NCERT Textbook: Digestion & Absorption Notes | Study Biology Class 11 - NEET

 Page 1


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of molecules,
supra molecular assemblies, cells, tissues, organisms and indeed,
populations and communities, each create emergent properties. In the
chapters under this unit, major human physiological processes like
digestion, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last two
chapters point to the coordination and regulation of body events at the
organismic level.
HUMAN PHYSIOLOGY
Chapter 16
Digestion and Absorption
Chapter 17
Breathing and Exchange
of Gases
Chapter 18
Body Fluids and
Circulation
Chapter 19
Excretory Products and
their Elimination
Chapter 20
Locomotion and Movement
Chapter 21
Neural Control and
Coordination
Chapter 22
Chemical Coordination
and Integration
2020-21
Page 2


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of molecules,
supra molecular assemblies, cells, tissues, organisms and indeed,
populations and communities, each create emergent properties. In the
chapters under this unit, major human physiological processes like
digestion, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last two
chapters point to the coordination and regulation of body events at the
organismic level.
HUMAN PHYSIOLOGY
Chapter 16
Digestion and Absorption
Chapter 17
Breathing and Exchange
of Gases
Chapter 18
Body Fluids and
Circulation
Chapter 19
Excretory Products and
their Elimination
Chapter 20
Locomotion and Movement
Chapter 21
Neural Control and
Coordination
Chapter 22
Chemical Coordination
and Integration
2020-21
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2020-21
Page 3


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of molecules,
supra molecular assemblies, cells, tissues, organisms and indeed,
populations and communities, each create emergent properties. In the
chapters under this unit, major human physiological processes like
digestion, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last two
chapters point to the coordination and regulation of body events at the
organismic level.
HUMAN PHYSIOLOGY
Chapter 16
Digestion and Absorption
Chapter 17
Breathing and Exchange
of Gases
Chapter 18
Body Fluids and
Circulation
Chapter 19
Excretory Products and
their Elimination
Chapter 20
Locomotion and Movement
Chapter 21
Neural Control and
Coordination
Chapter 22
Chemical Coordination
and Integration
2020-21
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2020-21
Food is one of the basic requirements of all living organisms. The major
components of our food are carbohydrates, proteins and fats. Vitamins
and minerals are also required in small quantities. Food provides energy
and organic materials for growth and repair of tissues. The water we take
in, plays an important role in metabolic processes and also prevents
dehydration of the body. Biomacromolecules in food cannot be utilised
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 substances to simple absorbable forms is
called digestion and is carried out by our digestive system by mechanical
and biochemical methods. General organisation of the human digestive
system is shown in Figure 16.1.
16.1 DIGESTIVE SYSTEM
The human digestive system consists of the alimentary canal and the
associated glands.
16.1.1 Alimentary Canal
The alimentary canal begins with an anterior opening – the mouth, and it
opens out posteriorly through the anus. The mouth leads to the buccal
cavity or oral cavity. The oral cavity has a number of teeth and a muscular
tongue. Each tooth is embedded in a socket of jaw bone (Figure16.2).
This type of attachment is called thecodont. Majority of mammals
including human being forms two sets of teeth during their life, a set of
temporary milk or deciduous teeth replaced by a set of permanent or
adult teeth. This type of dentition is called diphyodont. An adult human
DIGESTION AND ABSORPTION
CHAPTER  16
16.1 Digestive
System
16.2 Digestion of
Food
16.3 Absorption of
Digested
Products
16.4 Disorders of
Digestive
System
2020-21
Page 4


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of molecules,
supra molecular assemblies, cells, tissues, organisms and indeed,
populations and communities, each create emergent properties. In the
chapters under this unit, major human physiological processes like
digestion, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last two
chapters point to the coordination and regulation of body events at the
organismic level.
HUMAN PHYSIOLOGY
Chapter 16
Digestion and Absorption
Chapter 17
Breathing and Exchange
of Gases
Chapter 18
Body Fluids and
Circulation
Chapter 19
Excretory Products and
their Elimination
Chapter 20
Locomotion and Movement
Chapter 21
Neural Control and
Coordination
Chapter 22
Chemical Coordination
and Integration
2020-21
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2020-21
Food is one of the basic requirements of all living organisms. The major
components of our food are carbohydrates, proteins and fats. Vitamins
and minerals are also required in small quantities. Food provides energy
and organic materials for growth and repair of tissues. The water we take
in, plays an important role in metabolic processes and also prevents
dehydration of the body. Biomacromolecules in food cannot be utilised
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 substances to simple absorbable forms is
called digestion and is carried out by our digestive system by mechanical
and biochemical methods. General organisation of the human digestive
system is shown in Figure 16.1.
16.1 DIGESTIVE SYSTEM
The human digestive system consists of the alimentary canal and the
associated glands.
16.1.1 Alimentary Canal
The alimentary canal begins with an anterior opening – the mouth, and it
opens out posteriorly through the anus. The mouth leads to the buccal
cavity or oral cavity. The oral cavity has a number of teeth and a muscular
tongue. Each tooth is embedded in a socket of jaw bone (Figure16.2).
This type of attachment is called thecodont. Majority of mammals
including human being forms two sets of teeth during their life, a set of
temporary milk or deciduous teeth replaced by a set of permanent or
adult teeth. This type of dentition is called diphyodont. An adult human
DIGESTION AND ABSORPTION
CHAPTER  16
16.1 Digestive
System
16.2 Digestion of
Food
16.3 Absorption of
Digested
Products
16.4 Disorders of
Digestive
System
2020-21
258 BIOLOGY
Sigmoid Colon
Figure 16.1 The human digestive system
has 32 permanent teeth which are of four different types (Heterodont
dentition), namely, incisors (I), canine (C), premolars (PM) and molars
(M). Arrangement of teeth in each half of the upper and lower jaw in the
order I, C, PM, M is represented by a dental formula which in human
is 
2123
2123
. The hard chewing surface of the teeth, made up of enamel, helps
in the mastication of food. The tongue is a freely movable muscular organ
attached to the floor of the oral cavity by the frenulum. The upper surface
of the tongue has small projections called papillae, some of which bear
taste buds.
The oral cavity leads into a short pharynx which serves as a common
passage for food and air. The oesophagus and the trachea (wind pipe)
open into the pharynx. A cartilaginous flap called epiglottis prevents the
entry of food into the glottis – opening of the wind pipe – during swallowing.
The oesophagus is a thin, long tube which extends posteriorly passing
through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag
2020-21
Page 5


UNIT 5
The reductionist  approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of molecules,
supra molecular assemblies, cells, tissues, organisms and indeed,
populations and communities, each create emergent properties. In the
chapters under this unit, major human physiological processes like
digestion, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last two
chapters point to the coordination and regulation of body events at the
organismic level.
HUMAN PHYSIOLOGY
Chapter 16
Digestion and Absorption
Chapter 17
Breathing and Exchange
of Gases
Chapter 18
Body Fluids and
Circulation
Chapter 19
Excretory Products and
their Elimination
Chapter 20
Locomotion and Movement
Chapter 21
Neural Control and
Coordination
Chapter 22
Chemical Coordination
and Integration
2020-21
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2020-21
Food is one of the basic requirements of all living organisms. The major
components of our food are carbohydrates, proteins and fats. Vitamins
and minerals are also required in small quantities. Food provides energy
and organic materials for growth and repair of tissues. The water we take
in, plays an important role in metabolic processes and also prevents
dehydration of the body. Biomacromolecules in food cannot be utilised
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 substances to simple absorbable forms is
called digestion and is carried out by our digestive system by mechanical
and biochemical methods. General organisation of the human digestive
system is shown in Figure 16.1.
16.1 DIGESTIVE SYSTEM
The human digestive system consists of the alimentary canal and the
associated glands.
16.1.1 Alimentary Canal
The alimentary canal begins with an anterior opening – the mouth, and it
opens out posteriorly through the anus. The mouth leads to the buccal
cavity or oral cavity. The oral cavity has a number of teeth and a muscular
tongue. Each tooth is embedded in a socket of jaw bone (Figure16.2).
This type of attachment is called thecodont. Majority of mammals
including human being forms two sets of teeth during their life, a set of
temporary milk or deciduous teeth replaced by a set of permanent or
adult teeth. This type of dentition is called diphyodont. An adult human
DIGESTION AND ABSORPTION
CHAPTER  16
16.1 Digestive
System
16.2 Digestion of
Food
16.3 Absorption of
Digested
Products
16.4 Disorders of
Digestive
System
2020-21
258 BIOLOGY
Sigmoid Colon
Figure 16.1 The human digestive system
has 32 permanent teeth which are of four different types (Heterodont
dentition), namely, incisors (I), canine (C), premolars (PM) and molars
(M). Arrangement of teeth in each half of the upper and lower jaw in the
order I, C, PM, M is represented by a dental formula which in human
is 
2123
2123
. The hard chewing surface of the teeth, made up of enamel, helps
in the mastication of food. The tongue is a freely movable muscular organ
attached to the floor of the oral cavity by the frenulum. The upper surface
of the tongue has small projections called papillae, some of which bear
taste buds.
The oral cavity leads into a short pharynx which serves as a common
passage for food and air. The oesophagus and the trachea (wind pipe)
open into the pharynx. A cartilaginous flap called epiglottis prevents the
entry of food into the glottis – opening of the wind pipe – during swallowing.
The oesophagus is a thin, long tube which extends posteriorly passing
through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag
2020-21
DIGESTION AND ABSORPTION 259
like structure called stomach. A muscular
sphincter (gastro-oesophageal) regulates the
opening of oesophagus into the stomach.
The stomach, located in the upper left
portion of the abdominal cavity, has four
major parts – a cardiac portion into which
the oesophagus opens, a fundic region, body
(main central region) and a pyloric portion
which opens into the first part of small
intestine (Figure 16.3). Small intestine is
distinguishable into three regions, a ‘C’
shaped duodenum, a long coiled middle
portion jejunum and a highly coiled ileum.
The opening of the stomach into the
duodenum is guarded by the pyloric
sphincter. Ileum opens into the large
intestine. It consists of caecum, colon and
rectum. Caecum is a small blind sac which
hosts some symbiotic micro-organisms. A
narrow finger-like tubular projection, the
vermiform appendix which is a vestigial
organ, arises from the caecum. The caecum
opens into the colon. The colon is divided
into four parts – an ascending, a transverse,
descending part and a sigmoid colon. The
descending part opens into the rectum
which opens out through the anus.
The wall of alimentary canal from
oesophagus to rectum possesses four layers
(Figure 16.4) namely serosa, muscularis,
sub-mucosa and mucosa. Serosa is the
outermost layer and is made up of a thin
mesothelium (epithelium of visceral organs)
with some connective tissues. Muscularis is
formed by smooth muscles usually
arranged into an inner circular and an outer
longitudinal layer. An oblique muscle layer
may be present in some regions. The sub-
mucosal layer is formed of loose connective
tissues containing nerves, blood and lymph
vessels. In duodenum, glands are also
present in sub-mucosa. The innermost
layer lining the lumen of the alimentary
canal is the mucosa. This layer forms
irregular folds (rugae) in the stomach and
small finger-like foldings called villi in the
small intestine (Figure 16.5). The cells lining
the villi produce numerous microscopic
Figure 16.2 Arrangement of different types of
teeth in the jaws on one side and
the sockets on the other side
Figure 16.3 Anatomical regions of human
stomach
2020-21