Digestion is the breakdown of large insoluble food molecules into smaller soluble molecules that can be absorbed and used by the body. There are two stages of digestion: physical/mechanical digestion which breaks food into smaller pieces using teeth and peristalsis, and chemical digestion where enzymes break down food further. Digestion begins in the mouth and stomach and is completed in the small intestine, where enzymes from the pancreas and liver break proteins, fats and carbohydrates into amino acids, fatty acids, and sugars. The digested nutrients are then absorbed through the walls of the small intestine and transported to cells to be used for energy, building molecules, or stored. Undigested matter then moves to the large intestine where water

1. DIGESTION

2. Digestion is the process by
which
 insoluble food, consisting of large molecules is
broken down into soluble compounds

3. What are starch, proteins & fats
broken down into?
enzyme
STARCH
Glucose
enzyme
PROTEIN
Amino acids
enzyme
FAT
Fatty acids &
glycerol

4. Why must food be digested?
1
Food in
intestine
2
Blood
Food in
intestine
enzyme
Blood
Wall of
intestine
3 Food in
intestine
Blood
To be small enough
to be absorbed
through the wall of
intestine.

5. Digestion
starts at the
mouth and
ends at the
anus
……………….

6. Food is broken down by two actions:
1. PHYSICAL
/MECHANICAL
2. CHEMICAL

7. 1. PHYSICAL DIGESTION by:
 teeth
 peristalsis of the alimentary
canal
Chewing (mastication)

8. Why is it important to break down
food physically?
To increase the surface area for enzyme action.

9. Question: SEP, 2009
Give TWO reasons why food must be masticated
before swallowed. (4)
1. Increased surface area for enzymes to act
quickly.
2. Easier to manipulate by tongue to form a
bolus.

10. 2. CHEMICAL DIGESTION by:
 enzymes
Digestive enzymes are the chemicals that break
large insoluble food molecules into smaller soluble
molecules.

11. Four stages in feeding & digestion:
2. Digestion
3. Absorption into
the bloodstream
4. Egestion
1. Ingestion

12. Digested food is absorbed & then
assimilated by the cells
Absorption
Assimilation
Digestive
system
Body
cell
Circulatory
system

13. Digestive tract is also
called:
 Alimentary canal / tract
 Gut

14. Mucus is produced
throughout the gut:
 For lubrication
 To protect gut wall
from digestive enzymes

15. Fig. 1
The human
digestive
system.

16. The 1st stage of nutrition:
1. Ingestion
2. Digestion
3. Absorption
4. Egestion

17. DIGESTION
IN THE
MOUTH

18.  salivary glands produce SALIVA
(pH = 7-8)

19. Salivation is a reflex action which
is triggered by:
 Sight
 Smell
 Thinking
of food

20. Saliva contains:
 mucus
 water
 sodium hydrogen
carbonate
 salivary amylase
(ptyalin [old term])

21. Functions of mucus in saliva:
1. lubricates the food
2. sticks the food particles together
bolus

22. Salivary
amylase

23. In swallowing, food is directed into
the gullet / oesophagus

24. A person cannot breathe while
swallowing. Why?
1. The soft palate closes
the nasal cavity .
2. The epiglottis closes the
trachea.

25. Swallowing

26. Peristalsis:
 wave-like motions of the alimentary
canal which move the food forwards
Digestive tract
Bolus
1
Wave of
relaxation
Bolus
moves
2
2
Wave of
contraction

27. Peristalsis:

28. Peristalsis is possible as gut wall is
muscular
 contraction of circular & longitudinal
muscles alternate to push food

29. Food is pushed when:
Circular muscle
above bolus
CONTRACTS
What does the
longitudinal muscle
do at the same time?
Relaxes

30. Food reaches the
stomach

31. The stomach stores
food for some time &
releases it slowly
 Food stays for 2-6 hours in the
stomach
Chyme:
creamy liquid leaving
the stomach

32. Food remains inside stomach due to
contraction of two sphincters
Oesophagus
Cardiac sphincter
Pyloric sphincter
Lining of
mucus
Duodenum
Churning due to:
Layers of muscle

33. Which sphincter is contracted?
Cardiac sphincter:
Pyloric sphincter:
relaxes to let chyme
into the duodenum
contracted

34. Gastric glands
secrete
gastric juice

35. What happens to food inside the
stomach?

36. Gastric juice contains:
1. HYDROCHLORIC ACID
2. PEPSIN
3. RENNIN
What is the pH in
the stomach?
1-2

37. Functions of hydrochloric acid:
1. creates the optimum pH for pepsin
HCl
Pepsinogen
2.
(inactive form)
Pepsin
(active enzyme)
3. kills bacteria taken in
with the food

38. Excessive production of gastric
juice: burning sensation

39. Gastric juice enters oesophagus:
not enough mucus to protect wall

40. Remedy for burning sensation in
throat and chest:
1. Drink milk
2. Take antacids

41. Doctors can see the
stomach lining by using
an:
oesophagus
endoscope

42. Stomach ulcers due to:
little mucus in stomach

43. Pepsin is a protease:
 breaks:
Proteins
Polypeptides

44. Which conditions are needed for a
piece of meat to be digested?
 Pepsin in acidic conditions

45. Question: MAY,2009
The following diagram represents the protein found
in egg white. Each circle represents an amino acid.
Complete the diagram below to show how pepsin
breaks down the protein found in egg white.
[Idea is that short chains representing polypeptides are
made.]
(Do NOT accept individual amino acids represented as single
circles or dipeptides represented as a pair of circles)

46. Question: MAY, 2012
List TWO reasons why amylase does not act on
starch in the stomach. (2)
i. pH in stomach is not optimum for amylase
ii. there is no enzyme in the stomach to break
starch

47. Some substances that are absorbed
by the stomach:





ethanol
vitamins
certain salts and drugs
some water
glucose
How did I
get drunk so
quickly?

48. Rennin
 is an enzyme found in the stomach of young
children
 Function:
Pepsin then acts
on the insoluble
protein
Soluble protein
in milk
Changes into
insoluble protein

49. In cheese-making, rennet [contains
rennin] is added to milk

50. THE INTESTINES:
Small intestine
7 m long
Large intestine
1.5 m long

51. The small
intestine is
composed of:
duodenum &
ileum

52. Two functions of the small
intestine:
1. digestion is completed
2. digested food is absorbed
Bloodstream
Products of
digestion

53. Duodenum
 U-shaped
 first part of small intestine (most digestion
occurs here)
 about 25 cm long

54. The duodenum receives secretions
from the:
Liver
[bile via the
bile duct]
Pancreas
[pancreatic juice via
the pancreatic duct]

55. DIGESTION IN THE
DUODENUM

56. Where is bile:
i) made? ii)stored?

57. Made in
liver
Stored
in gall
bladder

58. Bile:
 has no enzymes
 consists of:
 water
 sodium chloride
 bile salts
 bile pigments
 pH = 8

59. Functions of Bile:
 dilutes contents from stomach
 creates optimum pH for pancreatic enzymes
to work in duodenum
 contains bile salts which emulsify fats
Bile salts

60. Bile salts emulsify fats i.e.:
 break large fat globules into smaller globules
What is the benefit of emulsification?
The surface area where lipase can act is increased.
Emulsification
of fats

61. A person had his gall bladder removed.
Explain why the doctor told him to limit
fats in the diet.
Fats cannot be emulsified;
difficult to digest fats by lipase.

62. Three enzymes in
pancreatic juice:
Pancreas
Pancreatic amylase
Trypsin
Lipase
Action of
each
enzyme:

63. amylase
maltose
starch molecule
trypsin
proteins &
polypeptides
peptides
lipase
fat molecule
+
glycerol
fatty
acids

64. Question: MAY, 2012
Bile and pancreatic juice are involved in
digestion of fats.
a) Describe the role of both secretions in this
process. (4)
b) Explain why bile must be released before
pancreatic juice for the efficient digestion of
fats. (2)

65. DIGESTION
IN THE
ILEUM

66. The wall of the ileum has
intestinal glands
that produce
intestinal juice

67. Enzymes in intestinal juice
SUBSTRATE
ENZYME
END PRODUCT
Peptides
Lipids
Peptidase
Lipase
Maltose
Sucrose
Lactose
Maltase
Sucrase
Lactase
Amino acids
Fatty acids &
glycerol
Glucose + glucose
Glucose + fructose
Glucose + galactose

68. Remember:
Proteins
Pepsin
Polypeptides
Trypsin
Peptides
Peptidase
Amino acids

69. Remember: Digestion of starch
Starch
Salivary Amylase
(In Mouth)
Maltose
No digestion of starch in
stomach (amylase is
denatured)
Maltose
(in ileum)
Maltase
Glucose
Pancreatic
Amylase

70. Digestion is complete in the
ileum. What happens to the
digested food?

71. Model of the ileum showing numerous villi
Villus: a finger-like projection

72. Villi contain blood & lymph vessels

73. Structure of a villus

74. A villus is adapted for absorption

75. The ileum is adapted to absorb
digested food
 by providing a LARGE surface area:
 Folded walls
 Has villi
 Has microvilli

76. Ileum is adapted for absorption:
1. Ileum is long:
 more time for digestion
 a greater surface area for absorption
2. Villi:
 increase surface area for absorption
3. Microvilli in epithelium:
 increase surface area for absorption
4. Epithelium is very thin:
 soluble products of digestion pass through quickly
5. Dense capillary network:
 removes the digested food

77. THE
LARGE INTESTINE

78. The large intestine is composed of:
4. Colon
1.Caecum
2. Appendix
3. Rectum
Anus

79. Material in the large intestine
consists of:




water
mucus
dead cells
undigested matter
Most of the undigested matter is
cellulose. Why is this so?

81. Function of the:
Colon:
absorbs water
Rectum:
stores FAECES before being
expelled through the ANUS

82. The 4th stage of feeding &
digestion:
4. Egesting

83. I really have
to go to the
bathroom!
Egestion / Defaecation
 the process of removing
faeces

84. Caecum & Appendix are
 Vestigial in humans:
[small & no function]
Caecum
Appendix
 Large in herbivores

85. Question: SEP, 2011
Explain why the caecum /
appendix in humans is small
and non-functional. (4)
Human
Rabbit
Humans are omnivores.
Do not depend only upon vegetation for
nutrients.
A large caecum is found in herbivores and is full
of bacteria that produce cellulase to digest
cellulose.

86. Ingestion
(2 L)
Salivary gland secretions
(1 L)
Gastric secretions
(2 L)
Bile
(0.7 L)
Small intestine
secretions
(2 L)
1% in
faeces
Pancreatic secretions
(1.2 L)
Although 8.9 L of
water are ingested
or secreted daily,
the faeces are dry.
Explain.

87. Ingestion
(2 L)
Salivary gland secretions
(1 L)
Gastric secretions
(2 L)
Bile
(0.7 L)
Small intestine
secretions
(2 L)
1% in
faeces
Pancreatic secretions
(1.2 L)
Absorbed in the
small intestine &
colon.

88. ASSIMILATION OF DIGESTED
FOOD
FOOD
USE
STORE
Glucose
Respiration
Glycogen or Fat
Fats
Respiration,
Fat
to build cell structure,
as a store of energy
Build proteins for cell CANNOT BE STORED
– ARE DEAMINATED
structures, enzymes
& USED IN
RESPIRATION
Amino
acids

89. Deamination occurs in the liver
AMINO ACID
is broken into two:
Part with
nitrogen forms:
AMMONIA
UREA
The rest is
used for
energy

90. Question: MAY, 2007
Write the correct term for each of the following
processes:
i) the breakdown of excess amino acids
Deamination
ii) rhythmical muscular contractions that push
food along the digestive system
Peristalsis

91. iii) the elimination of indigestible food from
the body
Egestion / Defaecation
iv) the passage of digested food through the
gut wall in the blood stream.
Absorption

92. BLOOD SUPPLY OF THE LIVER

93. Blood supply to and away from the liver
HEART
HEPATIC VEIN
to heart
HEPATIC
PORTAL VEIN
Blood from
digestive system
LIVER
HEPATIC ARTERY
GUT
Oxygenated blood
from heart

94. Write an account of a controlled experiment
you would perform to show the action of the
enzyme amylase on starch. (6) [MAY, 1997]
1
Two spotting tiles are
prepared with a drop of
iodine solution in each
hole.
2
An equal volume of starch
is placed into two separate
test tubes, labelled A and B.

95. 3
An equal volume of amylase is placed
into two separate test tubes, labelled C
and D.
Test tube C is boiled for 5 minutes to
denature the enzyme and so act as a
control.
4
All four test tubes are
placed in a water bath at
37C for 5 minutes to
acclimatise.

96. Starch in test tube A is
poured into amylase in test
tube D, mixed and the stop
watch is started.
5
6
After 30 seconds, a drop of the
mixture is taken and added to one
of the drops of iodine solution on
the spotting tile. This is repeated
until a yellow colour appears.

97. 7
8
Starch in test tube B is poured
into amylase in test tube C,
mixed and the same method is
repeated.
Contents in test tube:
i) D give a yellow colour with
iodine solution after a few
minutes
ii) C continues to give a blueblack colour after 30 minutes.

98. 9
10
The results show that amylase breaks down
starch.
If Benedict’s solution is added to
the contents of test tubes C and D
and heated for a few minutes, a
brick red colour is obtained only
in test tube D [unboiled amylase].
This shows that when starch
breaks down, reducing sugars are
formed.
C
D

99. THE END