Basic Science JSS3
Topic: Metabolism in the Human Body
Introduction
Metabolism is described has the chemical reaction in the body. Metabolism (from Greek: metabolē, “change”) is the set of life-sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Food substances are taken into the mouth. In the mouth, the food materials are chewed with the teeth, mixed with the saliva and rolled down the oesophagus into the stomach. The digested food diffuses into the bloodstream through the villus on the walls of the small intestine. The absorbed food circulates to various parts of the body through the bloodstream for metabolic processes. The unabsorbed food materials, however, go out of the body from the large intestine through the anus.
Metabolism can be conveniently divided into two categories:
- Catabolism – the breakdown of molecules to obtain energy
- Anabolism – the synthesis of all compounds needed by the cells
Metabolism is closely linked to nutrition and the availability of nutrients. Bioenergetics is a term which describes the biochemical or metabolic pathways by which the cell ultimately obtains energy. Energy formation is one of the vital components of metabolism.
Digestion of Food
Digestion is the process by which complex food materials are broken down into smaller particles, with the aid of enzymes and juice produced by specialized cells and glands so that the body can absorb them. Digestion of food starts from the mouth.
All the organs that food passes through, including those that secrete substances that act on it, such as gall bladder, pancreas, salivary gland, etc., make up the digestive system. Digestive system is also known as alimentary canal. The alimentary canal has five parts through which food pass through. These include:
- Mouth
- Gullets (Oesophagus)
- Stomach
- Small Intestine
- Large Intestine
Digestion Process
Digestion process begins with ingestion. Ingestion is the act of taking food into the alimentary canal. The food is sent to the mouth and the mouth chewed it into bits with the teeth and acted upon by the saliva that converts starch to maltose. This change is possible due to an enzyme called ptyalin in the saliva which acts on the starch. Enzymes are chemical substances produced in the body by specialized cells. They speed up digestion of food.
The food from the mouth passes through the oesophagus into the stomach where substances like gastric juice (which contains rennin and pepsin) produced in the stomach act on it. They act on protein, converting it to polypeptides in the case of pepsin, while rennin coagulates milk protein. Hydrochloric acid in the gastric juice neutralizes the alkali in food from the mouth. The food is churned in the stomach and passed into the small intestine. The small intestine consists of three parts—the duodenum (the first part), jejunum and the ileum. When the food enters into the duodenum, the gall bladder produces bile which breaks down fats and oil (emulsifies) into smaller molecules. The food also passes into the second and third parts of the small intestine where other enzymes act on it.
Absorption of Food in the Body
Absorption of food is the process whereby digested food particles diffuse into the bloodstream, which circulates them to where they are required for metabolic activities. Absorption of food takes place in the villi of the small intestine.
The digested food is absorbed into the bloodstream because it can pass easily through the walls of the small intestine through structures called the villi (singular: villus). The villus is a finger-like structure that contains a structure called the lacteal which transports fatty acids and glycerol. There are also blood vessels which transport digested proteins and carbohydrates through the capillaries.
The end products of digestion namely glucose, fatty acid and glycerol and amino acid are the substances that the body can readily use because they can pass through the small intestine directly into the bloodstream and taken to the liver. Energy from the absorbed food helps the muscles to carry out their activities as found in heartbeats, respiration, excretion, etc.
Forms in which Excess Foods are Stored
Most times, the foods we eat are in excess and so must be stored in the tissues to be used later. The ability of the body to store excess absorbed food substances that are not for immediate use is known as food storage. Of the six classes of food (i.e. carbohydrates, proteins, fats and oils, minerals, vitamins and water), only two are converted and stored in the body for use when needed. They are carbohydrates, fats and oils.
- Carbohydrates: These are energy – giving foods. Examples include cereals, yam, potatoes, cassava, etc. These are called starchy foods which are digested into glucose. Excess glucose is converted by insulin and stored in the liver as glycogen. Some others are oxidized to carbon (IV) oxide and water which are used up by the body.
- Fats and Oils: These are oily food substances, they are also called lipids. Fats and oils provide warmth to the system, and are also called energy – giving foods. Fats and oils are digested to glycerol and fatty acids. Excess fat is stored under the skin, around the abdomen, thigh and muscles.
- Proteins: are not stored in the body. Blood from the small intestine only contains amino acids needed by the body at a particular time. Excess amino acids are deaminated in the liver. This is the removal of the nitrogen – containing amino group from the amino acids. The amino group is then converted into urea which is excreted by the kidney as urine.
Places of Storage
Starch in the blood is in the form of glucose and is converted by insulin and stored in the liver as glycogen. Excess fat is stored under the skin, around the abdomen, cheek, hips/waist, thigh, muscles, etc. Proteins when in excess, are not stored in the body but are immediately excreted in the form of urea by the kidney.
Problems Associated with Food Storage
- Excess glucose is stored as glycogen in the liver by the insulin. In the absence of insulin, excess glucose or sugar is found in the bloodstream. This process causes diabetes.
- Excess protein is deaminated by the liver. In any case of malfunctioning of the liver, the presence of proteins in the bloodstream causes hepatitis.
- Excess fat is stored under the skin and gives energy. The accumulation of excess fat in the skin leads to obesity and other heart diseases.
