2.1 – Enzymes &

Digestion

By Piril Erel

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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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