• The pancreas is a long gland that extends from the duodenum to the spleen. The pancreas produces enzymes that digest fats, proteins, carbohydrates, and nucleic acids. The protein digesting enzymes are produced in inactive forms, which must be converted to active forms in the small intestine by other enzymes. The pancreas also produces large amounts of alkaline fluid, which neutralizes the chyme in the small intestine, thus protecting the lining of the digestive tract.
• These juices collect in a main duct that joins the common bile duct or empties into the duodenum near the common bile duct. Most persons also have an additional smaller duct that opens into the duodenum. Because they are usually confined to proper channels, pancreatic enzymes do not damage body tissues.
•  However, if the bile ducts become blocked, pancreatic enzymes back up into the pancreas. Also, in some cases of gallbladder disease, disease, infection may extend to the pancreas and cause abnormal activation of the pancreatic enzymes. In either circumstance, the pancreas suffers destruction by its own juice, and the outcome can be fatal; this condition is known as acute pancreatitis.
• The pancreas also functions as an endocrine gland, producing the hormones insulin and glucagon that regulate sugar metabolism. These secretions of the islet’s cells are released directly into the blood.

Digestion and Absorption of Carbohydrates, Fats, and Proteins

Digestion

• Digestion, a complex process that occurs in the alimentary canal, consists of physical and chemical changes that prepare food for absorption. Mechanical digestion breaks food into tiny particles, mixes them with digestive juices, moves them along the alimentary canal, and finally eliminates the digestive wastes from the body.
•  Chewing or mastication, swallowing or deglutition, peristalsis, and defecation are the main processes of mechanical digestion. Chemical digestion breaks down large, non-absorbable food molecules−molecules that are able to pass through the intestinal mucosa into blood and lymph.
• Chemical digestion consists of numerous chemical reactions catalyzed by enzymes in saliva, gastric juice, pancreatic juice, and intestinal juice.

Carbohydrate Digestion

• Very little digestion of carbohydrates (starches and sugars) occurs before food reaches the small intestine. Salivary amylase usually has little time to do its work because so many of us swallow our food so fast. Gastric juice contains no carbohydrate-digesting enzymes. But after the food reaches the small intestine, pancreatic and intestinal juice enzymes digest the starches and sugars. A pancreatic enzyme (amylase) starts the process by changing starches into a double sugar, namely, maltose. Three intestinal
• enzymes−rnaltase, sucrase, and lactase−digest double sugars by changing them into simple sugars, chiefly glucose (dextrose). Maltase digests maltose (malt sugar), sucrose digests sucrose (ordinary cane sugar), and lactase digests lactose (milk sugar). The end product of carbohydrate digestion is the so-called simple sugar; the most abundant is glucose.

Protein Digestion

• Protein digestion starts in the stomach. Two enzymes (rennin and pepsin) in the gastric juice cause the giant protein molecules to break up into somewhat simpler compounds. Pepsinogen, a component of gastric juice, is converted into active pepsin enzyme by hydrochloric acid (also in gastric juice). In the intestine, other enzymes (trypsin in the pancreatic juice and peptidases in the intestinal juice) finish the job of protein digestion.
• Every protein molecule is made up of many amino acids joined together. When enzymes have split up the large protein molecule into its separate amino acids, protein digestion is completed. Hence the end product of protein digestion is amino acids. For obvious reasons, the amino acids are also referred to as protein building blocks.

Fat Digestion

Very little carbohydrate and fat digestion occurs before food reaches the small intestine. Most fats are undigested until after emulsification by bile in the duodenum (that is, fat droplets are broken into very small droplets). After this takes place, pancreatic lipase splits up the fat molecules into fatty acids and glycerol (glycerine). The end products of fat digestion, then, are fatty acids and glycerol.

 the main facts about chemical digestion:

Enzyme names indicate the type of food digested by the enzyme. For example, the name amylase indicates that the enzyme digests carbohydrates (starches and sugars), protease indicates a protein- digesting enzyme, and lipase means a fat-digesting enzyme. When carbohydrate digestion has been completed, starches (polysaccharides) and double sugars (disaccharides) have been changed mainly to glucose, a simple sugar (monosaccharide).

The end products of protein digestion, on the other hand, are amino acids. Fatty acids and glycerol are the end products of fat digestion.

Absorption

After food is digested, it is absorbed; that is, it moves through the mucous membrane lining of the small intestine into the blood and lymph. In other words, food absorption is the process by which molecules of amino acids, glucose, fatty acids, and glycerol goes from the inside of the intestines into the circulating fluids of the body. Absorption of foods is just as essential as digestion of foods. The reason is fairly obvious. As long as food stays in the intestines, it cannot nourish the millions of cells that compose all other parts of the body. Their lives depend on the absorption of digested food and its transportation to them by the circulating blood.

Structural adaptations of the digestive tube, including;

•  Folds in the lining mucosa, villi, and micro villi, increase the absorptive surface and the efficiency and speed of transfer of materials from the intestinal lumen to body fluids.
•  Many salts such as sodium are actively transported through the intestinal mucosa.
• Water follows by osmosis. Other nutrients are also actively transported into the blood of capillaries in the intestinal villi.
• Fats enter the lymphatic vessels or lacteals found in intestinal villi.

Assignment : Give a summery of Chemical digestion