Digestion anD absorption

www.sciencetuts.com

Chapter Outline:

DIGESTION AND ABSORPTION

Prerequisites•Learning objectives•Digestive System•Digestion of Food•Absorption of Digested Products•Disorders of Digestive System•Summary•

Digestion anD absorption

www.sciencetuts.com

Prerequisites

To perform various activities, living organisms require a continuous supply of energy.1.

The energy comes from the food we eat.2.

The process of taking or consuming food is called Nutrition. 3.

The major components of food are carbohydrates, proteins & fats. Vitamins & minerals are 4.

also required in small quantities.

The water also plays an important role in the metabolic process.5.

Learning objectives

To know the parts of Digestive system.1.

To develop the skill of drawing with well labeled diagrams of human digestive system and 2.

its related organs.

To identify the arrangement of different types of teeth in the jaws on one side and the 3.

sockets on the other side.

To understand and interprets the biological equations of enzymes in the process of digestion 4.

and absorption.

To understand the process of digestion & absorption. 5.

To gain knowledge about digestive system and its disorders.6.

2

Digestion anD absorption

www.sciencetuts.com

Digestive system

Nutrition involves 5 steps

Alimentary canal

Mouth

1) Ingestion 2) Digestion 3) Absorption 4) Assimilation 5) Egestion.

The process of taking in food is called ingestion (the act of getting & eating food). The food cannot be utilized by our body in their original form. They have to be broken down and converted into simple substances in the digestive system. This process of conversion of complex food substance to simple absorbable forms is called digestion. Simplified substances must be absorbed through the living membranes (absorption), absorbed food must be incorporated into cell components (assimilation), and the undigested residual food must be removed (egestion).Higher organisms have special organs in their bodies for each of the steps. Mammals show (holozoic) mode of nutrition i.e., they consume the whole food (animal or plant or their parts) into their body.

The term digestive system is used for the alimentary canal along with the associated digestive glands, which produce enzymes necessary for the chemical break down of the food into simpler substances.

Alimentary canal begins with mouth and ends with anus. It is a muscular coiled tubular structure varying in length from 8-9m.The mouth cavity contains teeth and tongue.The various organs beginning from mouth are

The mouth leads into buccal cavity or oral cavity. Digestion starts in the buccal cavity.Teeth, tongue and opening of 3 parts of salivary glands are present in buccal cavity. Physical & chemical nature of the food changes when it is masticated (chewed) with the help of teeth & mixed with saliva. On the floor of the cavity a tongue bearing taste buds are present. The roof of the mouth is formed by the palate which separates the air channel from food channel. The cavity is supported by upper and lower jaws and the jaws are arranged with different kinds of teeth.

Mouth → Oesophagus → Stomach → Small intestine (Consisting of duodenum, jejunum & ileum) → Large intestine (Consisting of caecu, colon and rectum).Digestive glands – There are 3 digestive glands 1) Salivary glands 2) Pancreas and 3) Liver

Oesophagus

StomachPancreasJejunum

RectumAnus

Vermiform appendix

Ascending colonTransverse colon

Submaxillary andsublingual glands

DuodenumGall bladder

Liver

MouthOral cavity

CaecumIleum Descending colon

PharynxParotid gland

Digestive system

Teeth

The arrangement of teeth in the jaws is called dentition. The dentition is related to the diet.The teeth show variation in size & shape & is termed as heterodont. Each tooth is embedded in a socket of jaw bone.

3

Digestion anD absorption

www.sciencetuts.com

Socket of jaws

IncisorsCanines

123

45678

Premolars

Molars

TeethIncisors (I): There are 4 chisel shaped teeth & lie in the centre of each jaw. They are used for cutting & biting the food.Canines(C): On the sides of incisors are canines having conical shape & pointed ends. They are used for holding & tearing.Premolars (PM): Next to canines on each side of the jaw are the pre molars. They help in crushing & cheering the food.Molars (M): Behind the pre molars are the last 3 teeth on each side of the jaw. The last molars is called wisdom tooth which grows after the age of 18.upperjaw Lowerjaw Dental formula is i 2 , c 1 , pm 2 , m 3 = 8 × 2 =16 = 32

2 2 3 8 161

Pharynx

Oesophagus

Tongue

Tongue: It is a muscular organ found in the mouth.Functions: To taste the food. It helps in mastication.Swallowing of food.Mucus secretion.The mouth leads into;

This serves as a common passage for food & air. The food passage continues as oesophagus (food pipe). The air passage continues as larynx & trachea (wind pipe). The opening of larynx is guarded by muscular flap called epiglottis, which prevents the entry of food into wind pipe while swallowing.

It is a straight, collapsible & muscular tube which passes through the neck, thorax & through the diaphragm into the stomach.It has both voluntary & involuntary muscles.Internally the wall of oesophagus is lined with the mucous membrane which secretes mucous. Mucous acts as a lubricant and helps in easy and smooth passage of food. Swallowing is a voluntary act. Once food enters oesophagus, swallowing becomes an involuntary act. When food enters into oesophagus, the muscles present in its wall contract & relax alternately producing wave like movements. These are called peristaltic movements. They help in pushing the food down the Oesophagus into the stomach. peristaltic movements of Oesophagus are involuntary. There are no digestive enzymes in Oesophagus. Oesophagus is only a passage through which food enters into stomach. Hence food does not undergo any change in pharynx and oesophagus.

StomachStomach is a muscular bag. It is present on the left side in the abdominal cavity, below the diaphragm. The first part of the stomach is called cardiac stomach. The part of the stomach that opens into duodenum is called pyloric stomach. Opening of the pyloric stomach into duodenum is protected by pyloric sphincter.

This type of attachment is called thecodont. There are 2 sets of teeth during their life,a set of temporary (milk teeth) teeth which are decidiuous as they fall off later, they are replaced by a set of permanent teeth. This type of dentition is called diphyodont. An adult human has 32 permanent teeth which are of 4 different types namely

4

Digestion anD absorption

www.sciencetuts.com

Pyloric sphincter acts like a valve and does not open till the food in the stomach is fully mixed & churned by muscular contractions of the stomach wall. Muscles in the wall of the stomach are involuntary. These muscles contract in different directions. As a result food is churned in the stomach.Stomach has 3 important roles.1. It stores the food temporarily.2. Mixing of various components in the food thoroughly –This occurs due to contraction & relaxation of muscles.3. It brings about physical & chemical changes in the food.

FundusOesophagus

Cardiac

Pyloric

Superior portion

of duodenum

StomachSmall intestine

Jejunum

It is 6-7m long & 2.5 cm in diameter. It is narrow coiled tube having 3 parts.(a) Duodenum (b) jejunum (c) ileumDuodenum: Is ‘U’ shaped. Bile from the liver & pancreatic juice from pancreas reach duodenum through separate ducts.The pyloric sphincter remains closed until digestion of food in the stomach is completed.

The middle part is called jejunum. It is a short region of small intestine.

Ileum

It is the longest part of the small intestine. Ileum opens into the large intestine. The major part of the process of digestion takes place in small intestine. The inner lining of the small intestine is produced into a number of (Fig 16.5) finger like projections called villi. (Sing; villus). Each villus is covered by a single layer of epithelium and supplied with net work of capillaries and a large lymph vessel called the lacteal. The villi increase the surface area for absorption.The small intestine is so long that food is retained in it for a longer period and shows following character.a) The inner lining of small intestine has millions of villi throughout its length in order to increase the surface area of absorption.b) The epithelium of villi has mucus lining throughout its length for easy diffusion of digested food.

Large intestine

The small intestine opens into large intestine on the right lower side of the abdomen. It has 3 parts. Caecum, colon and rectum.

Caecum

The point where ileum joins the large intestine,a sac- like part called caecum is present which hosts some symbiotic micro organisms.A narrow finger like tubular projection the vermiform appendix which is a vestigial organ, arises from caecum.Large intestine does not play any part in the digestive process. No digestive juice & enzymes are poured in this region. Formation of faeces occurs in this region.

5

Digestion anD absorption

www.sciencetuts.com

ColonThe caecum opens into colon, the colon is divided into 3 parts an ascending a transverse & a descending part.

Rectum

The descending part opens into the rectum. Rectum is the last part of alimentary canal conserved with the storage of undigested food called faeces. The external opening of rectum is called anus which is kept closed by a ring of muscles called anal sphincter. It opens only during defaecation.The wall of alimentary canal from oesophagus to rectum possesses 4 layers.1. Serosa is the outermost layer & is made up of a thin mesothelium with some connective tissues.2. Muscularis is formed by smooth muscles usually arranged into an inner circular & outer longitudinal layer. An oblique muscle layer may be present in some regions.

Section of gut

Mucosa

SerosaInner circular

outer-longitudinalSub-mucosa

MuscularisLumen

Mucosa showing villi

Villi

Lacteal

Capillaries

ArteryVein

Crypts

The digestive glands associated with alimentary canal include salivary glands, the liver & the pancreas.

Three pairs of salivary glands are present in the mouth. They are parotid, sub maxillary & sub-lingual.

They are largest of salivary glands present on each side of face in front of the ears. The duct opens on the inside of the cheek.

They are located at the posterior part of the floor of mouth. The ducts open on the floor of the mouth cavity.

They are located in the anterior part below the tongue. The duct opens at the floor of the mouth. The glands secrete salivary juice into the buccal cavity.

Digestive Glands

Salivary glands

Parotid glands

Sub maxillary glands

Sub lingual glands

3. The Sub mucosa layer is formed of loose connective tissues containing nerves, blood & lymph vessels. On duodenum, glands are also present in the sub mucosa.4. Mucosa is the innermost layer lining the lumen of alimentary canal. Mucosal epithelium has goblet cells which secrete mucus & help in lubrication. Mucosa also forms glands in the stomach. All these 4 layers show modifications in different parts of alimentary canal.

6

Digestion anD absorption

www.sciencetuts.com

Liver

Liver is the largest gland of the body weighting about 1.2 to 1.5 kg. It is situated in the abdominal cavity just below the diaphram. It is dark brown in color. The bile secreted through hepatic duct and is stored in a thin sac called the gall bladder. The duct of gall bladder along with the hepatic duct from the liver forms a common Bile duct. Bile reaches the duodenum through a duct called bile duct.

It is much lobed glands situated in between the stomach & duodenum. Pancreas opens into the duodenum through a duct called common bile duct. Pancreas is both endocrine and exocrine gland. The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon.

Liver

Pancreas

The process of digestion is accompanied by mechanical & chemical processes.

Digestion of food begins in the mouth. The food ingested is masticated by teeth (chewed) and broken into smaller particles so that large surface area is provided for the action of enzymes. The food is mixed with saliva secreted by salivary glands which moistens and lubricates the food and aids in swallowing. The masticated food is rolled into a ball or bolus by the tongue and passed through the pharynx into oesophagus by swallowing or deglutition. During this process the epiglottis closes and prevents the food from entering the trachea (wind pipe). The food is passed along the oesophagus by contraction and relaxation of its muscular walls called peristalsis. Peristalsis forms a wave forcing the food to the stomach.

Digestion in mouth

Duct from gall bladder

Gall bladderCommon bile duct

Duodenum

Ducts from liver

Pancreas

Pancreatic duct

Digestion of Food

Digestion of Food

Saliva is alkaline in nature. It contains an enzyme called salivary amylase & lysozyme. Amylase converts starch into maltose. Lysozyme present in saliva acts as an antibacterial agent that prevents infections.Starch Salivary Amylase Maltose PH 6.8

Digestion in Stomach

Internally stomach wall is lined by mucous membrane.A number of glands called gastric glands are present in this membrane. Gastric glands have 3 major types of cells (i) Mucus cells or neck cells secrete mucus.(ii) Peptic cells secrete proenzyme pepsinogen and (iii) Oxyntic cells which secrete Hcl.

7

Digestion anD absorption

www.sciencetuts.com

The stomach stores the food for 4–5 hrs. The food mixes thoroughly with the acidic gastric juice of the stomach by churning movements of its muscular wall and is called chyme. The pepsinogen on exposure to Hcl gets converted to active pepsin. Pepsin converts proteins into proteoses and peptones. Pepsinogen Hcl pepsin Proteins pepsin proteoses & peptones. Rennin is proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. It causes curdling of milk. This enzyme disappears as the child grows.Hcl present in gastric juice kills the bacteria swallowed along with the food.The mucus & Bicarbonates presents in the gastric juice play on important role in lubrication & protection of mucosal epithelium from excoriation by the highly concentrated Hcl.Lipase converts fats into fatty acids & glycerol.

The bile, pancreatic juice & intestinal juice are the secretions released into the small intestine.The bile released into the duodenum contains bile pigments (bilirubin & biliverdin), bile salts, cholesterol & phospholipids but no enzymes. Bile helps in emulsification of fats (i.e.) breaking down of fats into very small micelles. Bile also activates lipases.

Duodenum

Pancreatic juice

Proteins, proteoses &peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice.Proteins Trypsin, / chymotrypsin dipetides.Peptones carboxypeptidase Proteoses

Amylase present in pancreatic juice acts on carbohydrates & converts into disaccharides. Starch Amylase Disaccharides. Fats Lipase Diglyceride Monoglyceride.Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides & nucleosides Nucleic acids nucleases nucleotides nucleosidesCells present in the intestinal wall secrete mucous and enzymes in the form of intestinal juice. This is called succus entericus. The juice contains variety of enzymes like disaccharidases, dipeptidases, lipases, nucleosidases etc.

These enzymes act on the end products of the above reactions to form simple absorbable forms.Peptidases convert peptides into amino acids.Dipeptides Dipeptidases amino acidsIntestinal lipase completely digests fats.

Di & Monoglycerides ________Lipases____________ fatty acids + glycerolEnzymes like sucrase, maltase and lactase, hydrolyze sucrose,maltose and lactose respectively converting them into glucose. Other sugars are also produced in this process.

8

Digestion anD absorption

www.sciencetuts.com

Maltose maltase Glucose + GlucoseLactose Lactase Glucose + galactoseSucrose sucrase Glucose + fructoseNucleotidase & nucleosidase complete the digestion of nucleic acids.Nucleotides nucleotidases Nucleosides nucleosidases sugars + basesThe breakdown of biomacromolecules occurs in the duodenum region of the small intestine. The simple substances thus formed are absorbed in the jejunum &ileum regions of the small intestine. The undigested & 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 water & minerals.ii) Secretion of mucus helps in adhering the waste particles together & lubricating it for easy passage. The undigested, unabsorbed substances called faeces enters into caecum of large intestine though ileo caecal valve, which prevents the back flow of faecal matter. It is temporarily stored in the rectum from where it is egested through the anus. The undigested waste is called faeces. The process of elimination of undigested food is called Defaecation. The roughage in the diet helps in promoting the movement of bowels

Absorption of Digested Products

Absorption

Diffusion

Facilitated transport

Active transport

Is the process by which the end products of digestion are taken into blood stream. It is carried out by passive, active or facilitated transport mechanism.

Fructose & amino acids are absorbed with the help of carrier ions like Na+. This mechanism is called the facilitated transport.

Occurs against the concentration gradient and hence requires energy. Various nutrients like amino acids, monosaccharides like glucose and electrolytes like Na+ are absorbed into the blood by this mechanism. Fatty acids & glycerol being insoluble cannot be absorbed into the blood. They are incorporated into small droplets called micelles which move into the intestine mucosa. They are reformed 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.The absorbed substances finally reach the tissues which utilize them for their activities. This process is called assimilation.The egestion of faeces to the outside through the anal opening is a voluntary process & is carried out by means of mass peristaltic movement.

Monosaccharides like glucose; amino acids of electrolytes like cl¯ are absorbed by simple diffusion. The passage of these substances into the blood depends upon the concentration gradients.

9

Digestion anD absorption

www.sciencetuts.com

Disorders of digestive system

1. Jaundice. French word jaune means yellow.It is yellowish pigmentation of the skin, eyes due to increased levels of bile pigment – bilirubin in the blood.2. Vomiting. Is the forceful expulsion of the contents of one’s stomach through the mouth & sometimes the nose. The feeling that one is about to vomit is called nausea which usually proceeds to vomiting. Vomiting may be caused due to wide variety of condition.3. Diarrhoea (Gk word Dia – through rheo “flow” – meaning flowing through). is a condition of abnormal bowel frequency & increased liquidity of the feacal discharge. It causes dehydration & salt imbalance. 4. Constipation. Refers to bowel movement that is frequent or hard to pass. It is common cause of painful defecation. Treatment includes change in dietary habits. Because it is a symptom, not a disease. Effective treatment may require first determining the cause.5. Indigestion – also called dyspepsia or an upset stomach is discomfort in upper abdomen. It is not a disease but a condition of symptoms including bloating, belching and nausea. Or heart burn. It leads to upper abdominal fullness and feeling full earlier than expected when eating. The causes of indigestion are inadequate enzyme secretion anxiety, food poisoning, over eating & spicy food.

Summary

The process of hydrolyzing complex food molecules into simple substances by the enzymes is • called digestion.Digestive system consists of mouth, buccal cavity, pharynx, oesophagus, stomach, duodenum, • small intestine, large intestine, rectum and anus. Teeth, tongue, openings of 3 pairs of salivary glands are present in the buccal cavity. parotid, sub • maxillary and sub – lingual glands secrete saliva.Saliva is alkaline and consists of water, salts, mucous and amylase. Amylase converts starch into • maltose and sugars.Tongue pushes food from pharynx into oesophagus. Peristaltic movements of muscles in • oesophagus move the food into the stomach.Stomach has 3 functions – stores food, mechanically mix the food by the action of muscles, food • undergoes chemical changes due to the action of digestive juices.Gastric glands secrete gastric juice. Hcl, pepsin and lipase are present in gastric juice.• Pepsin converts proteins into peptones and proteoses.• Lipase converts fats into fatty acids and glycerol. • Liver secretes bile. Bile helps in emulsification of fats.• Pancreas secretes pancreatic juice. • Trypsin and chymotrypsin are secreted in inactive form. Enterokinase converts them into active • form.Intestinal glands secrete intestinal juice.Digested food is absorbed into blood through the wall of • intestine.

10