DIGESTIVE SYSTEM In mammals, holozoic nutrition involves five main processes. These are: i. Ingestion Ii. Digestion iii. Absorption iv. Assimilation v. Egestion.

INGESTION AND DIGESTION Ingestion is the act of eating, taking food into the gut, where it is processed. Digestion breaks down the large biological molecules in food into smaller molecules: Polysaccharides are broken down into monosaccharide fats into glycerol and fatty acids proteins into amino acids nucleic acids into nucleotides.

ABSORPTION AND EGESTION Absorption takes place after digestion. Useful digestion products and other soluble substances such as vitamins and mineral are transported across the cells lining the wall into the bloodstream and lymph. Undigested food is eliminated from the gut through the anus. This process is called egestion.

ASSIMILATION After absorption, digested food molecules are carried by the bloodstream to the body cells. The molecules may be stored for future use, broken down further by respiration to generate energy, or used by cells to maintain good health, for growth, or to repair body tissues. The process of incorporating and using digested food molecules in the body is called assimilation.

HUMAN DIGESTIVE SYSTEM Humans have an alimentary canal, a tubular passage or gut divided into specific regions where different digestive processes take place. Other organs outside the alimentary canal that contribute to digestion include the salivary glands, pancreas and liver. These organs secrete digestive juice which enter the alimentary canal through ducts. Bile , the digestive secretion of liver, is stored in the gall bladder before it passes to the small intestine.

HUMAN DIGESTIVE SYESTEM

PARTS OF ALIMENTARY CANAL The main parts of the alimentary canal are: Mouth Tongue Pharynx Esophagus Stomach Small intestine Large intestine Rectum Anus.

DIGESTION IN THE MOUTH The human digestive system begins at the oral cavity. The teeth breaks up food by mechanical means. Tongue manipulates food and forms it into a semi-spherical ball (bolus) with the aid of saliva (produced from salivary gland) - Saliva also contains amylases (i.e. ptyalin) which break down starch into maltose. The tongue pushes the bolus into the pharynx which is the cavity where the oesophagus and trachea (wind pipe) meet. The larynx is raised against the epiglottis and the glottis is closed, preventing food from passing into the tracheae. The act of swallowing initiates the movement of food down through a tube connecting the mouth to the stomach called the oesophagus. Once inside the oesophagus, the food is moved by involuntary peristaltic waves towards the stomach. Peristalsis is the alternate contraction and relaxation of the smooth muscles lining the digestive tract, and helps to move the bolus along.

DIGESTIVE IN THE STOMACH The stomach is a highly elastic and muscular organ which can expand easily to hold a large meal. As the food nears the stomach, the sphincter muscles called cardiac sphincter controlling the opening of stomach relaxes and allows the food to enter the stomach. In the stomach, it is acted upon by the gastric juice, which works optimally in an acidic medium. Continual movement of the muscular stomach will churn the food, mixing it with gastric juice and helping to break it down mechanically. After about four hours in the stomach, the semi-digested food, or chyme, is released into the small intestine (duodenum) through the pyloric sphincter.

Composition of gastric juice in the stomach Gastric juice contains: 1. Hydrochloric acid (HCl), which acidifies the medium in order for the proteolytic enzymes to work optimally. HCl also ingests bacteria. 2. Pepsin, a proteolytic enzyme that cleaves along protein molecules into shorter fragments called peptides. Renin, an enzymes that solidifies casein, a milk protein , so that it can be retained in the stomach long enough for pepsin to act upon it.

Digestion in the intestine The intestine is of two parts: i. The small intestine – The first part of the small intestine is duodenum while the second part is the ileum The large intestine

Digestion in the duodenum Duodenum receives chyme from the stomach and secretions from the gall bladder and pancreases. The pancreas secretes pancreatic juice, a watery alkaline liquid containing several digestive enzymes. The liver produces bile, which is stored in the gall bladder. Bile is a greenish alkaline liquid which does not contain digestive enzymes. When chyme enters the duodenum, certain hormones stimulate the pancreas and gall bladder to send their digestive juice to the duodenum.

Composition of pancreatic juice Pancreatic juice contains three important enzymes (a) Amylopsin: which breaks down starch to maltose (b) Trypsin, a protease, which breaks down proteins to polypeptides. (c) Lipase that breaks down fat to fatty acids and glycerol. Enzymes of pancreatic juice requires an alkaline medium to work in.

ASSIGNMENT - MAKE A LABELLED DRAWING OF A VILLUS