Metabolism is the summation of all chemical reactions that occur in living cells to maintain life processes. There are two kinds of reactions: anabolic and catabolic. In anabolic reactions small molecules combine together to form larger molecules like DNA and proteins. In catabolic reactions larger molecules are broken down into smaller molecules to produce energy or building blocks.
Metabolic processes are the means by which the body derives energy and synthesizes the other molecules it needs from the lipids, carbohydrates and proteins we eat as food. The substances that are produced or used during metabolic processes are referred to as metabolites. Metabolic processes involve a complicated network of enzyme-catalyzed reactions which are helped by minerals and vitamins. Metabolism is essential to digestion, the elimination of waste, breathing, circulating blood, proper function of the brain and nerves, and controlling body temperature. 
Anabolism is also referred to as constructive metabolism. During this process, small molecules combine together to form larger molecules, which requires the use of ATP.  The process of anabolism helps to grow new cells, and body tissues, and it also stores energy for later use. When the smaller molecules form larger molecules they are forming the complex molecules known as carbohydrates, proteins, and fats.  Anabolic reactions that take place in our bodies create many different kinds of molecules. Through anabolism, muscle mass is increased and bone is grown. The process builds polymers using monomers. Polymers are large molecules that are made up of small molecules (monomers). For example, amino acids are considered monomers while proteins are considered polymers. Anabolic hormones include growth hormones, insulin, testosterone, and estrogen. The growth hormone, which is created in the pituitary gland, helps to stimulate growth through the release of somatomedin. Somatomedin is formed in the liver and causes a person to grow. IGF1 hormones help to stimulate the creation of sulfate and protein. Some of these hormones specifically target an organ to help it grow more rapidly. Insulin is a hormone that is needed to regulate the sugar glucose levels in the blood. The hormone is made in the pancreas by beta cells. The body can't use glucose properly without the use of insulin. 
Catabolism is also referred to as destructive metabolism. During catabolism, larger organic molecules break down into smaller molecules causing the release of ATP.  The energy that is produced through catabolism is needed by all cells in order to function properly. The larger molecules that are being broken down are usually carbohydrates and fats. The energy that is created through the breakdown of these molecules provides the fuel necessary for heating the body, anabolism, and the movement of muscles throughout the body. As the molecules are being broken down, the waste that is produced is expelled from the body through the skin, kidneys, lungs, and intestines.  Some catabolic hormones are cortisol, glucagon, adrenaline, and cytokines. Sometimes referred to as the stress hormone, cortisol responds to stress in the body. Cortisol is made by the adrenal cortex, which makes up part of the adrenal gland. Cortisol increases a person's blood sugar and pressure and also decreases the immune systems responses. Glucagon is a hormone that helps to breakdown glycogen, which causes the blood sugar levels rise. It is made in the pancreas by alpha cells. Glycogen is stored in the liver as a carbohydrate and is used to give a person energy throughout their day. Another hormone is adrenalin, also known as epinephrine. Adrenalin increases a person's heart rate, opens the lungs to allow more air, and allows the heart to forcefully contract. Cytokines are small proteins that effects how cells react with each other. Through catabolism the energy that is stored up is used to synthesize enzymes, hormones, and sugars. They also give energy to cells that are involved with cell growth, tissue repair, and reproduction. The excess energy is then stored as fat throughout the body. Compared to other more active cells fat cells need barely any energy, contrary to the other cells in the human body that are more active. 
Animals, microbes, and plants are made of three different kinds of molecules: carbohydrates, fats (or lipids), and amino acids. These three molecules are essential to sustain life. These molecules are the main focus in metabolic reactions when the body is building new cells and tissues, as well as being a source of energy. Combining these biochemicals creates polymers like proteins and DNA 
- Proteins are made up of amino acids joined together with peptide bonds. They are arranged in a linear chain. Most of the proteins created are catalysts used in different chemical reactions. Some of the other proteins are used to help construct the cytoskeleton.
- Lipids are part of the cell membrane as well as a source for energy. Fats include a lot of compounds made of different fatty acids as well as glycerol. Steroids are also considered to be a major group of lipids, cholesterol being one of them.
- Carbohydrates store and transport energy. They are made up of straight chain aldehydes with a lot of hydroxyl groups that make up rings or straight chains.
- Nucleotides are long chained polymers like DNA and RNA. These specific molecules are essential in the use of genetic information. Nucleotides are created when a nucleobase attaches to a ribose sugar.
- Coenzymes perform different chemical reactions. The most well known coenzyme is ATP. There isn't a lot of ATP in the cells, but because it is always creating more than it never runs out.
Molecules involved in the metabolism of proteins, fats, and carbohydrates are referred to as enzymes. Proteins also help to control the rate at which metabolic reactions take place. Enzymes in the digestive system break proteins down into different amino acids, carbohydrates are broken down into simple sugars, and fats are broken down into fatty acids. Following digestion these molecules are absorbed into the bloodstream, which transport them to cells throughout the body. The energy obtained from food is stored in the liver, muscles, or fat.
Hormones secreted by the endocrine system help control a persons metabolism. One of the main hormones that regulate metabolism is thyroxine, which helps to determine how quickly or slowly the chemical reactions should take place within cells. The pancreas produces a hormone that can tell whether a person needs to begin anabolic or catabolic reactions. This usually takes place after a meal when the increase in glucose tells the body that it needs insulin.  
Metabolic disorders are usually characterized by irregular chemical reactions within the cells of the body. These disorders are usually caused by abnormal enzymes or hormones. If the metabolism is not functioning properly, then toxic chemicals begin to build up in the body and can lead to serious complications. Although not all are genetic, some have been found to be inherited from parents or grandparents. One of the enzymes needed for the metabolism to function correctly is G6PD (or Glucose-6-phosphate dehydrogenase). G6PD is one of the main enzymes that is used in the breakdown of carbohydrates, and it is produced by red blood cells. If there is not enough G6PD in the blood, then specific substances that are harmful to the body will destroy the red blood cells. This disorder is known as hemolytic anemia. When red blood cells are destroyed at an early age, a person's bone marrow is unable to keep up with a person's growth and therefore puts them at risk for injury. This condition is referred to as hemolysis. People who have a G6PD deficiency have the following symptoms: 
- pale skin
- fatigue and tiredness
- rapid and shallow breathing
- abnormal/rapid heartbeat
- enlarged spleen
- yellowish tint to eyes
Another metabolism disorder is galactosemia which occurs in babies. Babies born with galactosemia don't have the enzymes needed to break down the sugar molecules in milk. The sugar is referred to as galactose. The enzyme is secreted from the liver, and if there is not enough produced, galactose will start to build-up in the babies blood. This buildup can lead to major health problems such as liver, brain, kidney, and eye damage. It can be diagnosed within the first couple days after birth. The symptoms include vomiting, swollen liver, and jaundice. 
Another serious metabolism condition is hyperthyroidism. Hyperthyroidism is the result of an overactive thyroid. If the thyroid secretes too much thyroxine, then that person's BMR (or basal metabolic rate) will increase. The major symptoms of hyperthyroidism are:
- rapid weight loss
- increased blood pressure
- increased heart rate
- swollen neck due to an enlarged thyroid gland
- enlarged eyes
Hyperthyroidism can be controlled as simply as with medications, or through surgery. It can also be treated through radiation treatments. 
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