Topic
X
4
Digestive
System
LEARNING OUTCOMES
By the end of this topic, you should be able to:
1.
Explain the process of physical digestion and chemical digestion;
2.
Describe the structure of the human digestive system;
3.
Explain the process of digestion in humans;
4.
Describe digestion in ruminants;
5.
Describe digestion in rodents; and
6.
Explain how excretion occurs in plants.
X INTRODUCTION
In Topic 3, we looked at the various classes of food that are needed by our body.
Have you ever wondered about what happens to these foods after we have eaten
them? The foods that we eat are in quite a different state from the one that can be
used by the cells in our body. Before the foods can be used, they need to be
converted into smaller units, so that they can be easily absorbed by our body
cells. This process is called digestion.
Basically, digestion is the process of breaking down food from complex
substances into simpler soluble molecules to be absorbed by the body. Starch,
protein and fat are large insoluble food molecules. At the end of digestion, starch
is broken down into glucose, protein is broken down into amino acids and fats
are broken down into fatty acids and glycerol. Glucose, amino acids, fatty acids
and glycerol are in the simplest form and are easily absorbed into the body cells.
But, how does digestion take place?
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X TOPIC 4 DIGESTIVE SYSTEM
This is what we are going to learn in this topic. We are going to learn how
digestion occurs and also about the digestion in ruminants and rodents.
Excretion in plants will also be discussed at the end of this topic.
4.1
DIGESTION IN HUMANS
Digestion in humans can be divided into two types. They are:
(a)
Physical or Mechanical Digestion
Physical or mechanical digestion involves physically breaking the food into
smaller pieces. By breaking up food into smaller pieces, mechanical
digestion increases the surface area of the food available for chemical
digestion. For example: teeth chop and grind food; stomach churns (mixes)
the food.
(b)
Chemical Digestion
Chemical digestion is the breaking down of large molecules, such as starch,
proteins and fats into smaller soluble molecules for easy absorption by the
body. Chemical digestion involves digestive enzymes. Enzymes break
insoluble molecules into smaller molecules. Examples of digestive enzymes
are proteases which break up proteins into amino acids; amylases which
break up carbohydrates into sugars and lipases which break up fats and
other lipids into fatty acids and glycerol.
4.1.1
Stages of Digestion
The digestive system begins with the mouth where the food enters. This stage is
called ingestion. After the food enters the alimentary canal, it is digested. This is
the breakdown of complex food into the simple subunits. The products of
digestion then enter the blood or lymph. This stage is known as absorption. After
the food is absorbed, the nutrients are brought to the body cells. Here,
assimilation occurs where the absorbed nutrients are converted into complex
molecules for growth and repair. Finally, the waste products which remain
behind must be removed from the body. This stage is called egestion. These five
stages of digestion are shown in Table 4.1.
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Table 4.1: The Stages of Human Digestion
Stages of Digestion
What Happens
Ingestion
Taking of food into the body (eating).
Digestion
Breaking down of complex insoluble food into simpler soluble
substances. Consists of physical digestion and chemical
digestion.
Absorption
Absorption of digested food into the blood or lymph.
Assimilation
The uptake and use of absorbed food in the body for metabolic
activities.
Egestion
Undigested food is egested (removed).
4.1.2
The Human Digestive System
The system of organs that carries out digestion is known as the digestive system.
The organs involved in the human digestive system can be divided into two
main groups:
(a)
The Alimentary Canal
The alimentary canal is a continuous muscular tube running from the
mouth to the anus. It is about 10 m long in adults and is further subdivided
into organs with specific functions. The alimentary system consists of the
mouth, oesophagus, stomach, small intestine, large intestine and anus.
(b)
Accessory Structures of the Digestive System
Accessory structures of the digestive system are organs that lie outside the
alimentary canal and either produce or store secretions which aid in the
digestion of food. Examples of such organs include the salivary glands,
liver, gall bladder and pancreas.
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X TOPIC 4 DIGESTIVE SYSTEM
Figure 4.1 shows the human digestive system.
ȱ
Figure 4.1: Human digestive system
Source: http://leavingbio.net
Now, study Table 4.2, which provides the information of the features and
functions of organs of our digestive system.
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Table 4.2: The Features and Functions of the Organs of the Digestive System
Organ
Features
Function
Mouth
Teeth and tongue, salivary
glands.
x Ingestion and mechanical
digestion (chewing of food).
x Digestion of starch.
Oesophagus
Long muscular tube which leads
from mouth to stomach.
x Moves food from mouth to
stomach through peristalsis.
Stomach
Thick walled sac that contains
gastric glands.
x Acidity kills some bacteria.
x Digestion of starch stops.
x Digestion of protein starts.
Liver
Produces bile which is stored in
the gall bladder. Bile enters the
duodenum via the bile duct. ȱ
x Bile creates an alkaline
environment for the enzyme
action in the duodenum.
x Bile salts emulsify lipids.
Pancreas
Pancreas secretes pancreatic juice.
This is secreted into the
duodenum by the pancreas via
the pancreatic duct.
x Pancreatic juice contains the
enzymes pancreatic amylase,
trypsin and lipase.
Gall
bladder
Small sac found on liver.
x Stores and concentrates bile
from the liver.
Small
intestine
Duodenum
Receives secretions
from the gall
bladder (bile) and
pancreas
(pancreatic juice).
x Digestion of starch, proteins
and fats.
Jejunum
and Ileum
Produce intestinal
juices.
x Completion of digestion and
absorption of food.
Caecum
Small pouch at the
junction of the
small and large
intestines.
Appendix projects
from caecum.
x No function in humans.
Colon
Consists of three
parts: ascending,
transverse and
descending limb.
x Absorption of water and salts.
Rectum
Short and muscular.
x Storage of faeces.
Large
intestine
Anus
External opening surrounded by
circular muscles.
x Egestion or defecation.
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What is the function of salivary glands? Salivary glands secrete saliva which
moistens and lubricates food. It also contains amylase enzymes which start the
breakdown of starch. Does the appendix have a function? The appendix is a
vestigial (functionless) organ in humans, but is large and functional in
herbivores. You will be learning more about this later in this topic.
SELF-CHECK 4.1
1.
Explain the following terms:
2.
(a)
Ingestion; and
(b)
Digestion.
Complete Table 4.3 to show what happens to each class of food as
they pass through the mouth cavity, stomach, duodenum and
ileum.
Table 4.3
Nutrient
Part of Alimentary Canal
Mouth cavity
Stomach
Duodenum
Ileum
Protein
Fat
Starch
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Describe the parts played in the digestion of food by the
following organs:
4.
(a)
Pancreas; and
(b)
Liver.
Label and state the function of each part of the human digestive
system in Figure 4.2.
Figure 4.2: Unlabelled diagram of the human digestive system
ȱ
4.1.3
The Process of Digestion
Now, let us go through the process of digestion in detail. These are:
(a)
Digestion in the Mouth
The digestive process starts in the mouth. The chewing action breaks the
food into smaller particles. The presence of food in the mouth stimulates
the secretion of saliva by the salivary glands. The tongue manipulates the
food while it is being chewed to ensure it is mixed well with the saliva.
Saliva contains the enzyme salivary amylase which begins the breakdown
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of starch to maltose. The chewed food is rolled into a mass called a bolus in
preparation for swallowing. The food then enters and moves down the
oesophagus by a process called peristalsis, which is a series of wave-like
muscular contractions. The food then enters the stomach. This process is
illustrated in Figure 4.3.
Figure 4.3: Peristalsis
Source: http://www.tutorvista.com
(a)
ȱ
Digestion in the Stomach
The stomach is a thick-walled, muscular sac with a highly folded inner
wall. The lining of the stomach wall contains gastric glands which secrete
gastric juice. Gastric juice contains dilute hydrochloric acid and the
digestive enzymes pepsin and rennin. The contents and functions of gastric
juice are shown in Table 4.4.
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Table 4.4: Contents and Functions of Gastric Juice
Contents of Gastric
Juice
Dilute hydrochloric
acid
Functions
x
x
x
Pepsin
x
Stops the action of salivary amylase, which needs an
alkaline medium.
Helps to kill bacteria in food.
Provides an acidic medium for the action of pepsin
and rennin.
Starts the breakdown of large protein molecules to
polypeptides.
pepsin
Proteins
Rennin
x
Polypeptides
Coagulates liquid milk into a solid form.
Food stays in the stomach for a number of hours. During this period, the
food is thoroughly churned and mixed with the gastric juice by the
peristaltic contractions of the stomach wall. Eventually, the contents of the
stomach become a semi-fluid called chyme. The food then enters the first
part of the small intestine, which is called the duodenum.
(b)
Digestion in the Small Intestine
The small intestine consists of the duodenum, jejunum and the ileum as
shown in Figure 4.4.
Figure 4.4: Parts of the small intestine
Source: http://www.fashion-reply.com
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Let us read about the three parts of the small intestine.
(i)
The Duodenum
The duodenum is the first part of the small intestine and is about
25 cm long. It receives chyme from the stomach and secretions from
the gall bladder and pancreas. Study Figure 4.5, which shows the
position of the duodenum, pancreas and liver.
Figure 4.5: The position of the duodenum, pancreas and liver
Source: http://digestive.niddk.nih.gov
The liver secretes bile, an alkaline greenish-yellow fluid which is
stored in the gall bladder. Bile creates an alkaline environment
for the enzyme action in the duodenum. Bile also emulsifies fats,
transforming large lumps of fats into tiny droplets. This increases the
surface area for lipid digestion.
The pancreas secretes pancreatic juice, which contains three types of
enzymes: pancreatic amylase, trypsin and lipase. The digestion of
starch, proteins and lipids takes place in the duodenum as shown
here:
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The Jejunum and Ileum
The duodenum leads on to the jejunum and ileum. The jejunum is
about 2 m long. The ileum, which is about 4 m long, is the longest part
and is coiled and twisted. Glands in the wall of the ileum secrete
intestinal juice, which contains the digestive enzymes: maltase,
lactase, sucrase and erepsin.These enzymes complete the digestion of
proteins and carbohydrates. They require an alkaline medium to act at
an optimal rate. The action of the intestinal juice enzymes are shown
here:
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4.1.4
Absorption of Digested Food
The final products of digestion are glucose, amino acids, fatty acids and glycerol.
The absorption of these digested products takes place in the small intestine. The
small intestine has several adaptations to increase its efficiency in the absorption
process. These are:
(a)
The small intestine is long and coiled. It is about 67 m long and is the
longest part of the alimentary canal. This increases the time for enzyme
action. The surface area for absorption is also increased.
(b)
The internal walls of the small intestine are folded to increase the surface
area for absorption.
(c)
The internal walls of the small intestines are covered with tiny finger-like
projections called villi (singular: villus). The structure of the villus is ideally
suited for the function of absorption of food. Figure 4.6 shows the structure
of a villus.
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ȱ
Figure 4.6: Structure of villus
Source: http://studentbiologist.blogspot.com
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What are the adaptations of the villus for the absorption of food? They are:
(a)
Numerous, thus increasing the internal surface area for absorption;
(b)
Thin walled (only one cell thick); thus, digested food can be absorbed
rapidly;
(c)
Contain a network of blood capillaries for the efficient transport of digested
food; and
(d)
Contain special structures called lacteals for absorbing fatty acids and
glycerols.
Simple sugars and amino acids are absorbed directly into the blood capillaries of
the villus. Fatty acids and glycerol are absorbed into the lacteal where they are
reconverted into lipids and move into the lymphatic system. You will learn more
about the lymphatic system in the next topic.
4.1.5
Assimilation of Digested Food
Assimilation refers to what happens to the products of digestion. Absorbed food
substances are brought directly to the liver by the blood stream. The liver turns
the building materials such as sugars and amino acids into substances that are
used by different cells of the body. For an example, amino acids are transformed
into proteins.
The liver acts as a checkpoint which controls the amount of nutrients released
into the blood system. Glucose is oxidised to produce energy during respiration.
Excess glucose is changed to glycogen and stored in the liver and muscles. Fats
are used to build cell membranes or act as an energy source when required.
Excess fats are deposited beneath the skin to reduce heat loss from the body.
Amino acids are used to form structural proteins for growth, repair and making
of enzymes and antibodies. Excess amino acids undergo a process called
deamination where they are broken down and form urea. Urea is carried by the
blood to the kidney to be excreted. This process is illustrated in Figure 4.7.
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Figure 4.7: Assimilation in the liver
Source: https://www.cdli.ca
4.1.6
Defaecation or Egestion
After the absorption of nutrients has taken place in the small intestine, the
intestinal contents enter the large intestine or colon. The intestinal contents
consist of a mixture of water, undigested food substances and indigestible fibre,
most of which is cellulose from plant cell walls. The movement of these
undigested materials along the colon is slow and helped by peristalsis. The colon
reabsorbs almost 90% of water and minerals into the bloodstream. Absorption of
water from the undigested that remains in the colon results in the formation of
faeces which are a semi-solid waste. After 1224 hours in the colon, the faeces
pass into the rectum for temporary storage. As the faeces accumulate, pressure in
the rectum increases, causing a desire to expel the faeces from the body. The
elimination of faeces is known as defaecation. This process is controlled by
muscles around the anus, which is the opening of the rectum. The problems
related to defaecation are shown in Table 4.5.
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Table 4.5: Problems Related to Defaecation
Problems
Explanation
Diarrhoea
Food passes through the large intestine too quickly. Not enough water
is absorbed by the intestine.
Constipation
Faeces move too slowly through the colon. As a result, too much water
is reabsorbed making the faeces hard. Taking sufficient amounts of
fibre in the diet and drinking a lot of water can prevent constipation.
Haemorrhoids
Abnormally swollen veins in the rectum and anus. Caused by too
much pressure in the rectum forcing blood veins to stretch, bulge and
sometimes rupture.
Colon cancer
Malignant tumours of the colon. Believed to be caused by diets high in
fats. Breakdown of products of fat metabolism leads to cancer-causing
chemicals (carcinogens). A diet high in vegetables and fibre may help
to reduce the risk of cancer.
What is the difference between defaecation and excretion? Defaecation or
egestion should not be confused with excretion. Defaecation is the elimination of
the waste products of digestion from the alimentary canal. Meanwhile, excretion
is the removal of waste products of metabolism from excretory organs such as
the skin, lungs and kidneys. We will deal with this in Topic 5.
SELF-CHECK 4.2
1.
List three ways in which the intestine increases the surface area
for absorption.
2.
Name the end products of digestion which are absorbed by:
(a)
Blood capillaries of intestinal villi; and
(b)
Lacteals.
3.
Explain the meaning of assimilation.
4.
State the effects of insufficient intake of dietary fibre.
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4.2
DIGESTION IN RUMINANTS AND
RODENTS
Ruminants and rodents are herbivores. The plant materials they feed on contain a
high percentage of cellulose. In Topic 2, you learnt that cellulose is an insoluble
polysaccharide. The digestive systems of ruminants and rodents have unique
adaptations that help them to digest cellulose. Ruminants include cattle, sheep,
goats, buffalo, deer, antelopes, giraffes and camels. Examples of ruminants are
shown in Figure 4.8.
Figure 4.8: Examples of ruminants
Examples of rodents are shown in Figure 4.9.
Figure 4.9: Examples of rodents
Source: http://visual.merriam-webster.com
Let us now study the digestive systems of ruminants and rodents.
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Digestion in Ruminants
A ruminant is an animal which has a complicated digestive system in which the
stomach has several chambers. This unique digestive system allows them to use
energy from cellulose plant materials effectively. Ruminant animals like cattle,
sheep and goats are hoofed mammals that feed on plants but do not produce
cellulase which is needed to digest cellulose. How does digestion of cellulose
occur?
Ruminant stomachs are made up of four chambers:
(a)
Rumen;
(b)
Reticulum;
(c)
Omasum; and
(d)
Abomasum.
This adaptation enables ruminants to regurgitate and chew food again. Study
Figure 4.10 which shows the digestive tract of a ruminant.
ȱ
ȱ
Figure 4.10: Digestive tract of a ruminant
As you can see in Figure 4.10, the first two chambers the rumen and reticulum
are specialised compartments which have large communities of bacteria and
protozoa. These microorganisms produce cellulase which digests cellulose into
simple sugars. The abomasum corresponds to the stomach in humans.
100 X TOPIC 4 DIGESTIVE SYSTEM
Still looking at Figure 4.10, partially chewed food is passed into the rumen. Here,
cellulose is broken down by the cellulase produced by the microorganisms.
The partially digested food called the „cud‰ is then passed on into the reticulum.
The cud is then regurgitated bit by bit into the mouth to be chewed again.
Regurgitation of materials from the reticulum, followed by re-chewing and
re-swallowing, is called rumination. Rumination provides effective mechanical
breakdown of cellulose and increases the surface area for microbe action.The
food is then re-swallowed and moves to the omasum. Here, large particles of
food are broken down into smaller pieces by peristalsis. The walls of the omasum
also reabsorb water from the cud. The food particles finally move into the
abomasum, the true stomach of the cow. Here, gastric juices containing the
digestive enzymes complete the digestion of the other food substances. The food
then passes through the small intestine to be digested and absorbed.
4.2.2
Digestion in Rodents
Rodents also feed on plants but their digestive systems are different from those
of ruminants. Firstly, the caecum and appendix of rodents are enlarged to store
cellulase-producing microorganisms (bacteria and protozoa). Secondly rodents
rely on double digestion, that is, their food passes through the alimentary canal
twice. Rodents eat their own faeces so as to obtain all the nutrients lost with the
faeces. The faeces in the first batch are soft and watery. These faeces are eaten
again to enable the animals to absorb the products of bacterial breakdown as they
pass through the alimentary canal for the second time. The second faeces
becomes drier and harder. This adaptation allows rodents to recover the
nutrients initially lost with the faeces. This process in illustrated in Figure 4.11.
Figure 4.11: Digestive tract of rodents
Source: http://www.petcaregt.com
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SELF-CHECK 4.3
1.
4.3
Cellulose is an insoluble carbohydrate. However, many
herbivorous mammals have special adaptations in their digestive
systems that help them to digest carbohydrates.
(a)
Describe the adaptations of ruminants that help in the
digestion of cellulose.
(b)
In what ways are the digestive systems of rodents different
from ruminants?
(c)
Explain what happens to cellulose in the human alimentary
canal.
EXCRETION IN PLANTS
Do you remember what excretion is? Excretion is the process by which an
organism removes the waste products of metabolism. Important differences
between plant and animal metabolism make the process of excretion in plants
less significant than excretion in animals. Plants do not have any specialised
organs of excretion as animals do. Most of the waste products diffuse out of the
plant through tiny openings called stomata in leaves.
Why is plant excretion different from animal excretion? It is because according to
Clegg, C. J. & Mackean, D. G (2000):
(a)
Plants are stationary so they have a lower metabolic rate and metabolic
waste products move more slowly.
(b)
Plants are producers and synthesise their own food as raw materials
become available. For example, nitrogenous compounds such as ammonia
and nitrate are resources for protein production rather than unwanted
substances to excrete. Carbon dioxide is used to synthesise sugar.
(c)
Plants do not need to break down large molecules as they make their own
food.
(d)
Much of the structure of plants is based on carbohydrates rather than
proteins.
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What are the waste products of plants? The following are the waste products of
plants:
(a)
Carbon Dioxide
Green plants use carbon dioxide for photosynthesis during the day.
However, in the dark, carbon dioxide produced by respiration becomes a
waste product and diffuses out through the stomata in leaves.
(b)
Oxygen
During the day, oxygen is a waste product of photosynthesis and diffuses
out through the stomata in leaves.
(c)
Water
Water is produced as a respiratory waste product. Plants get rid of the
excess water by the processes of transpiration and guttation. You will be
learning more about transpiration and guttation in Topic 5.
(d)
Excretory Products
Plants also excrete products such as secretions, alkaloids, oils and crystals.
Resin, tannin, quinine, nicotine, oil, morphine and latex are examples of
plant excretory products. This is shown in Table 4.6.
Table 4.66: Plant Excretory Products and their Uses
Source
Excretory Products
Uses
Bark of casuarina tree
Resin
Manufacture of paint and varnish
Bark of mangrove
Tannin
Manufacture of ink
Softening of leather
Bark of cinchona
Quinine
Medicine of malaria
Tobacco leaf
Nicotine
Medicine, drug, poison
Flowers/ Leaf
Oil
Perfume
Poppy leaf
Morphine
Medicine, drug
Bark of rubber tree
Latex
Manufacture of rubber goods
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SELF-CHECK 4.4
1.
Explain why plants do not have specialised excretory organs.
2.
State the uses of the following excretory products of plants: latex,
resin and tannin.
Digestion is the process where food is broken down from complex substances
into simpler soluble molecules.
x
Digestion can be divided into physical and chemical digestion.
x
The human digestive system can be divided into the organs of the alimentary
canal and the accessory structures like the salivary glands, liver, gall bladder
and pancreas.
x
The presence of food in the mouth stimulates the secretion of saliva by the
salivary glands.
x
Saliva contains the enzyme salivary amylase which begins the breakdown of
starch to maltose.
x
The food then enters and moves down the oesophagus into the stomach by a
process called peristalsis.
x
The lining of the stomach wall contains gastric glands which secrete gastric
juice.
x
Gastric juice contains dilute hydrochloric acid and the digestive enzymes
pepsin and rennin.
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x
The duodenum receives the contents from the stomach and secretions from
the gall bladder and pancreas.
x
The liver secretes bile, which is stored in the gall bladder.
x
Bile also emulsifies fats.
x
The pancreas secretes pancreatic juice which contains three types of enzymes:
pancreatic amylase, trypsin and lipase.
x
Glands in the wall of the ileum secrete intestinal juice which contains the
digestive enzymes: maltase, lactase, sucrase and erepsin which complete the
digestion of proteins and carbohydrates.
x
The final products of digestion, glucose, amino acids, fatty acids and glycerol
are absorbed in the small intestine through the villi.
x
The elimination of faeces known as defaecation is controlled by muscles
around the anus.
x
Ruminants and rodents are herbivores and need to digest cellulose.
x
The digestive systems of ruminants and rodents have unique adaptations that
help them to digest cellulose.
x
Plants do not have any specialised organs of excretion as animals do.
x
Most of the waste products diffuse out of the plant through tiny openings
called stomata in leaves.
x
The waste products of plants are carbon dioxide, oxygen, water and excretory
products.
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DIGESTIVE SYSTEM
Abomasum
Oesophagus
Assimilation
Omasum
Bolus
Pancreas
Caecum
Pancreatic amylase
Digestion
Pepsin
Duodenum
Reticulum
Egestion
Rectum
Erepsin
Rennin
Gall bladder
Rodent
Ileum
Ruminant
Ingestion
Salivary amylase
Jejunum
Salivary glands
Lactase
Sucrase
Lipase
Trypsin
Maltase
Villus
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Clegg, C. J., & Mackean, D. G. (2000). Advanced biology: Principles and
applications (2nd ed.). London: Hodder Murray.
Enchanted Learning. (2010). Human digestive system. Retrieved March 20, 2011
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e&id=123&Itemid=176
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Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., &
Jackson, R. B. (2010). Campbell biology (9th ed.). San Francisco: Pearson Benjamin Cummings Pub
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