DIGESTION AND ABSORPTION OF FOOD BIOLOGY FOR 11th CLASS ,NEET

Digestion And Absorption of Food

The Process of conversion of complex food substances to simple absorbable forms is called digestion.

Intracellular: When the process of digestion occurs within the cell in the food vacuole.Examples:Protozoa, Porifera,

Extracellular: When the process of digestion occurs outside the cell, Examples: Coelenterates and phylum Platyhelminthes to phylum Chordata.

Digestion in vertebrates occurs in the digestive tract or alimentary canal. The various parts involved in digestion can be broadly grouped in two groups –

(1) Digestive tract or alimentary canal

(2) Digestive glands

On the basis of the embryonic origin, the alimentary canal of vertebrates can be divided into three parts–

(1) Fore gut / Stomodaeum: Ectodermal. It includes buccal cavity / oral cavity, pharynx, oesophagus, stomach and small part of duodenum.

(2) Mid gut/Mesodaeum: Endodermal. It includes small intestine, and large intestine.

(3) Hind gut/Proctodaeum:Ectodermal. It includes anal canal and anus.

Human Digestive System

The human digestive system is a complex series of organs and glands that processes food. It converts ingested food so that it can be assimilated by the organism.

Alimentary Canal:

It is a long and muscular tube that runs from mouth to anus. The accessory glands of the digestive system like salivary glands, liver, and pancreas pour their secretions into the canal through their ducts.

The alimentary canal has the following distinct regions:

1. Buccal (oral) cavity

2. Pharynx                                                     

3. Oesophagus (food pipe)

4. Stomach

5. Small intestine

6. Large intestine:-

a)    Mouth and Buccal Cavity

Mouth is a slit like opening, bounded by two soft, movable lips. It opens into a small vestibule (space enclosed between lips and cheeks externally and gums and teeth internally), which inturn leads into the buccal or oral cavity.

Oral cavity further comprises of two main components

a.    Teeth-These are hard structures present in the mouth on both the jaws (i.e., upper and lower jaw). Each tooth is embedded in a socket of jaw bone.

b.    The dental formula is  2123/2123, it represents the upper and lower half of the jaw The hard chewing surface of the teeth is made up of a substance called enamel. It helps in the mastication of food.

Mammalian teeth are characterised by following three features

a)    Thecodont The teeth are fixed in sockets. They have very well-developed roots, which are implanted deeply in the jaw bone socket.

b)    Diphyodont Like other mammals, human beings also has two sets of teeth formed during lifetime. The first set of teeth is temporary and is known as milk or deciduous teeth.Milk teeth are 20 in number.

c)    The milk set is replaced by the second set known as permanent teeth or adult teeth. Permanent teeth last for whole life, if lost, cannot be replaced.

d)    Heterodont An adult human has 32 permanent teeth, but they are of different size, shape and type.

They are of following four types

·       Incisors (I) for cutting of food

·       Canine (C) for tearing the food

·       Premolars (Pm) Molars (M) for crushing, grinding and chewing the food

Tongue

It is a muscular organ, which is freely movable in the oral cavity. A fold called frenulum attaches the tongue to the floor of oral cavity. The upper surface of the tongue bears small projections (elevations) known as papillae. Some of the papillae bear taste buds.

Note:

Papillae provides a characteristic roughness to the tongue.

The hard visible chewing surface of tooth helps in the mastication of food and is covered by a thick, shiny and translucent substance called enamel (the hardest substance in the body).

Taste buds present at the surface of the tongue contain chemosensory cells. Human taste buds are sensitive to four basic tastes, i.e., sweet, bitter, salty and sour. These four taste buds are present at different locations of the tongue.

 

 

Pharynx

a)    It is a small funnel-shaped chamber located behind the oral cavity. It serves as a common passage for both food and air, i.e., it communicates with both oesophagus (food pipe) and trachea (wind pipe).

b)    The opening of trachea or wind pipe is called glottis, which is guarded by a cartilaginous flap or lid called epiglottis. The glottis normally remain open, but during swallowing of food it gets covered by epiglottis to prevent the entry of food in trachea..

It is classified as: - Nasopharynx 2) Oropharynx 3) Laryngopharynx

Oesophagus

It is the thin, long muscular tube that extend posteriorly passing through the neck, thorax and diaphragm and finally leads into a J-shaped bag- like structure called stomach. A muscular gastro-oesophageal sphincter regulates the opening of oesophagus into the stomach.Regulated by gastro-oesophageal sphincter.

Stomach: –

It is a J-shaped, muscular, hollow and dilated part of the digestive system. It is located between the oesophagus and the small intestine. It has 1 liter capacity. It secretes protein-digesting enzymes (proteases) and strong acids which aid in food digestion.

The stomach has three parts:

Cardiac: The part of the stomach into which oesophagus opens.

Fundus: It is the air filled portion of stomach.

Pyloric: The portion of the stomach that opens into the small intestine

Functions of Stomach

·       Acts as a short term storage reservoir.

·       The substantial chemical and enzymatic digestion is initiated here (especially of proteins).

·       Gastric smooth muscles mix and grind the foodstuff by vigorous contractions with gastric secretions.

·       Food become liquefied in the stomach and is released slowly in the small intestine.

Note:

o   The lymphatic tissues of the pharynx and oral cavity are arranged in a ring like manner, that are collectively called Waldeyer’s ring. This ring consists of lingual tonsils and palatine tonsils.

o   The lower – part of oesophagus has only involuntary muscles

Small Intestine:-It is the part of the gastrointestinal tract that comes after the stomach and is followed by the large intestine.  Small intestine distinguished into three parts:

(a) Duodenum: It is ‘U’ shaped first part of the small intestine.

(b) Jejunum: It is the longer, coiled middle portion.

(c) Ileum: Ileum is the highly coiled posterior part of the small intestine.

v  Large Intestine: -

·       Large intestine consists of three parts:

(a) Caecum: It is a small blind sac. Vermiform appendix is a finger-like blind tubular projection of caecum.

(b) Colon: The Caecum opens into colon. Colon has three distinct parts-

·       Ascending colon

·       Transverse colon

·       Descending colon

(c) Rectum: It is the final straight portion of the large intestine.

v  Histology of Alimentary canal-The wall of the alimentary canal from Oesophagus to rectum consists of four layers.

·       Serosa– it is the outermost layer made up of squamous epithelium and areolar connective tissue.

·       Muscularis– it is composed of outer longitudinal and inner circular muscle fibres. Muscles fibres are smooth and have network of nerve cells.

·       Submucosa– it consists of loose connective tissue richly supplied with blood and lymphatic vessels. Meissner’s plexus is present between the muscular coat and mucosa that controls the secretion of intestinal juice.

·       Mucosa– is innermost layer lining the lumen of the alimentary canal. It has irregular folding in stomach called rugae and villi in small intestine. Mucosa forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn).

 

 

·       Digestive Glands-To bring about the chemical simplification of food, digestive juices are secreted by the different glands. The digestive glands associated with the alimentary canal include majorly salivary glands, the liver and pancreas.

a)    Salivary Glands

These are exocrine glands that secretes saliva. There are three pairs of salivary glands in man. All three glands are situated just outside the buccal cavity and secrete salivary juice into the buccal cavity. These are as follows:

(a) Parotid Glands These are largest of the three glands present one on either side of the cheek on the upper palate.

(b) Sub-maxillary or Sub-mandibular Glands These are present at the angle of the lower jaw.

(c) Sub-lingual Gland These are situated beneath the tongue.

Each sublingual gland has about ten small duets called sub-lingual ducts or ducts of Rivinus, which open into the floor of mouth.).

b)    Liver-

It is the largest gland of the body, an exocrine gland. In adults, it weighs about 1.2-1.5 kg and lies in the abdominal cavity just below the diaphragm and has two lobes, i.e., left and right lobes.

It is a large organ and occupies most of the right side of abdominal cavity.

Liver is a double membrane structure. Interiorly, it is divided into many small units called hepatic or liver lobules (structural and functional units of liver) consisting of many hepatic cells (hepatocytes) that are arranged in the form of cords.Each lobule is also covered by a thin connective tissue sheath called the Glisson’s capsule. Hepatic cells secretes the bile juice, which passes through the hepatic duct into the gall bladder.

Functions of Liver

Liver serves the following functions

(a) It helps in producing RBCs in embryo.

(b) Bile secreted by the liver helps in emulsification of fats, i.e., breaking down of fats into very small micelles.

(c) Bile also activates lipases.

(d) It also produces heparin for preventing clotting of blood inside the blood vessels

v  Gall Bladder

It is a small pear-shaped, thin muscular sac-like organ situated just below the liver. It is attached by connective tissues to liver. The duct of gall bladder, i.e., cystic’duct along with the hepatic duct form a common bile duct, which regulates the amount of bile to be discharged into the duodenum.

After certain distance, the bile duct and pancreatic duct (duct of pancreas) form common hepato-pancreatic duct, which open into duodenum. It is guarded by a sphincter called the sphincter of oddi.

The common hepato-pancreatic duct carries both the bile (from liver) as well as pancreatic juice (from pancreas) into the duodenum.

 

 

iii. Pancreas

·       It is a compound elongated organ situated partly behind the stomach between the limbs of the U-shaped duodenum. As it is a mixed gland, it has both exocrine as well as endocrine activity.

·       An alkaline pancreatic juice containing enzymes is secreted by its exocrine portion and the endocrine portion is responsible for the secretion of hormones, insulin and glucagon.

Mechanism of Digestion

In human being, the digestion of food starts in the buccal cavity and continues till the anus of large intestine.The mechanism of digestion continues in the following steps

·       Digestion in Buccal or Oral Cavity

·       Digestion starts in the oral cavity by the chemical hydrolytic action of the carbohydrate splitting enzyme, salivary amylase.

·       The saliva secreted into oral cavity contains electrolytes (Na+,K+, Cl–, HCO3, etc.) and enzymes, i.e., salivaryamylase and lysozyme (acts as an antibacterial agent that prevents infections).

·       About 30% of the starch gets hydrolysed in the oral cavity by the action of salivary amylase (at optimum pH 6.8) into a disaccharide,i. e., maltose..

·       Digestion in Stomach

The stomach stores the food for around 4-5 hours. Internal mucosa of stomach contains gastric glands, which mainly comprises of three types ofcells

(а) Mucus or neck cells for secreting mucus.

(b) Peptic or chief or zymogenic cells for secreting proenzyme pepsinogen.

(c) Parietal or oxyntic cells for secreting HCl and intrinsic factor (essential for vitamin-B12 absorption).The gastro-oesophageal sphincter controls the passage of food into the stomach.

Food is slowly released from the stomach in small quantities into the small intestine, so that a slow process of digestion and absorption can occur.

The food mixes thoroughly with the acidic gastric juice secreted in the stomach by the churning movements of its muscles and becomes semi-digested, acidic, pulpy mass called chyme. The HCl and the enzymes of the gastric juice now helps in the chemical simplification of food

The enzymes of stomach and their actions are given below

a. Pepsin-On exposure to HCl, the proenzyme pepsinogen gets converted into pepsin (proteolytic enzyme of the stomach) that further converts proteins into proteases and peptones (peptides)

Pepsin usually attacks the peptide bonds between amino acids. It can attack all proteins except keratins, protamines, histones, etc.

b. Rennin-It is a proteolytic enzyme found in gastric juice of only infants, in its inactive form. Its secretion takes place in order to digest the milk proteins.

c. Gastric Lipases Small amounts of lipases are also secreted by the gastric glands. Activity of this enzyme is inhibited in the stomach by the acidic condition.It act on emulsified fats and also help in digesting around 25% of milk fat (in infants).

It is mainly the digestion of proteins that occurs in the stomach.

Apart from all these enzymes, the amount of mucus and bicarbonates present in the gastric juice plays an important role in the lubrication and protection of mucosal epithelium from excoriation by the highly acidic pH.

iii. Digestion in Small Intestine

To further facilitiate the digestion of food, muscularis layer of small intestine shows various types of movements which allows a thorough mixing up of food with various secretions in the intestine.

These contractions of muscles in the small intestine allows the further churning and kneading of the chyme and finally pushing it into the large intestine.

The respective digestive juices from the liver (bile), pancreas (pancreatic juice) and srtiall intestine (intestinal juices) are released into the small intestine to bring out the further chemical simplification of food. The , pancreatic juice from the pancreas and the bile from the liver are released through the hepato-pancreatic duct.

Enzymes from Pancreas

The pancreatic juice secreted from the pancreas contains the various inactive enzymes.These are as follows

(a) trypsinogen

(b) chymotrypsinogen

(c) procarboxypeptidases

(d) amylases

(e) lipases

(f) nucleases

Trypsinogen is activated by an enzyme enterokinase secreted by intestinal mucosa into active trypsin which in turn activates the other enzymes of pancreatic juice.

The proteins, proteases and peptones (partially hydrolysed form of proteins) present in the chyme (reaching the intestine) are acted upon by the proteolytic enzymes of pancreatic juice. These are given below as

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

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

v  Enzymes from Liver
The bile secreted from the liver is released into duodenum of small intestine. Bile contains the bile pigments, i.e., bilurubin and biliverdin, bile salts, cholesterol and phospholipids.
Thus, fats are broken down into di and monoglycerides by the action of lipases

Note:

·       Bile does not contain any enzymes as gastric juice. It helps in emulsifying fats, i.e., in breakdown of fats into very small micelles which are kept suspended in an aqueous medium.

·       The process of emulsification is basically carried out by the salts of bile. This increases the surface area of fat available for digestion by the lipase (as bile also activates lipases).

v  Enzymes from Intestine
Intestinal mucosal epithelium has goblet cells (secretes mucus). Thus, the secretions of the brush border cells of mucosa together with the goblet cell secretions forms the intestinal juice (also known as succus entericus).
The succus entericus thus, contains various enzymes
(a) Disaccharidases, e.g., Maltase for digestion of maltose into glucose.
(b) Dipeptidases
(c) Lipases
(d) Nucleosidases

v  Pancreatic and intestinal lipases together helps in the emulsification of fats.The mucus along with the bicarbonates from the pancreas helps in protecting the mucosal layer of intestine from the action of acid and also provides an alkaline medium (pH-7.8) for enzymatic activities.

 

·       Glands (Brunner’s gland) from sub-mucosal layer of intestine also helps in this. Hence, all the enzymes in the succus entericus acts on the end products of the above mentioned reactions in order to form their respective simpler forms.

·       All these final steps takes place very close to mucosal epithelial cells of the intestine.

·       All the biomacromolecules mentioned above breakdown in the duodenum region of small intestine, while the simpler forms are absorbed in the other two regions of small intestine, i.e., jejunum and ileum.

Digestion in Large Intestine

·       The last stage of chemical simplification of food occurs in the last part of the alimentary canal, i.e., large intestine. This is carried out by bacterial action. Glands of this region tends to secrete mucus, i.e., enzymes are not secreted into this part of the digestive system.

·       The undigested and the unabsorbed substances are finally passed on to the large intestine.

Note:

There are more than 500 species of bacteria found in the colon region of the large intestine which are not usually harmful as long as they remain in the large intestine.

Infact, all these bacteria synthesise vitamin-K and B12 , also helps in absorption of calcium, magnesium and zinc (by increasing the acidity of colon region).

Following functions are performed by the large intestine

(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.

No significant digestive activity occurs in this region of digestive tract.

The undigested, unabsorbed substances called faeces, enters into the caecum region of the large intestine (through the ileo-caecal valve, which prevents the back flow of faecel matter). It is temporarily stored in the rectum till defecation (egestion) through the anus.

Apart from absorbing vitamins secreted by various types of bacteria, large intestine also helps in absorbing water and electrolytes such as Na+, Cl–.

Neural and Hormonal Control of Digestion

For proper coordination- and functioning of different parts of the gastrointestinal tract, it should be under a proper, neural and hormonal control

Neural Control-Secretion of saliva is stimulated by the sight, smell and the presence of food in the oral cavity. Similarly, the gastric and intestinal secretions are also under the control of neural signals.

The muscular activities of different parts of the alimentary canal are also moderated by neural mechanisms (both local and through CNS).

Hormonal Control

The major hormones that control the functions of digestive system are produced and released by the cells in the mucosa of the stomach and the large intestine.

Disorders of Digestive System

Jaundice: In jaundice, the liver is affected. Also, skin and eyes turn yellow due to the deposition of bile pigments.

Vomiting is the ejection of stomach contents through the mouth. It is a reflex action controlled by the vomiting center in the medulla.

Diarrhoea: It is the abnormal movements of the bowel and increased liquidity of the faecal discharge.

Constipation: The bowel movements occur irregularly and faeces are retained in the large intestine.

Indigestion: It gives a feeling of fullness.

PEM: Protein-energy malnutrition (PEM) produces Marasmus and Kwashiorkor.

Marasmus: It occurs due to a deficiency of calories(carbohydrates).

Kwashiorkor happens due to protein deficiency.

Symptoms of Kwashiorkor include a wasting of muscles, thinning of limbs, failure of growth, and brain development