Digestion is the process by which food is broken down into smaller molecules that can be absorbed into the bloodstream and used by cells to provide energy and building materials. Various organs work together to mechanically and chemically break down food, including the mouth, stomach, and small intestine. Enzymes play a key role in digestion by speeding up chemical reactions to break nutrients like carbohydrates, proteins, and fats into simpler substances like glucose, amino acids, and fatty acids. The small intestine contains villi and microvilli that increase its surface area for absorption of digested nutrients into the bloodstream via diffusion and active transport.

1. Digestion

2. Learning Objectives
At the end of this lesson, you should be able to:
• explain what is meant by digestion
• explain why food must be digested
• define enzymes as biological catalysts that speed up the
rate of reactions without being chemically changed at the
end of the reaction
• explain the effects of temperature on enzyme activity
• explain the mode of enzyme action in terms of the lock-
and-key hypothesis

3. Why EAT?
We need to eat because…
Our body needs NUTRIENTS
CARBOHYDRATES PROTEINS FATS
Types of Nutrients the Body Needs

4. Why EAT?
CARBOHYDRATES PROTEINS FATS
Types of Nutrients the Body Needs

5. Why EAT?
CARBOHYDRATES PROTEINS FATS
Types of Nutrients the Body Needs

6. Why DIGESTION?

7. Why DIGESTION?
Digestion is necessary
- to break down large insoluble molecules into smaller
molecules
- so that the body can absorb the nutrients required for its
functioning
DIGESTION is the process whereby
large, insoluble food molecules are broken
down into smaller, soluble food molecules

8. Large Complex Insoluble Molecules DIGESTION
Small Simple Soluble Molecules
Starch Glucose
carbohydrase
Proteins protease Amino Acids
lipase
Fats Glycerol Fatty
Acids

9. DIGESTION
These small simple soluble
Glucose molecules are used for the
following purposes:
• Growth of new cells and tissues
• Provide energy for
Amino Acids
growth, movement and for
keeping warm
• Repair damaged tissues
• Keep us healthy
Glycerol Fatty
Acids

10. Food Tests
• To test whether certain nutrients are present
in a certain food, specific tests can be
conducted.
• Iodine solution can be used to test if there is
starch present in food.
• Benedict’s solution can be used to test if there
is reducing sugar (eg. Glucose) present in
food.

11. FOOD TESTS
• STARCH test
• Add about 5 drops of
iodine solution to the
sample to be tested.
• If starch is present, the
iodine will turn dark blue
• If starch is absent, the
iodine will remain brown

12. FOOD TESTS
• REDUCING SUGAR test
• Add 2cm3 of Benedict’s
Solution to 2cm3 of the sample
solution in a test-tube.
• Heat the test-tube in a
boiling water bath for 2-5
minutes.
• If reducing sugar is
present, the solution produces
a brick-red precipitate.
• If reducing sugar is
absent, the solution remains
blue.

13. ENZYMES
Enzymes are biological catalysts.
A A
Catalysts are substances that
speed up the rate of reactions, but
themselves remain chemically
enzyme unchanged at the end of the
reaction.
A A
A A
enzyme enzyme

14. Lock & Key Hypothesis
Substrate A
Enzyme Enzyme
Substrate B
Enzyme Enzyme

15. ENZYMES- Mechanism
substrate
LOCK and KEY hypothesis
• Specificity: One enzyme binds to
Enzyme only one substrate
• Reusable: They are chemically
unchanged at the end of the
reaction
products • Required in minute amounts
• Sensitive to temperature and pH
Enzyme

16. ENZYMES- Mechanism
substrate
Enzyme-substrate
Enzyme complex
products
Enzyme

17. ENZYMES- Mechanism
substrate • In an enzyme reaction, the reactants
are called substrates.
• Every enzyme has an active
site, which is the place where the
substrate will bind with the enzyme.
Enzyme • This active site has a specific 3-
dimensional shape.
• The active site only allows
substrates with the complementary
products shape to fit in, just like how a key fits
into the lock.
• Enzyme is the LOCK and substrate
is the KEY.
Enzyme

18. ENZYMES- Mechanism
substrate • The enzyme and substrate
molecules will combine to form a
temporary structure called the
enzyme-substrate complex.
• When substrate binds to the
Enzyme enzyme, the enzyme will then react
to convert the substrate into
product(s).
• The product(s) then separates from
products the enzyme leaving the enzyme free
to combine again with more
substrates.
Enzyme

19. ENZYMES- Mechanism
substrate • Product formed no longer fit into the
active site and are released.
• The active site is now free to receive
more substrate molecules.
• In conclusion, enzyme action is
Enzyme specific due to the shape of the
substrate and active site being
complementary to each other.
products
Enzyme

20. Lock & Key Hypothesis

21. ENZYMES- Factors
Enzymes work best at
their optimum
temperature.
Rate of Enzyme Activity
At low At high
temperatures, en temperatures, en
zymes are zymes are
inactivated. denatured.
Temperature/ °C

22. ENZYMES- Factors
Enzymes work best at
their optimum pH.
Rate of Enzyme Activity
At extreme
pH, enzymes are
denatured.
pH

23. Learning Objectives
At the end of this lesson, you should be able to:
• explain what is meant by digestion
• explain why food must be digested
• define enzymes as biological catalysts that speed up the
rate of reactions without being chemically changed at the
end of the reaction
• explain the effects of temperature on enzyme activity
• explain the mode of enzyme action in terms of the lock-
and-key hypothesis

24. LEARNING OBJECTIVES
You should now know how to:
- identify the main organs in the
human digestive system
- describe the process of
digestion within the human
body in detail

25. DIGESTIVE SYSTEM
Associated
(accessory)
organs

26. The PROCESS
Mechanical Digestion
Biting
 Tearing
 Slicing
 Chewing (Masticating)
Chemical Digestion
 Acid
 Enzymes

27. Mouth
1. Mechanical Digestion
 Food is broken down into
smaller pieces by the
chewing, tearing, slicing
and biting action of the
teeth
2. Enzymatic Digestion
 Starch is broken down into
maltose by amylase in
saliva
 No digestion of proteins
and fats in the mouth
Starch  Maltose

28. Oesophagus
Partially digested food is
pushed down from the
mouth into the stomach by
muscles in the oesophagus
that alternately contract
and relax. (peristalsis)
NO enzymes are present in
the oesophagus.

29. Stomach
Complex Proteins  Simpler
Proteins
Complex proteins are broken
down into simpler proteins
by proteases found in
gastric juices secreted by
the stomach.
No digestion of
carbohydrates or fats
occur in the stomach.

30. Small Intestines
Simpler Proteins  Amino Acids
Maltose Glucose
Fats  Glycerol + Fatty Acids
Three digestive juices are secreted into
the small intestines to complete
digestion:
1. Bile from the Liver
 This breaks fats into small droplets
for easier digestion
2. Pancreatic Juice from the Pancreas
 This contains
carbohydrase, protease and lipase for
digestion
3. Intestinal Juice from the Small
Intestines
 This also contains
carbohydrase, protease and lipase.

31. Large Intestines
Undigested food that reaches the
large intestines will be passed
to the rectum and anus for
removal by the body as faeces.
As it passes through, a lot of
water is absorbed by the large
intestines and taken back into
the body.

32. INQUIRY time!
Some obese people have had
part of their small intestines
removed.
Suggest with reasons the effects
of such a surgery?
Proteins are digested in the stomach.
The stomach wall consists of cells
which are made of protein. So why
does the stomach wall not get
digested?

33. LEARNING OBJECTIVES
You should now know how to:
- identify the main organs in the
human digestive system
- explain the process of
digestion within the human
body

34. RECALL
You have learnt about
• the meaning of digestion
• why food must be digested
• enzymes as biological catalysts that speed up the rate of
reactions without being chemically changed at the end of
the reaction
• the process of digestion and where specific nutrients
are digested in the body

35. Learning Objectives
At the end of this lesson, you should be able to:
• explain how digested food is absorbed into the
bloodstream

36. ABSORPTION
Small Intestine
Villi
cells
Blood Capillary

37. ABSORPTION
Absorption is the movement of digested food
molecules through the walls of the small intestine.
By what process do
the food molecules
Small Intestine
move through the
intestinal walls?
Why does food
molecules move into
Blood Capillary the blood stream but
not move out?

38. ABSORPTION
Absorption is the movement of digested food
molecules through the walls of the small intestine.
Why are there so many
finger-like projections
in the cells of the small
Small Intestine intestine?
Blood Capillary

39. ABSORPTION
Absorption is the movement of digested food molecules through
the walls of the small intestine.
Digested food molecules move through the walls of the small
intestine via diffusion and active transport.
Undigested food molecules are too large to move through the
walls of the small intestine.
Small Intestine Finger-like projections in the
wall of the small intestine
provide a large surface-area for
absorption of digested food
molecules.
Blood Capillary

40. ABSORPTION
1. Finger-like projections in the wall of
the small intestine provide a large
How is the rate surface-area for absorption of digested
of absorption food molecules.
made faster in
the small
intestine?
Small Intestine
2. The small intestine is long
and convoluted, thus increasing
the surface area for quicker
absorption.
Blood Capillary

41. ABSORPTION
3. The wall of the small intestine
How is the rate is only one-cell thick, shortening
of absorption the distance between the wall
made faster in and blood vessels so absorption
the small can be faster.
intestine?
4. A constant flow of blood
ensures a low concentration of
Small Intestine food molecules in the capillary,
so rate of diffusion of digested
food molecules into the blood is
increased.
Blood Capillary

42. Learning Objectives
At the end of this lesson, you should be able to:
• explain how digested food is absorbed into the
bloodstream

43. RECALL
You have learnt about
• the meaning of digestion
• why food must be digested
• enzymes as biological catalysts that speed up the rate of
reactions without being chemically changed at the end of
the reaction
• the process of digestion and where specific nutrients
are digested in the body
• how absorption occurs efficiently in the body