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IB BIOLOGY
DIGESTIVE SYSTEM
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animation digestive system
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6.1.1 Explain why digestion of large food molecules is essential.
The need for digestion: The food that humans eat contains
substances made by other organisms, many of which are
not suitable for human tissues.They must therefore be
broken down and reassembled in a form that is suitable.
A second reason for digestion is that many of the molecules
in foods are too large to be absorbed by the villi in the
small intestine. These large molecules have to be broken
down into small molecules that can be absorbed by
digestion, facilitated diffusion or active transport. The three
main types of food molecules that need to be digested are
starch, protein and triglycerides (fats and oils)
6.1.2 Explain the need for enzymes in digestion.
The need for enzymes in digestion:
Digestion of large molecules of food happens naturally at
body temperature, but only at a very slow rate. Enzymes
are essential to speed up the process.
Enzymes are necessary catalysts for healthy
chemical reactions inside the body. Enzymes are,
very simply put, protein based substances that
bind with other nutrients that bring about
changes in the body by speeding up tasks such as
digesting food, absorbing nutrients, and
maintaining and repairing tissue.
6.1.3 State the source, substrate, products and optimum
pH conditions for one amylase, one protease and one
lipase.
Enzymes of digestion:
Amylase
Examples Salivary
of this
amylase(ptya
enzyme
lin)
Source
Salivary
glands
Substrate Starch
Products
Maltose
Optimum PH 7
pH
protease
Pepsin
Lipase
Pancreatic
lipase
Wall of
stomach
Protein
Pancreas
Triglycerides(f
ats or oil)
Small
Fatty Acids
polypeptides and Glycerol
PH 1.5
PH 7
6.1.4 Draw and label a diagram of the digestive system
Label the given diagram of the digestive system.
6.1.5 Outline the functions of the stomach, small
intestine and large intestine.
Digestion in the mouth:
1. Mechanical and chemical digestion begin in
the oral cavity. Chewing cuts, smashes and
grinds food making it easier to swallow and
exposing more food surface to digestive
enzymes.
2. Incisors help in cutting food, canines help in
tearing the food, premolars and molars help in
grinding food.
3. The tongue helps in tasting and in manipulates
food and helps shape it into a ball called bolus.
It also helps in swallowing.
4. The salivary amylase converts starch into
maltose.
5. The food then passes to the esophagus by
peristalsis and reaches the stomach.
6. Questions:
Chewing functions in -------------- digestion,
and salivary amylase initiates the chemical
digestion of -----------.
How does food get from the pharynx to the
stomach?
Digestion in the stomach
1. The stomach has three major functions. First,
the stomach stores food and releases it
gradually into the small intestine at a rate
suitable for proper digestion and absorption.
Folds in the stomach wall increase its capacity,
allowing us to eat large infrequent meals.
2. A second function of the stomach is to assist in
the mechanical breakdown of food. In addition
to peristalsis, its muscular walls undergo a
variety of churning contractions that help
break apart large pieces of food.
3. The third function of the stomach is the
chemical breakdown of food.
4. Glands in the lining of the stomach secrete
enzymes and other substances, including
gastrin, hydrochloric acid, pepsinogen and
mucus.
5. Functions of HCl: The acid kills most bacteria
and other microbes that are swallowed with
the food. It gives the fluid in the stomach a
very acidic pH of 1 to 3. HCL converts inactive
pepsinogen into active pepsin which functions
best in an acidic environment.
6. Mucus lubricates and protects the cells lining
the stomach. It serves as a barrier to selfdigestion.
7. Pepsin is a protease, an enzyme that breaks
proteins into shorter chain of amino acids
called peptides. Pepsin is secreted in an
inactive form of pepsinogen to prevent it from
digesting the very cells that produce it.
8. Food in the stomach is gradually converted to
a thick, acidic liquid called chyme, which
consists of partially digested food and digestive
secretions.
9. The food passes from the stomach to small
intestine by peristalsis and the flow is
regulated by pyloric sphincter.
FUNCTIONS OF SMALL INTESTINE:
1. Most digestion occurs in the small intestine.
The small
Intestine is about 1 to 2 inches in diameter in
an adult human and with a length of 10 feet.
The small intestine functions to digest food
into smaller molecules and to absorb these
molecules into the bloodstream. The first role
of the small intestine ----- digestion ------- is
accomplished with the help of digestive
secretions from three sources: the liver, the
pancreas, and the cells of the small intestine
itself.
2. The role of liver in digestion is to produce bile,
a liquid
Stored and concentrated in the gallbladder
and released into the small intestine through a
tube called the bileduct. Bile helps in the
emulsification of fats, dispersing globs of fat in
the chyme into microscopic particles. These
particles expose a large surface area for attack
by lipases, lipid digesting enzymes produced
by the pancreas.
3. The pancreas secrete pancreatic juice, which is
released into
The small intestine through the hepato
pancreatic duct. Pancreatic juice neutralizes
the acidic chyme and digests carbohydrates,
proteins and lipids. Pancreatic juice contains
water, sodium bicarbonate and enzymes.
Sodium bicarbonate neutralizes the acidic
chyme in the small intestine, producing a
slightly basic ph. Pancraetic enzymes require a
more basic pH to function properly.
4. The pancreatic amylase hydrolyzes starch (a
polysaccharide) into the disaccharide maltose.
The enzyme maltase then splits maltose into
monosaccharide glucose. Sucrase acts on
sucrose and lactase acts on lasctose.
5. The enzymes Trypsin and chymotrypsin break
polypeptides into smaller polypeptides.
Peptidases then converts polypeptides into
amino acids.
6. The enzyme nucleases hydrolyzes the nucleic
acid in food into nucleotides which are then
broken down into nitrogenous bases, sugars
and phosphates.
7. Pancreatic lipases act on lipids and convert
them into fatty acid and glycerol.
6.1.7 Explain how the structure of the villus is
related to its role in absorption and
transport of the products of digestion.
The role of small intestine in the process of
absorption:
Around the inner wall of the small intestine
are large circular folds with numerous small,
finger like projetion called villi. Each of the
epithelial cells lining a villus has many tiny
surface projections called microvilli. The
microvilli extend into the lumen of the intestine
and greatly increase the surface area across
which nutrient is absorbed.
An epithelium, consisting of only one thin layer
of cells, is all that foods have to pass through to
be absorbed. Blood capillaries inside the villus
are very close to the epithelium so the distance
for diffusion of foods is very small. Some
nutrients are absorbed by simple diffusion other
nutrients are pumped against concentration
gradients into the epithelial cells. Absorbed
nutrients , such as amino acids and sugars, pass
out of the intestinal epithelium and then across
the thin wall of the capillaries into the blood.
A lacteal ( a branch of the lymphatic system) in
the center of the villus carries away fats after
absorption.
The capillaries that drain nutrients away from
the villi converge into larger veins and eventually
into main vessel, the hepatic portal vein, that
leads directly to the liver.
How is the structure of the villi adapted to its functions?
 The villus has a large surface area to volume ratio
 Microvilli increase surface area for absorption
 Thin epithelial layer so the products of digestion can easily pass
through
 Network of capillaries inside each villus (so only short distance) for
movement of absorbed products
 Capillaries transport absorbed nutrients/sugars and aminoacids away
from small intestine
 Central lacteal to transport absorbed fats away from small intestine
6.1.6 Distinguish between absorption and
assimilation
Distinguish between absorption and assimilation.

Absorption involves the passage of digested
nutrients into the blood from the gastro-intestinal
tract, glucose, fructose and amino acids go straight
to the blood capillaries, whereas fatty acids and
monoglycerides so first into the lymphatic system
and then the blood system.
Assimilation involves the integration of these absorbed
molecules into the living processes of the organism that
ingested them, using them to build new molecules that
are necessary for its normal functioning and survival.
Or using them to produce energy.
Role of large intestine in the process of digestion:
The large intestine or colon is 1.5m long and 5cm in
diameter.The colon’s main function is to absorb water from
the alimentary canal.The indigestible parts of the food,
together with a large volume of water, pass on into the
large intestine. Water is absorbed here leaving solid feces(
consists mainly of indigestible plant fibers) which are
eventually egested through the anus. The terminal portion
of the colon is the rectum, where feces are stored until they
can be eliminated.
Some of our colon bacteria such as E.coli, produce
important vitamins, including Biotin, Folic acid several B
vitamin and Vitamin K they are absorbed into the
bloodstream through the colon.
6.1.1 Digestion of macromolecules


Most food molecules are large polymers and insoluble
They must first be digested to smaller soluble molecules before they can be
absorbed into the blood
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6.1.2 Enzymes and digestion.

Enzymes are biological catalysts that increase the rate of reaction

Digestive enzymes are secreted into the lumen of the gut

Digestive enzyme increase the rate of reaction of the hydrolysis of insoluble food
molecules to soluble end products

Digestive enzymes
increase the rate of reaction at
body temperature

This image illustrates
the reduction in activation
energy that is achieved by the
use of an enzyme
 Notice that the normal
reaction requires a higher
activation energy which would
correspond to a high body
temperature. This is usually not
possible in living organisms.

The enzyme-catalysed reaction has a lower activation energy. This lower
activation energy would correspond to body temperature but is only possible in
the presence of an enzyme
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6.1.3 TYpes of digestive enzyme
Example 1 Pancreatic
amylase:
Conditions:

Source the
Pancreas
 Optimal pH 7.5-7.8
 Substrate is starch
(amylose)
 End product is the
disaccharide maltose
 Action: hydrolysis
of 1-4 glycosidic bonds
Example 2: Pepsin is a protease
produced in the stomach
Conditions:



Source is the stomach
Optimal pH is 2
Substrate is a polypeptide
chains of amino acids
 End product is small
polypeptides
 Action is the hydrolysis of
peptide bonds within the
polypeptide chain
(endopeptidase).
Example 3: Pancreatic lipases:

Source is the
pancreas
 The optimal
pH is 7.2
 The
substrate is a
triglyceride lipid
 The product
is glycerol and
fatty acid chains
 The action
of pancreatic
amylases also
requires the
presence of bile
salts that emulsify
the lipid. This emulsification has two effects:
1. Increases the surface area of the lipid for the digestion of fat
2. Exposes the glycerol 'head' structure to the enzyme

Action: hydrolysis of ester bonds between the glycerol molecules and the fatty
acid chains.
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6.1.4 Structure of the digestive system
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6.1.5 Functions of the stomach. small intestine and large intestine
1. Stomach:
The stomach stores the food from a meal and
begins protein digestion.
(a) Lumen of the stomach which stores the
food from a meal
(b) Gastric pits from which mucus , enzymes
and acid are secreted
(c) Mucus secreting cells. Mucus protects the
surface of the stomach from auto-digestion
(d) Parietal cells that produce HCL which kills
microorganism that enter the digestive system (food & tracheal mucus). This also
converts inactive pepsinogen to active pepsin
(e) Chief cells: produces pepsinogen a protease enzyme
2. small Intestine
In the small intestine digestion is completed.
The products of digestion are absorbed into
the blood stream.
(a) Villus which increase the surface area for
absorption of the products of digestion
(b) Microvilli border of the epithelial cell
increases the surface are for absorption.
(c) Lacteals are connect to the lymphatic
system for the transport of lipids.
(d) In the wall of the small intestine are the blood vessels to transport absorbed products
to the general circulation, There are also the muscle to maintain peristalsis
3. Large Intestine or colon:
The colon is responsible for the reabsorption
of water from the gut.
(a) The lumen of the colon
(b) The mucus producing goblet cells
(b) Muscular walls to maintain peristalsis
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6.1.6 Absorption and
assimilation.
Insoluble food molecules are
digested to soluble products
in the lumen of the gut.
Absorption:
1. The soluble products
are first taken up by various
mechanisms into the
epithelial cells that line the
gut.
2. These epithelial cells
then load the various
absorbed molecules into the
blood stream.
Assimilation:
1. The soluble products of digestion are then transported to the various tissues by the
circulatory system.
2. The cells of the tissues then absorb the molecules for use within this tissues
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6.1.7 Structure and function of the villus
The structure of the villus increases the surface are for the absorption of digested food
molecules.
(a) folds increase SA:VOL ration by X 3
(b) Villi project into the lumen of the gut increasing the surface area by X 10
(c) Microvilli are outward folds of the plasma membrane increasing the surface area
another X10
This sequence of light microscope and electron micrograph images show the same
sequence as the diagram above.
Histological adaptations within the villus.

Blood supply in the
villus which absorb the end
products of digestion from the
epithelial cells

The lacteals (green)
that receive the lipoproteins
before transporting them to the
circulatory system.

Muscular walls that
maintain the movement of chyme by peristalsis.
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