2. What happens to the food we eat
• Ingestion
Intake of food into the mouth
• Digestion
Breaking down large, insoluble food molecules into smaller soluble ones using
enzymes
• Absorption
Digested food molecules pass across the wall of the small intestine into the
blood or the lymph
• Assimilation
Uptake of food molecules by cells
• Egestion
Passing out of undigested food, in the form of faeces, from the anus
4. Foods must be digested before they can be absorbed:
Food in its natural form consists of large macromolecules like proteins
and carbohydrates. These molecules are too large to be absorbed
directly into cells in the body. They first must be broken down into their
component parts, called monomers. Then, individual amino acids and
sugar molecules can be absorbed through the digestive tract and into
the bloodstream where they can be used by the body for growth and
repair.
7. Teeth
There are different types of teeth, each specially shaped to perform a
particular job.
• Incisor
A broad flat sharp tooth found at the front of the mouth. Designed for biting and
cutting food.
• Canine
A sharp pointed tooth for piercing flesh and tearing.
• Pre-molar & molar
A broad flat tooth with many cusps. Its rough surface is used for crushing, grinding
and chewing food
8. Physical digestion
• Mechanical digestion is the breakdown of food into smaller pieces to
increase the surface area of the food so that enzyme can break them down
more quickly.
• It is mainly carried out by the chewing action of the teeth, the churning
action of the stomach by peristalsis and the emulsification of fats by bile in
the duodenum
9. Chemical Digestion
• The purpose of digestion is to break down large, insoluble molecules
(carbohydrates, proteins and lipids) into small, soluble molecules that can
be absorbed into the bloodstream
• Chemical digestion is controlled by enzymes which are produced in different
areas of the digestive system
10. In mouth
• Mechanical digestion includes Chewing food and mixing with saliva in
the mouth
• Saliva contains
• Amylase which is an enzyme that starts to digest starch into sugars
(maltose) known as chemical digestion
• Mucus helps soften the food making it easier to swallow
• The digestion of food can either be mechanical or chemical.
11. Stomach
• Stomach consists of gastric glands which secretes gastric juice. Gastric juice
consists of hydrochloric acid (highly acidic), renin and pepsin.
• The HCl converts pepsinogen (inactive) into pepsin. This pepsin then converts
protein into polypeptides.
• Another enzyme prorennin is converted to rennin by HCl. This rennin converts
soluble milk protein (caseinogen) into insoluble milk proteins (casein). This
milk protein is then converted to polypeptides by pepsin.
• The churning action of the stomach by peristalsis is the mechanical digestion.
12. Small intestine
• The small intestine consists of U- shaped duodenum, the jejunum and the
much coiled ileum. The lining of the walls of the small intestine contains
glands which secrete digestive enzyme.
• It is 6 meters long.
13. Liver
• The liver is the largest gland in the body. The liver secrete bile an alkaline
greenish-yellow liquid containing bile salts and bile pigments it does not
contain enzyme
• Bile is temporarily stored in the gall bladder . The gall bladder is a greenish-
yellow liquid bag attached to the liver. When the gall bladder contracts the bile
flows into the duodenum via the bile duct.
14. Pancreas
The pancreas produces pancreatic juice which contains digestive enzymes. The
pancreas also secretes the hormones insulin and glucagon which plays an
important part in the control of the blood sugar level in the body & utilization of
carbohydrates.
15. Large intestine
• The large intestine is shorter but much broader than the small intestine.
• The small intestine opens into the large intestine at the right side of the
abdominal cavity.
• It is about 1.5 meters long.
16. Peristalsis
Peristalsis is the rhythmic wavelike
movements of the longitudinal and
circular muscles, primarily in the
digestive tract but occasionally in
other hollow tubes of the body.
Peristaltic waves occur in the
oesophagus, stomach, and intestines
to move food along the gut.
17. In the Small intestine
The small intestine consists of duodenum, jejunum and ileum.
In the small intestine the chyme stimulates the following:
1.Intestinal juice which contains enterokinase, erepsin, maltase, sucrase, lactase
and intestinal lipase.
2.Pancreatic juice from the pancreas which consists of pancreatic amylase,
pancreatic lipase and trypsinogen.
3. The gallbladder to release bile.
Carbohydrate digestion in the small intestine:
1. Starch gets converted to maltose by pancreatic amylase. Maltose gets
converted to glucose by maltase.
2. Lactose to glucose and galactose by lactase
3. Sucrose to glucose and fructose by sucrase.
18. In the Small intestine
Fat digestion in the small intestine:
Bile at first emulsifies the fat that it lowers the surface tension of the fat and
breaks them down into minute fat globules suspended in water to form a stable
emulsion which increases the surface area. These emulsified fats are digested by
intestinal and pancreatic lipases to fatty acids and glycerol which are the end
products of fat digestion.
Protein digestion in the small intestine:
Some protein digestion begins in the stomach, the rest of the undigested protein
enter the small intestine where the protein digestion is completed.
1.Trypsinogen is converted to trypsin by intestinal enterokinase (an enzyme).
2.Active trypsin converts proteins to polypeptides.
3.Polypeptides are converted to amino acids by the enzyme erepsin.
19. Absorption
The small intestine is well adapted for the absorption of digested food the simple sugars
(glucose, fructose, galactose), amino acids and fatty acids and glycerol. Absorption of food
mainly takes place in the ileum and jejunum of the small intestine. The wall of the small
intestine is extensively folded, the inner wall of it is thrown into folds and furrows, these folds
and furrows bear finger-like projections called the villi(one cell thick), the villi in turn bears
microvilli (microscopic). These villi and microvilli greatly increase the surface area.
20. Absorption
The villi and the intestine are densely supplied with lymphatic vessel (absorbs fat)
and blood vessel( absorbs amino acids and simple sugars). Usually amino acids,
simple sugars and mineral salts are absorbed by active transport.
Glycerol is soluble in water so it diffuses into the epithelium of the villi while the
fatty acids react with bile salts to form soluble soaps which then diffuse through
the epithelium. In the epithelium both the soaps and glycerol combine to form fat
globules which then gets absorbed by the lymphatic vessel.
Large intestine:
No digestion takes place in the large intestine. Mainly water and mineral salts are
absorbed, the rest of the undigested matter is stored temporarily in the rectum
before being discharged as faeces through the anus which is known as egestion or
defecation.
21. Assimilation
• Hepatic portal vein
transports absorbed food
from the small intestine to
the liver
• After a meal, high
concentrations of glucose
and amino acids are in the
blood
• The liver reduces the levels
back to normal
22. Assimilation
• Excess glucose arriving in the liver is converted to glycogen for
storage
• Excess amino acids can not be stored in the body
• The liver removes these from the blood and breaks them down to form an energy
source and urea.
• Fatty acids and glycerol are absorbed into the lacteals
• Body cells take up the re-formed fat molecules
• Used in respiration as a source of energy
• Stored
24. Deamination of amino acids
Deamination is the removal of an amino group from a molecule.
Enzymes that catalyse this reaction are called deaminases. In the
human body, deamination takes place primarily in the liver, however it
can also occur in the kidney.
