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Digestion in Animals
Digestion
• Digestion is the chemical decomposition of food
into simple substances which the body of cells of
an animal can absorb.
• Food particles need to be broken down so they
can be transported to all parts of the body.
• To be absorbed by blood, the particles need to
soluble and small enough to pass through the
walls of blood vessels and cell membranes.
Digestion
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The whole process of taking in and using food particles
is called nutrition.
In animals nutrition involves five stages.
Ingestion – the intake of complex organic foods into
the body.
Digestion – the breaking down of food into simpler,
soluble substances inside the organism.
Absorption – the taking in of soluble substances into
the bloodstream.
Assimilation – the movement of soluble substances
from the bloodstream into the cells.
Egestion – the elimination from the body of undigested
food and wastes.
The digestive system
• Many animals have a digestive system to
process food material.
• These systems consists of a series of organs
connected to a tube, called the alimentary
canal that runs from the mouth to the anus.
• Glands along the alimentary canal secrete
chemicals, which help digest the food.
• Food is moved along the alimentary canal by a
muscular action called peristalsis.
The human digestive system
• Food enters through the mouth.
• Here it is chewed (mastication) into smaller pieces.
• Then swallowed down part of the alimentary canal called
the oesophagus.
• The oesophagus takes the food to the stomach, where
chemicals break down the food further into a sludge.
• Food is then moved into the small intestine, where more
chemicals act on it.
• Soluble food particles are absorbed through the walls of
the small intestine into the bloodstream.
• Undigested food travels through the large intestine and
is eliminated through the anus.
The Mouth
• Describe the involvement of the mouth in
the digestive process
The Mouth
The stomach
• The stomach is a muscular organ situated in the left side of the
abdominal cavity just below the diaphragm.
• It has a capacity of about 1.5 litres.
• Contraction of the stomach muscles churns the food into a sludge.
• Glands in the lining of the stomach secrete a gastric juice which breaks
down the food further.
• Gastric juice contain three main substances:
– The enzyme pepsin which breaks down long chains of proteins into shorter
chains of amino acids called peptides.
– The enzyme rennin which coagulates milk, causing its to stay in the
stomach until pepsin breaks down the milk proteins.
– Hydrochloric acid, which provides a suitable pH for pepsin activity. It also
dissolves minerals and kills bacteria.
• In addition gastric juices also contains lipase (which digests fats to fatty
acids and glycerol) and mucus (to protect the lining of the stomach from
the acid.
• The stomach is a temporary store of food.
• It delivers food in small amounts at a time to the duodenum.
• Mineral ions and simple sugars, also drugs and alcohol, are absorbed
through the stomach wall into the bloodstream.
Duodenum
• What role does the Duodenum play in
digestion?
Duodenum
Small Intestine
• Summarise the function of the small
intestine’s role in digestion. – Remember
to talk about its structure.
Small Intestine
Summary of Chemical Digestion
• Copy the Summary of chemical digestion
of page 113.
Draw a cross section of a vilius
(pg115)
Large Intestine
• Undigested material from the small intestine
passes along the alimentary canal into the large
intestine.
• The main part of the large intestine is the colon.
• The main function of the colon is to absorb water
from the undigested matter.
• Remaining semi-solid waste (faeces) is moved
into the rectum by peristalsis.
• The rectum stores faeces until they are egested
through the anus by muscular action.
Enzyme action in the human digestive system
Body part
Source of
fluid
Name of fluid
Contents
Mouth
Salivary gland
Saliva
Salivary amylase
Stomach wall
Gastric Juice
Protease (pepsin)
Protease
(Rennin)
Hydrochloric Acid
Breaks down protein.
Coagulates Milk.
Breaks down carbo
Breaks down lipids
Breaks down proteins
Stomach
Pancreas
Pancreatic juice
Amylase
Lipase
Protease (trypsin)
Liver
Bile
Bile Salts
Intestine Wall
Intestine
Protease (erepsin)
Duodenum
Maltase
Small intestine
Lactase
Sucrase
Function
Converts starch to
maltose
Activates pepsin, kills
bacteria, dissolves
minerals.
Emulsifies fats
Breaks down peptides to
amino acids.
Changes maltose to
glucose
Changes lactose to
glucose
Changes sucrose to
glucose
Important things to realize
• Humans lack certain enzymes to digest some
substances, such as plant cellulose. However
indigestible substances are important because they help
move food through the alimentary canal.
• Herbivorous mammals are able to digest plant cellulose
and convert it into glucose. Rats, cows and koalas are
examples of mammals that do this in an area called the
caecum. This large sac contains cellulose-digesting
bacteria and is situated between the small and large
intestines.
• The elimination of faeces by the anus is not the same
type of process as the elimination of metabolic wastes by
the kidneys, lungs and skin.
Questions
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What is digestion? Why is it necessary?
Summarise the five main stages of nutrition in an animal.
What is the alimentary canal?
How is food moved along this canal?
Draw a labelled diagram of the human digestive system. Identify
these parts on a human torso.
In which part of the digestive system is most food absorbed into
the bloodstream?
What is the role of the villi? How does their structure suit their
function?
What does bile do? Where is it produced and stored?
Which of the following organs do not secrete digestive enzymes?
(Stomach, oesophagus, liver, small intestine, large intestine)
Name two important functions of the large intestine.
Where does the digestion process begin?
Account for the fact that stomach juice is acidic, and juice in the
small intestine is alkaline.