NEET Biology Notes Digestion

NEET Biology Notes Digestion 

Digestion

Digestion is a process which involves breakdown of complex food substances by hydrolysis into smaller molecules that can be absorbed through the epithelium of the gastro-intestinal tract. Human and other animals have holozoic nutrition, i.e. intake of solid or liquid form of food.

Human Digestive System

The system which helps in the complete process of digestion by mechanical and biochemical methods is called digestive system.
The human digestive system consists of the alimentary canal and the digestive glands.

Alimentary Canal

The anterior opening of alimentary canal is mouth and posterior are in the anus. Alimentary canal consists of different structures which are described below:

1. Oral Cavity
   The cavity which opens through mouth is called oral cavity, it contains teeth and muscular tongue.
2.  Teeth
   Each tooth is embedded in a socket of jaw bone. This attachment is called thecodont. Humans and majority of mammals have two sets of teeth during their life, a set of temporary teeth and a set of permanent 32 adult teeths. This type of dentition is called diphyodont.
   Adult human has heterodont dentition. The arrangement of teeth in each half of the upper and lower jaw is represented by dental formula. The arrangement order is, i.e. Incisors (I), Canine (C), Premolar (Pm) and Molar (M). The dental formula in human isElephant tusks are modified incisers, tusks of nabus are modified canines.
3.  Tongue or Lingua
   The muscular organ tongue is attached to the floor of oral cavity by the frenulum. It has small projections called papillae on its upper surface, some of them contain taste buds.
4.  Oesophagus
   The oesophagus and the trachea open into the pharynx which is a common passage for food and air.
   The oesophagus is 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.
5. Stomach
   The stomach, located in the upper 1 ft portion of the abdominal cavity has three major parts, i.e. a cardiac portion, a fundic portion and a pyloric portion
6. Small Intestine
   It is distinguishable into three regions, a U-shaped duodenum, a long coiled middle portion jejunum and a highly coiled ileum. It is the widest, shortest and most flexed part of small intestine .
   Ileum opens into the large intestine.
7.  Large Intestine
   Although it. is shorter but, is called large intestine, because it is wider in diameter than small intestine.It consists of following parts :(a) Caecum It hosts some symbiotic microorganisms.
   (b) Colon It is divided into three parts, i.e. an ascending, a transverse and a descending part.
   (c) Rectum The wall of alimentary canal from oesophagus to rectum possesses four layers, i.e. serosa, muscularis, sub-mucosa and mucosa.

Internal Structure of Alimentary Canal

The innermost layer lining the lumen of the alimentary canal is the mucosa. This layer forms irregular folds in the stomach and small finger-like foldings called villi in the small intestine. The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance.
Mucosa also forms glands in the stomach and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn).

Digestive Glands

The digestive glands are salivary glands, liver, pancreas and Brunner’s gland.

•  The salivary glands are three pairs, i.e. the parotids
  (cheek), the sub-maxillary/sub-mandibular (lower jaw) and the sub-lingual (below the tongue) which secrete salivary juice into the buccal cavity.
• Liver is the largest gland of the body weighing about 1.2-1.5 kg in an adult human. 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 in gall bladder.
  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 sphincter of Odii.
•  The pancreas is a compound organ (both exocrine and endocrine) situated between the U-shaped duodenum. The exocrine part secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon.
• Brunner’s glands are branched, tubuloalveolar submucosal glands in the duodenum, whose, secretion is rich in bicarbonate ions and alkaline glucoproteins, hence, it neutralizes the acidic chyme entering the duodenum from the stomach.

Process of Digestion

The process of digestion is defined as ‘the process in which large macromolecules of food are broken down into smaller usable molecule with the help of enzymes.’ It takes place in following steps:

• Ingestion of Food

Food is taken through mouth cavity. It is masticated by teeth and. swallowed. Ingestion takes place in buccal cavity. Salivary glands lubricate the food and bind the food particles together to form bolus. Salivary glands have starch splitting enzyme ptyalin.

• Digestion of Food

Process of converting complex, insoluble, food particles into simple, soluble and absorbable form is called digestion.

• Digestion in buccal cavity In buccal cavity, salivary amylase acts on starch.
•  Digestion in stomach The food passes down through the oesophagus into stomach. Now, food is mixed with gastric juice and hydrochloric acid, which disinfect the food and create acidic medium. Pepsin digests proteins and converts them into peptones and proteoses. Rennin converts milk to curd. Digested food now is called chyme.
•  Digestion in small intestine Chyme moves to duodenum. Food is mixed with bile (liver) to breakdown fats into smaller globules. Trypsin acts upon proteins and breaks them into peptides. Amylase converts starch into simple sugar. Lipase converts fats into fatty acids and glycerol. Food passes into ileum and mixes with intestinal juice. Maltase converts maltose into glucose. Lactase converts lactose into glucose and galactose. Sucrase converts sucrose into glucose and fructose. Trypsin digests the peptides into amino acids. Now, this digested food is called as chyle.

Absorption and Assimilation of Digested Food

Ileum’s internal surface has finger-like folds called villi. There is a dense network of blood capillaries and lymph capillaries in each villi. It helps in absorption of food.
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.
Transport of water depends upon the osmotic gradient. Active transport occurs against the concentration gradient and requires energy. Nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.
Fatty acids and glycerol are re-formed into very small protein coated fat globules called the chylomicrons, which are transported into the lymph vessels (lacteals) in the villi. The absorbed substances finally reach the tissues, which utilise them for their activities. This process is called assimilation.

The Summary of Absorption in Different Parts of Digestive System

Stomach Absorption of water, simple sugars and alcohol, etc., takes place. Cellulose is not digested in human beings.
Small Intestine Principal organ for absorption of nutrients where digestion is completed and the final products of digestion such as glucose, fructose, fatty acids, glycerol and amino acids are absorbed through the mucosa into the blood stream and lymph.
Large Intestine Absorption of water, some minerals and drugs takes place. Some additional points related with absorption are following:
Tea/Coffee inhibit the absorption of iron from the diet. Prolonged consumption of these can lead to anaemia.
Saccharine has a sweet taste but it is not a sugar.
Rennin is present in the infants stomach. It is absent in the adults.

Egestion of Undigested Food

Digested food passes into large intestine. Large intestine cannot absorb food but absorbs much of the water. The remaining semi-solid waste is called faeces and is passed into rectum. The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal. It is expelled out through the anus.
High temperature shuts off the appetite centre that is why one does not feel like taking meals during high fever.

Role of Digestive Enzymes

Digestive enzymes are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body.
Digestive enzymes are found in the digestive tracts of animals (including humans) and in the traps of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival.

• Gastric juice of infants contains pepsin lipase and rennin.
• Rennin helps in the digestion of milk.
• Carbohydrates

Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.

• Fats

Fats are broken down by lipases with the help of bile into di and monoglycerides

• Proteins

Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice.

• Nucleic Acids

Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides

The enzymes in the succus entericus (intestinal juice) act on the end products of above reactions to form following simple absorbable forms :

 

Gastrointestinal Hormones Related to Digestion

The endocrine cells present in different parts of gastrointestinal tract secrete following hormones. All these hormones are proteinaceous in nature.

• Gastrin It acts on gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen.
• Secretin It acts on exocrine pancreas and stimulates secretion of water and bicarbonate ions.
• Cholecystokinin (CCK) It stimulates the secretion of pancreatic enzymes and bile juice.
• Pancreozymin It stimulates pancreas to secrete enzymes.
• Duocrinin It is an intestinal hormone. It stimulates the secretion of mucus andHC03 from Brunner’s glands.
• Enterogastrone It inhibits the secretion of gastric juice.

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