Digestion
Principle of Biology
is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy.
The digestive system
The digestive system is mainly a long tube that runs from the mouth to the anus. This tube is called the gut or alimentary canal. The alimentary canal is associated with the organs of the liver, gall bladder, and pancreas.
Digestion: Digestion is the mechanical and chemical breakdown of food.
Food: is any solid, semi-solid, liquid which is broken down in the body to produce energy (ATP), nutrients, stimulate growth, and maintain life.
There are two forms of digestion:
1) Physical digestion – this involves the mechanical process of chewing or mastication of food in the mouth and the churning due to backwards motion of the stomach
2) Chemical digestion- this is the use of enzymes to break down the food into smaller particles that can be readily absorbed into the body.
Digestion in the mouth
- Digestion begins in the mouth
- This process begins with ingestion which is the placing of food in the mouth.
- We use our teeth to bite, slice and grind food into smaller pieces.
- Food is mix with saliva which contains enzyme called salivary amylase
- Salivary amylase breaks down the starch chemically into simpler sugar such as maltose and glucose.
- The tongue rolls the food into a ball called bolus, which is swallowed
- The bolus enters the oesophagus and is push downward by peristalsis
- Peristalsis which is a wave-like motion cause by contraction and relaxation of circular muscles in the wall of the alimentary canal The bolus is pushed down to the stomach
In leaving the mouth a bolus of food must cross the respiratory tract (trachea is anterior to oesophagus) by a complicated mechanism known as swallowing or deglutition which empties the mouth and ensures that food does not enter the windpipe.
Swallowing involves co-ordinate activity of tongue, soft palate, pharynx and oesophagus. The first phase is voluntary, food being forced into the pharynx by the tongue. After this the process is reflex. The tongue blocks the mouth, soft palate closes off the nose and the larynx rises so that the epiglottis closes off the trachea. Food thus moves into the pharynx and onwards by peristalsis aided by gravity. The esophagus is like a stretchy pipe that's about 10 inches (25 centimeters) long. It moves food from the back of your throat to your stomach.
Digestion in the stomach
The stomach is like a mixer, churning and mashing together all the bolus into smaller pieces. The stomach has muscular wall that is lined with many gastric glands. These glands secrete gastric juices and a small amount of hydrochloric acid. Hydrochloric acid kills most contaminating microorganisms and begins break down of some food, and chemical alteration of some. The hydrochloric acid has a low pH, which allows enzymes to work more efficiently. After some time (typically an hour or two in humans), the resulting thick liquid is called chyme.
The gastric juices contain an enzyme, pepsin, which breaks protein in the food into smaller units called polypeptides. Pepsin works best in an acidic medium hence reason for HCl acid. The chyme is then passed from the stomach into the small intestine a little at a time. This process is regulated by a ring of muscle located at the base of the stomach called the pyloric sphincter.
Digestion in the small intestine
The small intestine is a long that has three parts: the Duodenum, Jejunum, and Ileum; this tube that's about 1½ inches to 2 inches (about 3.5 to 5 cm) around, and it's packed inside you beneath your stomach. If you stretched out an adult's small intestine, it would be about 22 feet long (6.7 meters). Unlike the stomach, the small intestine is alkaline since digestive juices released here cannot function under acidic conditions. As chyme enters the duodenum, pancreatic juices is secreted from the pancreas and bile release from the liver into the duodenum.
Pancreatic juice contains three enzymes:
1. Trysin – this continues digestion of protein into polypeptides and also into even smaller units called amino acids
2. Pancreatic amylase- this continues the digestion of starch which began in the mouth. Any starch that is still present is digested to maltose and glucose.
3. Lipase – this begins the digestion of fats and oils into smaller units called fatty acid and glycerol.
The bile from the liver is very alkaline and so it neutralizes the chyme. By the time the food reaches the ileum, most of the complex food substances have been broken down into very small particles. Enzymes released in the ileum complete the digestion process: The enzymes include maltase, lactase and sucrose.
Vitamins, minerals, and water in the food do not need to be broken down. In the ileum the process of absorption begins. The walls of the ileum are covered by finger- like projections called villi. The villi increase the surface area for absorption
Nutrients are absorbed easily because each villus is only one cell thick. Inside the villus is a network of many blood vessels and a central lymph vessel called the lacteal which transport fats to the liver.
The liver
This functions as a storage house for digested products and the conversion of excess products
1) Excess glucose is converted to glycogen
2) Minerals, need to make blood, as well as vitamin A,B and D, are stored until need
3) Excess amino acids are converted into useful substance. The left of molecules are converted into urea to be excreted.
Large intestine:
Digestion and absorption have finished by the time the food bolus reaches the large intestine. All that is left is undigested food, including fibre or roughage, and some water. As this mixture pass along the colon, water is absorbed leaving behind a solid waste called faeces. This is stored in the rectum until it is eliminated or egested from the body via the anus. This process is called defecation.
No comments:
Post a Comment