Digestion and Absorption Test - 28

Digestion of starch normally begins in the mouth of humans, where an enzyme, salivary amylase, is secreted, catalyzing the breakup of the starch by hydrolysis. The starch is digested in mouth by chewing and the action of the amylase. Amylase works in the pH of 6.8. 

In herbivores, the caecum stores food material where bacteria are able to break down the cellulose. This function no longer occurs in the human caecum, so in humans it is simply a dead-end pouch forming a part of the large intestine.

The pylorus is a narrow cone shaped constriction present at the end of the stomach and the beginning of the small intestine. It contains two region antrum which connects the parts of the stomach and pyloric canal which connects the stomach and small intestine.  Thus, the correct answer is option B.

Saliva contains salivary amylase also known as ptyalin for the digestion of carbohydrates such as starch into maltose, isomaltose and limit dextrins. 

Trypsinogen is an important protein splitting proenzyme present in pancreatic juice. Chymotrypsinogen and procarboxypeptidase are the other protein splitting proenzymes present in pancreatic juice. The trypsinogen is activated by the enzyme enterokinase. The active trypsin in turn activates chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase.

• After food is being swallowed, from the mouth it is transported to the stomach. No absorption of nutrients takes place in the mouth and oesophagus. Hence, No absorption of food takes place in the mouth and oesophagus.
• Active glucose absorption is thought to depend on a gradient of sodium ion concentration across the brush border membrane of intestinal epithelial cells. Specifically, glucose absorption was minimally reduced when intraluminal sodium concentration was reduced. Hence, Absorption of glucose depends on sodium ion.
• Pinocytosis is the absorption of liquids, whereas phagocytosis is the absorption of solid objects which are essentially food for the cell. Phagocytosis engulfs whole particles, which are later broken down by enzymes, such as cathepsins, and absorbed into the cells. Hence, Whole protein particles cannot be engulfed by pinocytosis.

Rennin's primary function is to coagulate the milk. The stomachs of infants are not properly developed. That is why they cannot digest solid food in the first months of life. Also, because the digestive tract is very sensible, it might eliminate the milk young children usually ingest. Therefore, its very important that milk stays in the stomach and within the body as much as possible. This doesnt happen, when the milk is in its liquid form, but when its curdled. Thats the reason, why rennin is that essential for babies. 

The chyle entering the large intestine consists of about 75% water and some fatty and proteinaceous substances. Water is absorbed into the blood as the waste materials move through the colon. The symbiotic bacteria E. coli and Streptococcus faecalis normally residing in the human healthy colon, feed on protein contents of chyle, breaking them into amino acids which are absorbed into the blood. The colon bacteria also synthesize vitamin B1, B2, biotin, folic acid and K which are absorbed. They convert bile pigments into brown pigments that give colour to faeces. Intestinal bacteria also produce foul-smelling compounds and gases (methane. hydrogen sulphide) which give the characteristic odour to intestinal gas and faeces. The intestinal bacteria also help prevent pathogenic microorganisms from establishing in the intestine. 

In a zymogen, a peptide blocks the active site of the enzyme. Cleaving off this peptide activates the enzyme. The peptidase in the stomach is pepsin. Pepsin works optimally in the acidic environment of the stomach, being active at pH 2 - 3, but becoming inactivated, when the pH is above 5. The chief cells at the base of the gastric glands secrete the zymogen, which is called pepsinogen. Activation of pepsinogen starts, when hydrocholoric acid (HCl), which is secreted by the parietal cells partially activates pepsinogen. This partially active enzyme, then cleaves the peptide from other pepsinogen molecules to form active pepsin.  

Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. It is carried out by passive, active or facilitated transport mechanisms. Small amounts of monosaccharides like glucose, amino acids and some of electrolytes like chloride ions are generally absorbed by simple diffusion. Active transport occurs against the concentration gradient and hence requires energy. Various nutrients like amino acids, monosaccharides like glucose are absorbed into the blood by this mechanism. Fatty acids and glycerol being insoluble, cannot be absorbed into the blood. They are first incorporated into small droplets called micelles, which move into the intestinal mucosa. They are re-formed into very small protein coated fat globules, called the chylomicrons, which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream.