Biology 12
Chapter 12 pg 213-225
Mr. Kruger
DIGESTIVE SYSTEM
1. What are the functions of the Digestive System?
1. Ingestion: taking food in to the system.
2. Secretion: enzymes, bile, and HCl are secreted in to the digestive tract to assist in
digestion.
3. Digestion: breaking large biological molecules in to small organic molecules.
4. Absorption: absorb the digested nutrients in to the blood stream.
5. Elimination: getting rid of the undigestable wastes.
a. Definitions:
Anatomy is the study of structures.
Physiology is the study of functions.
*We will study both as we learn about all of the systems in our body.
2. Physiology of the Digestive System
THE MOUTH:
Functions:
a) Ingestion
b) To begin digestion
Structures:
a) Teeth: take in the food and begin mechanical digestion by breaking the food in to
smaller pieces.
b) Salivary glands: ducted glands that produce saliva, which:
1) liquifies food
2) Contains amylase and begins chemical digestion
Amylase
Starch
+ H2O
maltose
3) Lubricates and softens the BOLUS of food.
c) Tongue: The tongue has 3 functions…
1) Contains taste buds, which are located at the back of the tongue. This protects us
against poisons as they most often taste bitter.
2) Moves the food around in the mouth to mix the food and the saliva.
3) Pushes the BOLUS of the food to the back of the throat to the ‘swallow reflex center’.
Biology 12
THE PHARYNX:
Chapter 12 pg 213-225
Mr. Kruger
Structure:: This is the back of the throat, which opens to both the respiratory and
digestive systems.
Function: When food is placed on the ‘swallowing reflex
center’ by the tongue, the following things happen:
a) the soft palate covers the opening to the nose
b) the epiglottis covers the trachea
c) peristalsis of the esophagus begins
THE ESOPHAGUS:
Structure: This is a 30cm long tube that connects the pharynx to the stomach.
 It is the only part of the digestive system where no digestion occurs.

A ring of muscle called the cardiac sphincter (located at the beginning of the
stomach) ensures that no food re-enters the esophagus.
Function:

Food moves through the esophagus (and the rest of the
digestive tract) by a process called PERISTALSIS,
which is a slow, rhythmic contraction that pushes the
BOLUS along.
THE STOMACH:
Structure: The stomach is a ‘J’ shaped organ.
 It can hold 2.3 Litres of food.
 It also contains the circular and longitudinal smooth muscle layers, but also has a
third muscle layer in the muscularis (transverse layer).
Biology 12
Chapter 12 pg 213-225
Mr. Kruger
Functions:
1. Churns food and liquifies it (mechanical digestion).
 This process is aided by the ridges in the mucosa layer of the stomach.
2. Begins the chemical digestion of proteins. In the mucosa there are three types of
cells which aid this process:
a) Columnar epithelium produces an inactive enzyme called PEPSINOGEN.
 These cells contain many mitochondria for active transport.
b) Chief Cells produce 3M Hydrochloric Acid (HCl).
c) Mucous cells produce mucous to protect the mucosa cells (inner stomach
lining) from the HCl.
HCl
Gastric Juices
contain:
Pepsinogen
(inactive)
HCl and
Pepsinogen
Proteins + H2O
pH 2-3
Pepsin
Pepsin
(active)
Peptides + Polypeptides

Hydrochloric acid (HCl) is released when proteins enter the stomach.
 This transforms pepsinogen into an active hydrolytic enzyme (pepsin),
which begins the digestion of the proteins into peptides and polypeptides.
HCl will also be released, however, when you are stressed and there is chronic stimulation
of your autonomic nervous system.
 This dissolves the mucosa layer of the stomach lining, and results in an
ULCER.
The stomach empties within 4 hours. What leaves the stomach is an acidic liquid called
CHYME.
 The pyloric sphincter at the base of the stomach will meter out the chyme
into the duodenum at a slow, controlled rate.
THE SMALL INTESTINE
Functions: To complete digestion and begin absorption of nutrients.
Structure:
A) There are 3 regions of the small intestine
1. Duodenum: completes the chemical digestion
2. Jejenum: finishes digestion and begins
absorption
3. Ileum: this is the longest section and its
function is to absorb all of the nutrients into the
circulatory and lymphatic systems.

The small intestine has an increased rate of
absorption (speeds up diffusion) due to its
Biology 12
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Chapter 12 pg 213-225
Mr. Kruger
highly convoluted (folded) walls.
There are folds in the mucosa layer of the small intestine called VILLI.
These villi also have smaller folds called MICROVILLI.
Both of these increase the surface area of absorption. The absorption takes place
through the collumnar cells of the microvilli. This involves active transport and
requires much energy.
The total surface area of the
small intestine is 180m2 (that’s the
size of a tennis court).
o Sugars and amino acids are
absorbed by the capillaries
(blood stream).
o Glycerol and fatty acids are
absorbed by the lymph
lacteals.
3. The Three Accessory Glands:
o These are glands that produce materials for digestion and empty these into the
duodenum.
1) The Liver:


the largest internal organ.
It has over 500 functions. Some of these
functions that are important to digestion are:
a) Destroys old red blood cells (converts
hemoglobin to bile)

The liver recycles the Hemoglobin and
produces a waste product called “Bilirubin”
which is a yellow pigment. Excessive bilirubin
in the blood leads to JAUNDICE.

The liver gets rid of the bilirubin by
1) The bilirubin is exported in bile to the gall bladder
2) It is also put into the blood stream where it is removed by the kidneys
b) Produces bile (inorganic salts).
 The function of bile is to emulsify fats (break them down in to small, water
soluble droplets).
 This increases the surface area of the lipids and makes it easier for lipid enzymes
to digest the fats.
bile
Fats -------> small fat droplets
Bile is transported via the Hepatic Duct and Cystic Duct to the gall bladder for storage.
Biology 12
Chapter 12 pg 213-225
Mr. Kruger
3) The Pancreas: (this is a dual organ)




Half of it is an endocrine gland, which
produces the hormones insulin and
glucagon.
These hormones are produced the specialized
cells of the pancreas called ‘the islets of
Langerhans. If there is more than 0.1%
glucose in the blood, the pancreas will release
insulin. This will cause the liver to convert
the glucose the glycogen for storage,
promotes the formation of fats and proteins, and causes the protein gates in cell
membranes to open and allow the entrance of glucose into cells. This all results in
lowering blood sugar levels.
If there is less than 0.1% glucose in the blood, the pancreas will release the
hormone glucagon, and this will cause the liver to convert glycogen into glucose to
be released into the blood.
The other half of the pancreas is an exocrine gland that produces various
digestive juices.
The pancreatic juices contain:
a) SODIUM BICARBONATE: NaHCO3 (base); used to neutralize the acidic
chyme.(changes pH of small intestine to slightly basic 8-9)
b) TRYPSIN: this enzyme continues the digestion of proteins into peptides, dipeptides, and tri-peptides.
Proteins
Trypsin
polypeptides
Trypsin
peptides
c) LIPASE: breaks down lipids into fatty acids and glycerol
Lipase
Lipids
Fatty acids + Glycerol
d) AMYLASE: converts carbohydrates into maltose
Carbohydrates
Pancreatic Amylase
Maltose
Biology 12
Chapter 12 pg 213-225
Mr. Kruger
4) The Walls of the Duodenum :

The glands of the submucosa and the columnar cells of the mucosa produce and
release a variety of enzymes.
The juices from the Duodenum contain:
a) Peptidases:
Peptidase
Polypeptides
b) Nucleases:
Amino acids
Nucleases
Nucleic acids
c) Maltase:
Maltose
Nucleotides
Maltase
Glucose
d) Sucrase and lactase:
Sucrase/Lactase
Sucrose/Lactose
Simple sugars
Control of Digestion
Enzymes are energetically expensive to make. We want enzymes in the gut only when
food is present. This is controlled by 3 hormones.
A hormone is a chemical messenger that is produced in the glandular tissue. It travels to
some other target location in the body via the blood stream, where it has a desired effect.
1. Gastrin:
 When food with protein is present in the stomach, the submucosa of the lower
stomach releases GASTRIN and this causes the upper stomach mucosa to release
pepsinogen.
 As mentioned earlier, the HCl causes the pepsinogen to be transformed into pepsin
and this is the enzyme that digests the proteins.
2. Secretin:
 When there is HCl in the duodenum (a LOW pH) this causes the pancreas to release
the pancreatic juices which include NaHCO3.
 This sodium bicarbonate enzyme then neutralises the chyme to a pH of ~8.5.
Biology 12
Chapter 12 pg 213-225
Mr. Kruger
3. CCK (Cholecystokinin):
 When partially digested lipids and proteins enter the duodenum, the CCK hormone
is released into the blood stream and it causes the gall bladder to secrete bile and
the pancreas to release its pancreatic juices with digestive enzymes.
 These ‘juices’ are sent to the duodenum where the enzymes can begin the digestion
of lipids and proteins.
THE LARGE INTESTINE
Structure: The large intestine is large in diameter, but
is much shorter in length than the small intestine.
 It consists of 4 parts:
a) Ascending Colon
b) Transverse Colon
c) Descending Colon
d) Rectum
Function:
1. Absorption of the water and salts that were
used in the digestive process
2. The anaerobic bacteria that live in the large
intestine do 4 things
i)
They slow the movement of waste through the colon, which allows
time for the water to be re-absorbed.
ii)
They eat the wastes and produce useful things that we need to
survive. (ie: vitamins and amino acids)
iii)
They produce growth factors (proteins that stimulate cell growth)
iv)
Produce waste of their own (methane gas) Phew!


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By the end of the large intestine wastes are transformed into pasty ‘feces’.
If these waste moves through the large intestine too quickly, it can not absorb
enough water and you have diarrhea (liquified feces).
If these materials move through the large intestine too slowly, it will absorb too
much water, and you have constipation (hard feces).
The entire process of digestion from the mouth to the anus lasts 24
hours.
THE LIVER… more information


All of the blood from the villi of the gut travels via the hepatic portal vein to the
liver.
The liver acts as a ‘gatekeeper’ to the blood by keeping levels of various food in the
blood constant.
What happens to the blood in the liver? (Function)
1. Detoxifies any poisons or harmful substances that were absorbed by the digestive tract
(ie: alcohol is metabolised into fatty acids).

Over time this can cause scarring of the liver tissue which gives rise to cirrhosis.
Biology 12
Chapter 12 pg 213-225
Mr. Kruger
2. Regulates the blood glucose level at ~0.1% of plasma.
Insulin
Excess blood glucose in LIVER
glycogen
Glucagon
3. Deamination of amino acids.




If necessary the liver can convert amino acids into glucose to maintain glucose
concentration of plasma (this is called GLUCONEOGENESIS).
This process releases the amino acids groups which the liver converts into urea.
The urea is released into the blood, where it becomes one of the nitrogenous
wastes removed by the kidneys in the production of urine.
We have to do this because NH3 (ammonia)is very toxic..
4. Destroys old red blood cells (after ~ 4 months) and recycles hemoglobin.

Most of the Hb is reused by the bone to make new RBC, the rest is ‘worn out’ and
is converted into bilirubin and biliverdin (the components of bile)
5. Makes blood clotting proteins (fibrinogen and prothrombin) and the protein albumin
which helps to maintain the osmotic pressure of the blood.
6. Produces bile which emulsifies fats by breaking fats into smaller pieces, thus increasing
their surface area for digestion.
 This makes the enzyme lipase much more efficient.
Hormone Summary Chart
Hormone
Chemoreceptors
detect presence
of what?
Source
Organ
Target
Organ
Effect
Gastrin
Presence of
BOLUS in stomach
Stomach
Stomach
Release of gastric
juice to kill
bacteria and
digest protein
Secretin
Presence of
CHYME in
duodenum
Duodenum
Pancreas
Release of
pancreatic juice to
digest food and
neutralize chyme
CCK
Presence of fatty
acids in duodenum
Duodenum
Pancreas
and gall
bladder
Release of bile and
pancreatic juice to
emulsify and
digest fats
Biology 12
Chapter 12 pg 213-225
Mr. Kruger
Digestive Enzyme Summary Chart
Enzyme
Salivary
amylase
Pancreatic
amylase
Maltase
Source
Target
Salivary
glands
Pancreas
Mouth
Pepsin
Sm.
Intestine
Stomach
Trypsin
Peptidases
Lipase
Nuclease
Substrate
Product
Cooked
starch
Starch
Maltose
Sm.
Intestine
Stomach
Maltose
Glucose
Protein
Polypeptides
Pancreas
Duodenum
Peptides
Peptides
Sm.
Intestine &
Pancreas
Pancreas
Sm.
Intestine
Peptides
Amino acids
Duodenum
Lipids
Pancreas &
Sm.
Intestine
Duodenum
Nucleic
acids
Fatty acids &
glycerol
Nucleotides
Duodenum
*****remember: bile is NOT and enzyme!
Maltose
Conditions
pH 7
37o
pH 8.5
38o
pH 8.5
38o
pH 2.5
38o
pH 8.5
38o
pH 8.5
38o
pH 8.5
38o
pH 8.5
38o