Important Notes for NEET: Digestion & Absorption | Biology for JAMB PDF Download

Food is a vital component for all living organisms as it provides energy and essential building materials for growth and tissue repair. It primarily consists of carbohydrates, proteins, and fats, which have a calorific value. Additionally, vitamins and minerals are essential components of a healthy diet, despite not containing any calorific value. Water intake is also crucial for the proper functioning of our metabolism and preventing dehydration. However, the biomacromolecules present in food cannot be utilized by our body in their original form.

What is Digestion?

• The process of converting complex food substances into simple, absorbable forms is vital for our body to make the most of the nutrients present in the food we eat. This process of conversion, known as digestion, is carried out by our digestive system through mechanical and biochemical methods. 
• Mechanical digestion involves physical movements such as chewing and stomach contractions that help break down food into smaller pieces, while biochemical digestion involves enzymes and acids that break down the chemical bonds in food. 
• These processes work together to convert food into simple substances that can be absorbed by the body and utilized for energy and growth. Without this intricate process, our body would not be able to obtain the necessary nutrients from food, leading to deficiencies and poor health.

The Human Digestive System

1. The alimentary canal.
2. The associated glands.

Alimentary Canal

• The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus. 
• The mouth leads to the buccal cavity or oral cavity. 
• The oral cavity has a number of teeth and a muscular tongue. 
• Each tooth is embedded in a socket of jaw bone → Thecodont teeth.
• Human being forms two sets of teeth during their life → Diphyodont teeth.
  ⇒ A set of temporary milk or deciduous teeth.
  ⇒ A set of permanent or adult teeth.
• An adult human has 32 permanent teeth.
• Four different types → Heterodont dentition.
   ⇒ Incisors (I), Canine (C), Premolars (PM) and Molars (M).
• Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is 2123 / 2123.
• Enamel → The hard chewing surface of the teeth → Helps in the mastication of food (Mechanical Process).

(i) Tongue

• Freely movable muscular organ.
• Attached to the floor of the oral cavity by the frenulum. 
• The upper surface has small projections called papillae, some of which bear taste buds. 

(ii) Pharynx

• The oral cavity leads into a short pharynx.
• Serves as a common passage for food and air.
• The oesophagus and the trachea (wind pipe) open into the pharynx.
• A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the wind pipe – during swallowing.

(iii) Oesophagus

• A thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm.
• Leads to a ‘J’ shaped bag like structure called stomach.
• A muscular sphincter (gastro-oesophageal) regulates the opening of oesophagus into the stomach.

(iv) Stomach

• Located in the upper left portion of the abdominal cavity.
• Has three major parts:
  ⇒ A Cardiac portion into which the oesophagus opens.
  ⇒ A fundic region.
  ⇒ A pyloric portion which opens into the first part of small intestine.

(v) Small Intestine

• Divided into three regions:
  ⇒ A ‘C’ shaped duodenum.
  ⇒ A long coiled middle portion jejunum.
  ⇒ A highly coiled ileum.

Small Intestine

• The opening of the stomach into the duodenum is guarded by the pyloric sphincter.
• Ileum opens into the large intestine.

(vi) Large Intestine

• It consists of:
  ⇒ Caecum
  ⇒ Colon
  ⇒ Rectum
  
  Large Intestine
• Caecum is a small blind sac which hosts some symbiotic micro-organisms. 
• Vermiform appendix
  ⇒ A narrow finger-like tubular projection.
  ⇒ A vestigial organ, arises from the caecum.
• The caecum opens into the colon.
• The colon is divided into four parts:
  ⇒ An ascending part
  ⇒ A transverse part
  ⇒ A descending part
  ⇒ Sigmoid colon
• The descending part opens into the rectum which opens out through the anus.

(vii) The wall of Alimentary Canal

• Possesses four layers namely:
  ⇒ Serosa
  ⇒ Muscularis
  ⇒ Sub-mucosa
  ⇒ Mucosa

I. Serosa

• The outermost layer.
• Made up of a thin mesothelium (epithelium of visceral organs) with some connective tissues.

II. Muscularis

• Formed by smooth muscles usually arranged into
  ⇒ An inner circular layer.
  ⇒ An outer longitudinal layer.
•  An oblique muscle layer may be present in some regions.

III. Submucosal layer

• Formed of loose connective tissues containing nerves, blood and lymph vessels.
• In duodenum, glands are also present in sub-mucosa.

IV. Mucosa

• The innermost layer.
• Lining the lumen of the alimentary canal.
• This layer forms
  ⇒ Irregular folds (rugae) in the stomach.
  ⇒ Small finger-like foldings called villi in the small intestine.
• The cells lining the villi produce numerous microscopic projections.
• Microvilli
  ⇒ Giving a brush border appearance.
  ⇒ Increase the surface area enormously.
• Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal.
• Mucosal epithelium has goblet cells.
  ⇒ Goblet cells secrete mucus → help in lubrication.
• Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn).
• All the four layers show modifications in different parts of the alimentary canal.

Digestive Glands 

• Associated with the alimentary canal
• These are:
  ⇒ The salivary glands
  ⇒ The liver
  ⇒ The pancreas

(i) Salivary Glands 

• Three pairs of salivary glands:
  ⇒ The parotids (cheek)
  ⇒ The sub-maxillary/sub-mandibular (lower jaw)
  ⇒ The sublinguals (below the tongue)
• These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity. (Saliva is mainly produced).

(ii) Liver

• Largest gland of the body.
• Weighing about 1.2 to 1.5 kg in an adult human.
• It is situated
  ⇒ In the abdominal cavity.
  ⇒ Just below the diaphragm.
• Liver has two lobes.
• The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords.
• Each lobule is covered by a thin connective tissue sheath called the glisson’s capsule.
• The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder.
• The duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct.
• The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of oddi.

(iii) Pancreas

• A compound elongated organ.

Pancreas

• Situated between the limbs of the ‘C’ shaped duodenum.
• Have both exocrine and endocrine portions
• The exocrine portion 
  ⇒ Secretes an alkaline pancreatic juice containing enzymes.
• The endocrine portion
  ⇒ Secretes hormones, insulin and glucagon. 

Digestion of Food 

• Involves mechanical and chemical processes.
• The buccal cavity performs two major functions:
  ⇒ Mastication of food.
  ⇒ Facilitation of swallowing.
• The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly.
• Role of 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 per cent 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:
  ⇒ Mucus neck cells which secrete mucus.
  ⇒ Peptic or chief cells which secrete the proenzyme pepsinogen.
  ⇒ Parietal or oxyntic cells which secrete HCl and intrinsic factor. Intrinsic 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 HCl 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
  ⇒ A proteolytic enzyme.
  ⇒ Found in gastric juice of infants.
  ⇒ Helps in the digestion of milk proteins.
• Small amounts of lipases are also secreted by gastric glands.
• Small intestine movements are generated by the muscularis layer
  ⇒  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
  ⇒ 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.
• The bile contains:
  ⇒ bile pigments (bilirubin and bili-verdin)
  ⇒ bile salts
  ⇒ cholesterol and
  ⇒ phospholipids
• The bile → enzymes absent
  ⇒ Bile helps in emulsification of fats i.e., breaking down of the fats into very small micelles.
• Bile → activates lipases.
• The intestinal mucosal epithelium has goblet cells which secrete mucus.
• Intestinal juice or succus entericus 
  ⇒ The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells.
  ⇒ 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.
• 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.
• The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms.
• These final steps in digestion occur very close to the mucosal epithelial cells of the 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:
  ⇒ Absorption of some water, minerals and certain drugs;
  ⇒ 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.
• Faeces enters into the caecum of the large intestine through ileocaecal valve, which prevents the back flow of the faecal matter.Faeces temporarily stored in the rectum till defaecation. 

Control of Digestive System

• Under neural and hormonal control.
• 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 Products

• Absorption → 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. 
• Simple diffusion → Small amounts of monosaccharides like glucose, amino acids and some electrolytes like chloride ions.
• Facilitated transport → Some substances like glucose and amino acids are absorbed with the help of carrier proteins.
• Transport of water depends upon the osmotic gradient.
• Active transport → occurs against the concentration gradient and hence requires energy.
  Example: 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. 
• The absorbed substances finally reach the tissues which utilise them for their activities. This process is called assimilation.

Defaecation

• The egestion of faeces to the outside through the anal opening.
• Faeces in the rectum initiate a neural reflex.
• 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, threadworm, hookworm, pin worm, etc.

• Jaundice
  ⇒ The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.
• Vomiting
  ⇒ Ejection of stomach contents through the mouth.
  ⇒ This reflex action is controlled by the vomit centre in the medulla.
• Diarrhoea
  ⇒ The abnormal frequency of bowel movement.
  ⇒ Increased liquidity of the faecal discharge.
  ⇒ It reduces the absorption of food.
• Constipation
  ⇒ The faeces are retained within the rectum.
  ⇒ The bowel movements occur irregularly.
• Indigestion
  ⇒ The food is not properly digested.
  ⇒ Leading to a feeling of fullness.
  ⇒ The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.