Digestion involves two kinds of processes:
(a) Mechanical process which includes cutting and, grinding thus breaking the food into small particles; swallowing the food and then pushing the food along the food canal. Smaller particles expose greater surface area for action by enzymes.
(b) Chemical process which includes the enzymatic breakdown of complex food constituents (nutrients) into simpler absorbable form.
Digestion involves hydrolysis, i.e. spliting by addition of water (H+ and OH– ions) to a molecule resulting in its breakdown into two or more simpler molecules. The enzymes act only as catalysts to accelerate the reaction.
Mechanical Process in Digestion
The lips hold the food within the mouth and help in sucking it in and sipping liquids.
The teeth cut, tear and grind the food.
The tongue manipulates food while chewing, mixes saliva in it, rolls it into a ball termed as bolus and helps in swallowing.
The oesophagus conducts the food (bolus) down into the stomach by a wave of contraction of the circular muscles in the wall of alimentary canal. This wave of contraction is called peristalsis.
The stomach churns the food mixing it with gastric juice and produces a creamy chyme (partially digested food).
The peristaltic movements keep pushing the food from stomach to the intestine and finally pushing it into the rectum.
Chemical Processes in Digestion
1. In Mouth
Saliva contains only a single enzyme Amylase which acts on starch in two ways:
- Raw uncooked starch → Dextrins
- Cooked starch → Maltose
2. In Oesophagus
Food as bolus moves through obsophagus into the stomach by peristalsis. Salivary amylase continues digesting starch.
3. In Stomach
Initial digestion of starch by salivary amylase continues till the contents of stomach become acidic by presence of HCl. The gastric juice produced from the lining of the stomach is a colourless highly acidic liquid (pH 1-2). It contains water (98%), some salts, hydrochloric acid (0.5%), the lubricant mucin and two enzymes pepsin and lipase.
Hydrochloric acid is secreted by Oxyntic (parietal) cells of the stomach wall. It performs the following functions:
- kills bacteria entering along with food
- loosens fibrous material in food
- activates the inactive pepsinogen to its active form pepsin
- maintains acidic medium for action by pepsin
- curdles milk so that it does not flow out and stays for action by pepsin
Pepsin is secreted in its inactive form or the proenzyme called pepsinogen secreted from the chief cells of the stomach wall. In the presence of HCl it turns into the active pepsin which acts on proteins and breaks them down into proteoses and peptones.
Protein → Proteoses and Peptones
4. Small Intestine
In the small intestine the food which is partially digested in the stomach, and called chyme is acted upon by three main digestive juices.
- Bile juice from the liver
- Pancreatic juice from the pancreas
- Intestinal juice secreted from special cells in the intestinal epithelium at the base of intestinal villi
The bile juice and pancreatic juice are poured into the duodenum by their respective ducts which join together to form a common hepato pancreatic duct. The intestinal juice directly mixes with the food.