Physiology of GIT
Gastrointestinal physiology is the
branch of human physiology that addresses the physical function of the
gastrointestinal (GI) tract. The function of the GI tract is to process
ingested food by mechanical and chemical means, extract nutrients and excrete
waste products. The GI tract is composed of the alimentary canal, that runs
from the mouth to the anus, as well as the associated glands, chemicals,
hormones, and enzymes that assist in digestion. The major processes that occur
in the GI tract are: motility, secretion, regulation, digestion and
circulation. The proper function and coordination of these processes are vital
for maintaining good health by providing for the effective digestion and uptake
of nutrients.
Motility: The gastrointestinal
tract generates motility using smooth muscle subunits linked by gap junctions.
These subunits fire spontaneously in either a tonic or a phasic fashion. Tonic
contractions are those contractions that are maintained from several minutes up
to hours at a time. These occur in the sphincters of the tract, as well as in
the anterior stomach. The other type of contractions, called phasic
contractions, consist of brief periods of both relaxation and contraction,
occurring in the posterior stomach and the small intestine, and are carried out
by the muscularis externa. Motility may be overactive (hypermotility), leading
to diarrhea or vomiting, or underactive (hypomotility), leading to constipation
or vomiting; either may cause abdominal pain.
Stimulation: The stimulation for these
contractions likely originates in modified smooth muscle cells called
interstitial cells of Cajal. These cells cause spontaneous cycles of slow wave
potentials that can cause action potentials in smooth muscle cells. They are associated
with the contractile smooth muscle via gap junctions. These slow wave
potentials must reach a threshold level for the action potential to occur,
whereupon Ca2+ channels on the smooth muscle open and an action potential
occurs. As the contraction is graded based upon how much Ca2+ enters the cell,
the longer the duration of slow wave, the more action potentials occur. This,
in turn, results in greater contraction force from the smooth muscle. Both
amplitude and duration of the slow waves can be modified based upon the
presence of neurotransmitters, hormones or other paracrine signaling. The
number of slow wave potentials per minute varies based upon the location in the
digestive tract. This number ranges from 3 waves/min in the stomach to 12
waves/min in the intestines.
Contraction
patterns:
The patterns of GI contraction as a whole can be divided into two distinct
patterns, peristalsis and segmentation. Occurring between meals, the migrating
motor complex is a series of peristaltic wave cycles in distinct phases
starting with relaxation, followed by an increasing level of activity to a peak
level of peristaltic activity lasting for 5–15 minutes.[5] This cycle repeats
every 1.5–2 hours but is interrupted by food ingestion. The role of this
process is likely to clean excess bacteria and food from the digestive system.
Peristalsis: Peristalsis is one of the
patterns that occur during and shortly after a meal. The contractions occur in
wave patterns traveling down short lengths of the GI tract from one section to
the next. The contractions occur directly behind the bolus of food that is in
the system, forcing it toward the anus into the next relaxed section of smooth
muscle. This relaxed section then contracts, generating smooth forward movement
of the bolus at between 2–25 cm per second. This contraction pattern depends upon
hormones, paracrine signals, and the autonomic nervous system for proper
regulation.
Segmentation: Segmentation also occurs
during and shortly after a meal within short lengths in segmented or random
patterns along the intestine. This process is carried out by the longitudinal
muscles relaxing while circular muscles contract at alternating sections thereby
mixing the food. This mixing allows food and digestive enzymes to maintain a
uniform composition, as well as to ensure contact with the epithelium for
proper absorption.
Secretion: Every day, seven liters
of fluid are secreted by the digestive system. This fluid is composed of four
primary components: ions, digestive enzymes, mucus, and bile. About half of
these fluids are secreted by the salivary glands, pancreas, and liver, which
compose the accessory organs and glands of the digestive system. The rest of
the fluid is secreted by the GI epithelial cells.
Ions: The largest component of
secreted fluids is ions and water, which are first secreted and then reabsorbed
along the tract. The ions secreted primarily consist of H+, K+, Cl−, HCO3− and
Na+. Water follows the movement of these ions. The GI tract accomplishes this
ion pumping using a system of proteins that are capable of active transport,
facilitated diffusion and open channel ion movement. The arrangement of these
proteins on the apical and basolateral sides of the epithelium determines the
net movement of ions and water in the tract.
H+ and Cl− are secreted by the parietal cells into the lumen of the
stomach creating acidic conditions with a low pH of 1. H+ is pumped into the
stomach by exchanging it with K+. This process also requires ATP as a source of
energy; however, Cl− then follows the positive charge in the H+ through an open
apical channel protein.
HCO3− secretion occurs to
neutralize the acid secretions that make their way into the duodenum of the
small intestine. Most of the HCO3− comes from pancreatic acinar cells in the
form of NaHCO3 in an aqueous solution.[5] This is the result of the high
concentration of both HCO3− and Na+ present in the duct creating an osmotic
gradient to which the water follows.[4]
Digestive
enzymes:
The second vital secretion of the GI tract is that of digestive enzymes that
are secreted in the mouth, stomach and intestines. Some of these enzymes are
secreted by accessory digestive organs, while others are secreted by the
epithelial cells of the stomach and intestine. While some of these enzymes
remain embedded in the wall of the GI tract, others are secreted in an inactive
proenzyme form.[4] When these proenzymes reach the lumen of the tract, a factor
specific to a particular proenzyme will activate it. A prime example of this is
pepsin, which is secreted in the stomach by chief cells. Pepsin in its secreted
form is inactive (pepsinogen). However, once it reaches the gastric lumen it
becomes activated into pepsin by the high H+ concentration, becoming an enzyme
vital to digestion. The release of the enzymes is regulated by neural,
hormonal, or paracrine signals. However, in general, parasympathetic
stimulation increases secretion of all digestive enzymes.
Mucus: Mucus is released in the
stomach and intestine, and serves to lubricate and protect the inner mucosa of
the tract. It is composed of a specific family of glycoproteins termed mucins
and is generally very viscous. Mucus is made by two types of specialized cells
termed mucus cells in the stomach and goblet cells in the intestines. Signals
for increased mucus release include parasympathetic innervations, immune system
response and enteric nervous system messengers.
Bile: Bile is secreted into the
duodenum of the small intestine via the common bile duct. It is produced in
liver cells and stored in the gall bladder until release during a meal. Bile is
formed of three elements: bile salts, bilirubin and cholesterol. Bilirubin is a
waste product of the breakdown of hemoglobin. The cholesterol present is
secreted with the feces. The bile salt component is an active non-enzymatic
substance that facilitates fat absorption by helping it to form an emulsion
with water due to its amphoteric nature. These salts are formed in the
hepatocytes from bile acids combined with an amino acid. Other compounds such
as the waste products of drug degradation are also present in the bile.
Regulation: The digestive system has
a complex system of motility and secretion regulation which is vital for proper
function. This task is accomplished via a system of long reflexes from the
central nervous system (CNS), short reflexes from the enteric nervous system
(ENS) and reflexes from GI peptides working in harmony with each other.
Long
reflexes:
Long reflexes to the digestive system involve a sensory neuron sending
information to the brain, which integrates the signal and then sends messages
to the digestive system. While in some situations, the sensory information
comes from the GI tract itself; in others, information is received from sources
other than the GI tract. When the latter situation occurs, these reflexes are
called feedforward reflexes. This type of reflex includes reactions to food or
danger triggering effects in the GI tract. Emotional responses can also trigger
GI response such as the butterflies in the stomach feeling when nervous. The
feedforward and emotional reflexes of the GI tract are considered cephalic
reflexes.
Short
reflexes:
Control of the digestive system is also maintained by ENS, which can be thought
of as a digestive brain that can help to regulate motility, secretion and
growth. Sensory information from the digestive system can be received,
integrated and acted upon by the enteric system alone. When this occurs, the
reflex is called a short reflex.[4] Although this may be the case in several
situations, the ENS can also work in conjunction with the CNS; vagal afferents
from the viscera are received by the medulla, efferents are affected by the
vagus nerve. When this occurs, the reflex is called vagovagal reflex. The
myenteric plexus and submucosal plexus are both located in the gut wall and
receive sensory signals from the lumen of the gut or the CNS.
Gastrointestinal
peptides:
GI peptides are signal molecules that are released into the blood by the GI
cells themselves. They act on a variety of tissues including the brain,
digestive accessory organs, and the GI tract. The effects range from excitatory
or inhibitory effects on motility and secretion to feelings of satiety or
hunger when acting on the brain. These hormones fall into three major
categories, the gastrin and secretin families, with the third composed of all
the other hormones unlike those in the other two families. Further information
on the GI peptides is summarized in the table below.
|
|
Secreted by |
Target |
Effects on endocrine secretion |
Effects on exocrine secretion |
Effects on motility |
Other effects |
Stimulus for release |
|
Gastrin |
G Cells in stomach |
ECL cells; parietal cells |
None |
Increases acid secretion, increases mucus growth |
Stimulates gastric contraction |
None |
Peptides and amino acids in lumen; gastrin releasing peptide and
ACh in nervous reflexes |
|
Cholecystokinin (CCK) |
Endocrine I cells of the small intestine; neurons of the brain and
gut |
Gall bladder, pancreas, gastric smooth muscle |
None |
Stimulates pancreatic enzyme and HCO3- secretion |
Stimulates gallbladder contraction; inhibits stomach emptying |
Satiety |
Fatty acids and some amino acids |
|
Secretin |
Endocrine S cells of the small intestine |
Pancreas, stomach |
None |
Stimulates pancreatic and hepatic HCO3- secretion; inhibits acid
secretion; pancreatic growth |
Stimulates gallbladder contraction; Inhibits stomach emptying |
None |
Acid in small intestine |
|
Gastric inhibitory Peptide |
Endocrine K cells of the small intestine |
Beta cells of the pancreas |
Stimulates pancreatic insulin release |
Inhibits acid secretion |
None |
Satiety and lipid metabolism |
Glucose, fatty acid, and amino acids in small intestine |
|
Motilin |
Endocrine M cells in small intestine |
Smooth muscle of stomach and duodenum |
None |
None |
Stimulates migrating motor complex |
Action in brain, stimulates migratory motor complex |
Fasting: cyclic release every 1.5–2 hours by neural stimulus |
|
Glucagon-like peptide-1 |
Endocrine cells in small intestine |
Endocrine pancreas |
Stimulates insulin release; inhibits glucagon release |
Possibly inhibits acid secretion |
Slows gastric emptying |
Satiety; various CNS functions |
Mixed meals of fats and carbohydrates |
DIGESTION
AND ABSORPTION OF CARBOHYRATES, LIPID, PROTEIN
1. DIGESTION OF CARBOHYDRATES
The term is most common in
biochemistry, where it is a synonym of saccharide, a group that includes
sugars, starch, and cellulose. The saccharides are divided into four chemical
groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
Monosaccharides and disaccharides, the smallest (lower molecular weight)
carbohydrates, are commonly referred to as sugars. The word saccharide comes
from the Greek word σάκχαρον (sákkharon), meaning "sugar". While the
scientific nomenclature of carbohydrates is complex, the names of the
monosaccharides and disaccharides very often end in the suffix -ose, as in the
monosaccharides fructose (fruit sugar) and glucose (starch sugar) and the
disaccharides sucrose (cane or beet sugar) and lactose (milk sugar).
Carbohydrates perform
numerous roles in living organisms. Polysaccharides serve for the storage of
energy (e.g. starch and glycogen) and as structural components (e.g. cellulose
in plants and chitin in arthropods). The 5-carbon monosaccharide ribose is an
important component of coenzymes (e.g. ATP, FAD and NAD) and the backbone of
the genetic molecule known as RNA. The related deoxyribose is a component of
DNA. Saccharides and their derivatives include many other important
biomolecules that play key roles in the immune system, fertilization,
preventing pathogenesis, blood clotting, and development.
Carbohydrates are central
to nutrition and are found in a wide variety of natural and processed foods.
Starch is a polysaccharide. It is abundant in cereals (wheat, maize, rice),
potatoes, and processed food based on cereal flour, such as bread, pizza or
pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted
from sugarcane or sugar beets), lactose (abundant in milk), glucose and
fructose, both of which occur naturally in honey, many fruits, and some
vegetables. Table sugar, milk, or honey are often added to drinks and many
prepared foods such as jam, biscuits and cakes.
Cellulose, a
polysaccharide found in the cell walls of all plants, is one of the main
components of insoluble dietary fiber. Although it is not digestible, insoluble
dietary fiber helps to maintain a healthy digestive system by easing
defecation. Other polysaccharides contained in dietary fiber include resistant
starch and inulin, which feed some bacteria in the microbiota of the large
intestine, and are metabolized by these bacteria to yield short-chain fatty
acids.
Among carbohydrates, only
the monosaccharide forms are absorbed. Hence, all carbohydrates must be
digested to glucose, galactose and fructose for absorption to proceed. There
are two types of carbohydrates in the human food which need to be digested in the
alimentary canal of man. These are:
a) Disaccharides like maltose (malt
sugar), sucrose (cane sugar), and lactose (milk sugar). Carbohydrate digestion
involves the hydrolysis of glycosidic bonds of polymeric and dimeric
carbohyrates to form monosaccharides. The carbohydrate splitting enzymes are
called amylolytic or carbohyrolases or glycosidases. Different glycosidases hydrolyse different glycosidic
bonds.
b) Polysaccharides like starch, glycogen
and cellulose.
Digestion of
Starch:
Starch is a storage polysaccharides of the plants. It is found in the potato
tubers; cereals like rice, wheat, etc. and fruits like mango, banana, etc. It
is formed of two complex polymeric compounds-α-amylose and amylopectin, both
are formed of D-glucose molecules interlinked by α(1 4) glycosidic bonds.
1 – In Buccal Cavity: There are three pairs of salivary glands (one
pair of each of parotid glands just in front and below the pinna; submaxillary
glands below the jaw angle, and sublingual glands below the tongue. There are
no infraorbital glands founds in most of
the mammals) in the buccal cavity of man (four pairs of salivary glands in
rabbit). These salivary glands secrete a slightly acidic secretion called
saliva which contains an amylolytic enzyme called salivary amylase or ptyalin
or diatase. Composition of saliva is 99.5% water, 0.2% minerals like Cl-,
HCO3-CO32- and phosphates of Na+, K+,
Ca++, and Mg++; and 0.3% organic compounds like mucin and enzymes, especially
salivary amylase. Saliva also contains antibacterial thiocyanate ions. It is an
endoamylase and hydrolyses α(1 4) glycosidic bonds of starch into double
sugars, maltose and isomaltose and small dextrins called ‘limit’ dextrins. The
ptyalin is most active at pH 6.8 and in the presence of Cl- .
Some enzymatic digestion
of starch occurs in the mouth, due to the action of the enzyme salivary
amylase. This enzyme starts to break the long glucose chains of starch into
shorter chains, some as small as maltose. (The other carbohydrates in the bread
don't undergo any enzymatic digestion in the mouth.), but in catalyzed reactions, that is,
facilitated by hydrolytic enzymes, secreted by exocrine pancreas and/or present
on the surface of the intestinal mucosal brush border cells (enterocytes).
Starch digestion begins in
mouth by salivary alpha-amylase, therefore the rate of mastication and the time
of permanence in mouth, however relatively short, are the first factors that
affect the interaction between starch and the enzyme and that can improve
digestion. Digestion of carbohydrate begins at oral cavity level and then goes
on in the next parts of the gastrointestinal tract, particularly in the small
intestine
Fig. The enzyme salivary amylase breaks starch
into smaller polysaccharides and maltose.
Illustration showing that the enzyme
salivary amylase breaks starch into smaller polysaccharides and maltose. The
image shows a long chain of starch (shown as green hexagons) that is then
broken into shorter lengths, including maltose, by salivary amylase
2 - Stomach
Once in the stomach, that essentially
acts as a tank, gastric acidity inactivates salivary alpha-amylase, whose
optimal pH is about 7, though the presence of starch may partly protects the
enzyme from gastric degradation, allowing the passage with meal into the
duodenum, where it may support pancreatic alpha-amylase in the digestive
process.
The low pH in the stomach inactivates
salivary amylase, so it no longer works once it arrives at the stomach.
Although there's more mechanical digestion in the stomach, there's little
chemical digestion of carbohydrates here.
3 - Small intestine
When we pass from the stomach into
the small intestine, bicarbonate ion secreted by pancreas (under stimulation of
secretin hormone) neutralizes gastric acidity leading pH to about 7, an optimal
value for the action of pancreatic enzymes, including alpha-amylase, and
intestinal enzymes, and for the residual salivary alpha-amylase. So starch
digestion, which occurs mostly in the duodenum, begins again by the action of
pancreatic alpha-amylase, secreted in amounts greatly exceeding than the
digestive needs (in reply to meals the enzyme is secreted in amounts at least
10 times greater than that needed for optimal starch digestion).
Long
reflexes:
Long reflexes to the digestive system involve a sensory neuron sending
information to the brain, which integrates the signal and then sends messages
to the digestive system. While in some situations, the sensory information
comes from the GI tract itself; in others, information is received from sources
other than the GI tract. When the latter situation occurs, these reflexes are
called feedforward reflexes. This type of reflex includes reactions to food or
danger triggering effects in the GI tract. Emotional responses can also trigger
GI response such as the butterflies in the stomach feeling when nervous. The
feedforward and emotional reflexes of the GI tract are considered cephalic
reflexes.
Short
reflexes:
Control of the digestive system is also maintained by ENS, which can be thought
of as a digestive brain that can help to regulate motility, secretion and
growth. Sensory information from the digestive system can be received,
integrated and acted upon by the enteric system alone. When this occurs, the
reflex is called a short reflex.[4] Although this may be the case in several
situations, the ENS can also work in conjunction with the CNS; vagal afferents
from the viscera are received by the medulla, efferents are affected by the
vagus nerve. When this occurs, the reflex is called vagovagal reflex. The
myenteric plexus and submucosal plexus are both located in the gut wall and
receive sensory signals from the lumen of the gut or the CNS.
Gastrointestinal
peptides:
GI peptides are signal molecules that are released into the blood by the GI
cells themselves. They act on a variety of tissues including the brain,
digestive accessory organs, and the GI tract. The effects range from excitatory
or inhibitory effects on motility and secretion to feelings of satiety or
hunger when acting on the brain. These hormones fall into three major
categories, the gastrin and secretin families, with the third composed of all
the other hormones unlike those in the other two families. Further information
on the GI peptides is summarized in the table below.
|
|
Secreted by |
Target |
Effects on endocrine secretion |
Effects on exocrine secretion |
Effects on motility |
Other effects |
Stimulus for release |
|
Gastrin |
G Cells in stomach |
ECL cells; parietal cells |
None |
Increases acid secretion, increases mucus growth |
Stimulates gastric contraction |
None |
Peptides and amino acids in lumen; gastrin releasing peptide and
ACh in nervous reflexes |
|
Cholecystokinin (CCK) |
Endocrine I cells of the small intestine; neurons of the brain and
gut |
Gall bladder, pancreas, gastric smooth muscle |
None |
Stimulates pancreatic enzyme and HCO3- secretion |
Stimulates gallbladder contraction; inhibits stomach emptying |
Satiety |
Fatty acids and some amino acids |
|
Secretin |
Endocrine S cells of the small intestine |
Pancreas, stomach |
None |
Stimulates pancreatic and hepatic HCO3- secretion; inhibits acid
secretion; pancreatic growth |
Stimulates gallbladder contraction; Inhibits stomach emptying |
None |
Acid in small intestine |
|
Gastric inhibitory Peptide |
Endocrine K cells of the small intestine |
Beta cells of the pancreas |
Stimulates pancreatic insulin release |
Inhibits acid secretion |
None |
Satiety and lipid metabolism |
Glucose, fatty acid, and amino acids in small intestine |
|
Motilin |
Endocrine M cells in small intestine |
Smooth muscle of stomach and duodenum |
None |
None |
Stimulates migrating motor complex |
Action in brain, stimulates migratory motor complex |
Fasting: cyclic release every 1.5–2 hours by neural stimulus |
|
Glucagon-like peptide-1 |
Endocrine cells in small intestine |
Endocrine pancreas |
Stimulates insulin release; inhibits glucagon release |
Possibly inhibits acid secretion |
Slows gastric emptying |
Satiety; various CNS functions |
Mixed meals of fats and carbohydrates |
DIGESTION
AND ABSORPTION OF CARBOHYRATES, LIPID, PROTEIN
1. DIGESTION OF CARBOHYDRATES
The term is most common in
biochemistry, where it is a synonym of saccharide, a group that includes
sugars, starch, and cellulose. The saccharides are divided into four chemical
groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
Monosaccharides and disaccharides, the smallest (lower molecular weight)
carbohydrates, are commonly referred to as sugars. The word saccharide comes
from the Greek word σάκχαρον (sákkharon), meaning "sugar". While the
scientific nomenclature of carbohydrates is complex, the names of the
monosaccharides and disaccharides very often end in the suffix -ose, as in the
monosaccharides fructose (fruit sugar) and glucose (starch sugar) and the
disaccharides sucrose (cane or beet sugar) and lactose (milk sugar).
Carbohydrates perform
numerous roles in living organisms. Polysaccharides serve for the storage of
energy (e.g. starch and glycogen) and as structural components (e.g. cellulose
in plants and chitin in arthropods). The 5-carbon monosaccharide ribose is an
important component of coenzymes (e.g. ATP, FAD and NAD) and the backbone of
the genetic molecule known as RNA. The related deoxyribose is a component of
DNA. Saccharides and their derivatives include many other important
biomolecules that play key roles in the immune system, fertilization,
preventing pathogenesis, blood clotting, and development.
Carbohydrates are central
to nutrition and are found in a wide variety of natural and processed foods.
Starch is a polysaccharide. It is abundant in cereals (wheat, maize, rice),
potatoes, and processed food based on cereal flour, such as bread, pizza or
pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted
from sugarcane or sugar beets), lactose (abundant in milk), glucose and
fructose, both of which occur naturally in honey, many fruits, and some
vegetables. Table sugar, milk, or honey are often added to drinks and many
prepared foods such as jam, biscuits and cakes.
Cellulose, a
polysaccharide found in the cell walls of all plants, is one of the main
components of insoluble dietary fiber. Although it is not digestible, insoluble
dietary fiber helps to maintain a healthy digestive system by easing
defecation. Other polysaccharides contained in dietary fiber include resistant
starch and inulin, which feed some bacteria in the microbiota of the large
intestine, and are metabolized by these bacteria to yield short-chain fatty
acids.
Among carbohydrates, only
the monosaccharide forms are absorbed. Hence, all carbohydrates must be
digested to glucose, galactose and fructose for absorption to proceed. There
are two types of carbohydrates in the human food which need to be digested in the
alimentary canal of man. These are:
a) Disaccharides like maltose (malt
sugar), sucrose (cane sugar), and lactose (milk sugar). Carbohydrate digestion
involves the hydrolysis of glycosidic bonds of polymeric and dimeric
carbohyrates to form monosaccharides. The carbohydrate splitting enzymes are
called amylolytic or carbohyrolases or glycosidases. Different glycosidases hydrolyse different glycosidic
bonds.
b) Polysaccharides like starch, glycogen
and cellulose.
Digestion of
Starch:
Starch is a storage polysaccharides of the plants. It is found in the potato
tubers; cereals like rice, wheat, etc. and fruits like mango, banana, etc. It
is formed of two complex polymeric compounds-α-amylose and amylopectin, both
are formed of D-glucose molecules interlinked by α(1 4) glycosidic bonds.
1 – In Buccal Cavity: There are three pairs of salivary glands (one
pair of each of parotid glands just in front and below the pinna; submaxillary
glands below the jaw angle, and sublingual glands below the tongue. There are
no infraorbital glands founds in most of
the mammals) in the buccal cavity of man (four pairs of salivary glands in
rabbit). These salivary glands secrete a slightly acidic secretion called
saliva which contains an amylolytic enzyme called salivary amylase or ptyalin
or diatase. Composition of saliva is 99.5% water, 0.2% minerals like Cl-,
HCO3-CO32- and phosphates of Na+, K+,
Ca++, and Mg++; and 0.3% organic compounds like mucin and enzymes, especially
salivary amylase. Saliva also contains antibacterial thiocyanate ions. It is an
endoamylase and hydrolyses α(1 4) glycosidic bonds of starch into double
sugars, maltose and isomaltose and small dextrins called ‘limit’ dextrins. The
ptyalin is most active at pH 6.8 and in the presence of Cl- .
Some enzymatic digestion
of starch occurs in the mouth, due to the action of the enzyme salivary
amylase. This enzyme starts to break the long glucose chains of starch into
shorter chains, some as small as maltose. (The other carbohydrates in the bread
don't undergo any enzymatic digestion in the mouth.), but in catalyzed reactions, that is,
facilitated by hydrolytic enzymes, secreted by exocrine pancreas and/or present
on the surface of the intestinal mucosal brush border cells (enterocytes).
Starch digestion begins in
mouth by salivary alpha-amylase, therefore the rate of mastication and the time
of permanence in mouth, however relatively short, are the first factors that
affect the interaction between starch and the enzyme and that can improve
digestion. Digestion of carbohydrate begins at oral cavity level and then goes
on in the next parts of the gastrointestinal tract, particularly in the small
intestine
Fig. The enzyme salivary amylase breaks starch
into smaller polysaccharides and maltose.
Illustration showing that the enzyme
salivary amylase breaks starch into smaller polysaccharides and maltose. The
image shows a long chain of starch (shown as green hexagons) that is then
broken into shorter lengths, including maltose, by salivary amylase
2 - Stomach
Once in the stomach, that essentially
acts as a tank, gastric acidity inactivates salivary alpha-amylase, whose
optimal pH is about 7, though the presence of starch may partly protects the
enzyme from gastric degradation, allowing the passage with meal into the
duodenum, where it may support pancreatic alpha-amylase in the digestive
process.
The low pH in the stomach inactivates
salivary amylase, so it no longer works once it arrives at the stomach.
Although there's more mechanical digestion in the stomach, there's little
chemical digestion of carbohydrates here.
3 - Small intestine
When we pass from the stomach into
the small intestine, bicarbonate ion secreted by pancreas (under stimulation of
secretin hormone) neutralizes gastric acidity leading pH to about 7, an optimal
value for the action of pancreatic enzymes, including alpha-amylase, and
intestinal enzymes, and for the residual salivary alpha-amylase. So starch
digestion, which occurs mostly in the duodenum, begins again by the action of
pancreatic alpha-amylase, secreted in amounts greatly exceeding than the
digestive needs (in reply to meals the enzyme is secreted in amounts at least
10 times greater than that needed for optimal starch digestion).
Although pancreatic alpha-amylase
acts primarily in the polar milieu of intestinal content, where therefore the
most part of the starch digestion occurs, a part adheres to the intestinal
mucosa on the brush border surface of enterocytes. It has been proposed that
this topographic disposition could be favorable because it would cause the
release of the cleavage products of the starch (maltose, maltotriose and
alpha-limit dextrins) at the lumen-membrane interface, where the final part of
the digestion occurs by the action of brush border enzymes (see below).
Ileum, the final part of the small
intestine, is able to digest and absorb carbohydrates, but in a less extend
than jejunum and obviously duodenum. In the presence of illness affecting
jejunum or of a surgical removal of the upper tract of the small intestine, the
ileum can adapt to the new condition and assume an important role in
carbohydrate digestion and absorption. Most carbohydrate digestion occurs in
the small intestine, thanks to a suite of enzymes. Pancreatic amylase is
secreted from the pancreas into the small intestine, and like salivary amylase,
it breaks starch down to small oligosaccharides (containing 3 to 10 glucose
molecules) and maltose.
Fig. The enzyme pancreatic amylase breaks starch
into smaller polysaccharides and maltose.
Illustration showing that the enzyme
pancreatic amylase breaks starch into smaller polysaccharides and maltose. The
image shows a long chain of starch (shown as green hexagons) that is then
broken into shorter lengths, including maltose, by pancreatic amylase.
The rest of the work of carbohydrate
digestion is done by enzymes produced by the enterocytes, the cells lining the
small intestine. When it comes to digesting your slice of pizza, these enzymes
will break down the maltase formed in the process of starch digestion, the
lactose from the cheese, and the sucrose present in the sauce.
Maltose
is digested by maltase, forming 2 glucose molecules
Illustration showing maltose
(represented by two green hexagons linked together) being broken into two
glucose molecules by the enzyme maltase.
Lactose
is digested by lactase, forming glucose and galactose
Illustration showing lactose
(represented by a green hexagon linked to a blue hexagon) being broken into one
glucose molecule and one galactose molecule by the enzyme lactase.
Sucrose
is digested by sucrase, forming glucose and fructose
Fig.
Action of the enzymes maltase, lactase, and sucrase.
Illustration showing sucrose
(represented by a green hexagon linked to a purple pentagon) being broken into
one glucose molecule and one fructose molecule by the enzyme sucrase.
(Recall that if a person is lactose
intolerant, they don't make enough lactase enzyme to digest lactose adequately.
Therefore, lactose passes to the large intestine. There it draws water in by
osmosis and is fermented by bacteria, causing symptoms such as flatulence,
bloating, and diarrhea.)
Fructose and galactose are converted
to glucose in the liver. Once absorbed carbohydrates pass through the liver,
glucose is the main form of carbohydrate circulating in the bloodstream.
a-Amylases (salivary and pancreatic)
hydrolyze 1,4-glycosidic bonds in starch, yielding maltose, maltotriose, and
α-limit dextrins.
Maltase, a-dextrinase, and sucrase in
the intestinal brush border then hydrolyze the oligosaccharides to glucose.
Lactase, trehalase, and sucrase
degrade their respective disaccharides lactose, trehalose, and sucrose to
monosaccharides.
Lactase degrades lactose to glucose
and galactose.
Trehalase degrades trehalose to
glucose.
Sucrase degrades sucrose to glucose
and fructose.
By the end of this process of
enzymatic digestion, we're left with three monosaccharides: glucose, fructose,
and galactose. These can now be absorbed across the enterocytes of the small
intestine and into the bloodstream to be transported to the liver.
Digestion and absorption of
carbohydrates in the small intestine are depicted in a very simplified
schematic below. (Remember that the inner wall of the small intestine is
actually composed of large circular folds, lined with many villi, the surface
of which are made up of microvilli. All of this gives the small intestine a huge
surface area for absorption.
Monosaccharides
are then absorbed into the bloodstream and travel to the liver.
Fig. Digestion and
absorption of carbohydrates in the small intestine.
4 - Large Intestine or Colon
Any carbohydrates that
weren't digested in the small intestine -- mainly fiber -- pass into the large
intestine, but there's no enzymatic digestion of these carbohydrates here.
Instead, bacteria living in the large intestine, sometimes called our gut microbiota,
ferment these carbohydrates to feed themselves. Fermentation causes gas
production, and that's why we may experience bloating and flatulence after a
particularly fibrous meal. Fermentation also produces short-chain fatty acids,
which our large intestine cells can use as an energy source. Over the last
decade or so, more and more research has shown that our gut microbiota are
incredibly important to our health, playing important roles in the function of
our immune response, nutrition, and risk of disease. A diet high in whole food
sources of fiber helps to maintain a population of healthy gut microbes.
Digestion of
di- and oligosaccharides
The final step of carbohydrate
digestion is yielded by enzymes synthesized in enterocytes and localized on the
brush border surface of the same cells.
They are glycoproteins with
hydrolasic activity that act on the products of the alpha-amylase action,
maltose, maltotriose and alpha-limit dextrins, and even more on two other
carbohydrates, the disaccharides sucrose and lactose.
The capacity of synthesize these
enzymes is acquired during foetal period prior to birth, therefore newborn
infants have all these enzymes.
Several glycosidases can act only on
alpha-glycosidic bonds that is bonds in which the “bridge” made up by oxygen
atom is below the plan individuated by the ring structure of the sugar; so they
are called alpha-glycosidases and, in particular:
sucrase;
glucoamylase;
alpha-dextrinase.
It should be noted that glycosidases
present in our body can’t act on carbohydrates in which glucose is linked by
beta-glycosidic bonds, as e.g. cellulose).
All the alpha-glycosidases present on
the brush border surface of enterocytes are specific for the α-(1→4) glycosidic
bond that links, at the non-reducing end of the chain, the last to the last but
one residue of glucose. What differentiates them, and which is at the base of
their nomenclature, is the degree of affinity for glycosidic bonds present at
the nonreducing end of the saccharidic chain.
It is clear that alpha-glycosidases
do not work in a separate manner on substrates because of in every step of the
digestive process one or more of them will have an high specificity for the
alpha-glycosidic bond currently closest to non-reducing end of the
oligosaccharide on duty.
Only the final products of the
catalytic activities of alpha-glycosidases, lactase and trehalase, that is
glucose, fructose and galactose, will be transported across the intestinal
membrane barrier and flowed into the bloodstream to be distributed to liver and
then to the several tissues.
Summary of
Carbohydrate Digestion: The primary goal of carbohydrate digestion is to break
polysaccharides and disaccharides into monosaccharides (glucose, galactose and
fructose), which can be absorbed into the bloodstream.
1. After eating, nothing needs to
happen in the digestive tract to the monosaccharides in a food like grapes,
because they are already small enough to be absorbed as is.
2. Disaccharides in that grape or in
a food like milk are broken down (enzymatically digested) in the digestive
tract to monosaccharides (glucose, galactose, and fructose).
3. Starch in food is broken down
(enzymatically digested) in the digestive tract to glucose molecules.
4. Fiber in food is not enzymatically
digested in the digestive tract, because humans don't have enzymes to do this.
However, some dietary fiber is fermented in the large intestine by gut
microbes.
Fig.
Digestion of carbohydrates: Digestion of carbohydrates is performed by several
enzymes. Starch and glycogen are broken down into glucose by amylase and
maltase. Sucrose (table sugar) and lactose (milk sugar) are broken down by
sucrase and lactase, respectively.
2. DIGESTION OF PROTEINS
Dietary proteins are a
source of amino acids that are utilized for the formation of various cellular
substances. Mostly, proteins must be broken down into amino acids for
absorption. Digestive products of protein can be absorbed as amino acids,
dipeptides, and tripeptides
Proteins are the polymers of amino
acids interlinked by peptide bonds formed by the loss of water molecules
between adjacent amino acids. The proteins are digested by proteolytic enzymes
called proteases or peptidases. The peptidases, on the basis of their
site of action are divided into two categories:
i)
Endopeptidases are hydrolyse internal
peptide bonds and include pepsin, trypsin, etc.
ii)
Exopeptidases are hydrolyse terminal
peptide bonds and separate individual amino acids. These include carboxypeptidases and aminopeptidases.
Both endopeptidases enzymes which
degrade proteins by hydrolyzing interior peptide bonds and exopeptidases
enzyme that hydrolyzes one amino acid at a time from the C-terminus of
proteins and peptides are involved in the digestion of proteins.
Digestion takes place in the stomach
and the small intestine.
a) In Buccal cavity. Saliva has no proteolytic enzyme so no digestion of
proteins in the buccal cavity.
b) In Stomach. Both mechanical (churning of food, mixing of food with
gastric juices) and chemical actions occur on food in the stomach.
Chemical
actions in the stomach, food is mixing with gastric juices of the gastric glands
secreted by argentaffin cells of pyloric mucosa controlled by gastrin hormone.
Gastric juice contains mucus (goblet cells), HCl (oxyntic cells), two
proenzymes- prorennin and pepsinogen (peptic or chief cells) and a weak gastric
lipase enzyme (peptic cells).
Role of HCl.
HCl forms
about 0.3% of gastric juices. It helps in-
1. Killing the bacteria in
the food.
2. Stopping the action of
saliva.
3. Activating the pepsinogen to pepsin and prorennin to rennin.
4. Providing optimum pH
(1.2-1.8 pH) for pepsin
5. Softening of food.
Enzymes
Involved
Role of
Pepsin: Pepsin
is secreted in its zymogen form as pepsinogen by the chief cells of the
stomach. Pepsinogen is activated to pepsin by gastric H+. The optimum pH for
pepsin is between 1 and 3. Pepsin
hydrolyzes proteins into peptones and
proteoses. When the pH is >5,
pepsin is denatured. Thus, in the intestine, as HCO3− is
secreted in pancreatic fluids, duodenal pH increases, and pepsin is
inactivated.
The proteoses are the largest fragments of proteins.
Role of Rennin: The proenzyme prorennin is mainly
present in infants and is first activatedby HCl in acidic medium to rennin
(also called as rennets or chymosin) which is a very strong milk protein
coagulating factor (proteinase). It hydrolyses milk soluble protein casein to
paracasein. Paracasein is precipitated spontaneously as calcium paracaseinate
in the form of curd. This is called curdling of milk.
Curdling of milk increases the period
of action of pepsin on the milk-proteins especially casein, in the stomach for
their proper digestion. Amount of rennin decreases with the age so it is absent
in most of adult mammals including man and cow then the curdling of milk is
done by pepsin and chymotrypsin.
c) In intestine: The food entered in the intestine is semi-digested
called chyme, which is mixed with three alkaline juices: bile (pH 8.0) from liver; pancreatic
juices (pH 8.8) from pancreas; and intestine
juices (pH 8.3) from intestinal glands. These function in alkaline medium.
1. Bile has no enzyme so it has no chemical action on proteins.
2. Pancreatic juices contain three alkaline proteoses:
Pancreatic
proteases: The digestion is completed in the small intestine by the action of
pancreatic and intestinal juice. The
proteases include trypsin, chymotrypsin,
elastase, carboxypeptidase A, and carboxypeptidase
B. They are secreted in inactive
forms that are activated in the small intestine as follows:
Trypsinogen
is activated to trypsin by a brush border enzyme, enterokinase.
Trypsin then
converts chymotrypsinogen, proelastase, and procarboxypeptidase A and B to
their active forms.
Trypsin is an endoproteolytic
enzyme and hydrolyses the peptones and proteoses into peptides by hydrolysing
the peptide bonds on C-terminus side of arginine and lysine amino acids.
Trypsin is found from protozoans to mammals so is also called universal enzyme.
Trypsin also activates other
proenzymes:
Chymotrypsin hydrolyses the peptide
bonds on C-terminus side of tyrosine,
tryptophan and phenylalanine amino acids.
It also has the power of clotting milk in alkaline medium.
Carboxypeptidase:
It is an
exopeptidase and separates individual amino acids from C-terminus.
3. Intestinal juices: These contain two alkaline
proteases;
Aminopeptidase
(Erepsin)
hydrolyses the terminal peptide bond at N-terminus of the peptide chain to
release the amino acids one by one.
Dipeptidase hydrolyses the dipeptides
to release the amino acids.
Intestinal juice also contain a
non-digestive protease enterokinase which activates trypsinogen to trypsin.
In biology and
biochemistry, a lipid is a macrobiomolecule that is soluble in nonpolar
solvents. Non-polar solvents are typically hydrocarbons used to dissolve other
naturally occurring hydrocarbon. Lipid molecules that do not (or do not easily)
dissolve in water, including fatty acids, waxes, sterols, fat-soluble vitamins
(such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides,
and phospholipids.
Fig. Structures of
some common lipids. At the top are cholesterol[1] and oleic acid.[2] The middle
structure is a triglyceride composed of oleoyl, stearoyl, and palmitoyl chains
attached to a glycerol backbone. At the bottom is the common phospholipid
phosphatidylcholine.
Scientists sometimes
define lipids as hydrophobic or amphiphilic small molecules; the amphiphilic
nature of some lipids allows them to form structures such as vesicles,
multilamellar/unilamellar liposomes, or membranes in an aqueous environment.
Biological lipids originate entirely or in part from two distinct types of
biochemical subunits or "building-blocks": ketoacyl and isoprene
groups. Using this approach, lipids may be divided into eight categories: fatty
acids, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and
polyketides (derived from condensation of ketoacyl subunits); and sterol lipids
and prenol lipids (derived from condensation of isoprene subunits).
Fats not being soluble in
water by their nature are both difficult to digest and absorb. They do not mix
with the stomach or intestinal contents. Almost the entire dietary fat consists
of neutral fats (triglycerides). A
triglyceride is the combination of three fatty acids molecules condensed with a
molecule of glycerol by ester bonds formed by the loss of water molecules. The
enzymes involved in the hydrolysis of fats are esterases. Most important
esterase is lipase.
Fig. Lipid Digestion
The first step in lipid digestion is
emulsification, which is the transformation of large lipid droplets into much
smaller droplets.
The emulsification process increases
the surface area of the lipid-exposed to the digestive enzymes by decreasing
the droplet size.
Fat digestion started in the stomach
and is nearly completed in the small intestine.
a) In the Buccal Cavity: Lingual lipases digest
some of the ingested triglycerides to monoglycerides and fatty acids. Saliva
has no lipolytic enzyme, so no digestion of fats in the buccal cavity.
b) In the Stomach: Gastric juice of gastric glands of stomach has no
fat-emulsifying enzyme but has a weak gastric lipase enzyme which hydrolyses a
small amount of fats. It is so that gastric lipase has an optimum pH 4.0 to 5.0
and is inactivated by strong acidic conditions. However, most of the ingested
lipids are digested in the intestine by pancreatic lipases.
c) In Small Intestine:
(1)
Action of Bile. Bile of liver is an
alkaline, yellowish-green and non-enzymatic digestive juices has no chemical
action on food. It is formed of water (86%), sodium bicarbonates, bile pigments
(green coloured biliverdin and yellow coloured bilirubin), two bile salts (sodium
glycocholate and sodium taurocholate). About 90% of bile salts show
enterohepatatic circulation between intestine and liver. These are absorbed in
portal blood and carried to liver and reused in bile formation again and again.
Bile
increases the absorption of fat, it is an important part of the absorption of
the fat-soluble substances, such as the vitamins A, D, E, and K. Besides its
digestive function, bile serves also as the route of excretion for bilirubin, a
byproduct of red blood cells recycled by the liver. Bile salts or bile juice
creates an alkaline medium in small intestine for the action of enzymes named;
pepsin and trypsin which are used for digestion of protien also bile juice
convert big fat molecules into smaller molecules for the action of lypase on
fats. Large amounts of bile acids are secreted into the intestine every day,
but only relatively small quantities are lost from the body. This is because
approximately 95% of the bile acids delivered to the duodenum are absorbed back
into blood within the ileum.
Bile acids emulsify lipids in the small intestine, increasing the surface
area for digestion. The hydrophobic products of lipid digestion are solubilized
in micelles by bile acids. Bile acids are lipid carriers and are able to
solubilize many lipids by forming micelles - aggregates of lipids such as fatty
acids, cholesterol and monoglycerides - that remain suspended in water. Bile
acids are also critical for transport and absorption of the fat soluble
vitamens.
(2)
Action of pancreatic
lipases (Steapsin): Pancreatic lipase, also known as pancreatic
triacylglycerol lipase or steapsin, is an enzyme secreted from the pancreas. As
the primary lipase enzyme that hydrolyzes (breaks down) dietary fat molecules
in the human digestive system, it is one of the main digestive enzymes,
converting triglyceride substrates like 1 found in ingested oils to
monoglycerides 3 and free fatty acids 2a and 2b.
Fig. Hydrolysis of a
triglyceride 1
So, the pancreatic lipase hydrolyses
the dietary fats and oils into glycerol, fatty acids, monoglyceride and
diglyceride. It is so that the fat-digestion is a very slow process which takes
about a few hours but is never complete.
During, digestion of fats, only 50% fat is completely digested to fatty acids
and glycerol while remaining 50% are mono- and di-glycerides.
The schematic digestion of long chain
triglycerides by pancreatic lipases has been shown in above figure.
Pancreatic lipases hydrolyze lipids
to fatty acids, monoglycerides, cholesterol, and lysolecithin. The enzymes are
pancreatic lipase, cholesterol ester hydrolase, and phospholipase A2.
The role of
the colipase in the action of pancreatic lipase.
Colipase is a small protein cofactor
needed by pancreatic lipase for efficient dietary lipid hydrolysis. Efficient
absorption of dietary fats is dependent on the action of pancreatic
triglyceride lipase. Colipase binds to the C-terminal, non-catalytic domain of
lipase, thereby stabilising an active conformation and considerably increasing
the hydrophobicity of its binding site. Structural studies of the complex and
of colipase alone have revealed the functionality of its architecture.
Colipase, abbreviated CLPS, is a
protein co-enzyme required for optimal enzyme activity of pancreatic lipase.
... Its function is to prevent the inhibitory effect of bile salts on the
lipase-catalyzed intra-duodenal hydrolysis of dietary long-chain triglycerides.
In humans, the colipase protein is encoded by the CLPS gene. Colipase is also a
family of evolutionarily related proteins.
Fig. Structure of the pancreatic lipase-colipase
complex inhibited by a C11 alkyl phosphonate.
Symbol Colipase
Pfam PF01114
InterPro IPR001981
PROSITE PDOC00111
SCOP2 1lpb / SCOPe / SUPFAM
CDD cd00039
Pancreatic juice has also two more
lipolytic enzymes: Phospholipase and cholesterol-esterase which hydrolyse the
phospholipids and free cholesterol respectively.
(3)
Action of Intestinal
juice: Lipase,
any of a group of fat-splitting enzymes found in the blood, gastric juices,
pancreatic secretions, intestinal juices, and adipose tissues. Lipases
hydrolyze triglycerides (fats) into their component fatty acid and glycerol
molecules.
Initial lipase digestion
occurs in the lumen (interior) of the small intestine. Bile salts reduce the
surface tension of the fat droplets so that the lipases can attack the
triglyceride molecules. The fatty acid and glycerol molecules are then taken up
into the epithelial cells that line the intestinal wall, where they are
resynthesized into triglycerides for transport to muscles and adipose tissues.
At these sites lipases in the bloodstream hydrolyze the triglycerides, and the
resulting fatty acids and glycerol are taken up by the cells of these tissues.
In the adipose tissues triglycerides are re-formed for storage until the energy
needs of the animal increase under conditions of stress or exercise. Lipases in
the cells of adipose tissues break down the triglycerides so that fatty acids
can reenter the bloodstream for transport to energy-requiring tissues.
The term absorption has been derived
from L. absorbere = suck in. When the food is present in the lumen of
alimentary canal , it is supposed to be out of body because it has not entered
the living tissues of the. These digested food materials must pass on to the
body tissue so that these can be used in
cellular functions.
It is
process by which diffusible nutrients are transferred from the lumen of gut into the blood or lymph by
certain physic-chemical processes and active transport.
The absorptive power varies greatly in different regions of
the alimentary canal. No appreciable absorption of nutrients occurs through the
mucus membrane of buccal cavity and
oesophagus except of adrenaline chloride and methyl testosterone. Absorption of
only certain drugs occurs in buccal cavity. Very little absorption takes place
through the gastric mucosa e.g. of water, alcohol, simple salts, glucose and
chlorides. The mucosa of colon and rectum is involved in the absorption of
water, glucose and inorganic salts.
So, the principal site of the
absorption of digested and diffusible nutrients is the small intestine.
To ensure maximum absorption, small
intestine shows following adaptations:
i)
Enormously long sized small intestine (6 metres or 20 feet).
ii)
Mucosal layer of small intestine is thrown into circular
folds which cause a three folds increase in its surface area. These mucosal
folds are called folds of Kerckring (also called plicae circulares) and raised
into about 4 million finger like projections, called villi, which cause a
further ten-folds increase of its surface area (total 30 folds increase). Each
villus is about 0.5 to 1 mm long and there are 20-40 villi per mm2
of intestinal mucosa.
iii)
The free surface of each epithelial cell of a villus has
electron microscopic evaginations called microvilli (3,000 per intestinal cell) which forms a brush border. These
microvilli increase the absorptive surface area by 20 to 30 folds. The villi
are numerous in ileum of small intestine.
iv)
Each villus is with a lymph capillary called lacteal in the
centre which is surrounded by a network of blood capillaries and about 1.4 ltrs
of blood flows through this capillary network per minute which is increased by
1-3rd during
digestion of food.
v)
Absorption is further increased by the movements of villi
stimulated by villikinin hormone secreted by the mucosa of small intestine.
These movements of villi help to pump lymph into the lacteals of the
sub-mucosa.
Fig. Absorption
in Small Intestine
Fig. Chemical digestion
and absorption(Unlike amino acids and simple sugars, lipids are transformed as
they are absorbed through epithelial cells)
The absorption across plasma membrane
of intestinal cells depends upon two types of processes:
(A) Passive absorptiom and (B) Active absorption
Passive absorption. In this, the nutrients are absorbed
along the concentration gradient (higher concentration inside the lumen of the
small intestine while lower concentration inside the intestinal blood
capillaries). It depends upon the physical processes like diffusion, osmosis
and facilitated diffusion (movement of molecules like fructose, mannose, etc.
along concentration gradient with the help of some carrier molecules). It does
not depend upon the energy so is a slow process. It continues till the concentration
becomes equal on both sides of cell membrane so the substances cannot be
absorbed completely. Water, some water soluble substsances, most vitamins,
purines, pyrimidine and fructose are absorbed by passive absorption.
Active absorption. In this, the nutrients are absorbed
through the intestinal mucosa against concentration gradient. This is a rapid
process as it depends upon the energy provided by the ATP. By active
absorption, the nutrients can be absorbed completely from the intestinal lumen.
If the cells are poisoned with cyanide or depressed by cold, active absorption
stops. Active absorption occurs by two processes:
i)
Active transport is that active absorption which involves the carrier
molecules called permease or translocases which are generally proteinaceous in
nature. Glucose, galactose, amino acids, Na+ etc. are absorbed by
active transport. For active transport of Na+, a sodium pump
operates in the cell membrane.
ii)
Endocytosis is also an active process
by which large sized liquid or solid nutrients are taken in some vesicles
through the plasma membrane.
There are some evidence that the
leucocytes found in the intestinal mucosa, actually pass through the intestinal
wall and engulf the food particles and when become loaded with food, carry them
back into the blood and lymph.
1. ABSORPTION OF CARBOHYDRATES
Carbohydrates are chiefly absorbed by
the small intestine in the form of monosaccharides like glucose (80%),
galactose (10%), and fructose (10%) formed by the hydrolysis of polysaccharides
and oligosaccharides. Fructose is absorbed mainly by facilitated diffusion
while glucose and galactose are absorbed actively.
Different monosaccharides show
differential absorption (Cori, 1925). He reported that rate of absorption of
monosaccharides depends upon:
a) Size of the sugar. The pentoses (ribose, deoxyribose)
are small sized so are absorbed more rapidly than hexoses.
b) Configuration structure. The rapid absorption of galactose and
glucose than fructose has been explained on the basis that carbohydrates having
D-pyranose ring form having an intact –OH at the position 2-carbon undergo
active transport while fructose having furanose ring form is absorbed by
facilitated diffusion. The faster absorption of fructose than mannose was
explained on the basis of its conversion into lactic acid in the epithelial
cells (Wilson and Weismann, 1954) in
rat and hamster; and glucose (Hers and
Kusaka, 1953) in guinea pig and hamster.
Monosaccharides are absorbed by
carrier mediated transport.
At least two types are known:
1. Na+ monosaccharide
transporter
2. Na+ independent,
diffusion type monosaccharide transport system
Glucose
and Galactose
They are transported from the
intestinal lumen into the cells by a Na+- dependent co-transport
(SGLT 1) in the luminal membrane. The sugar is transported “uphill” and Na+
is transported “downhill.”
They are then transported from cell
to blood by facilitated diffusion (GLUT 2).
The Na+–K+ pump
in the baso-lateral membrane keeps the intracellular [Na+] low, thus
maintaining the Na+ gradient across the luminal membrane.
Fig – Sodium moves down its concentration
gradient, bringing in glucose to the the cell.
Glucose and galactose are absorbed
across the apical membrane by secondary active transport (along with Na+)
through the Sodium-Glucose cotransporter (SGLT1). Both glucose and galactose
exit the cell via GLUT2 receptors across the basolateral membrane into the
blood. Fructose enters the cell by facilitated diffusion via GLUT5 and is
transported into the blood via GLUT2 receptors.
Fructose: It differs from that of
glucose and galactose:
i)
Fructose is transported exclusively by facilitated
diffusion;
ii)
It cannot be absorbed against a concentration gradient;
iii)
It is partly converted into glucose in the intestinal cells
and then released in the portal blood.
Absorption
of Disaccharides
Millner and Crane (1960) showed that
the disaccharides like sucrose (cane sugar), maltose (malt sugar), and lactose
(milk sugar) first enter the enterocytes. Sucrose is mainly absorbed in the
jejunum in man. Inside enterocytes of intestinal mucosa, these are hydrolysed
by the disaccharidases like sucrose, maltase, and lactase respectively into
their monosaccharides. These are then passed from enterocytes into the blood
capillary within villus. So the earlier belief that dietary disaccharides are
first broken into monosaccharides in intestinal lumen and then absorbed, is not
applicable. Out of these disaccharides, lactose is less taken by the
enterocytes so lactose reaches the lower part of the intestine and promotes the
growth of highly beneficial certain lactose- fermenting micro-organism.
Fig. Digestion of carbohydrates: Digestion of
carbohydrates is performed by several enzymes. Starch and glycogen are broken
down into glucose by amylase and maltase. Sucrose (table sugar) and lactose
(milk sugar) are broken down by sucrase and lactase, respectively.
2. ABSORPTION
OF PROTEINS
In the gastro-intestinal tract,
exogenous proteins (dietary) as well as endogenous proteins (enzymatic proteins
and epithelial cellular proteins) are hydrolyzed by proteases into tri and
dipeptides or free amino acids. In all,
about 98% of the dietary proteins
are hydrolyzed into amino acids. Amino acids are 2 types: L-amino acids
(naturally occurring) and D-amino acids (optical isomers). The
L-amino acids are absorbed by active transport in the presence of
vitamin B6 while D-amino acids are absorbed passively by diffusion. The small
intestine also shows differential absorption of amino acids which depends upon:
i)
Molecular weight. Small sized amino acids with low molecular
weight are absorbed faster e.g.,glycine is absorbed more rapidly followed by alanine, cysteine, glutamic acid,
valine and so on.
ii)
Form of amino acids. L-amino acids are
absorbed rapidly than the corresponding D-amino acids (Gibson and Weismann,
1951).
Free amino
acids. Almost
all the L-amino acids are too large to diffuse through the pores of the plasma
membrane, so amino acids are mainly absorbed by facilitated or active transport
involving carrier mechanisms. The carrier molecules have been reported in the brush border of the enterocytes. Four
types of transport systems (because different amino acids have different
binding properties) have been reported to be involved in the transport of amino
acids. Each system being involved with a specific subset of amino acids with
specific molecular characteristics and all being dependent upon high
concentration of Na+ in the intestinal lumen because amino acids and Na+
show symport as in glucose transport.
Na+-dependent amino acid
cotransport occurs in the luminal membrane. It is analogous to the cotransporter
for glucose and galactose. The amino acids are then transported from cell to blood
by facilitated diffusion. There are four separate carriers for neutral, acidic,
basic, and imino amino acids, respectively.
Dipeptides
and tripeptides. They are absorbed faster than free amino acids but in
different ways. These first enter the enterocytes where these are hydrolyzed
into amino acids by peptidases present
in the brush border of the enterocytes and are then released into the
portal blood.
The intestinal cells of the new born
can absorb the whole proteins by pinocytosis so is able to absorb the
proteinaceous antibodies from the mother’s milk to get immunity against the
pathogenic microbes. But this power is lost within a few week.
H+-dependent
co-transport of dipeptides and tripeptides also occurs in the luminal membrane.
After the dipeptides and tripeptides are transported into the intestinal cells,
cytoplasmic peptidases hydrolyze them to amino acids. The amino acids are then
transported from cell to blood by facilitated diffusion.
3. ABSORPTION OF FATS
The bulk of dietary lipid
is neutral fat or triglyceride, composed of a glycerol backbone with each
carbon linked to a fatty acid. Foodstuffs typically also contain phospholipids,
sterols like cholesterol and many minor lipids, including fat-soluble vitamins.
Finally, small intestinal contents contain lipids from sloughed epithelial
cells and considerable cholesterol delivered in bile. The end products of fats
are a mixture of monoglycerides, fatty acids and glycerol. Glycerol is water
soluble so is directly absorbed by the mucosal cells of small intestine. The short chain fatty
acids (less than 14-carbon atoms) are directly absorbed into the portal circulation because these are more
water-soluble so are readily diffusible into the blood capillaries of the villi
through the epithelial cells. But the long chain fatty acids and monoglycerides
are insoluble in water.
In order for the
triglyceride to be absorbed, two processes must occur:
Large aggregates of
dietary triglyceride, which are virtually insoluble in an aqueous environment,
must be broken down physically and held in suspension - a process called
emulsification.
Triglyceride molecules
must be enzymatically digested to yield monoglyceride and fatty acids, both of
which can efficiently diffuse or be transported into the enterocyte
The key players in these
two transformations are bile acids and pancreatic lipase, both of which are
mixed with chyme and act in the lumen of the small intestine. Bile acids are
also necessary to solubilize other lipids, including cholesterol.
Emulsification,
Hydrolysis and Micelle Formation
Bile acids play their
first critical role in lipid assimilation by promoting emulsification. As
derivatives of cholesterol, bile acids have both hydrophilic and hydrophobic
domains (i.e. they are amphipathic). On exposure to a large aggregate of
triglyceride, the hydrophobic portions of bile acids intercalate into the
lipid, with the hydrophilic domains remaining at the surface. Such coating with
bile acids aids in breakdown of large aggregates or droplets into smaller and
smaller droplets.
Hydrolysis of triglyceride
into monoglyceride and free fatty acids is accomplished predominantly by
pancreatic lipase. The activity of this enzyme is to clip the fatty acids at
positions 1 and 3 of the triglyceride, leaving two free fatty acids and a 2-monoglyceride.
The drug orlistat (Xenical) that is promoted for treatment of obesity works by
inhibiting pancreatic lipase, thereby reducing the digestion and absorption of
fat in the small intestine.
Lipase is a water-soluble
enzyme, and with a little imagination, it's easy to understand why
emulsification is a necessary prelude to its efficient activity. Shortly after
a meal, lipase is present within the small intestine in rather huge quantities,
but can act only on the surface of triglyeride droplets. For a given volume of
lipid, the smaller the droplet size, the greater the surface area, which means
more lipase molecules can get to work.
As monoglycerides and
fatty acids are liberated through the action of lipase, they retain their
association with bile acids and complex with other lipids to form structures
called micelles. Micelles are essentially small aggregates (4-8 nm in diameter)
of mixed lipids and bile acids suspended within the ingesta. As the ingesta is
mixed, micelles bump into the brush border of small intestinal enterocytes, and
the lipids, including monoglyceride and fatty acids, are taken up into the
epithelial cells.
Transport of lipids into
the circulation is also different from what occurs with sugars and amino acids.
Instead of being absorbed directly into capillary blood, chylomicrons are
transported first into the lymphatic vessel that penetrates into each villus
called the central lacteal. Until recently, it was not understood how the large
chylomicrons are taken up into the lacteals. As it turns out, there are patches
of the lacteal in which endothelial cells are held together through specialized
"button junctions" that are much more permeable to chylomicrons than
normal cellular junctions. Chylomicron-rich lymph then drains into the system
lymphatic system, which rapidly flows into blood. Blood-borne chylomicrons are
rapidly disassembled and their constitutent lipids utilized throughout the
body.
SUMMARY OF
LIPID ABSORPTION
Lipids within the
digestive system will tend to hydrophobically aggregate to form large fat
globules.
Bile salts, secreted from
the gall bladder, emulsify these fat globules and break them up into smaller
droplets.
Hydrolytic enzymes called
lipases then digest the fats into their component parts.
When the fatty acids are
absorbed into the epithelial cells of the intestinal lining, they are combined
to form triglycerides.
The triglycerides are
combined with proteins inside the Golgi apparatus to form chylomicrons.
Chylomicrons are released
from the epithelial cells and are transported via the lacteals to the liver.
While in the liver,
chylomicrons may be modified to form a variety of lipoproteins.
Low density lipoproteins
will transport lipids via the bloodstream to cells.
High density lipoproteins
will scavenge excess lipids from the bloodstream and tissues and return them to
the liver.
Fig. Lipid
absorption in small intestine
Cholesterol
Another lipid of
importance that is absorbed in the small intestine is cholesterol. Cholesterol
homeostatis results from a balance of cholestrol synthesis, absorption of
dietary cholesterol, and elimination of cholesterol by excretion in bile. Years
ago it was shown that cholesterol, but not plant sterols, is readily absorbed
in the intestine. More recently, a specific transport protein (NPC1L1) has been
identified that ferries cholesterol from the intestinal lumen into the
enterocyte. From there, a bulk of the cholesterol is esterified, incorporated
into chylomicrons and shuttled into blood by the mechanisms described above. In
enterocytes, the cholesterol is re-esterified and finally slowly released into
the lacteals.
4. ABSORPTION OF OTHER SUBSTANCES
Nucleic Acid
Absorption
The products of nucleic
acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are
transported by carriers across the villus epithelium via active transport.
These products then enter the bloodstream.
Mineral
Absorption
The electrolytes absorbed
by the small intestine are from both GI secretions and ingested foods. Since
electrolytes dissociate into ions in water, most are absorbed via active
transport throughout the entire small intestine. During absorption, co-transport
mechanisms result in the accumulation of sodium ions inside the cells, whereas
anti-port mechanisms reduce the potassium ion concentration inside the cells.
To restore the sodium-potassium gradient across the cell membrane, a
sodium-potassium pump requiring ATP pumps sodium out and potassium in.
In general, all minerals
that enter the intestine are absorbed, whether you need them or not. Iron and
calcium are exceptions; they are absorbed in the duodenum in amounts that meet
the body’s current requirements, as follows:
Figure Absorption is a complex
process, in which nutrients from digested food are harvested.
Iron—The ionic iron needed for the production of hemoglobin is
absorbed into mucosal cells via active transport. Once inside mucosal cells,
ionic iron binds to the protein ferritin, creating iron-ferritin complexes that
store iron until needed. When the body has enough iron, most of the stored iron
is lost when worn-out epithelial cells slough off. When the body needs iron
because, for example, it is lost during acute or chronic bleeding, there is
increased uptake of iron from the intestine and accelerated release of iron
into the bloodstream. Since women experience significant iron loss during
menstruation, they have around four times as many iron transport proteins in
their intestinal epithelial cells as do men.
Calcium—Blood levels of ionic calcium determine the absorption of
dietary calcium. When blood levels of ionic calcium drop, parathyroid hormone
(PTH) secreted by the parathyroid glands stimulates the release of calcium ions
from bone matrices and increases the reabsorption of calcium by the kidneys.
PTH also upregulates the activation of vitamin D in the kidney, which then
facilitates intestinal calcium ion absorption.
Vitamin
Absorption
The small intestine
absorbs the vitamins that occur naturally in food and supplements. Fat-soluble
vitamins (A, D, E, and K) are absorbed along with dietary lipids in micelles
via simple diffusion. This is why you are advised to eat some fatty foods when
you take fat-soluble vitamin supplements. Most water-soluble vitamins
(including most B vitamins and vitamin C) also are absorbed by simple
diffusion. An exception is vitamin B12, which is a very large molecule.
Intrinsic factor secreted in the stomach binds to vitamin B12, preventing its
digestion and creating a complex that binds to mucosal receptors in the
terminal ileum, where it is taken up by endocytosis.
Water
Absorption
Each day, about nine liters of fluid enter the small intestine. About 2.3 liters are ingested in foods and beverages, and the rest is from GI secretions. About 90 percent of this water is absorbed in the small intestine. Water absorption is driven by the concentration gradient of the water: The concentration of water is higher in chyme than it is in epithelial cells. Thus, water moves down its concentration gradient from the chyme into cells. As noted earlier, much of the remaining water is then absorbed in the colon.
References
- Animal Physiology and Biochemistry by Agarwal, R A., Srivastava, A K and K.
Kumar
-Animal Physiology by Verma, P S and Aggarwal, V K
-Principles of
Biochemistry by W.H. Nelson, D. L., Cox,
M. M. and Lehninger, A.L. (2009) IV Edition
-Textbook of Medical
Physiology by Guyton, A.C. and Hall,
J.E. (2011) XII Edition
-Animal physiology by Eckert –
- A text book of Animal
Physiology by Nagabhushnam
-Wikipaedia site of
internet also consulted.
