Human Digestive System

Protein in invertebrates is a breakdown by four different enzymes known as a proteinase, carboxypolypeptidase, aminopolypeptidase, and dipeptidase, which can be separated by adsorption but vertebrates have one extra enzyme trypsin is also found that helps in activation of the given four and digestion of protein.

Amylase, rennin and trypsin are protein enzymes.

Pancreatic amylase is an enzyme secreted by the pancreas into the small intestine that specifically breaks down complex carbohydrates like starch into simpler sugars.

Various types of movements are generated by the muscularis layer of the small intestine. These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion. The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine. Pancreatic juice and bile are released through the hepato-pancreatic duct. The intestinal mucosal epithelium has goblet cells, which secrete mucus. The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice or succus entericus.

Small intestine has three parts - duodenum, jejunum and ileum. When food leaves the stomach through its pyloric end and enters the duodenum it is called chyme (acidic). The digestion of macro molecules ingested is completed in small intestine. The digested products are absorbed in the small intestine and comes to the blood.

• Small intestine has three parts - duodenum, jejunum, and ileum. 
• The maximum digestion of food takes place in the duodenum. While the digestion of food is completed in jejunum.
•  The maximum absorption of food takes place in ileum.

Oxyntic cells or parietal cells, are the stomach epithelium cells that secrete gastric acid intrinsic factor. These cells secrete hydrochloric acid (HCl), which makes the gastric juice acidic (pH =2 -3). 

Protein digestion occurs in stomach, duodenum, jejunum and ileum. It is completed in ileum.

Pancreatic juice contains a variety of enzymes, including pancreatic amylase for starch and trypsin/chymotrypsin for proteins, making it capable of digesting both.

Lactase is a brush-border enzyme located in the small intestine that breaks down lactose into glucose and galactose.

Pancreatic juice is liquid secetrted by pancreas . It is alkaline in nature and it has following enzymes lipase, for digesting fats, amylase for digesting starch, trypsinogen for digesting proteins.

1. Amino acids and proteins are the building blocks of life. Amino acids are organic compounds that combine to form proteins.
2.  When proteins are digested or broken down, amino acids are left. Zymogenic cell or peptic cell in the stomach that releases pepsinogen and chymosin. 
3. This inactive pepsinogen, on exposure to hydrochloric acid, gets converted to the active enzyme pepsin.
4.  Pepsin is produced in the stomach and is one of the main digestive enzymes, which helps in digestion of the proteins to polypeptide chains. The end product of protein is it must be broken down into amino acids. Hence, the end product of protein digestion is amino acids.

Enteropeptidase is an enzyme produced by the cells of the duodenum and is involved in the digestion in humans and other animals. Enteropeptidase converts trypsinogen (a zymogen) into its active form trypsin, resulting in the subsequent activation of pancreatic digestive enzymes

Parietal cells secrete HCl and intrinsic factor. Parietal glands also secrete pepsinogen to which hydrochloric acid acts to convert it into pepsin. Pepsin in return causes digestion of protein. If parietal cells become non-functional, it will directly affect protein digestion. So, the correct answer is (d).