Mechanical and
chemical digestion
What is Mechanical Digestion?

Mechanical digestion: the movement and breakdown of food (for
example, tearing, smashing).

There are two basic types of mechanical digestion.

•
Mastication: chewing begins the process of breaking down
food into nutrients.

•
Peristalsis: is the involuntary contractions responsible for the
movement of food through the esophagus and intestinal tracts.
Watch this video clip

http://highered.mheducation.com/sites/0072495855/student_view0/
chapter26/animation__organs_of_digestion.html
What is Chemical Digestion?

enzymes are needed to chemically digest the large molecules:
carbohydrates  monosaccharides
lipids  fatty acids & glycerol
proteins  amino acids

once digested, the nutrients can be absorbed

the other types of nutrients (vitamins, minerals, water) can be
absorbed directly without being digested
The mouth- salivary glands

secrete saliva

saliva contains:
lubrication


mucins for
amylase for digesting starch
the chewed bolus is pushed into
the pharynx and swallowed
Stomach



the stomach is lined with millions of
gastric glands
parietal cells:
secrete hydrochloric acid (pH = 1 to 2)

The acid kills some bacteria,
denatures protein, breaks down
tissue, activates the enzyme pepsin

chief cells:

secrete pepsinogen – which is
activated by the acid to form the
enzyme pepsin

pepsin digests proteins into smaller
polypeptide chains
Small intestine
 long,
thin, muscular tube (7 m long)
 three major sections:
duodenum – receives secretions
from pancreas and liver
jejunum – most digestion occurs
here; some absorption
ileum – most absorption occurs here
Duodenum
bile:
produced in liver,
stored in gall bladder
composed of salts,
pigments and
cholesterol (NOT an
enzyme!)
emulsifies fats:
physically breaks
down large fat
droplets into smaller
fat droplets
digestive enzymes:
produced in the
pancreas and
sodium
bicarbonate:
from pancreas
neutralizes acid,
raises pH to 8
activates intestinal
enzymes
Jejunum & Ileum

secrete more digestive enzymes from intestinal glands

also is the major site for nutrient absorption
How Long Does it Take to Digest
Food?

In healthy adults, this process can range from 24-72 hours but the
average is about 36.

Typically, after ingestion, food remains in the stomach and small
intestine from 6-8 hours.

The large intestine is capable of holding undigested food waste for
days.