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Explain the process of digestion in human beings.

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Hint: Humans consume food to generate energy and it is also used for the growth and maintenance of the body. Human food consists of carbohydrates, fats, proteins, minerals, vitamins, and water, which break down with the help of enzymes and get distributed to various parts of the body.

Complete answer:
> Digestion starts in the oral cavity where food gets mixed with saliva which consists of a mixture of water, electrolytes, mucus, serous fluid, salivary amylase, and lysozyme. 
> Salivary amylase helps to break down the starch into maltose, isomaltose, and dextrin with the help of the enzyme salivary amylase. 
> The mucus of saliva helps to moisten the food and pass it into the oesophagus. 
> Digestion continues as the bolus passes down from the oesophagus to the stomach. 
> Bolus gets to enter into the stomach through the cardiac orifice. In the stomach, food gets mixed with gastric juice and enzyme-like pepsinogen, prorenin, lipases, and mucus.
> Gastric juice helps to maintain the pH of the stomach and it also converts pepsinogen and prorenin into pepsin and renin which help to break down protein into proteases, peptones, and large peptides. Rennin is only found in the infant which breaks milk protein casein to paracasein and this paracasein gets converted into calcium paracaseinate. 
> The partially digested food then enters the duodenum through the pyloric sphincter. Duodenum receives digestive juice through the pancreas which is known as pancreatic juice and bile from the liver. 
> Pancreatic juice consists of trypsin, chymotrypsin, carboxypeptidase, pancreatic amylase, and lipase. This enzyme is secreted in inactive form trypsinogen, chymotrypsinogen, and procarboxypeptidase. 
> Trypsinogen gets converted into trypsin by enterokinase which is an intestinal juice, chymotrypsinogen gets converted into chymotrypsin by the enzyme trypsin and procarboxypeptidase get converted into carboxypeptidase by trypsin. 
> Trypsin and chymotrypsin help to break down the protein into large peptides. Carboxypeptidase converts large peptides into dipeptides and amino acids. 
> Pancreatic amylase converts starch into maltose, isomaltose and dextrin. Fat gets broken down with the help of bile into emulsified fat and this emulsified fat breaks down into fatty acid, diglyceride, monoglyceride, and glycerol with the help of pancreatic lipase. 
> Nucleotides get converted into nucleosides, inorganic phosphate, nitrogen bases, and pentose sugar. 
> The small intestine is the principal organ for digestion and absorption and digestion also gets completed here. In the large intestine, the only absorption of water and minerals takes place.

Note: Gastric juice gets mixed with food and forms chyme when this chyme gets poured into the oesophagus and if it is in huge amounts (that is overeating or spicy food) it can cause the burning of some cells. This burning causes heartburn or acidity. It is called heartburn because pain occurs near the heart but it does not cause any damage to the heart.