Topic 6.1: Digestion
Assessment Statements
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6.1.1: Explain why digestion of large food molecules is
essential
6.1.2: Explain the need for enzymes in digestion
6.1.3: State the source, substrate, products and optimum pH
conditions for one amylase, one protease, and one lipase
6.1.4: Draw and label a diagram of the digestive system
6.1.5: Outline the function of the stomach, small intestine
and large intestine
6.1.6: Distinguish between absorption and assimilation
6.1.7: Explain how the structure of the villus is related to its
role in absorption and transport of the products of digestion
Why do we digest?
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When you eat a snack or a meal, you
begin a set of events that leads to your
body cells being provided with needed
nutrients
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Ingestion
Digestion
Absorption
Transport
Digestion
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Digestion solves a problem of molecular
size
◦ Food is too large to pass through cell
membranes
◦ Food must be chemically digested to a
suitable size
Molecule type
Molecular form ingested
Molecular form after
digestion
Protein
Protein
Amino acids
Lipids
Triglycerides
Glycerol and fatty acids
Carbohydrate
Polysaccarides, disaccharides, and
monosaccharides
Monosaccharides
Nucleic acids
DNA, RNA
Nucelotides
Digestion
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Digestion allows you to turn molecules
into ‘your own’
◦ All the food that you eat is composed of plant
or animal cells, thus containing molecules
characteristics of a living organisms that is not
a human being
 Each type of living organism has its own set of
proteins, nucleic acids, and carboydrates
Digestion
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When we digest food molecules, we
break them down (hydrolyze them) into
smallest components.
◦ The components can be reassembled into
larger molecules that are useful to you!!
Digestion
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As food moves through your alimentary
canal, many digestive enzymes are added
along the way
◦ Each digestive enzyme is specific for a specific
food type
◦ Examples:
 Lipase: is an enzyme is specific for lipid molecules
 Amylase: is specific for amylose (starch)
Digestion
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The real function of enzyme is to lower
the activation energy of reactions that
they catalyze
◦ Digestive enzymes all help to catalyze
hydrolysis reactions
Examples of digestion enzymes
Salivary Amylase
Pepsin (a Protease)
Pancreatic
Lipase
Source
Salivary glands
Stomach cells
Pancreas cells
Substrate
Amylose (starch)
Proteins (polypeptides)
Lipids
Products
Maltose and glucose
Amino acids
Glycerol and fatty
acids
Optimum
Neutral (pH 7)
Acidic (pH 3)
Neutral (pH 7)
Human Digestive System
Much of the human digestive
system is a tube called the
alimentary canal
 Alimentary canal consists of:
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Mouth
Esophagus
Stomach
Small Intestine
Large Intestine (colon)
Rectum
Any foods that you ingest must
either be digested and absorbed
for use by the body or remain
undigested and be eliminated as
solid waste
Stomach
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Food is brought to
your stomach by a
muscular tube called
the esophagus
◦ When you swallow,
the food is forced
down to your
stomach by a
sequential series of
smooth muscle
contractions called
peristalsis
Stomach
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Once in the stomach, the food is held for
a period of time in order to mix it with a
variety of secretions collectively known as
gastric juice
Stomach
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Gastric juice is a mixture of three
secretions form the cells of the stomach
inner lining:
◦ Pepsin- a protease enzyme most active in
acidic pH
◦ Hydrochloric acid- helps degrade and break
down foods and creates the acidic pH
necessary for pepsin to be active
◦ Mucus- lines the inside the stomach wall to
prevent stomach damage from the
hydrochloric acids
Stomach
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The muscular wall of the stomach creates
a churning motion in order to mix food
with the gastric juice
◦ After a period of time, a valve at the lower
end of the stomach opens and the food
enters the small intestine
Small Intestine
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The first portion of the small
intestine is called the
duodenum
◦ Three different accessory
organs secrete juices into the
small intestine in order to
continue the digestive process
◦ These secretions include:
 Bile from the liver and gall bladder
 Trypsin (a protease), lipase, amylase,
and bicarbonate from the pancreas
Small Intestine
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As the digestive process continues in the
small intestine, molecules are produced
that are small enough to be absorbed
◦ The inner wall of the small intestine is made
up of thousand of finger-like extensions called
villi
Small Intestine
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Each villus contains a capillary bed and a lacteal
◦ Lacteal is a small vessel of your lymphatic
system
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If the inner lining of your small intestine
were smooth, you would have a fairly
limited membrane surface area for
absorption
◦ The function of the villi is to greatly increase
the surface area for absorption of molecules
such as glucose, amino acids, and fatty acids
Small Intestine
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Most molecules absorbed are taken into
the capillary bed within each villus
◦ Except fatty acids which are more efficiently
absorbed into the lacteal
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All absorbed molecules are taken to a
wide variety of body cells by the
circulatory system
Large Intestine
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The vast majority of useful nutrients are
absorbed while food is still inside the
small intestine
◦ Much of the water that we drink or that is
naturally contained in many food is also
present
Large Intestine
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The primary
function of the
large intestine is
water absorption
◦ Leaving water in
the alimentary canal
as long as possible
is beneficial because
it keeps the moving
food in a fluid
environment
Large Intestine
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The large intestine is also home to a very
large number of naturally occurring
bacteria including Escherichia coli.
Large Intestine
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These bacteria are examples of
mutualistic organisms within us.
◦ We provide nutrients, water, and a warm
environment for them while they synthesize
vitamin K and maintain a healthy overall
environment for us in our large intestine
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Any food undigested by us or the bacteria
is eliminated from the body as solid waste
or feces