Digestive System
Parts and Function
Digestion
All organisms are composed of four complex
biological molecules: lipids (or fats), proteins,
carbohydrates, and nucleic acids.
For consumers such as humans, these molecules
must be broken down into their component parts.
Lipids to fatty acids, proteins to individual amino acids,
and carbohydrates into simple sugars
The chemical breakdown of complex biological
molecules into their component parts is the
process of digestion.
The digestive system
In most animals the digestive system is made up
of a tube (alimentary canal) that runs more or less
the length of the body.
Generally the food moves in one direction and
different parts are responsible for doing different
jobs in the digestive process.
There are also accessory organs that are important
in digestion that connect to the alimentary canal
via ducts.
Principle Parts of Alimentary
Canal
Mouth- mechanical breakdown of food; tasting; secretion of
salivary glands (salivary amylase)
Esophagus- muscular tube that connects the mouth with the
stomach
Stomach- large muscular storage organ; functions in storage,
mixing, some secretions (acid and pepsinogen)
Small intestine (3 parts)
Duodenum, jejunum, ileum
Receives bile, pancreatic amylase, other secretions;
absorption of nutrients (most sugars absorbed here)
Large intestine- reabsorption of water; bacteria in colon
produce Vit. K
Anus- external opening surrounded by sphincter muscle
Accessory Organs Connected to
Digestive System
Liver- has many functions including
regulation of amino acids in blood,
production of glycogen (a storage molecule)
and bile, converting ammonia to urea
Pancreas- secretion of amylase and insulin
(lack on insulin may cause diabetes)
Gall Bladder- storage of bile
A. Salivary Glands
B. Esophagus
C. Stomach
D. Pancreas
E. Large Intestine
F. Appendix
G. Small intestine
H. Gall Bladder
I. Liver
Connections to the Circulatory
System
Mesenteric Veins- veins connected to capillary bed
closely tied with the intestine
Hepatic Portal System- major blood vessel that
takes blood from the intestine to the capillary bed
in the liver
Circulatory system- major system that transports
nutrients to the rest of the body
Absorption of nutrients from the small intestine occurs
with the aid of villi and microvilli in the small intestine
which increase surface area for diffusion
The Villi of the small intestine
Enzymes
They are biological catalysts which greatly
increase the rate of a chemical reaction but are not
themselves changed during the process
Enzymes are central in the digestion of many
substances including carbohydrates, fats and
proteins.
In most animals, the digestive enzymes are
secreted into a special extracellular (outside of the
cells) cavity called a gut where digestion actually
takes place
These smaller molecules can then be absorbed by the
circulatory system and distributed to cells throughout
the body.
Importance of Proteins
Proteins are important as a structural
element in bones, cartilage, hair, feathers,
nails, and cell membranes.
They are also important as enzymes, hormones,
antibodies, and in oxygen transport in red blood
cells.
Proteins are formed by the linkage of amino
acids into polypeptides.
Digestion of Proteins
Any enzyme that digests proteins is called a
protease
Chemical digestion of proteins begins in the
stomach
The stomach is very acidic (has a low pH,
1.5 - 7)
Pepsin is the primary digestive enzyme in the
stomach
The small intestine carries out further digestion
with trypsin, which is secreted by the pancreas
As proteins are digested, the polypeptide chains
unravel and break up into small chains of amino
acids called peptides
Importance of Lipids
They are fats and oils which are a fundamental
component of cell membranes and may be used
for energy storage or insulation
A characteristic feature is that they do not dissolve
in water
Digestion of Lipids
It begins in the small intestine by making the molecules more
compatible with water so that the digestive enzymes can
access them.
This is accomplished by breaking up the lipid into small droplets
which can be distributed in the water of the small intestine
• This process is referred to as emulsification
Bile which is produced by the liver, stored in the gall bladder and
pumped into the small intestine when lipids are present is responsible
for emulsification
Once emulsified, they may be digested into their subunits
(glycerol and fatty acids) by digestive enzymes called lipases.
They are produced in the pancreas and secreted into the small
intestine
Importance of Carbohydrates
These include simple sugars such as glucose and sucrose
and polysaccharides such as starch and cellulose
They are important as structural compounds and as a
source of energy that can be used as ATP
Starch is a complex polysaccharide made in plants cells for
the storage of energy
Foods such as potatoes and pumpkins are rich in starch
and can be good sources of energy
Cellulose is one of the most common carbohydrates and
can be found in the cell walls of plants
Human digestive system is unable to break down
cellulose and is the largest component of dietary fiber
Digestion of carbohydrates
The digestion begins by converting
polysaccharides (long chains of simple sugars) and
disaccharides (two sugars linked together) into
monosaccharides (simple sugar units) that can be
absorbed by body cells
It begins in the mouth and is completed in the small
intestine (they are not digested in the stomach)
Amylase is the enzyme responsible for digesting
starch
It can be found in the mouth in one’s saliva as well as in
the small intestine secreted by the pancreas