BIOLOGY | Digestion.. Absorption.. Assimilation

May 24, 2023 / 11:47 AM IST

DIGESTION OF FOOD

The process of digestion is accomplished by mechanical and chemical processes.
The buccal cavity performs two major functions, mastication of food and facilitation of swallowing.
The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly.
Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.
The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition.
The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.
The gastro-oesophageal sphincter controls the passage of food into the stomach.
The saliva secreted into the oral cavity contains electrolytes and enzymes, salivary amylase and lysozyme.
The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase. About 30 percent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose.
Lysozyme present in saliva acts as an antibacterial agent that prevents infections.
The mucosa of stomach has gastric glands. Gastric glands have three major types of cells namely –
(i) mucus neck cells which secrete mucus;
(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and
(iii) parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for absorption of vitamin B12).
The stomach stores the food for 4-5 hours.
The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme.
The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach.
Pepsin converts proteins into proteoses and peptones (peptides).
The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid.
HCl provides the acidic pH (pH 1.8) optimal for pepsins.
Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins.
Small amounts of lipases are also secreted by gastric glands.
Various types of movements are generated by the muscularis layer of the small intestine. These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.
The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine.
Pancreatic juice and bile are released through the hepato-pancreatic duct.
The pancreatic juice contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases.
Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice.
The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases.
The intestinal mucosal epithelium has goblet cells which secrete mucus. The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice or succus entericus. This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.
The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner’s glands) also help in this.
Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice (Trypsin/Chymotrypsin, Corboxypeptidase etc.) into Dipeptides.
Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.
Fats are broken down by lipases with the help of bile into di-and monoglycerides.
Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides.
Dipeptides convert into the Amino acids by the enzyme dipeptidase, Maltose converts into the two glucose molecules by the enzyme maltase, Lactose converts into the one glucose and one galactose molecules by the enzyme lactase, sucrose converts into the one glucose and one fructose molecule by the enzyme sucrase. Di and mono glycerides converts into the fatty acids and glycerol by the enzyme lipase.
The breakdown of biomacromo lecules mentioned above occurs in the duodenum region of the small intestine.
The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine.
The undigested and unabsorbed substances are passed on to the large intestine.
No significant digestive activity occurs in the large intestine.
The functions of large intestine are:
(i) absorption of some water, minerals and certain drugs;
(ii) secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.
The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter. It is temporarily stored in the rectum till defaecation.
The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts.
The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.
Gastric and intestinal secretions are also, similarly, stimulated by neural signals.
The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS.
Hormonal control of the secretion of digestive juices is carried out by local hormones produced by the gastric and intestinal mucosa.

ABSORPTION OF DIGESTED FOOD PRODUCTS

Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.
It is carried out by passive, active or facilitated transport mechanisms.
Small amounts of monosac charides like glucose, amino acids and some electrolytes like chloride ions are generally absorbed by simple diffusion.
The passage of these substances into the blood depends upon the concentration gradients. However, some substances like glucose and amino acids are absorbed with the help of carrier proteins. This mechanism is called the facilitated transport.
Transport of water depends upon the osmotic gradient.
Active transport occurs against the concentration gradient and hence requires energy. Various nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.
Fatty acids and glycerol being insoluble, cannot be absorbed into the blood.They are first incorporated into small droplets called micelles which move into the intestinal mucosa. They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream.
Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. However, maximum absorption occurs in the small intestine.
A summary of absorption (sites of absorption and substances absorbed) is given in Table.

Biology

The absorbed substances finally areach the tissues which utilise them for their activities. This process is called assimilation.
The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal.
The egestion of faeces to the outside through the anal opening (defecation) is a voluntary process and is carried out by a mass peristaltic movement.
DISORDERS OF DIGESTIVE SYSTEM
The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections.
The infections are also caused by the parasites of the intestine like tapeworm, roundworm, thread worm, hookworm, pin worm, etc.
Jaundice
The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.
Vomiting
It is the ejection of stomach contents through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling of nausea precedes vomiting.
Diarrhoea
The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.
Constipation
In constipation, the faeces are retained within the colon as the bowel movements occur irregularly.
Indigestion
In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating, and spicy food.
Protein-Energy Malnutrition (PEM)
PEM Dietary deficiencies of proteins and total food calories are widespread in many underdeveloped countries of South and South-east Asia, South America, and West and Central Africa.
Protein-energy malnutrition (PEM) may affect large sections of the population during drought, famine and political turmoil.
This happened in Bangladesh during the liberation war and in Ethiopia during the severe drought in mid-eighties.
PEM affects infants and children to produce Marasmus and Kwashiorkar.
Marasmus
Marasmus is produced by a simultaneous deficiency of proteins and calories.
It is found in infants less than a year in age, if mother’s milk is replaced too early by other foods which are poor in both proteins and caloric value.
This often happens if the mother has second pregnancy or childbirth when the older infant is still too young.
In Marasmus, protein deficiency impairs growth and replacement of tissue proteins; extreme emaciation of the body and thinning of limbs results, the skin becomes dry, thin and wrinkled. Growth rate and body weight decline considerably. Even growth and development of brain and mental faculties are impaired.
Kwashiorkar
Kwashiorkar is produced by protein deficiency unaccom panied by calorie deficiency.
It results from the replacement of mother’s milk by a high calories low protein diet in a child more than one year in age.
Like marasmus, kwashiorkor shows wasting of muscles, thinning of limbs, failure of growth and brain development.
But unlike marasmus, some fat is still left under the skin; moreover, extensive oedema and swelling of body parts are seen.