Plus One Zoology Notes Chapter 5 Digestion and Absorption is part of Plus One Zoology Notes. Here we have given Kerala Plus One Zoology Notes Chapter 5 Digestion and Absorption.
|Text Book||NCERT Based|
|Chapter Name||Digestion and Absorption|
|Category||Kerala Plus One|
Kerala Plus One Zoology Notes Chapter 5 Digestion and Absorption
Food is one of the basic requirements of all living organisms and provides energy and organic materials for growth and repair of tissues.
- Digestion: The process of conversion of complex food substances into simple absorbable forms.
- It is carried out by our digestive system by mechanical and biochemical methods.
Human digestive system
• The human digestive system consists of an alimentary canal and the associated glands.
- It is a long tube with muscular walls, extending from mouth to anus.
- The alimentary canal consists of following parts,
♦ Buccal cavity
- The mouth leads to a buccal cavity or oral cavity. It consists of teeth and a muscular tongue.
Each tooth is embedded in a socket cavity of a jaw bone. This type of attachment is called Thecodont.
- Humans have’two sets of teeth during their lifetime. This type of dentition is called diphyodont.
- They are of two type
1. Milk or deciduous teeth
They are weak temporary teeth which develop in childhood.
2. Permanent teeth
- Milk teeth are replaced by permanent teeth at the age of 6 7.There are 32 permanent teeth in humans. !
- Heterodont: Four different kinds of teeth, this dentition is called heterodont.
• The different types of teeth are
i. Incisors for cutting
ii. Canines for tearing
Premolars and molar for masticating the food.
The arrangement of different types of teeth in the jaws on one side and the sockets on the other side
- The arrangement of teeth in each half of the upper jaw and the lower jaw in the order I, C, PM, M is represented by Dental formula.
- The dental formula for human is
- The hard chewing surface of the teeth, made up of enamel helps in the mastication of food.
- A tongue is a freely movable muscular organ.
- It is attached to the floor of the oral cavity by the frenulum.
- The upper surface of the tongue has small projections called papillae. Some papillae bear taste buds.
- Oral cavity leads into a short pharynx which serves as a common passage for food and air.
- The esophagus and the trachea (windpipe) opens into the pharynx.
- A cartilaginous flap called epiglottis prevents the entry of food into the glottis.
- Glottis is the opening of pharynx into the trachea.
- It is a thin, long tube which extends posteriorly through the neck, thorax, and diaphragm.
- It leads to a” J” shaped bag like structure called “stomach”.
- A muscular sphincter (gastro-oesophageal sphincter) regulates the opening of the esophagus into the stomach.
- Located in the upper left portion of the abdominal cavity.
- It is divided into three major parts.
- Cardiac region Upper part into which esophagus opens.
- Fundic region
- Pyloric region
Anatomical Regions of Human Stomach
Pyloric region opens into the first part of the small intestine
♦ Small intestine
- It is a coiled, tubular structure forms the longest part of the alimentary canal.
- The small intestine is distinguishable into three regions:
a. Duodenum “C “shaped.
b. Jejunum Long coiled middle portion.
c. The ileum is highly coiled, it opens into the large intestine.
♦ Large intestine
It consists of the caecum, colon, and rectum.
- Small blind like sac which hosts some symbiotic microorganisms.
- Narrow finger like a tubular projection of the caecum is called vermiform appendix.
- Tn man vermiform appendix is a vestigial organ.
- The caecum opens into the colon.
The largest part of the large intestine. It is divided into three parts:
- The ascending colon
- Transverse colon
- Descending colon
Descending part opens into the rectum.
Thinnest and last part of the alimentary canal leading to the anus.
♦ Histology of alimentary canal
The wall of the alimentary canal from the esophagus to rectum and is made up of four layers: serosa, muscular, submucosa, and mucosa.
- It is the outer most layer.
- Made up of thin mesothelium(epithelium of visceral organs) with some connective tissue.
- It is formed by smooth muscles arranged into an inner circular and outer longitudinal layer.
- An oblique muscle layer may be seen in some
Diagrammatic representation of the transverse section of a gut.
- It is formed of loose connective tissue containing nerves, blood vessels, and lymph vessels
- In the duodenum, glands are also present in the submucosa.
- Innermost layer lining the lumen of the alimentary canal.
- This layer forms irregular folds in the stomach called rugae and small finger-like foldings called villi in the small intestine.
- The cells lining villi produce numerous microscopic projections called microvilli.
- Villi are supplied with a network of blood capillaries and a large lymph vessel called lacteal.
- Mucosal epithelium has goblet cells, which secrete mucus that helps in lubrication.
- Mucosa also forms glands in the stomach called gastric glands.
In between the bases of villi in the intestine, there are invaginations in the mucosa are called crypts of Lieberkuhn. All four layers show modifications in the different parts of the alimentary canal.
The digestive gland associated with alimentary canal includes,
- Salivary gland
♦ Salivary gland
- The salivary gland secretes saliva. It is mainly produced by three pairs of salivary glands,
- These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.
- It is the largest gland of the human body, weighing about 1.2 1.5 kg in an adult human.
- It is located in the abdominal cavity, just below the diaphragm.
- It 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 is stored in a thin muscular sac called gall bladder. (Bile is an alkaline, watery, and a greenish fluid)
- The duct of the 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 hepatopancreatic duct.
- This duct is guarded by a sphincter called Sphincter of Oddi.
- It is a compound gland formed of an exocrine part and endocrine part.
- It is an elongated organ situated between the limbs of the ‘ U ‘ shaped duodenum.
- The exocrine portion secretes alkaline pancreatic juice containing digestive enzymes.
- The endocrine portion secretes two hormones
Digestion of food
Includes mechanical and chemical process.
♦ Digestion in the bucall cavity
Buccal cavity performs two major functions:
i. Mastication of food
ii. Facilitation of swallowing
- As the food enters the mouth, it gets mixed with saliva.
- Mucus in saliva lubricates and adhere food particles into a bolus.
- Deglutition or swallowing conveys the bolus to the pharynx and then to the esophagus.
- Peristalsis conveys the bolus from esophagus to stomach.
- The saliva secreted by the salivary glands, contains electrolytes (Na+, H+ CL, HCO3) and enzymes, salivary amylase and lysozyme.
- Starch is hydrolyzed by this enzyme (salivary amylase) into disaccharide maltose.
- Lysozyme present in the saliva acts as an antibacterial agent that prevent infections.
♦ Digestion in the stomach
The mucosa of the stomach has gastric gland. These glands have three major types of cells.
Stomach stores the food for about 4 – 5 hours.
- The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movement of its muscular wall and forms chyme.
- The proenzyme pepsinogen, on exposure to hydrochloric acid, gets converted into pepsin.
- Pepsin converts proteins to Proteases & Peptones (peptides).
- Mucus & bicarbonates present in the gastric juice play an important role in:
- Protection of mucosal epithelium from excoriation by the highly concentrated HCI. HCI provides the acidic pH (pH 1.8) optimal for pepsins.
- Rennin is a proteolytic enzyme, found in gastric juice of infants. It helps in the digestion of milk proteins
- Small amounts of lipases are also secreted by gastric glands.
♦ Digestion in the small intestine
- Various types of movements are generated by the muscular layer of the small intestine.
- These movements help in mixing up of food with various secretions in the intestine and thereby facilitate digestion.
- Secretion released into the small intestine are:
- Pancreatic juice and bile are released through a hepatopancreatic duct.
- Pancreatic juice contains inactive enzymes Trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases.
- The enzyme enterokinase secreted by intestinal mucosa coverts trypsinogen into active trypsin, which in turn activates the other enzymes.
- The bile released into the duodenum contains Bile pigments (bilirubin & biliverdin), bile salts, cholesterol, and phospholipids but no enzymes.
Functions of bile
- Helps in emulsification of fat. (break down of the fat into small micelles)
- 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.
- Intestinal juice contains enzymes like, disaccharidases (Maltase, lactase and sucrase), dipeptidases, lipases, nucleosidases.
- The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as an alkaline medium for enzymatic activities. Submucosal glands (Brunner’s gland) also help in this.
- Partially hydrolyzed proteins in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice.
- Pancreatic amylase hydrolyze carbon drate in the chyme into disaccharides.
- Lipase enzyme breaks down fats into diglycerides and monoglycerides.
- Nucleases in the pancreatic juice act on the nucleic acid to form nucleotides and nucleosides.
- The enzymes in the succus entericus act on end products of the above reactions to form simple absorbable forms.
- These final steps in digestion occur very close to the mucosal epithelial cells of the intestine
- Break down of biomacromolecules takes place here.
♦ Jejunum & Ileum
- Absorption of the simple substances takes place.
- Undigested and unabsorbed substances are passed on to the large inte\stine.
♦ Large intestine
No significant digestive activity occurs in the large intestine.
Functions of Large intestine
1. Absorptionofsomewater.mineralsand certain drugs.
2. Secretion of mucus, which helps in the easy passage of undigested particles. Lubricating it for easy passage of materials.
- Caecum: The feces enters through an ileocaecal valve, which prevents the backflow of the fecal matter.
- Rectum: Faecal matter is temporarily stored here till defaecation.
♦ Neural and hormonal control of digestion
- The activities of the gastrointestinal tract are under the neural and hormonal control for proper coordination of different body parts.
- The sight, smell or the presence of food in the oral cavity can stimulate the secretion of saliva.
- Gastric and intestinal secretions are also, 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 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.
♦ Passive transport
Small amounts of monosaccharides 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 gradient.
♦ Facilitated transport
It occurs along the concentration gradient. Substances like glucose, amino acids are absorbed with the help of carrier proteins. Eg., Na+
♦ Active transport
Take place against the concentration gradient. It requires energy. Nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.
♦ Absorption of fat
- Fatty acids and glycerol, being insoluble cannot be absorbed into the blood.
- Fatty acids and glycerol incorporated with water-soluble droplets called micelles. Micelles are actively absorbed into the intestinal mucosa.
- In the intestinal cells, micelles are reformed into very small protein coated fat globules called chylomicrons.
- Chylomicrons are transported into the lymph vessels (lacteals) in the villi.
- Lymph vessels release the absorbed substances into the bloodstream.
♦ The Summary of Absorption in different parts of the Digestive System
|Mouth||Stomach||Small Intestine||Largo Intestine|
|Certain drugs coming in contact with the mucosa: of mouth and tower side of the tongue are absorbed into the blood capillaries lining them.||Absorption of water, simple sugars, and alcohol etc. takes place||Principal organ for absorption of nutrients, The digestion is completed here and the final products of digestion such as glucose, fructose, fatty acids, glycerol, and amino acids are absorbed through the mucosa into the bloodstream and lymph.||Absorption of water, some minerals and drugs take place.|
The absorbed substances finally reach the tissues which utilize them for their activities this process is called assimilation.
♦ Defaecation or Egestion
- The undigested food solidifies into a semisolid reduce called feces.
- The process of elimination of undigested residue from the alimentary canal is called egestion.
- it is a voluntary process and carried out by a mass peristaltic movement.
Disorders of the 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, pinworm, etc.
The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.
- It is the ejection of stomach contents through the mouth.
- This reflex action is controlled by the vomiting center in the medulla.
- A feeling of nausea precedes vomiting.
The abnormal frequency of bowel movement and increased liquidity of the fecal discharge is known as diarrhoea.lt reduces the absorption of food.
In constipation, the feces are retained within the rectum as the bowel movements occur irregularly.
In this condition, the food is not properly digested leading to a feeling of fullness. The causes of digestion are inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.
■ Layers of an alimentary canal
The wall of the alimentary canal from the esophagus to rectum prossesses four layers. They are given in the table.
|Serosa||Serosa is the outermost layer and is made up of a thin mesothelium with Serosa some connective tissues.|
|Muscular||Made up of smooth muscles usually arranged into an inner circular and an outer longitudinal layer.|
|Sub-mucosa||Inner to muscular. Made up of loose connective tissue. Nervous, blood and lymph vessels are present.|
|Mucosa||Innermost layer lining the lumen of the alimentary canal. Made up of a mucous membrane.|
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