Describe four mechanisms of digestion

1.Digestion in the Oral Cavity
When food is taken in through the mouth, chewing and mixing of the food occurs. There is also a chemical breakdown of carbohydrates, due to the action of saliva from the salivary glands.
30% of the starch is hydrolyzed by the action of amylase, which is a salivary enzyme.
The other enzyme, lysozyme is an antibacterial agent that prevents infections.

Starch + Salivary amylase ? Maltose

Mastication of food and swallowing of food are the important activities that take place here in the oral cavity. Food is broken down into smaller particles by the chewing action of teeth.
As saliva is added, it mixes with the food particles, slowly moistening and lubricating the food. This small ball is called a bolus, which is then swallowed. The pharynx helps in the movement of the bolus into the oesophagus, from where it moves to the stomach through the peristaltic movements of the oesophagus.

2.Digestion in the Stomach
When food reaches the stomach, it stays for approximately 4 to 5 hours. There are various gastric glands in the mucosa lining of the stomach. 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.
Food in the stomach gets mixed thoroughly with the gastric juices through the churning movements of the stomach muscle.
This mass of food that is semi-digested, acidic and pulpy is called the chyme. It is mostly the proteins that get digested in the stomach.
The mucus and the bicarbonates of the gastric juice help in protecting the mucosal epithelium from the highly acidic HCl. Mucus also helps in lubricating the food.
Gastric juices and enzymes:
-HCl provides the acidic pH.
-Pepsinogen(proenzyme) is converted into Pepsin by HCl
-Pepsin, in turn, converts protein into peptones & proteoses.
-Prorenin (proenzyme) is converted into Renin by HCl.
-Casein (milk protein) is converted into peptides by Renin.
-After the action of the gastric juices and enzymes, food then enters the small intestine.

3.Digestion in the Small Intestine
In the small intestine, further digestion takes place. Due to the various movements of this organ, the chyme is further mixed and churned.
There are many enzymes that are secreted into the small intestine from organs such as pancreas, liver; apart from the intestinal juices. All these react with the food particles and digest them into smaller particles that can be absorbed into the bloodstream.
The different chemical reactions that occur are summarised below:
Pancreatic juices:
-Amylase converts starch into Maltose.
-Enterokinase converts Trypsinogen into Trypsin
-Trypsin converts proteins into Dipeptides
-Trypsin converts Chymotrypsinogen into Chymotrypsin.
-Chymotrypsin converts peptones into Dipeptides.
-Trypsin converts Procarboxypeptidase into Carboxypeptidase.
-Carboxypeptidase converts proteoses into Dipeptides.
-Trypsin converts Proelastase into Elastase.
-Elastase converts elastin into Dipeptides.
-Pancreatic amylase converts polysaccharides (Starch) into Disaccharides.
-Nucleases in the pancreatic juice, act on nucleic acids and form nucleotides and nucleosides.

Intestinal juices:
-Maltase converts maltose into glucose.
-Sucrase converts sucrose into glucose & fructose.
-Lactase converts lactose into glucose & galactose.
-Aminopeptidases convert peptides into amino acids.
-Dipeptidases convert dipeptides into amino acids
-Bile -Bile converts fat globules into fat droplets through a process called emulsification. Fats are broken down into diglycerides and monoglycerides.
-Pancreatic lipase – It converts triglycerides into fatty acids & glycerol.
The biomacromolecules are broken down in the duodenum region. All the simpler forms of the digested food are absorbed in the jejunum and ileum regions. Any leftover undigested, unabsorbed food particles are then passed on to the large intestine.

4.Digestion in the Large Intestine
In the large intestine, the digestion activity is significantly less. Here, bacterial action on the leftover food particles occurs.
Minerals, water, and certain drugs are absorbed in the large intestine. The mucus secreted by the large intestine helps in holding the waste particles, apart from lubricating it.
Any undigested and unabsorbed waste particles called as the faecal matter, are then passed to the rectum, from where it is eliminated through the anus.

---

Titany answered the question on September 13, 2021 at 07:54