Mammalian Digestive System
2.2.5: Trace the digestion of foods in a mammalian digestive system, including:
– physical digestion
– chemical digestion
– absorption of nutrients, minerals and water
– elimination of solid waste
The table shows the main structures and associated organs of the human digestive system
(alimentary canal) and their functions.
The diagram shows the main structures of the digestive system.
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The Process of Digestion
Digestion is the breaking down of large and complex food particles into much smaller and
simpler particles. There are two types of digestion: physical (mechanical) and chemical. The
overall aim of digestion is to break down the particles into substances that are small enough
to be absorbed through the intestinal walls into the bloodstream.
Physical Digestion
This involves the physical breakdown of food particles. It begins in the mouth when the
different types of teeth break food into smaller pieces by cutting, tearing, chewing and
grinding the food. The churning motion of the stomach continues the process of physical
digestion. The aim of this physical digestion id to start the process of breaking food into
smaller pieces so that its surface area is increased and it can then be acted on by enzymes in
chemical digestion.
Chemical Digestion
Chemical digestion is the process of using digestive enzymes to chemically break down the
large, complex molecules in the food that has been ingested into their smaller, simpler
forms. Some of the simple substances obtained are glucose from complex carbohydrates,
amino acids from proteins, glycerol and fatty acids from lipids and nucleotides from nucleic
acids.
Pathway Through Digestive System
1. Mouth
After food enters the mouth, physical digestion begins the process of the breakdown
of the food. Teeth break the food up into smaller pieces with greater surface area for
the more efficient action of enzymes. Salivary amylase is released into the mouth,
and is mixed with the food by the tongue and the action of chewing. This enzyme
begins the chemical breakdown of the complex carbohydrate starch into the
simpler sugar maltose. Once the food has been chewed into small pieces and mixed
with saliva, the tongue forms it into a ball shape called the bolus. This is then
swallowed and enters the oesophagus.
2. Oesophagus
Once the bolus enters the oesophagus, it travels along the soft-walled, muscleringed tube to the stomach. As it passes the entrance to the trachea, a flap of skin,
the epiglottis, closes over this entrance to prevent the entry of food into the
respiratory system. The bolus of food does not move down the oesophagus just due
to gravity. Muscular contractions also move the bolus by a process called peristalsis.
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3. Stomach
diaphragm
At the entry and exit of the stomach, are
narrow openings whose opening and closing
are controlled by circular sphincter muscles.
This controls the movement of substances
into and out of the stomach. Once inside the
stomach, relaxation and contraction of the
stomach walls continue physical digestion.
The bolus breaks up into pieces that
combine with gastric juices contained within
the stomach to form a mixture of known as
chyme. Gastric juices, secreted from the wall
of the stomach, contain water, hydrochloric
acid, pepsinogen and pepsin. The acid causes
the pH of the interior of the stomach to be
at 2.0 – 3.0. Mucus lining the stomach
prevents the acid from ‘eating away’ the
walls of the stomach.
oesophagus
Cardiac sphincter
Pyloric
sphincter
Stomach
lining
Diagram showing the structure of the
stomach
The enzyme pepsinogen is converted into an active form called pepsin in the acidic
environment and begins the chemical breakdown of the long chained proteins into
shorter chained peptides. Pepsin also breaks down nucleic acids (DNA and RNA) in
the food to their component nucleotides. The chyme remains in the stomach for
about 6 hours.
4. Small intestine
The chyme from the stomach enters the
small intestine gradually through a small
muscular opening, the pyloric sphincter.
The highly folded small intestine is
approximately 7m long in an adult and
contains 3 main regions: the duodenum
(at the start of the small intestine), the
jejunum (middle section) and the ileum
(end region).
Diagram showing 3 parts of small intestine
As the chyme enters the duodenum, it
stimulates the release of a hormone,
which in turn stimulates the release of
pancreatic juices into the area.
Pancreatic juices are secreted by the
pancreas and contain a mixture of the
digestive enzymes amylase, trypsin and
lipase as well as bicarbonate ions. The
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bicarbonate ions act to neutralise the
acidic chyme leaving the stomach.
Amylase and trypsin continue the
chemical breakdown of carbohydrates
and proteins.
When there are lipids present in the chyme, bile is released into the duodenum. Bile
is produced by the liver and is stored in the gall bladder. Bile is not a digestive
enzyme. It acts in the same way as detergent acts on fats when washing a greasy
saucepan- it breaks down (emulsifies) the fats into smaller pieces or fat droplets.
This increases the surface are for the action of the digestive enzyme lipase to
chemically break the lipids into fatty acid and glycerol molecules.
From the duodenum, food enters the jejunum where most of the absorption of the
digestive products occurs.
Absorption in the digestive tract
The absorption of substances mostly occurs in the jejunum section of the small
intestine. Some substances, such as alcohol and drugs, are absorbed quickly into the
stomach. The products of digestion including amino acids, glucose, fatty acids and
glycerol, move into the transport systems of the body in the small intestine. These
products are moved by diffusion
or active transport through tiny
projections called villi, which line
the intestinal wall. These
projections greatly increase the
surface area for much more
efficient diffusion. Villi walls are
moist and are one cell thick. They
have a rich blood supply in the
tiny capillaries that are wrapped
around a lacteal. Lacteals are
connected to another transport
system in the body- the lymph
Diagram of villi in small intestine
system. Glucose and amino acids
are absorbed into the capillaries,
while fatty acids and glycerol move into the lacteal. Some water absorption will also
occur here.
5. Liver
Digested food, once absorbed into the bloodstream, travels to the liver, which is the
centre of food metabolism. It plays an important role in keeping sugars, glycogen,
and protein levels in balance in the body. It also detoxifies blood.
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6. Large Intestine
When all the required
digestive products have
been absorbed in the
small intestine, the
remaining undigested
material moves to the
large intestine. This
material is composed of
substances such as water,
salts and dietary fibre. The
large intestine has two
main sections: the colon
and the rectum. In the
colon, the water and
Diagram showing the parts of the large intestine
some salts are absorbed
back into the
bloodstream, with the undigested material compacting into a more solid substance.
Vitamins A and K, which are produced by bacteria in the colon acting on the
undigested matter, are also absorbed into the bloodstream.
Elimination of Solid Waste
The remaining waste material, known as faeces, is moved into the rectum by
peristalsis and then egested, or eliminated, from the body through the anus.
The end products of digestion can be built up by the body into useful substances, as
either new biological material or an energy source. For example, in mammals such as
humans, blood transports the products of digestion to where they are needed in the
body. They can then be reassembled by the cells of the body into structural parts
(for example, lipids and proteins form structural part of cell membranes, and protein
fibres in muscle tissue) or into energy storage (for example, fatty tissue or fat
beneath the skin, or the carbohydrate glycogen, a form of ‘animal starch’ in the liver
and muscles). Protein cannot be stored.
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Questions
Using the information in this booklet, answer the following questions:
1. Define the term digestion.
2. Outline the main functions of the digestive system.
3. Distinguish between physical and chemical digestion and identify where in the
digestive system each of these occurs. Use a table with suitable headings to
answer this question.
4. Identify and draw a flow chart to show, the structures that food must pass
through in a typical mammalian digestive system.
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5. Complete the table below.
Table of nutrients and the simple digestive products they form
Nutrient
Digestive Products formed
proteins
nucleic acids
carbohydrates
lipids
6. Outline the function of bile and identify where it is stored and where it is
produced.
7. Outline the function of the following enzymes:
a. Amylase
b. Pepsin
c. Trypsin
d. Lipase
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8. Identify the structures that increase the surface area of the internal wall of the
small intestine and explain how these structures assist the absorption of the
products of digestion.
9. Determine whether the material egested from the digestive system is part of the
internal or external environment. Justify your answer.
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10. Construct a summary table identifying each organ of the digestive system and its
function.
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