The digestive system breaks down ingested food into smaller molecules that can be absorbed and used by the body. It consists of the gastrointestinal tract - a long tube running from the mouth to the anus, including the mouth, esophagus, stomach, small intestine, large intestine, rectum and anus. Accessory organs like the teeth, tongue, liver, gallbladder and pancreas aid in digestion by secreting enzymes and digestive juices. The system mechanically and chemically breaks down food, absorbs nutrients, and eliminates waste through a multi-step process involving both organs and hormones.

1. The Digestive
System
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2. The system by which ingested food is acted upon by physical and chemical means to provide the
body with absorbable nutrients and to excrete waste products is called digestive system.
OR
• The organs involved in the breakdown of food—collectively called the digestive system.
• It breakdowns the larger molecules present in food into molecules that are small enough to enter body
cells by a process known as digestion.
• The digestive system is a tubular system which extends from the mouth to the anus.
• The medical specialty that deals with the structure, function, diagnosis, and treatment of diseases of the
stomach and intestines is called Gastroenterology.
• The medical specialty that deals with the diagnosis and treatment of disorders of the rectum and anus
is called Proctology

3. Organs of the Digestive System
A)Alimentary canal
• Also known as the gastrointestinal (GI) tract, this is essentially a long tube through which
food passes. It commences at the mouth and terminates at the anus, and the various organs along its
length have different functions
• The length of the GI tract is about 5–7 meters.
. The parts are:
Mouth
Pharynx
Oesophagus
Stomach
Small intestine
Large intestine
Rectum and anal
canal
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4. B) Accessory organs
The teeth help in the physical breakdown of food, and the tongue assists in chewing and
swallowing.
The other accessory digestive organs are not come in direct contact with food.
They produce or store secretions that flows into the GI tract through ducts; the secretions
aid in the chemical breakdown of food.
Teeth
Tongue
Salivary Glands
Liver
Gallbladder
Pancreas
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5. Functions of the Digestive System
 Ingestion – the oral cavity allows food to enter the digestive tract
and have mastication (chewing) occurs , and the resulting food bolus
is swallowed .
 Digestion:
 Mechanical digestion – muscular movement of the digestive tract
(mainly in the oral cavity and stomach) physically break down food
into smaller particles .
 Chemical digestion – hydrolysis reactions aided by enzymes
(mainly in the stomach and small intestine) chemically break
down food particles into nutrient molecules , small enough to
be absorbed .

6. Functions of the Digestive System
 Secretion – enzymes and digestive fluids secreted by the
digestive tract and its accessory organs facilitate chemical
digestion .
 Absorption – passage of the end – products (nutrients) of
chemical digestion from the digestive tract into blood or
lymph for distribution to tissue cells .
 Elimination – undigested material will be released through
the rectum and anus by defecation .

7. Fig: The digestive system
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9. • Stomach is a ‘J’ shaped enlargement
of GI Tract which lies below the
diaphragm in the upper left region of
the abdominal cavity.
• Stomach serves as mixing chamber
and holding reservoir for food.
• Functions of the stomach
• Receive food from the
esophagus, mix food with gastric
juice.
• Start protein digestion.
• Move food into the small
intestine.

10. Anatomy of stomach
• The stomach has four main regions: the cardia, fundus,
body and pylorus.
The cardia surrounds the superior opening of the stomach.
The fundus is the rounded portion superior and left to the
cardia.
The body is inferior to the fundus and is the large central
portion of the stomach.
The pylorus is the region of the stomach that connects to
the duodenum. (pyl = gate; orus = guard). Pylorus has two
parts, the pyloric antrum which connects to the body of the
stomach, and the pyloric canal, which leads into the
duodenum.

12. Structure of the stomach
 Greater and Lesser Curvatures: The medial and lateral borders of the
stomach are curved, forming the lesser and greater curvatures:
 Greater curvature – forms the long, convex, lateral border of the
stomach.
• Arising at the cardiac notch, it arches backwards and passes inferiorly to
the left.
• It curves to the right as it continues medially to reach the pyloric antrum.
• The short gastric arteries and the right and left gastro-omental arteries
supply branches to the greater curvature.
 Lesser curvature – forms the shorter, concave, medial surface of the
stomach.
• The most inferior part of the lesser curvature, the angular notch, indicates
the junction of the body and pyloric region.
• The lesser curvature gives attachment to the hepatogastric ligament and is
supplied by the left gastric artery and right gastric branch of the hepatic
artery.

13. Histologic Structure of the Stomach
• The stomach wall is composed of 4 basic layers:
Mucosa: Mucosa contains several glands called gastric glands. The gastric
glands contain three types of exocrine gland cells and one type of endocrine cells
that secrete their products into the stomach and bloodstream respectively.
1. Mucous cells. Produce alkaline mucous, which protects the stomach wall
itself from being damaged by the acid.
2. Parietal cells. Produce HCl – hydrochloric acid, which makes the stomach
content acidic.
3. Chief cells. Produce enzymes, mostly pepsinogen.
4. G-cells. Produce Gastrin hormone.
5. D-cells. Secrete Somatostatin.

16. Submucosa: is made up of areolar connective tissue.
Muscularis: is composed of 3 layers of smooth layers;
oblique muscles, circular muscles and longitudinal
muscles.
Serosa: forms outer most layer of stomach.

18. Digestion & absorption in the stomach
Carbohydrate digestion is continued with gastric amylase, resulting in
disaccharides .
Protein digestion begins with pepsin (activation of pepsinogen by HCl) ,
resulting in peptides (small chains of protein).
Lipid digestion begins with gastric lipases which can only break down
certain lipids such as butterfat , resulting in fatty acids .
Absorption in the stomach is limited, where only small and fat soluble
substances can be absorbed—water , alcohol, aspirin , and certain drugs .
The result of all these mixing , chemical digestion , secretion, and
absorption is a yellowish paste called chyme , which will be passed on to
the small intestine .

19. Regulation of Gastric Secretion
Regulation of gastric secretion and activities is by both nervous and hormonal
mechanisms – food moving along the oral cavity and esophagus stimulates the
parasympathetic nerves to activate the secretion in gastric glands , the gastric
hormone from G cells in turn stimulates the gastric glands for more activities
("positive feedback").
On the other hand , when food is emptying from the stomach , sympathetic
nerves inhibit the gastric glands and a hormone called intestinal gastrin
(released by small intestine) inhibits other gastric activities.
The above regulations occur in 3 overlapping phases:
Cephalic Phase, Gastric Phase, & Intestinal Phase.

20. Cephalic phase
 This gastric secretion phase happens just before food enters the stomach, and this phase prepares
the body for eating and digestion.
In the cephalic phase, the taste or smell of food, food sensation in the mouth, or thought of food
generates and sends nervous signals to the hypothalamus and medulla oblongata.
This signal causes parasympathetic neurons through the vagus nerves to stimulate hydrochloric acid
and pepsin secretion in the stomach.
The parasympathetic stimulation also secretes gastrin by the stomach.
This hormone taken through the bloodstream and stimulates the secretion of hydrochloric acid and
pepsin.
The stomach acidity is not buffered at this stage by the food thus inhibits parietal (acid secretion) and
G cell (gastrin secretion) activity through D cell secretion (somatostatin).

21. Gastric phase
 This phase takes about three to four hours.
In gastric phase, food has entered and distended the stomach.
This distention stimulates a parasympathetic reflex through medulla oblongata, and it
directly stimulates the gastric glands and releases more gastric juices.
If protein is available in the stomach, it binds with hydrogen ions and raises the
stomach pH. It, furthermore, lifts the inhibition of gastrin and gastric acid secretion.
This stimulates G cells to release gastrin, which stimulates parietal cells to secrete
gastric acid. This causes a continued secretion of HCl (hydrochloric acid) and pepsin,
which lowers the pH to 1-3 (acidic).

22. Intestinal Phase
Partially digested food fills the duodenum. This phase has two parts, the excitatory, and the
inhibitory.
In the intestinal phase, chyme has entered the duodenum.
If the chyme has lipids (digested fat) or has pH below 2, gastric secretion is inhibited.
The lipids and acidic pH (hydrogen ions) inhibits gastric secretion by three stages:
firstly, signals medulla oblongata to inhibit stimulation of the gastric glands.
Secondly local reflexes through neurons in the gut wall to decrease gastric secretion.
thirdly releases three local hormones that pass through the circulation to the gastric glands
to inhibit its secretion.

23. Fig: The three phases of secretion of gastric juice.
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25. • The small intestine is continuous with the stomach at the pyloric sphincter. The small
intestine is about 2.5 cm in diameter, a little over 5 metres long and leads into the
large intestine at the ileocaecal valve.
• It lies in the abdominal cavity surrounded by the large intestine.
• In the small intestine the chemical digestion of food is completed and absorption
of most nutrients takes place.
 The small intestine comprises three continuous parts….
• Duodenum. This is about 25 cm long and curves around the head of the pancreas.
• Jejunum. This is the middle section of the small intestine and is about 2 metres long.
• Ileum. This terminal section is about 3 metres long and ends at the3 ileocaecal valve,
which controls the flow of material from the ileum to the caecum, the first part of the
large intestine, and prevents backflow.

26. Structure of the small intestine
Histologically, the small intestine has four layers. From internal to external, they
are mucosa, submucosa, muscularis , and serosa. These layers are easy to
remember ( M.S.M.S).
There are several unique features in the small intestine, which act to significantly
increase its absorptive surface:
Circular folds (valves of Kerckring, plicae circulares) are the transverse folds of
mucosa found predominantly in the distal duodenum and proximal jejunum

27. Intestinal villi are finger like extensions of intestinal mucosa
which project into the lumen of the small intestine. Between the
villi are intestinal glands (crypts of Lieberkuhn) which secrete
intestinal juice rich in digestive enzymes.
Microvilli are projections found on the apical surface of each
intestinal cell (enterocyte)

28. There are also features of the small intestine which are segment-
specific:
Peyer's patches are part of gastrointestinal associated lymphoid tissue
(GALT). They are found in ileum.
Brunner glands are found in the submucosa of the duodenum. They
produce mucus rich in alkalines which protects the duodenum from
the corrosive effects of gastric acid.

29. Fig: Small intestine and its parts. Fig: Villus of small intestine.
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30. Functions of the small intestine
 Onward movement of its contents by peristalsis, which is increased by
parasympathetic stimulation.
 Secretion of intestinal juice.
 Completion of chemical digestion of carbohydrates, protein and fats in the
enterocytes of the villi.
 Protection against infection by microbes that have survived the
antimicrobial action of the hydrochloric acid in the stomach.
 Secretion of the hormones cholecystokinin (CCK) and secretin.
 Absorption of nutrients.
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31. Role of intestinal juice and brush border enzyme
About 1-2 lt of intestinal juice (pH 7.6) is secreted every day which contained water
and mucous.
Intestinal juice mixes with pancreatic juices and provides the liquid medium for
absorption of substance from chyme into small intestine.
Brush border enzymes secreted from absorptive cells in small intestine contain
following types of enzymes:
 Four carbohydrate-digesting enzymes called α-dextrinase, maltase, sucrase and
lactase
 Two Protein-digesting enzymes called peptidases (aminopeptidase and dipeptidase)
 Two types of nucleotide-digesting enzymes, nucleosidases and phosphatases.

32. Mechanical and chemical digestion in small intestine:
Mechanical Digestion:
Two types of movements occur in small intestine which result in mechanical
digestion:
Segmentation: These are localized mixing contractions that occur in portions of
small intestine distended with large volume of chyme. Segmentation helps in mixing
and absorption of chyme, but it does not push chyme forward.
Migrating motality complex (MMC): This is a type of peristaltic movement which
occurs when volume of chyme in distended portion of small intestine decreases.
This pushes the chyme forward. Chyme remains in small intestine for 3-5 hours.

33. Chemical digestion:
 Chyme entering the small intestine contains partially digested carbohydrates, proteins, and lipids
by the enzymes in mouth and stomach. The completion of the digestion of carbohydrates, proteins,
and lipids occurs in small intestine and it is a collective effort of pancreatic juice, bile, and intestinal
juice.
 Digestion of carbohydrates: Starches are broken into maltose, maltriose and α-dextrin units by
pancreatic amylase.
 Following brush border enzymes act on these and convert it even smaller units.
 α-Dextrinase acts on α-dextrin to produce glucose.
 Maltase splits maltose and maltriose into 2-3 units of glucose.
 Lactase digests lactose into a glucose and galactose.
 Sucrase breaks sucrose into molecule of glucose and fructose.
 Cellulose (a polysaccharide) is not digested by amylase enzymes and hence it is called roughage.

34. Digestion of proteins: Trypsin, chymotrypsin, carboxypeptidase and elastase convert
proteins into peptide units. These peptides are converted into small amino acids by two
enzymes aminopeptidase and Dipeptidase which break amino acids into single amino
acids.
Digestion of lipids: Most of triglycerides in food are broken into long chain or short
chain fatty acids and monoglycerides by pancreatic lipase. Long chain fatty acids are
emulsified into short chain fatty acid by bile salts in small intestine.
Digestion of nucleic acids: Pancreatic juice contains two nucleases (nucleic acid
digesting enzymes) ribonuclease (digests RNA) and deoxyribonuclease (digests DNA)
into nucleotides. These nucleotides are further digested by brush-border enzymes called
nucleosidases and phosphatases into pentoses, phosphates, and nitrogenous bases.

35. The primary function of the ileum is to absorb vitamin B12, bile salts, and whatever
products of digestion were not absorbed by the jejunum.
 Digestive Hormones that secreted from the small intestine and Their Functions
Hormone Origin Function
Cholecystokinin (CCK) Duodenum contraction of gallbladder
Gastric inhibitory
peptide (GIP)
Duodenum Stimulate the release of insulin
Gastrin Stomach Stimulate the production of acid
and enzyme
Secretin Duodenum increase bile secretion

37. The large intestine also
known as the colon, extends
from the distal end of the
ileum to the anus, a
distance of approximately
1.5 m in adults (5 ft) long,
making up one-fifth of the
length of the
gastrointestinal (GI) tract
and 6.5 cm (2.5 in.) in
diameter. The large
intestine is named for its
relatively large diameter,
not its length.
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38. The large intestine is
divided into the ….
I. Caecum
II.Colon
III.Sigmoid colon
IV.Rectum
V.Anal canal
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39.  The caecum
• First part of the large intestine, It is about 8-9 cm long.
• The small intestine feeds into the cecum through a small channel in the side of it (the ileocecal
valve), so the end of the cecum is actually closed like a pouch.
• This pouch, the first 6 inches of the colon, is also the widest portion of the large intestine. This
is the reservoir where food from the small intestine arrives in the large intestine.
• When the cecum is full, it triggers the muscle movements of the colon to begin.
 The colon : The colon has four parts which have the same structure and functions.
• The ascending colon
• Transverse colon
• Descending colon
• The sigmoid colon

40.  The rectum
• This is a slightly dilated section of the large intestine about 13 cm long. It
leads from the sigmoid colon and terminates in the anal canal.
 The anal canal
• This is a short passage about 3.8 cm long in the adult and leads from the
rectum to the exterior.
• Two sphincter muscles control the anus; the internal sphincter, consisting of
smooth muscle, is under the control of the autonomic nervous system and
the external sphincter, formed by skeletal muscle, is under voluntary control.

42. Histology of Large Intestine
 Large intestine is made up of four tissue layer they are:
• The innermost layer of the large intestine is known as the mucosa and it is made up of
simple epithelial tissue. Here mucosa lacks villi which is found in the small intestine.
Many mucosal glands secrete mucus which helps in the lubrication of food.
• Above the mucosal layer, there is a layer of blood vessels, nerves and connective tissues
known as submucosa, which provides support to other layers of the intestine.
• The further submucosal layer is surrounded by the muscularis layer which contains
several visceral muscle cells, which help in the contraction and relaxation of the large
intestine.
• Outermost layer of the large intestine is known as the serosa layer, they are made up of
squamous epithelial tissue and their secretion helps in the lubrication process.

43. Functions of the large intestine, rectum and anal canal
There are so many major function of digestion which are performed by the large
intestine:
 Formation and Elimination of Feces: When food is consumed by the small intestine then
undigested food moves to the large intestine where absorption of remaining water takes place.
Under this process conversion of liquid chyme into feces takes place. These feces consists of
food, bacteria, inorganic salt, unabsorbed substance, etc.
 Absorption of Vitamins: Large intestine absorbs various vitamins produced by its own
bacteria. There are more than 700 types of bacteria present in our large intestine. The major
function of these bacteria is to breakdown left over fiber and polysaccharides and further convert
them into short-chain fatty acid, which are absorbed by the large intestine by diffusion process.

44. Absorption of Water and Electrolytes: During this process, water is
absorbed by the large intestine by osmosis process as during this process
diffusion is happening as per concentration gradient. Sodium ion is absorbed
by the sodium-potassium pump in the colon region.
Antibody Production: The large intestine is a house for many lymphoid
tissue, and these lymphoid tissue is crucial for immunity. They help in the
creation of various antibodies, which are helpful against various harmful
bacteria.
Acid Reduction: bicarbonates which are present in the large intestine
neutralize acidity caused by the synthesis of fatty acids.

46. The salivary glands are exocrine glands that are positioned in the head, in and around
the oral cavity and secrete their salivary contents into the mouth. Their function is to
help keep the oral mucosa protected and lubricated. They also help in the initial stages
of digestion during mastication of food, so that a food bolus is created and ready to be
swallowed for further processing.
Fig: Salivary glands.
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47.  Parotid glands
• These are situated one on each side of the face just below the external acoustic meatus. Each
gland has a parotid duct opening into the mouth at the level of the second upper molar tooth.
 Submandibular glands
• These lie one on each side of the face under the angle of the jaw. The two submandibular
ducts open on the floor of the mouth, one on each side of the frenulum of the tongue.
 Sublingual glands
• These glands lie under the mucous membrane of the floor of the mouth in front of the
submandibular glands. They have numerous small ducts that open into the floor of the
mouth.
 Secretion of saliva
• Secretion of saliva is controlled by the autonomic nervous system. Parasympathetic stimulation
causes profuse secretion of watery saliva with a relatively low content of enzymes and other
organic substances. Sympathetic stimulation results in secretion of small amounts of saliva rich in
organic material, especially from the submandibular glands. Reflex secretion occurs when there is
food in the mouth and the reflex can easily become conditioned so that the sight, smell and even
the thought of food stimulates the flow of saliva.

48. Function
 The purpose of the salivary glands is to make saliva and to help:
• Keep the mouth moist
• Chewing
• Swallowing
• Digestion
• Keep the teeth and mouth clean
• Maintain pH (acid/base) balance in the mouth

50. • The pancreas is a pale grey gland weighing about 60 grams. It is about 12-15
cm long and is situated in the epigastric and left hypochondriac regions of the
abdominal cavity.
• It consists of a broad head, a body and a narrow tail. The head lies in the curve
of the duodenum, the body behind the stomach and the tail lies in front of the left
kidney and just reaches the spleen.
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51. Functions of the Pancreas
• A healthy pancreas produces the correct chemicals in the proper quantities, at
the right times, to digest the foods we eat.
 Exocrine Function:
• The pancreas contains exocrine glands that produce enzymes important to
digestion. These enzymes include trypsin and chymotrypsin to digest proteins;
amylase for the digestion of carbohydrates; and lipase to break down fats.
• When food enters the stomach, these pancreatic juices are released into a
system of ducts that culminate in the main pancreatic duct. The pancreatic
duct joins the common bile duct to form the ampulla of Vater which is
located at the first portion of the small intestine, called the duodenum. The
common bile duct originates in the liver and the gallbladder and produces
another important digestive juice called bile. The pancreatic juices and bile
that are released into the duodenum, help the body to digest fats,
carbohydrates, and proteins.

52. Endocrine Function:
• The endocrine component of the pancreas consists of islet cells
(islets of Langerhans) that create and release
important hormones directly into the bloodstream.
• Two of the main pancreatic hormones are insulin, which acts to
lower blood sugar, and glucagon, which acts to raise blood sugar.
Maintaining proper blood sugar levels is crucial to the functioning of
key organs including the brain, liver, and kidneys.

54. Fig:Anterior view of liver. Fig: The liver lobules.
• The liver is the largest gland in the body, weighing between 1 to 2.3 kg. It is
situated in the upper part of the abdominal cavity. Its upper and anterior surfaces are
smooth and curved to fit the under surface of the diaphragm; its posterior surface is
irregular in outline.
• The liver is enclosed in a thin inelastic capsule and incompletely covered by a layer
of peritoneum. Folds of peritoneum form supporting ligaments that attach the liver to
the inferior surface of the diaphragm.
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55. Functions of the Liver
• The liver is an essential organ of the body that performs over 500 vital
functions. These include removing waste products and foreign substances from
the bloodstream, regulating blood sugar levels, and creating essential nutrients.
Here are some of its most important functions:
Albumin Production: Albumin is a protein that keeps fluids in the
bloodstream from leaking into surrounding tissue. It also carries hormones,
vitamins, and enzymes through the body.
Bile Production: Bile is a fluid that is critical to the digestion and absorption
of fats in the small intestine.
Filters Blood: All the blood leaving the stomach and intestines passes through
the liver, which removes toxins, byproducts, and other harmful substances.
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56. Regulates Amino Acids: The production of proteins depend on
amino acids. The liver makes sure amino acid levels in the
bloodstream remain healthy.
Regulates Blood Clotting: Blood clotting coagulants are created
using vitamin K, which can only be absorbed with the help of bile,
a fluid the liver produces.
Resists Infections: As part of the filtering process, the liver also
removes bacteria from the bloodstream.
Stores Vitamins and Minerals: The liver stores significant
amounts of vitamins A, D, E, K, and B12, as well as iron and
copper.
Processes Glucose: The liver removes excess glucose (sugar) from
the bloodstream and stores it as glycogen. As needed, it can
convert glycogen back into glucose.

57. Muscular movement involves the gastrointestinal tract /
digestive system:-
Peristalsis
• It is a wavelike movement that occurs from the oropharynx to the rectum
which allowing the gastrointestinal tract to push food particles toward the
anus.
Mixing
• Mixing is a motion in the oral cavity and stomach that allows the
gastrointestinal tract to repeatedly break down food into smaller particles
using mechanical digestion.
 Segmentation
• In the regions of the small intestine contracting and relaxing occurs
independently that allowing the small intestine to digest and absorb more
efficiently.

58. Digestion and
Absorption of
Nutrients

60. Energetics

61. ATP (Adenosine triphosphate)
• ATP is a nucleotide composed of (1) the nitrogenous base adenine,
(2) the pentose sugar ribose, and (3) three phosphate radicals.
• Under the physical and chemical conditions of the body, each of these
high energy bonds contains about 12,000 calories of energy per mole
of ATP, which is many times greater than the energy stored in the
average chemical bond, thus giving rise to the term high-energy bond.
• The high-energy phosphate bond is very labile so that it can be split
instantly on demand whenever energy is required to promote other
intracellular reactions.
• When ATP releases its energy, a phosphoric acid radical is split away
and adenosine diphosphate (ADP) is formed.

62. • This released energy is used to energize virtually many of the cell’s other
functions, such as synthesis of substances and muscular contraction.
• To reconstitute the cellular ATP as it is used up, energy derived from
the cellular nutrients causes ADP and phosphoric acid to recombine to
form new ATP, and the entire process repeats over and over again.
• For these reasons, ATP has been called the energy currency of the cell
because it can be spent and remade continually, having a turnover time of
only a few minutes.

63. • The initial event is removal of an electron from the hydrogen atom, thus
converting it to a hydrogen ion.
• The terminal event is combination of hydrogen ions with oxygen to form water
plus the release of tremendous amounts of energy to large globular proteins,
called ATP synthetase, that protrude like knobs from the membranes of the
mitochondrial shelves.
• Finally, the enzyme ATP synthetase uses the energy from the hydrogen ions to
cause the conversion of ADP to ATP.
• The newly formed ATP is transported out of the mitochondria into all parts of
the cell cytoplasm and nucleoplasm, where its energy is used to energize
multiple cell functions.

64. Role of ATP
 Energy from ATP is used to promote three major categories of cellular
functions:
• Transport of substances through multiple membranes in the cell,
• Synthesis of chemical compounds throughout the cell, and mechanical work.
• To supply energy for the transport of sodium through the cell membrane,
• To promote protein synthesis by the ribosomes,
• To supply the energy needed during muscle contraction.
• In addition to membrane transport of sodium, energy from ATP is required
for membrane transport of potassium ions, calcium ions, magnesium ions,
phosphate ions, chloride ions, urate ions, hydrogen ions, and many other
ions and various organic substances. Membrane transport is so important to
cell function that some cells—the renal tubular cells, for instance—use as
much as 80 percent of the ATP that they form for this purpose alone.

65. • In addition to synthesizing proteins, cells make phospholipids, cholesterol, purines,
pyrimidines, and a host of other substances. Synthesis of almost any chemical
compound requires energy.
• For instance, a single protein molecule might be composed of as many as several
thousand amino acids attached to one another by peptide linkages; the formation of
each of these linkages requires energy derived from the breakdown of four high-energy
bonds; thus, many thousand ATP molecules must release their energy as each protein
molecule is formed. Indeed, some cells use as much as 75 percent of all the ATP
formed in the cell simply to synthesize new chemical compounds, especially protein
molecules; this is particularly true during the growth phase of cells.
• The final major use of ATP is to supply energy for special cells to perform mechanical
work. Each contraction of a muscle fiber requires expenditure of tremendous quantities
of ATP energy.
• Other cells perform mechanical work in other ways, especially by ciliary and ameboid
motion. The source of energy for all these types of mechanical work is ATP.

66. CREATINE PHOSPHATE
• Phosphocreatine, also known as Creatine phosphate (CP) or PCr (Pcr), is a
phosphorylated creatine molecule that serves as a rapidly mobilizable reserve of
high-energy phosphates in skeletal muscle, myocardium and the brain to recycle
adenosine triphosphate, the energy currency of the cell.
• In the kidneys, the enzyme AGAT(Arginine Glycine amidinotransferase) catalyzes
the conversion of two amino acids — arginine and glycine — into guanidinoacetate
(also called glycocyamine or GAA), which is then transported in the blood to the
liver. A methyl group is added to GAA from the amino acid methionine by the
enzyme GAMT, forming non-phosphorylated creatine.
• This is then released into the blood by the liver where it travels mainly to the muscle
cells (95% of the body's creatine is in muscles), and to a lesser extent the brain, heart,
and pancreas.

67. • Once inside the cells it is transformed into phosphocreatine by the enzyme
complex creatine kinase, which makes it able to donate its phosphate group to
convert adenosine diphosphate (ADP) into adenosine triphosphate (ATP).
• This process is an important component of all vertebrates' bioenergetic systems.
For instance, while the human body only produces 250 g of ATP daily, it
recycles its entire body weight in ATP each day through creatine phosphate.
• Creatine phosphate can be broken down into creatinine, which is then excreted
in the urine. A 70 kg man contains around 120 g of creatine, with 40% being the
unphosphorylated form and 60% as creatine phosphate. Of that amount, 1–2% is
broken down and excreted each day as creatinine.
• Phosphocreatine can anaerobically donate a phosphate group to ADP to form
ATP during the first two to seven seconds following an intense muscular or
neuronal effort.

68. • Conversely, excess ATP can be used during a period of low effort to convert
creatine to phosphocreatine. The reversible phosphorylation of creatine (i.e., both
the forward and backward reaction) is catalyzed by several creatine kinases.
• The presence of creatine kinase (CK-MB, MB for muscle/brain) in blood plasma
is indicative of tissue damage and is used in the diagnosis of myocardial
infarction.
• The cell's ability to generate phosphocreatine from excess ATP during rest, as
well as its use of phosphocreatine for quick regeneration of ATP during intense
activity, provides a spatial and temporal buffer of ATP concentration.
• In other words, phosphocreatine acts as high-energy reserve in a coupled
reaction; the energy given off from donating the phosphate group is used to
regenerate the other compound - in this case, ATP.
• Phosphocreatine plays a particularly important role in tissues that have high,
fluctuating energy demands such as muscle and brain.

69. BMR
• The Basal Metabolic Rate (BMR) is the energy required by an
awake individual during physical, emotional and digestive rest. It
is the minimum amount of energy required to maintain life or
sustain vital functions like the working of the heart, circulation,
brain function, respiration, etc.
• The metabolic rate during sleep is less than BMR. Basal
metabolic energy required to support the basic processes of life,
including circulation, respiration, temperature maintenance, etc.
• It excludes digestion and voluntary activities. BMR constitutes
the largest proportion (2/3) of a person’s daily expenditure.

70. FACTORS THAT AFFECT BMR
• Age –metabolism slows with age due to loss of muscle tissue, but also due
to due to hormonal and neurological changes.
• Growth – infants and children have higher energy demands per unit of
body weight due to the energy demands of growth
• Body composition – more lean muscle tissue (muscle burns kilojoules
rapidly) higher BMR
• Environmental temperature : if temp is very low or very high, , body has
to work harder to maintain its normal body temp. which inc. the BMR
• Fasting/starvation, lowers BMR
• Malnutrition, lowers BMR
• Thyroxine – regulates BMR

71. NORMAL VALUE FOR BMR
• For adult men normal value for BMR is 34-37 kcal/square
meter/hour, and
• For adult women, 30-35 kcal/Sq.m./hour.
• For easier calculations, BMR for an adult is fixed as 24 kcal/ kg
body weight/day.

72. DISORDERS OF THE DIGESTIVE SYSTEM
• Jaundice - It is the excess accumulation of bile pigments in the
bloodstream. In jaundice, there is a yellow discoloration of the
skin, the whites of the eyes as well as the mucous membranes.
This is caused by the increased amount of bilirubin in the blood,
which is a byproduct of the breakdown of RBC’s in the body.
Normally liver is involved in the metabolism and excretes
bilirubin, in the form of bile. Any disruption in this metabolism
causes jaundice. It is a most common form of the disease
hepatitis.

73. • Nausea and Vomiting - When there is irritation in the stomach,
sometimes the stomach contents get ejected from the mouth. This is a
reflex action and is called as vomiting. There is a forceful discharge of
the stomach contents that come out of the mouth. The uneasiness that
usually comes before vomiting is nausea. Vomiting and nausea are
generally the symptoms of conditions such as food poisoning, stomach
flu, gallbladder disease, ulcers, overeating etc.
• Peptic Ulcers - Peptic ulcers are holes or breaks in the protective
lining of the duodenum or stomach. Duodenal ulcers, stomach ulcers
are the types of ulcers, according to the position where they occur.
Oesophagal ulcers also occur at times, due to alcohol abuse or exposure
to certain antibiotics.

74. • Diarrhoea - It is the abnormal frequency of the bowel movement. There is an
increased liquidity of faecal discharge. The stools are loose and watery. It is
generally caused by viruses that cause the infection of the gut.
• Gallstones - These are small pieces of solid material that we find in the
gallbladder. When they block the bile duct, it causes immense pain and needs to
treated immediately.
• Gastroesophageal Reflux Disease (GERD) - GERD is a gastroesophageal reflux
disease. In this disease, the lower esophageal sphincter that does not allow the
food bolus to come back to the food pipe gets weakened, due to which stomach
acid gets leaked and comes up into our esophagus. This causes heartburn, and in
the long term, the lining of the esophagus gets affected.
• Crohn’s disease - Crohn's disease is an inflammatory disease of the digestive tract
that affects approximately half a million Americans. It is a type of inflammatory
bowel disease (IBD). It is most commonly found in the small intestine, but it can
affect any part of the digestive tract.

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