Digestion of Food

Digestion is the biological process by which complex food molecules are broken down into simpler substances that the body can absorb and utilize for energy, growth, and repair. It involves both mechanical and chemical processes that occur in a coordinated manner within the digestive system.

1.0Digestion In Oral Cavity 

• The buccal cavity has two primary functions: chewing (mastication) of food and aiding in swallowing. When food enters the mouth, it is tasted in the oral cavity and combined with saliva. 
• The tongue helps mix the food with saliva, forming a soft mass called the bolus. Saliva, which has a pH of 6.8–7.0, is composed of 99.5% water and contains electrolytes such as sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻).
• Digestion is accomplished by mechanical and chemical digestion. Mechanical digestion takes place in the mouth and small intestine.

Mechanical digestion 

• The mouth, teeth, tongue, and lips play important roles in mechanical digestion through chewing or mastication.

Ptyalin (Salivary amylase) 

• Starch Ptyalin  pH 6.8  Maltose 
• Ptyalin is found in human saliva because human food is mainly made up of starch. 
• Ptyalin digests only ripe and cooked starch. 30% starch in the buccal cavity is digested by ptyalin. 
• Ptyalin is absent in the saliva of rabbits and carnivorous animals. 
• Bolus is pushed inward through the pharynx into the Oesophagus, a process called swallowing or deglutition. It is a coordinated activity of the tongue, soft palate, pharynx, and Oesophagus.  
• The tongue blocks the mouth, part of the soft palate and uvula close off the internal nasal opening, and the nose and the larynx rise so that the epiglottis closes off the trachea and moves downward into the Oesophagus. 
• A travelling wave of constrictions called peristalsis pushes the Bolus (food) downward. 
• Digestive enzymes are absent in the Oesophagus, but salivary amylase continues to function up to the end of the Oesophagus. 
• The cardiac sphincter/Gastroesophageal sphincter opens, allowing Bolus to the stomach. The gastroesophageal sphincter remains typically closed and does not allow the stomach's food contents to move back. 

2.0Digestion In Stomach

• The stomach stores food for 4-5 hours, during which it mixes thoroughly with the acidic gastric juice through the churning movements of its muscular walls, forming a semi-liquid substance called chyme. 
• Hydrochloric acid activates the proenzyme pepsinogen, converting it into pepsin, a proteolytic enzyme responsible for breaking down proteins into proteoses and peptones (peptides).
• Mucus and bicarbonates in the gastric juice help lubricate and protect the stomach's mucosal lining from damage caused by the highly concentrated hydrochloric acid. 
• Upon the arrival of food in the stomach, G-cells release the hormone gastrin, which stimulates the secretion of gastric juice. The regulation of gastric juice secretion involves neural, hormonal, and chemical mechanisms.

Composition of Gastric juice 

• Water = 99.5% 
• pH = 1.8 (very acidic) 
• rest = mucus, water, HCl and gastric enzymes (Pepsinogen, Prorennin, Gastric Lipase, etc.). 

Functions of HCl 

• The main function of HCl (activator) is to convert inactive enzymes (zymogens) into active enzymes. 

• It destroys the bacteria present in the food. 
• HCl stops the action of saliva on food. In the stomach, the medium is highly acidic. 
• It dissolves the hard portions of the food and makes it soft.  
• Pepsinogen and Prorennin are inactive enzymes.

Digestion by Rennin (Chymosin)

• Rennin is secreted in mammals' childhood stages. It converts milk into a curd-like substance (clots the milk) and then digests it. It is absent in the adult stages.  
• Rennin acts on milk protein casein. Casein is a soluble protein.  
• In the presence of Rennin, casein gets converted into insoluble Ca-paracaseinate. 
• This process is termed curdling of milk. 
• After becoming insoluble, milk can remain in the stomach longer. (clotting/coagulation/curdling of milk is done by HCl, pepsin and chymotrypsin in adult humans). 
  

Digestion by Pepsin

• Inactive pepsinogen on getting proper pH converts into active pepsin. 
• Proteins ^Pepsin  _{pH 1.8}  Peptones + Proteoses [Peptides]

Digestion by Gastric Lipase

• It converts fats into fatty acids and monoglycerides. Because it is secreted in a smaller amount, less fat digestion takes place here.  
• This lipase acts on emulsified fat and converts it into fatty acids and monoglycerides. One per cent of emulsified fat is present in food.  
• Peristalsis (Churning movement) continues during digestion, so the gastric juice mixes properly with the food. 
• Due to peristalsis, the food is converted into a paste. This form of food, which is thick, acidic, and semi-digested in the stomach, is called chyme.  
• After short intervals, the pyloric orifice keeps opening and closing so the chyme is fed into the intestine in installments.

3.0Digestion of Food In Small Intestine

• In the small intestine mechanical and chemical digestion occurs.   
• The bile, pancreatic juice and intestinal juice are the secretions released into the small intestine.

Mechanical Digestion : Food reaches to different parts of the alimentary canal by peristalsis.

(A) Digestion in Duodenum 

(i) Pancreatic,  Alpha - Amylase

• Amylase dissociates starch into Maltose. The majority of starch breaks up into the duodenum.

(ii) Protein digestion in duodenum

(iii) Fat digesting enzyme

• In pancreatic-juices various Fat-digesting enzymes are found which are collectively called steapsin.  
• Pancreatic Lipase - It converts triglyceride into monoglyceride and diglyceride.  
• Cholesteryl Esterase - It digests cholesteryl esters. These esters are made up of cholesterol and fatty-acids Like- Lanolin, (cholesterol and Palmitic acid).  
• Phospholipase - These digest phospholipids.

(iv) DNase and RNase (Nucleases) 

• Digestion of DNA and RNA. 
• In the duodenum, biomacromolecules are digested in the presence of bile juice and pancreatic juice, which are released through the hepatopancreatic duct.

(B) Digestion in jejunum and ileum

• Final steps in digestion occur very close to the mucosal epithelial cells of the intestine. Some hormones stimulate the crypts of Leiberkuhn to secrete Succus-entericus or intestinal juice. 
• The secretions of the brush border cells of the mucosa along the secretion of goblet intestinal juice. This succus entericus mainly contains water (99%) and digestive enzymes (<1%). 
• Succus-entericus mainly contains the following enzymes-

(i) Peptidase or Erepsin 

• Dipeptides Dipeptidases  Amino acid. 
• It converts oligopeptides into amino-acids. 

(ii) Disaccharidases     

• Sucrase (Invertase) SucroseGlucose + Fructose     
• Maltase Maltose Glucose + Glucose      
• Lactase Lactose Glucose + Galactose  

(iii) Intestinal Lipase 

• This fat-digesting enzyme converts fats into fatty-acids and glycerol. 
• Lipase Di and monoglyceride Fatty acid Glycerol   

(iv) Nucleotidase and Nucleosidase - These act in the following way :- 

(a) Nucleotides Nucleotidase  Nucleosides + Phosphate 

(b) Nucleosides Nucleosidase  Pentose + Nitrogen base 

• The simple substances thus formed are absorbed in the Jejunum and ileum region of the small intestine. 
• The undigested and unabsorbed substances are passed into the large intestine. 

4.0Digestion In Large Intestine

• In herbivores, the symbiotic bacteria and protozoans present in the caecum help in digestion of cellulose into glucose. 
• So the digestion of cellulose takes place in caecum by the process of decomposition. 
• This decomposition process is very slow. So very less amount of cellulose is digested at a time in caecum.  
• In the last part of the large intestine faeces is temporarily stored. 
• Maximum digestion of food – Duodenum. 
• While digestion of food is complete in – Ileum. 
• Maximum absorption of food in – Jejunum. 

Regulation of digestion

• The gastrointestinal tract's activities are controlled by both neural and hormonal mechanisms to maintain coordination between its various parts. 
• The sight, smell, or presence of food in the mouth stimulates saliva secretion. 
• Likewise, gastric and intestinal secretions are activated by neural signals. 
• Neural regulation, whether through local reflexes or the central nervous system (CNS), also governs the muscular movements of different sections of the alimentary canal. 
• Additionally, hormones released by the gastric and intestinal mucosa play a key role in managing the secretion of digestive juices.

Role of some major gastrointestinal hormones