acorn, use energy from sunlight to make sugar and other organic molecules. Both plants and animals (like squirrel) use cellular respiration to derive energy from the organic molecules originally produced by plants”
Metabolism of Lipids/ Fatty acids (Lipid Metabolism)
Lipids are universal biological molecules. Not only does this broad class of compounds represent the primary structural component of biological membranes in all organisms, they also serve a number of vital roles in microorganisms. Among these, lipids can be metabolized by microbes for use as a primary energy source. Although not stated explicitly, the “Organic Acid Metabolism” atom in this module introduces the concept of lipid metabolism by describing the process of fatty acid metabolism through β-oxidation. This atom will expand on the metabolic pathway that enables degradation and utilization of lipids. Fatty acids are the building blocks of lipids. They are made of a hydrocarbon chain of variable length that terminates with a carboxylic acid group (-COOH). The fatty acid structure is one of the most fundamental categories of biological lipids. It is commonly used as a building block of more structurally complex lipids (such as phospholipids and triglycerides). When metabolized, fatty acids yield large quantities of ATP, which is why these molecules are important energy sources. Lipids are an energy and carbon source. Before complex lipids can be used to produce energy, they must first be hydrolyzed. This requires the activity of hydrolytic enzymes called lipases, which release fatty acids from derivatives such as phospholipids. These fatty acids can then enter a dedicated pathway that promotes step-wise lipid processing that ultimately yields acetyl-CoA, a critical metabolite that conveys carbon atoms to the TCA cycle (aka Krebs cycle or citric acid cycle) to be oxidized for energy production.
Metabolism of Proteins
Much of the body is made of protein, and these proteins take on a myriad of forms. They represent cell signaling receptors, signaling molecules, structural members, enzymes, intracellular trafficking components, extracellular matrix scaffolds, ion pumps, ion channels, oxygen and CO2 transporters (hemoglobin). That is not even the complete list! There is protein in bones (collagen), muscles, and tendons; the hemoglobin that transports oxygen; and enzymes that catalyze all biochemical reactions. Protein is also used for growth and repair. Amid all these necessary functions, proteins also hold the potential to serve as a metabolic fuel source. Proteins are not stored for later use, so excess proteins must be converted into glucose or triglycerides, and used to supply energy or build energy reserves. Although the body can synthesize proteins from amino acids, food is an important source of those amino acids, especially because humans cannot synthesize all of the 20 amino acids used to build proteins.
The digestion of proteins begins in the stomach. When protein-rich foods enter the stomach, they are greeted by a mixture of the enzyme pepsin and hydrochloric acid (HCl; 0.5 percent). The latter produces an environmental pH of 1.5–3.5 that denatures proteins within food. Pepsin cuts proteins into smaller polypeptides and their constituent amino acids. When the food-gastric juice mixture (chyme) enters the small intestine, the pancreas releases sodium bicarbonate to neutralize the HCl. This helps to protect the lining of the intestine. The small intestine also releases digestive hormones, including secretin and CCK, which stimulate digestive processes to break down the proteins further. Secretin also stimulates the pancreas to release sodium bicarbonate. The pancreas releases most of the digestive enzymes, including the proteases trypsin, chymotrypsin, and elastase, which aid protein digestion. Together, all of these enzymes break complex proteins into smaller individual amino acids, which are then transported across the intestinal mucosa to be used to create new proteins, or to be converted into fats or acetyl CoA and used in the Krebs cycle.
Metabolism of Carbohydrates
Carbohydrates are one of the major forms of energy for animals and plants. Plants build carbohydrates using light energy from the sun (during the process of photosynthesis), while animals eat plants or other animals to obtain carbohydrates. Plants store carbohydrates in long polysaccharides chains called starch, while animals store carbohydrates as the molecule glycogen. These large polysaccharides contain many chemical bonds and therefore store a lot of chemical energy. When these molecules are broken down during metabolism, the energy in the chemical bonds is released and can be harnessed for cellular processes.
“All living things use carbohydrates as a form of energy.: Plants, like oak tree and acorn, use energy from sunlight to make sugar and other organic molecules. Both plants and animals (like squirrel) use cellular respiration to derive energy from the organic molecules originally produced by plants”
Metabolism of Lipids/ Fatty acids (Lipid Metabolism)
Lipids are universal biological molecules. Not only does this broad class of compounds represent the primary structural component of biological membranes in all organisms, they also serve a number of vital roles in microorganisms. Among these, lipids can be metabolized by microbes for use as a primary energy source. Although not stated explicitly, the “Organic Acid Metabolism” atom in this module introduces the concept of lipid metabolism by describing the process of fatty acid metabolism through β-oxidation. This atom will expand on the metabolic pathway that enables degradation and utilization of lipids. Fatty acids are the building blocks of lipids. They are made of a hydrocarbon chain of variable length that terminates with a carboxylic acid group (-COOH). The fatty acid structure is one of the most fundamental categories of biological lipids. It is commonly used as a building block of more structurally complex lipids (such as phospholipids and triglycerides). When metabolized, fatty acids yield large quantities of ATP, which is why these molecules are important energy sources. Lipids are an energy and carbon source. Before complex lipids can be used to produce energy, they must first be hydrolyzed. This requires the activity of hydrolytic enzymes called lipases, which release fatty acids from derivatives such as phospholipids. These fatty acids can then enter a dedicated pathway that promotes step-wise lipid processing that ultimately yields acetyl-CoA, a critical metabolite that conveys carbon atoms to the TCA cycle (aka Krebs cycle or citric acid cycle) to be oxidized for energy production.
Metabolism of Proteins
Much of the body is made of protein, and these proteins take on a myriad of forms. They represent cell signaling receptors, signaling molecules, structural members, enzymes, intracellular trafficking components, extracellular matrix scaffolds, ion pumps, ion channels, oxygen and CO2 transporters (hemoglobin). That is not even the complete list! There is protein in bones (collagen), muscles, and tendons; the hemoglobin that transports oxygen; and enzymes that catalyze all biochemical reactions. Protein is also used for growth and repair. Amid all these necessary functions, proteins also hold the potential to serve as a metabolic fuel source. Proteins are not stored for later use, so excess proteins must be converted into glucose or triglycerides, and used to supply energy or build energy reserves. Although the body can synthesize proteins from amino acids, food is an important source of those amino acids, especially because humans cannot synthesize all of the 20 amino acids used to build proteins.
The digestion of proteins begins in the stomach. When protein-rich foods enter the stomach, they are greeted by a mixture of the enzyme pepsin and hydrochloric acid (HCl; 0.5 percent). The latter produces an environmental pH of 1.5–3.5 that denatures proteins within food. Pepsin cuts proteins into smaller polypeptides and their constituent amino acids. When the food-gastric juice mixture (chyme) enters the small intestine, the pancreas releases sodium bicarbonate to neutralize the HCl. This helps to protect the lining of the intestine. The small intestine also releases digestive hormones, including secretin and CCK, which stimulate digestive processes to break down the proteins further. Secretin also stimulates the pancreas to release sodium bicarbonate. The pancreas releases most of the digestive enzymes, including the proteases trypsin, chymotrypsin, and elastase, which aid protein digestion. Together, all of these enzymes break complex proteins into smaller individual amino acids, which are then transported across the intestinal mucosa to be used to create new proteins, or to be converted into fats or acetyl CoA and used in the Krebs cycle.