Question

a) Describe changes and locations of protein digestion

b) Describe changes and locations of fat/lipid digestion

c) Describe changes and locations of starch/carbohydrate digestion

d) Describe what happens to fiber

*Include where changes take place ; the specifics of digestion; where (and how) absorption takes place, and how these nutrients enter the blood stream. Include physical/mechanical digestion/contact digestion; and include names of all enzymes/agents and where these enzymes are produced.

Answer 1

Ans (a): Protein Digestion:

• Protein nutrients taken as food follows the path of gastrointestinal tract starting with ingesting it from buccal cavity to the area where completely digested proteins gets absorbed. No protein is digested in the buccal cavity as the saliva does not contain any protein digesting enzyme. Proteins are only digestible with the help of enzymes present in gastric juice of stomach, pancreatic juice and intestinal juice present in intestine.
• Gastric juice contains twoproenzymes: propepsin and prorennin. Propepsin is converted into pepsin by HCl which convert the protein into peptides in an acidic medium. Prorennin is converted into rennin by HCl which convert the soluble milk protein (casein) into insoluble compound called paracasein which is further converted into peptides by pepsin.
• From the stomach, food reaches into the intestine where it is mixed with the pancreatic juice and intestinal juice, and intestinal juice show its action which digests the protein in an alkaline medium created by the bile juice.
• Pancreatic juice contains two proenzymes - trypsinogen and chymotrypsinogen and one enzyme - carboxypolypeptidases. Trypsinogen is converted into trypsin by a non-digestive enzyme enterokinase which digests the protein into peptide. Chymotrypsinogen is converted into chymotrypsin by trypsin which converts the protein into peptides. Then, carboxypolypeptidases hydrolyse the last peptide bond of the polypeptide chain to release its last amino acid thereby changing the polypeptide into dipeptide or peptide.
• Intestinal juice contains aminopeptidases and dipeptidases which completes the digestion of protein by converting the dipeptides into amino acids. Finally, from the site of large intestine, absorption of amino acids into the bloodstream takes place.

Ans (b): Fat/ Lipid Digestion:

• No fat is digested in the buccal cavity as saliva does not contain any fat digesting enzymes.
• Fat is digested by lipase enzyme which is soluble in water while the fats and lipids are insoluble in water. Therefore, they must be emulsified i.e. broken into small droplets which is done by bile juice.
• Gastric juice contain mild lipase enzyme which hydrolyze little fat into fatty acid and glycerol.
• In the intestine, the fats are first emulsified by the bile juice which are then emulsified progressively by the pancreatic and intestinal lipase into diglyceride then into monoglyceride and finally into fatty acid and glycerol in an alkaline medium.

Ans (c): Starch/ Carbohydrate Digestion:

• Carbohydrates are the major contents of the animal diet. In our food diet, the carbohydrates are sucrose, disaccharides, lactose and starch which are digested in the alimentary canal with the help of some digestive enzymes.
• Approx. 30% od starch is digested in the buccal cavity with the help of ptyalin and maltase present in the saliva in the presence of Cl-. Starch gets converted to maltose with the help of ptyalin enzyme and further maltose is converted to glucose using enzyme maltase.
• Semi-digested food reach into stomach. Here no carbohydrate digesting enzymes are present.
• In small intestine, carbohydrates are mixed with the enzymes secreted by the pancreas and intestinal gland. Intestinal juices contain amylase, maltase, isomaltase, limitdextrinase which convert starch into glucose. Amylase converts the semi-digested starch into double sugar i.e. maltose, isomaltose and limitdextrin which further converted into glucose with the help of maltase, isomaltase and limitdextrinase respectively.

Ans (d):

Fiber is important for keeping the digestive tract working smoothly. Unlike other nutrients such as proteins, carbohydrates and fats, which our body digests and absorbs, fibers are not being digested by our body. Instead, it passes relatively intact through intestinal tract out of the body by absorbing water. Soluble fiber passes through the small intestine unchanged until it reaches colon, or large intestine, where fiber absorbs water from indigestible part of food. Eating fiber helps to prevent constipation.