Slides specifically for AQA syallbus during A-Levels, this is for unit 1 - biology of disease - chapter 2 (specifically 2.1 and 2.4) I believe these chapters go hand in hand, I have made these for my students and they have found them very useful. Slides aimed for teachers, but can be used as revision slides for students also. More than welcome to download, good luck with exams!

1. 2.1 – Enzymes &
Digestion
By Piril Erel
1

2. 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.
2

3. 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
3

4. 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.
4

5. 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

6. 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

7. 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.
7

8. 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
8

9. 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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10. Roles:
10

11. 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.

12. 2.4 – Carbohydrate
Digestion
12

13. 13

14. 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
14

15. 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)
15

16. 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
16

17. 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)
17

18. 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
18

19. 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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20. 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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21. 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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22. 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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23. 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

24. 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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