Question

What is meant by digestion?

Answer 1

Digestion is a process in which the large food particles are broken down into small absorbable particles in the digestive GI tract. It starts from the mouth and ends up at the rectum-anus.

Complete step by step answer:
Digestion begins from the mouth by chewing the food particles. The salivary amylase enzyme present in the saliva breaks complex carbohydrates into comparatively simpler forms. The chewed food particle then runs down the esophagus from the buccal cavity. The epiglottis present allows the food to pass through the esophagus and prevent it from entering the wind-pipe. The food particles pass through the esophagus due to the downward peristalsis movement.
The food particles now enter the stomach through a sphincter. In stomach cells of stomach lining that are parietal cells, goblet cells and chief cells produce HCl, pepsinogen, which gets activated in pepsin in the presence of acid, and mucus to protect the epithelial lining. Hormone like gastrin is also produced in the stomach. The food is now called the chime and passes to the small intestine, where the bile duct secretes enzyme cholecystokinin that helps in the emulsification of lipid particles in chime. Pancreas secretes pancreatic amylase, lipases, peptidases, and elastases to the small intestine and allows the digestion of lipids, complex carbohydrates, and proteins. They are broken down into fatty acids, simple sugars, and amino acids. The pancreatic enzymes are initially in the zymogen form but later get converted into the active form of enzymes along with bicarbonate to neutralize the chime.
Now the digested food particles enter the large intestine, where the molecules get absorbed through the villi present on the epithelial lining. The biomolecules are then assimilated in all the cells and finally, the unabsorbed food particles are excreted out through the anus.

Note:
The digested food particles are absorbed in the form of amino acids, monosaccharides, and fatty acids. These then allow the mitochondria to provide ATP, or energy for the cell to function.