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Digestion and Absorption
DIGESTION AND ABSORPTION
Chapter Outline:
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Prerequisites
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Learning objectives
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Digestive System
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Digestion of Food
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Absorption of Digested Products
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Disorders of Digestive System
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Summary
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Digestion and Absorption
Prerequisites
1. To perform various activities, living organisms require a continuous supply of energy.
2. The energy comes from the food we eat.
3. The process of taking or consuming food is called Nutrition.
4. The major components of food are carbohydrates, proteins & fats. Vitamins & minerals are
also required in small quantities.
5. The water also plays an important role in the metabolic process.
Learning objectives
1. To know the parts of Digestive system.
2. To develop the skill of drawing with well labeled diagrams of human digestive system and
its related organs.
3. To identify the arrangement of different types of teeth in the jaws on one side and the
sockets on the other side.
4. To understand and interprets the biological equations of enzymes in the process of digestion
and absorption.
5. To understand the process of digestion & absorption.
6. To gain knowledge about digestive system and its disorders.
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Digestion and Absorption
Nutrition involves 5 steps
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.
Digestive system
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
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
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
Mouth
Oral cavity
Mouth
Submaxillary and
sublingual glands
Parotid gland
Pharynx
Oesophagus
Liver
Gall bladder
Duodenum
Transverse colon
Ascending colon
Ileum
Caecum
Vermiform appendix
Stomach
Pancreas
Jejunum
Descending colon
Rectum
Anus
Digestive system
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.
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.
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Digestion and Absorption
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
8
7
Molars
Socket
of jaws
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Premolars
5
4
3
Canines
2 1
Incisors
Teeth
Incisors (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
3 8
1
2
2
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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;
Pharynx
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.
Oesophagus
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.
Stomach
Stomach 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.
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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
Oesophagus
Fundus
muscles contract in different directions. As a result food is
Cardiac
churned in the stomach.
Stomach has 3 important roles.
Superior portion
1. It stores the food temporarily.
of duodenum
2. Mixing of various components in the food
thoroughly –This occurs due to contraction & relaxation
Pyloric
of muscles.
3. It brings about physical & chemical changes in the food.
Stomach
Small intestine
It is 6-7m long & 2.5 cm in diameter. It is narrow coiled tube having 3 parts.
(a) Duodenum (b) jejunum (c) ileum
Duodenum: 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.
Jejunum
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.
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Colon
The 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.
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.
Serosa
Inner circular
Muscularis
outerlongitudinal
Lumen
Sub-mucosa
Mucosa
Section of gut
Villi
Crypts
Lacteal
Artery
Vein
Capillaries
Mucosa showing villi
Digestive Glands
The digestive glands associated with alimentary canal include salivary glands, the liver & the pancreas.
Salivary glands
Three pairs of salivary glands are present in the mouth. They are parotid, sub maxillary & sub-lingual.
Parotid glands
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.
Sub maxillary glands
They are located at the posterior part of the floor of mouth. The ducts open on the floor of the mouth cavity.
Sub lingual glands
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.
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Liver
Ducts from 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.
Pancreas
Duct from
gall bladder
Pancreatic duct
Gall bladder
Common bile duct
Duodenum
Liver
Pancreas
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.
Digestion of Food
The process of digestion is accompanied by mechanical &
chemical processes.
Digestion in mouth
Digestion of Food
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.
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.
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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.
Duodenum
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.
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 nucleosides
Cells 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 acids
Intestinal lipase completely digests fats.
Di & Monoglycerides ________Lipases____________ fatty acids + glycerol
Enzymes like sucrase, maltase and lactase, hydrolyze sucrose,maltose and lactose respectively converting them
into glucose. Other sugars are also produced in this process.
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Maltose maltase Glucose + Glucose
Lactose Lactase Glucose + galactose
Sucrose sucrase Glucose + fructose
Nucleotidase & nucleosidase complete the digestion of nucleic acids.
Nucleotides nucleotidases Nucleosides nucleosidases sugars + bases
The 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
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.
Diffusion
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.
Facilitated transport
Fructose & amino acids are absorbed with the help of carrier ions like Na+. This mechanism is called the facilitated
transport.
Active 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.
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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
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The process of hydrolyzing complex food molecules into simple substances by the enzymes is
called digestion.
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Digestive system consists of mouth, buccal cavity, pharynx, oesophagus, stomach, duodenum,
small intestine, large intestine, rectum and anus.
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Teeth, tongue, openings of 3 pairs of salivary glands are present in the buccal cavity. parotid, sub
maxillary and sub – lingual glands secrete saliva.
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Saliva is alkaline and consists of water, salts, mucous and amylase. Amylase converts starch into
maltose and sugars.
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Tongue pushes food from pharynx into oesophagus. Peristaltic
movements of muscles in
oesophagus move the food into the stomach.
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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.
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Gastric glands secrete gastric juice. Hcl, pepsin and lipase are present in gastric juice.
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Pepsin converts proteins into peptones and proteoses.
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Lipase converts fats into fatty acids and glycerol.
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Liver secretes bile. Bile helps in emulsification of fats.
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Pancreas secretes pancreatic juice.
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Trypsin and chymotrypsin are secreted in inactive form. Enterokinase converts them into active
form.
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Intestinal glands secrete intestinal juice.Digested food is absorbed into blood through the wall of
intestine.
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