Question

Describe the process of acid-digestion in the stomach. Identify in your answer, the major cells, enzymes and other secretions involved in the process and describe their interactions. Please also include in your answer a description of the three different phases of gastric secretion and the way in which the digestive secretions of the stomach, gall bladder and pancreas are regulated.

Answer 1

Digestion is the process of breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the body cells. During the process of digestion , proteins are converted into amino acids, carbohydrates are converted into simple sugars and fats are broken down into fatty acids and glycerol. Digestion involves the use of digestive enzymes and hormones. It takes place in different parts of the body such as oral cavity, stomach , small and large intestine.

In oral cavity, food is chewed , broken down into small particles and mixed properly. The breakdown of carbohydrates starts in mouth due to action of salivary amylase and lysozyme protect from infections.

In stomach, there are various gastric glands in the mucosa lining.The mucus neck cells secrete mucus. The Peptic Cells secrete the proenzyme pepsinogen. The Parietal or Oxyntic Cells secrete HCl (Hydrochloric acid) and intrinsic factor that is essential for vitamin B12 absorption. When food reaches stomach from oral cavity it gets mixed with gastric juices.The protein digestion takes place in stomach.In stomach the following reactions takes place

• HCl provides the acidic pH.
• Pepsinogen which is proenzyme is converted into Pepsin by HCl
• Pepsin acts on protein and convert them into peptones & proteoses.
• Prorenin which is also a proenzyme is converted into Renin by HCl.
• Renin acts on Casein (milk protein) and convert them into peptides.

After stomach food enters the small intestine. Organs such as pancreas release enzyme into instestine despite intestinal juice which already exist . The different chemical reactions are summarized as follows:

Action of pancreatic juice

• Amylase converts starch into Maltose.
• Enterokinase converts Trypsinogen which is a proenzyme into Trypsin
• Trypsin acts on  proteins and convert into Dipeptides
• Trypsin converts Chymotrypsinogen which is a proenzyme into Chymotrypsin.
• Chymotrypsin act on peptones and convert it into Dipeptides.
• Trypsin converts Procarboxypeptidase which is also a proenzyme into Carboxypeptidase.
• Carboxypeptidase converts proteoses into Dipeptides.
• Trypsin converts Proelastase into Elastase.
• Elastase converts elastin into Dipeptides.
• Pancreatic amylase act on  polysaccharides (Starch) and convert them into Disaccharides.
• Nucleases in the pancreatic juice act on nucleic acids and form nucleotides and nucleosides.

Action of intestinal  juice

• Maltase is an intestinal enzyme and converts maltose into glucose.
• Sucrase is an intestinal enzyme which converts sucrose into glucose & fructose.
• Lactase is an intestinal enzyme which converts lactose into glucose & galactose.
• Aminopeptidases convert peptides intomonomer amino acids.
• Dipeptidases convert dipeptides into monomer amino acids

Bile juice convert fat into diglycerides and monoglycerides..

Pancreatic lipase convert triglycerides to fatty acids and glycerol.