Digestion
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6: Human Health and Physiology
6.1 Digestion
Orange book: pg. 203-207
Green book: pg. 95-97
6.1.1 Explain why digestion of large food molecules is essential (pg. 203, 95)
6.1.2 Explain the need for enzymes in digestion (pg. 204, 95)
6.1.3 State the source, substrate, products and optimum pH conditions for one
amylase, one protease and one lipase (pg. 204, 95)
6.1.4 Draw and label a diagram of the digestive system (pg. 205, 96)
6.1.5 Outline the function of the stomach, small intestine and large intestine
(pg. 206, 96)
6.1.6 Distinguish between absorption and assimilation (pg. 207, 96)
6.1.7 Explain how the structure of the villus is related to its role in absorption
and transport of the products of digestion (pg. 97).
Role of Digestion
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6.1.1 Explain why digestion of large food molecules is essential
Orange book  pg. 203
Green book  pg. 95
To do:
 Read and highlight the notes below, read the relevant sections in the green
book.
 Discuss your findings
 Complete the table below




Summary in Green book to include:
list of events in digestive system
compounds and their monomers
sentence to explain the importance of digestion
Why do we digest food?
When you eat a snack or meal, you begin a set of events that leads to your body
cells being provided with needed nutrients. In very basic format, here is the
series of events in order:





Ingestion – eating food
Digestion – series of chemical reactions, whereby large complex insoluble
molecules are broken down to small simple soluble molecules.
Absorption – small molecular forms are absorbed through cells of your
digestive system and pass into nearby blood or lymphatic tissue.
Transport – your circulatory system delivers the small molecular nutrients to
your body cells
Assimilation – the components of digestions are ‘reassembled’ by our cells
into larger molecules that are needed by the body.
Digestion solves a problem of molecular size.
Many of the foods that we ingest have very large molecules – too large to pass
across the cell membrane. Yet to get into your bloodstream, molecules must
pass through the cell membranes of your intestines and then through the cell
membrane of a capillary. Any food that we eat must therefore be chemically
digested to a suitable size.
Compound
Carbohydrate
Monomers
Protein
Lipid
Carbohydrates
Notes:
This symbol usually represents a single glucose molecule.
When two glucose molecules join together they form a disaccharde.
Carbohydrates are polymers – they are formed from lots of single glucose
molecules joined together. Both starch and glycogen are formed from long
chains of glucose molecules.
Proteins
Proteins are complex molecules that are made from 20 basic monomers called
amino acids.
The order in which the amino acids are arrange determines the function of the
protein.
Each of these symbols represents a different amino acid (I have only used 6
instead of the 20 that are available):
The number of amino acids and the sequence in which they are arranged varies
between different proteins.
Lipids
Lipids are composed of two different monomers called fatty acids and glycerol.
G
FATTY ACID
L
Y
FATTY ACID
C
E
FATTY ACID
R
O
Key Importance
L is important because it breakdown large, complex, insoluble molecules
Digestion
into small, simple, soluble molecules.
Carbohydrates, proteins and lipids are too large to pass across the intestinal
wall and into the blood. They must be digested in order to be absorbed into the
blood.
Enzymes
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6.1.2 Explain the need for enzymes in digestion
Orange book  pg. 204
Green book  pg. 95
To do:
 Read and highlight the notes below, read the relevant sections in the green
book.
 Discuss your findings
 Summary in Green book to include:




explanation of specificity
function of enzymes in relation to activation energy
sentence to explain the importance of digestion
importance of enzymes being globular proteins

explanation of bond weakening in active site
The role of enzymes during digestion
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.
For example, lipase is an enzyme specific for lipid molecules and amylase is
specific for amylose (starch).
The real function of enzymes is to lower the activation energy of the reaction
they catalyse. This means that reactions occurring with an enzyme can occur
with a lower input of energy than the same reaction without the aid of an
enzyme. The input of energy is typically in the form of heat. Enzyme-catalysed
reactions proceed at higher reaction rates at a lower temperature than the
same reaction without an enzyme. This is a tremendous advantage for living
organisms. Many of the reactions which represent the digestive process would
need far higher temperatures than we are able to maintain safely if enzymes
were not involved. Humans maintain a stable body temperature of 37oC. This
temperature is warm enough to maintain good molecular movement and, with the
aid of enzymes, it provides enough activation energy for metabolic reactions
including digestion.
Globular Proteins
Enzymes are globular proteins (spherical/ globe like) that function as biological
catalysts (a catalyst speeds up a biological reaction without being altered or
used up in the reaction itself). They permit biological reactions to occur very
rapidly at normal body temperatures. The globular shape of the protein plays a
key role in the action of the enzyme. Due to the specific globular shape the
enzyme will have an active site, which will accommodate only one type of
substrate and so carry out a very specific function.
Digestive enzymes all help to catalyse hydrolysis reactions. A typical example is
the digestion of starch by amylase into glucose.
The role of amylase is to temporarily hold the starch in its active site and put
stress on the covalent bonds that bind the glucose molecules together within
the polysaccharide. When these bonds are stressed, it is more likely that the
surrounding thermal energy (body temperature) will provide enough molecular
motion to break the bonds.
Without enzymes it would take too long to try and totally digest our food
mechanically (e.g. by chewing in the mouth and mixing/ churning in the stomach
and small intestine).
Chewing and mixing will not break down food into small enough particles.
Enzymes are need to break it down to small, simple, soluble molecules.
Naming
The modern system of naming enzymes involves identifying the substance the
enzyme acts upon, called its substrate and adds the suffix –ase. Thus lipase
digests lipids and protease digests protein.
Enzyme Examples
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6.1.3 State the source, substrate, products and optimum pH conditions for one
amylase, one protease and one lipase
Orange book  pg. 204
Green book  pg. 95
To do:
 Read and the notes below, read the relevant sections in the green book.
Use your green book to fill in the table below
 Copy the table into Green exercise book
Answer the IB practice questions in your green exercise books
Examples of digestive enzymes
There are many enzymes that help us to digest food. Some of these are the
individual enzymes that are specific to many of the types of carbohydrates we
ingest. We also produce many different protease enzymes that collectively help
us to digest proteins. Some of these protease enzymes work within a protein by
recognizing specific amino acid pairs, and some digest proteins from the outer
ends and work ‘inwards’.
Use your book to fill in the following table:
Enzyme
Source
Substrate
Products
Optimum pH
IB Practice Questions
1. Describe the role of enzymes in the process of digestion of proteins,
carbohydrates and lipids in humans. (6)
2. Which organ secretes enzymes that are active at a low pH? (1)
A.
Mouth
B.
Pancreas
C.
Stomach
D.
Liver
3. State the sources, substrate, product, and optimum pH conditions for the
enzyme amylase. (4)
4. Which of the following is correct regarding the enzymes listed in the
table? (1)
Enzyme
Amylase
Lipase
Protease
A. Substra polysaccharide
te
emulsified fat
dipeptide or
polypeptide
B. Substra emulsified fat
dipeptide or
polysaccharide
te
C. Product amino acids
polypeptide
small
polysaccharides or
monosaccharides
D. Product small
amino acids
polysaccharides or
monosaccharides
fatty acids and
glycerol
fatty acids and
glycerol
5. Describe the role of enzymes in digestion with reference to two named
examples. (5)
6. State the source, substrate, products and optimum pH for any two
named digestive enzymes.
Digestive System
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6.1.4 Draw and label a diagram of the digestive system
Orange book  pg. 205
Green book  pg. 96
To do:
Study the diagrams of the digestive system
Visit each of the three websites
 Without using a book or your notes, on a piece of scrap paper draw a labeled
diagram of the human digestive system.
 In your green exercise books draw a labeled diagram of the human digestive
system
Much of the human digestive system is a tube called the alimentary canal. In
order the alimentary canal consists of:
Mouth
Oesophagus
Stomach
Small intestine
Large intestine (colon)
Rectum
Any food that you ingest must either be digested and absorbed for use by the
body or remain undigested and eliminated as solid waste (faeces).
Study each of the diagrams of the digestive system:
Websites
 Animation: Organs of Digestion
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter26/animation__organs_of_
digestion.html
Label the Digestive System
http://www.mhhe.com/biosci/genbio/maderbiology7/graphics/mader07b/mader
_labeling/mader_labeling_source/mi12-01b.dcr
http://www.mhhe.com/biosci/genbio/maderbiology7/graphics/mader07b/mader
_labeling/mader_labeling_source/mi12-01c.dcr
Functions of Digestive System
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6.1.5 Outline the function of the stomach, small intestine and large intestine
Orange book  pg. 206
Green book  pg. 96
To do:
Visit each of the website
 Read and highlight the notes below, read the relevant sections in the green
book.
 Complete the table at the end of the document




Summary in Green book to include:
movement of food in oesophagus
composition and function of gastric juice
churning and chyme





pancreatic secretion
bile
increased surface area in small intestine
absorption of water in large intestine
mutualistic bacteria
 Animation: Digestive System
http://www.bioanim.com/CellTissueHumanBody6/index.html
Stomach
Food is brought to your stomach by a muscular tube called the oesophagus.
When you swallow, the food is forced down to your stomach by a sequential
series of muscular contractions called peristalsis.
The stomach is an expandable bag where the food is stored for up to a few
hours in order to mix it with a variety of secretions collectively known as
gastric juice. The stomach wall has no villi, but numerous gastric pits, which
secrete the gastric juice. Gastric juice contains:
hydrochloric acid (pH 1) to kill bacteria (the acid does not help digestion, in fact
it hinders it by denaturing most enzymes); however, it creates the pH necessary
for pepsin to be active.
mucus to lubricate the food and to line the epithelium to protect it from the
acid;
pepsin a protease enzyme digest proteins.
It has a muscular wall which contracts and relaxes to churn the food into a
liquid called chyme. This is gradually released in to the small intestine by a
sphincter, a region of thick circular muscle that acts as a valve.
Small Intestine
This is about 6.5 m long, and can be divided into two sections:
The duodenum
Although this is short, almost all the digestion takes place here, due to two
secretions from different accessory glands.
Pancreatic juice is secreted by the pancreas through the pancreatic duct. This
contains numerous carbohydrases (including maylase), protease (trypsin) and
lipase enzymes and bicarbonate.
Bile, secreted by the liver, stored in the gall bladder, and released through the
bile duct into the duodenum. Bile contains bile salts to aid lipid digestion, and
the alkali sodium hydrogen carbonate to neutralise the stomach acid. Without
this, the pancreatic enzymes would not work. The bile duct and the pancreatic
duct join just before they enter the duodenum.
The ileum (4 m long).
This is the site of final digestion and all absorption. There are numerous glands
in the ileum lining which secrete enzymes. The digestion produces molecules,
which are small enough to be absorbed into the blood.
The internal surface area is increased enormously by three levels of folding:
large folds of the lining, villi, and microvilli. Don't confuse these: villi are large
structures composed of many cells that can clearly be seen with a light
microscope, while microvilli are small subcellular structures formed by the
folding of the plasma membrane of individual cells. Microvilli can only be seen
clearly with an electron microscope, and appear as a fuzzy brush border under
the light microscope.
Circular and longitudinal muscles propel the liquid food by peristalsis, and mix
the contents by pendular movements - bi-directional peristalsis. This also
improves absorption.
Large Intestine
The vast majority of useful nutrients are absorbed while food is still inside the
small intestine. What remains of the original food at the end of the small
intestine in undigested (and therefore unabsorbed). The primary function of the
large intestine is to absorb water. The lining contains villi but no microvilli, and
there are numerous glands secreting mucus. Food can spend 36 hours in the
large intestine, while water is absorbed to form semisolid faeces.
The large intestine is also home to a very large number of naturally occurring
bacteria including Escherichia coli. 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 intestines.
Faeces is made up of plant fibre (cellulose mainly), cholesterol, bile, mucus,
mucosa cells (250g of cells are lost each day), bacteria and water, and is
released by the anal sphincter.
Look at the diagram below and fill in the table
Keep functions very brief to act as an overview.
Name
A
B
C
D
E
F
Function
G
Absorption and Assimilation
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6.1.6 Distinguish between absorption and assimilation
Orange book  pg. 207
Green book  pg. 96
To do:
Do as it says in the objective 
 Write a couple of sentences in your green exercise book explaining the
difference between absorption and assimilation
Villi
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6.1.7 Explain how the structure of the villus is related to its role in absorption
and transport of the products of digestion.
Orange book  pg. 208
Green book  pg. 97
To do:
 Visit the website
 Read and highlight the notes below, read the relevant sections in the green
book.




Draw and label a diagram of a villus in vertical section.
Summary in Green book to include:
function of villi
microvilli

brush border enzymes
 Label Villi Structure
http://www.mhhe.com/biosci/genbio/maderbiology7/graphics/mader07b/mader
_labeling/mader_labeling_source/mi12-06.dcr
If the lining of the small intestine is examined closely it appears fuzzy. This is
due to finger like projections called villi, about 0.5 to 1.0mm high. Villi are very
thin to allow for absorption of the soluble products of digestion. If the inner
lining of your small intestine were smooth, you would have a fairly limited
membrane surface 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.
The core of a villus contains an arteriole, a capillary network, a venule and a
lymphatic capillary called a lacteal. Most nutrients are absorbed by the blood
capillaries, but most fat is absorbed by the lacteal and gives its contents the
milky appearance for which the lacteal is named. The core of the villus also has
a few fibres of smooth muscle that contract periodically. This enhances mixing
of the digested food in the intestine.
Each absorptive cell of a villus has a fuzzy brush border of microvilli about 1m
high. The brush border increases the absorptive surface area of the small
intestine and contains brush border enzymes, integral proteins of the plasma
membrane. The enzymes carry out the final stages of digestion. They are not
releases into the lumen; instead, the chime must contact the brush border for
digestion to occur. This process, called contact digestion, is one reason that
thorough mixing of the chime is so important.