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2.1 – Enzymes &
Digestion
By Piril Erel
1
Major Parts of The Digestive System
- Part I
The digestive system has many
organs and associated glands
ensuring complete digestion of
food.
Glands produce enzymes that
breakdown large molecules into
small ones ready for absorption.
The digestive system therefore
provides an interface with the
environment because food
substances enter the body through
it.
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Major Parts of The Digestive System
- Part II
OESOPHAGUS:
Carries food from the mouth to the stomach.
Adapted for transport
Thick muscular wall
STOMACH:
Muscular sac with an inner layer producing enzymes.
Storage and digestion of food (especially proteins)
Some glands that produce enzymes that digest protein
Some glands that produce mucus in order to product the stomach wall
Mucus prevents the stomach being digested by its own enzymes
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Major Parts of The Digestive System
- Part III
SMALL INTESTINE:
Long muscular tube
Further digestion via enzymes
produced by its own walls and by
accompanying glands (liver and
pancreas)
Inner walls of the small intestine
folded into villi (large surface area
to volume ratio)
Further increase in surface area as
villi microvilli on the epithelial
cells of each villus.
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Major Parts of The Digestive System
- Part IV
LARGE INTESTINE:
Absorbs water which are derived from the secretions of many digestive glands
Food within the large intestine becomes drier and thicker in consistency and forms faeces
RECTUM:
Final section of the intestines, Faecal storage before being removed via the anus in a process called egestion
Note:
EGESTION is when your body removes undigested food by the process of defecation.
EXCRETION is when your body removes metabolic waste which has entered your cells. 5
Major Parts of The Digestive System
- Part V
Associated glands which support the major organs further aiding digestion.
These include:
SALIVARY GLANDS:
Near the mouth
Pass their secretions via a duct into the mouth
Secretions contain amylase
PANCREAS:
Large gland below the stomach
Secretes pancreatic juice which contains:
Proteases, Lipase and amylase
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Enzyme Digests:
Proteases Proteins
Lipase Lipids
Amylase Starch maltose
What is Digestion?
- Part I
Digestion is the breakdown of food via two processes:
1. Physical breakdown
2. Chemical breakdown
PHYSICAL BREAKDOWN
If the food is large, broken down into smaller pieces via teeth
This makes food possible to ingest and provides a larger surface area for chemical digestion
Physical breakdown also occurs in the stomach wall where it is churned by the muscles.
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What is Digestion?
- Part II
CHEMICAL DIGESTION
Breaks down large, insoluble molecules into smaller soluble ones via
enzymes.
Enzymes digest molecules via hydrolysis (splitting up of molecules with
the addition of water) These enzymes are called hydrolases
Enzymes are specific; numerous enzymes and different types of enzymes
may be needed to break down a polysaccharide. These include:
CARBOHYDRASE – break down of carbohydrates monosaccharides
LIPASES – break down lipids (fats and oils) into glycerol and fatty acids
PROTEASES – break down of proteins, ultimately to amino acids
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What is Digestion?
- Part III
Once large food molecules have been hydrolyzed into
monosaccharides, glycerol, fatty acids and amino acids, they are
absorbed by various means from the small intestine into the blood
Transported to different parts of the body and are used in various
processes where the monosaccharides are required.
These molecules are incorporated into body tissues or used in
processes within the body.
The incorporation of absorbed molecules into body tissue is called assimilation
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Roles:
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Chapter Questions:
1. State one way in which the stomach is adapted:
a. To churn food
b. To prevent the enzymes it produces from digesting the surface of the stomach
2. What is hydrolysis?
3. Which two structures produce amylase?
4. Suggest a reason why the stomach does not have villi or microvilli
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By having a muscular wall
By secreting mucus
The breakdown of molecules by the addition of water to the bonds that hold
these molecules together
Salivary glands and pancreas
Villi and microvilli increase surface area to speed up the absorption of soluble
molecules. As the food in the stomach has not yet been broken down into soluble
molecules they cannot be absorbed and so villi and microvilli and unnecessary
For extra brownie points: The stomach is adapted to have a low pH (acidic)
compared to the small intestine where villi and microvilli are found where the
pH is much more neutral. Therefore, if villi and microvilli were to be present in
the stomach it would not withstand the acidic pH.
2.4 – Carbohydrate
Digestion
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Starch Digestion
- Part I
Numerous enzymes are required to breakdown molecules into smaller sections
and further into their monomers
Enzymes will be specifically adapted to the organs and the pH within this
organ
It is therefore obviously very important that enzymes are produced in the correct
sequence
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Starch Digestion
- Part II
The digestion of starch begins in the mouth, food is chewed; this breaks it
into smaller pieces giving it a large surface area
SALIVARY GLANDS in the mouth will secrete saliva which contains:
Amylase – digestion of starch to the (disaccharide) maltose
Mineral salts – helps to maintain the pH at around neutral in order to work at the optimum
pH of 7.5
Food is then swallowed becomes a form of bolus and enters the stomach; here the
conditions are acidic (pH 1-3)
The acidic HCl secreted in the stomach denatures the amylase and prevents further
hydrolysis of the starch (bolus mixed with HCl = Chyme)
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Starch Digestion
- Part III
Chyme is passed into the small intestine. The pancreas secretes pancreatic
juice containing:
Pancreatic amylase – continuation of hydrolysis of starch maltose
Alkaline salts –maintain the pH at around neutral so that amylase can function
After secretions from the pancreas, the muscular epithelial intestinal wall in
the small intestine secrete:
Maltase – enzyme hydrolyses the maltose from starch α-glucose
Alkaline salts –maintain the pH at around neutral so that amylase can function
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Disaccharide Digestion
- Part I
In addition to the digestion of starch and maltose there are two other common disaccharides in mammalian diets that need to be broken down:
SUCROSE – Table sugar (natural sweetener)
Sucrose is found within cells and these must be physically broken down by the teeth in order to release it
Sucrose passes through the stomach and into the small intestine, whose epithelial lining produces the enzyme sucrase
Sucrase – hydrolyses the single glycosidic bond in the sucrose molecule to produce the two monosaccharides it is made up from (glucose and fructose)
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Disaccharide Digestion
- Part II
LACTOSE – found in dairy products
Lactose is digested in the small intestine where the epithelial lining produces the
enzyme lactase.
Lactase – hydrolyses the glycosidic bond and produces the two monomers in which
it is made up from – glucose and galactose
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DISORDERS: Lactose Intolerance
- Part I
We saw above that lactose is the sugar found in milk and that it is hydrolysed by the enzyme lactase
Babies have a general diet of milk and as small babies our body naturally produces a lot of lactase in order to digest the high consumption of milk(lactose)
As we grow older the consumption of milk becomes minimal and therefore our body will adapt to this and decrease the production and amount of lactase.
However in some people this decrease becomes so profound that they end up producing little or no lactase! Therefore Lactose Intolerance
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DISORDERS: Lactose Intolerance
- Part II
Modern mass consumption of diary products makes it impossible for us to not
have lactose containing foods.
As a result some people do not produce sufficient lactase to digest all the
lactose that they consume
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DISORDERS: Lactose Intolerance
- Part III
Symptoms of Lactose intolerance include:
Flatulence (Wind)
Biological Mode of Action:
When undigested lactose reaches the large intestine, microorganisms break it down, giving rise to small soluble molecules and a large volume of gas
Diarrhoea
Biological Mode of Action:
In Normal Person: As waste material enters the large intestine it contains a high amount of water, the endothelial lining absorbs the water from the large intestine leaving only faecal matter. Water via osmosis from an area of high water potential (Ψ) to an area of low water potential. Therefore water is moving from the large intestine (high Ψ) to the endothelial lining (low Ψ)
In an Lactose Intolerant Person: Waste material entering the large intestine also contains small molecules from the digestion of lactose via microorganisms. The large intestine now has a lowered Ψ and therefore water enters the large intestine from the endothelial lining. The faecal matter now becomes watery and results in diarrhoea.
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DISORDERS: Lactose Intolerance
- Part IV
People who are lactose intolerance therefore cannot consume milk at all, while others can drink only a little
Lactose intolerance is not life-threatening in adults and can be managed by avoiding foods containing lactose
The main difficulty is taking in sufficient calcium in the absence of milk
Consumption of foods rich in calcium (seafood, legumes) can resolve this issue
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Summary of All Reactions:
Digestion of Starch:
In salivary glands, pancreas and small intestine:
Starch amylase maltose
In small intestine:
Maltose maltase α-glucose
Disaccharide digestion:
In small intestine:
Sucrose sucrase glucose + fructose
In small intestine:
Lactose lactase glucose + fructose
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Key:
Pink Enzymes in
the reaction
Chapter Questions:
1. What is the final product of starch digestion in the gut?
2. Name three enzymes produced by the epithelium of the small intestine.
3. In lactose-intolerant people, microorganisms in the large intestine convert
undigested lactose into gas, which accumulates and causes discomfort. By
which process do microorganisms probably produce this gas?
4. Suggest a reason why the gas is unlikely to be carbon dioxide.
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α-glucose
Maltase, sucrase, lactase
Anaerobic respiration
Carbon dioxide is formed as a result of aerobic respiration. The conditions in the
large intestine are anaerobic and no oxygen is present. Flatulence contains 90%
nitrogen
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