Human Digestive System

There are many bacteria that live in the large intestine of human. They live in symbiotic association. These are Lactobacillus, Bifidobacterium, Methanogens, Bacteroids, Enterococcus, Escherichia coli. Lactobacillus help in digestion, maintaining the pH balance and replacing the healthy bacteria in the gastrointestinal tract. It acts as a probiotic. Bifidobacterium is also a probiotic. It helps in metabolism of undigested carbohydrates. It also prevents pathogen colonization. Methanogen helps in the fermentation of organic matter to obtain energy. They help in breakdown of complex indigestible carbohydrates. Bacteroids helps in amino acid biosynthesis. Enterococcus helps in digestion of undigested carbohydrates and fibres.

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 major function of the large intestine is to absorb water from the remaining indigestible food matter and transmit the useless waste material from the body.

The large intestine also secretes K+ and Cl-. Chloride secretion increases in cystic fibrosis. Recycling of various nutrients takes place in colon. Examples include fermentation of carbohydrates, short chain fatty acids and urea cycling. 

The wall of the large intestine is lined with simple columnar epithelium. Instead of having the evaginations of the small intestine (villi), the large intestine has invaginations (the intestinal glands). While both the small intestine and the large intestine have goblet cells, they are abundant in the large intestine. 

Protein digestion begins in the stomach with pepsin and continues in the duodenum with pancreatic enzymes like trypsin and chymotrypsin.

• Digestion of protein is initiated in the stomach by the enzyme pepsin.
  
• In the small intestine, further digestion is done by trypsin, chymotrypsin, and carboxypeptidase into dipeptidase that releases from the pancreas.
• Dipeptidase are broken into amino acids by intestinal dipeptidase.
  
• These amino acids are absorbed actively by walls of intestine and release in the bloodstream.
• These are utilized by tissues and broken down into carbon dioxide and ammonia.
• Proteins require maximum energy to digest.

 Parietal cells of stomach secrets hydrochloric acid which convert inactive pepsinogen to partially active pepsinogen, partially activates pepsinogen then cleaves the peptide from other pepsinogen molecules to form active pepsin which converts protein into proteases and peptides.

(i) Pepsin digest protein in the stomach.

A) Amylase is produced by buccal cavity, pancreas and intestine and digest starch.

If pH of the stomach is made 7 protein digestion will affect as pepsin works ar pH of 2 to 3 and enzymes are highly precise about there function so it does not activate.

The HCl is secreted by the oxyntic cells in the stomach. The HCl provides the acidic medium in the stomach that is responsible for converting the inactive pepsinogen to the active pepsin. Pepsin is required for protein digestion. The acidic medium provides defence against bacteria entering the digestive system.

• Protein digestion occurs in the stomach and duodenum in which 3 main enzymes: Pepsin, trypsin and chymotrypsin.
•  pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. 
• Hence, Lipase enzymes are not useful in the digestion of proteins.
• So, the correct answer is 'Lipase'.

The digestion of the food starts at the mouth where the enzyme amylase is secreted which digests the start that is present in the food and converts it into maltose. From there through the esophagus the food arrives in the stomach where it is stored for 4 hours and mixed with acid. The food when arrives at the stomach where the wall of the stomach secrets HCl with a pH of 1.8. This acid activates the inactive pepsinogen enzyme and forms active pepsin which digests proteins and converts them into proteases and peptides. From here the food moves to the small intestine where secretion from the liver and pancreas reach through a hepatopancreatic duct which is alkaline in nature and neutralizes the acidic food that comes from the stomach. Here majority of the digestion of protein to amino acids, fat to fat droplets, polysaccharides to monosaccharides etc takes place.