Biology 20
Digestive System
2
DIGESTIVE SYSTEM

HETEROTROPHIC NUTRITION
 Unable to manufacture food from inorganic
molecules.
 Depend on autotrophs.

AUTOTROPHIC NUTRITION
 Organisms capable of synthesizing organic
molecules from simple inorganic material.
 Types: photosynthesizers and
chemosynthesizers.
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4 Steps of Heterotrophic Nutrition
1) Ingestion
• eating or drinking
• Heterotrophs need:
– Sugars
– Amino Acids
– Fatty Acids
– Glycerol
– Minerals
– Vitamins
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4 Steps of Heterotrophic Nutrition
2) Digestion
• Breakdown of macromolecules into smaller
ones that can be absorbed by the cells
– Mechanical -- chewing
– Chemical -- digestive enzymes
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4 Steps of Heterotrophic Nutrition
3) Absorption
• Nutrients are absorbed into the blood
– Digested monomers
– Water
– Minerals
– Vitamins
4) Egestion
• Elimination of undigested material
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Parts

Organs
• Mouth
• Pharynx
• Esophagus
– Cardiac sphincter
• Stomach

Glands
•
•
•
•
Salivary Glands
Pancreas
Liver
Gall Bladder
– Pyloric sphincter
• Small Intestine
– Ileal-caecal sphincter
• Large Intestine
• Rectum
– Anal sphincter
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Pathway of Food
The food will flow through the organs
in the order listed.
 The sphincters are muscular rings that
open and close to let food through.
 The food does not go through the
glands. The secrete important fluids
into the organs to help with digestion.

8
Order of Events
Digestive System
•Overview –
Labeling of the
Digestive system!
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Digestive System
1. salivary glands
2. Oral cavity
3. Kind of pointing to the epiglottis and pharynx area
4. Esophagus
5. Stomach
6. Cardiac sphincter
7. Pancreas
8. Pancreatic duct (tube carrying pancreatic fluid)
9. Large intestine
10. Large intestine
11. Rectum
12. Large intestine (you don’t have to know the different parts like the colon)
13. Appendix
14. Caecum (one part of the large intestine that you do need to know)
15. Small intestine
16. Small intestine
17. Small intestine (you do need to know the order of the 3 parts, but they aren’t shown well on this diagram)
18. Liver
19. Gallbladder
20. Bile duct (tube carrying bile fluid)
21. Diaphragm (respiratory system)
22. Trachea (respiratory system – wind pipe)
23. Tongue
24. Mouth opening
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Order of Events
Mouth

Hard Palate
• hard part of roof of mouth

Soft Palate
• back of roof of mouth

Pharynx
• chamber in throat where
– nasal cavity and mouth meet
– esophagus and trachea meet

Mucus Membranes
• lubricates walls of mouth for easy
passage of food
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Mouth

Purpose
• Mechanical digestion by teeth and tongue
– increases surface area for action by enzymes
– Mixes food thoroughly
• Chemical digestion by enzymes in saliva

Secretions
• Saliva
– 1 to 2 liters per day
– from 3 pairs of glands in the mouth
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Salivary Glands
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Mouth

Composition of Saliva
• Water
– moistens food
• Amylase
– begins STARCH digestion
• Mucin
– lubricant
– binds food together for easier swallow
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Mouth

Control of Salivary Secretions
• Mechanical pressure
– food in the mouth
• Nervous system
– Brain thinking of food in the mouth
– Signal sent to salivary glands from the brain
– Mmmmmmm. Fooooood….
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Order of Events
Pharynx
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Pharynx
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Pharynx
Throat
 Common tube through which both air
and food pass
 Four openings

• Nasal Cavity
• Trachea

Mouth Cavity
Esophagus
Contains a flap of tissue called the
EPIGLOTTIS
• This prevents food from entering the
trachea
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Order of Events
Esophagus


Hollow muscular tube
Connects pharynx to the stomach
What makes the food go to the stomach?

Movement of food by peristalsis
• rhythmic waves of contraction and relaxation of
muscular walls
• food is squeezed through esophagus into stomach
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Peristalsis
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Peristalsis
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Esophagus


Hollow muscular tube
Connects pharynx to the stomach
What makes the food go to the stomach?

Movement of food by peristalsis
• rhythmic waves of contraction and relaxation of
muscular walls
• food is squeezed through esophagus into stomach

Epiglottis shunt food into esophagus during
swallowing
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Esophagus

Structure
• thick walled and muscular
• from pharynx to stomach
• walls contain mucus glands which
secrete mucin
– mucin lubricates food for easy passage
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Order of Events
Stomach
a hollow muscular pouch
 located high in the abdominal cavity,
just under the diaphragm
 The esophagus connects to the
stomach by the cardiac sphincter.

– Also called the lower-esophageal sphincter
• Heartburn
• muscular ring surround esophagus at this
point
• acts like a valve to open and close the
tube
• keeps the food from falling out of your
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stomach when standing on your head.
Stomach
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Stomach

FUNCTIONS
• temporary storage of food
• liquefaction of food
• beginning of protein digestion
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Secretions -- Gastric Juice

Secreted into the stomach cavity by
stomach cells
• churning of the stomach mixes juice with
food

Contains
• Hydrochloric Acid – HCl(aq)
– pH of 1-2
– destroys bacteria present in food
– liquefies food.
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Secretions -- Gastric Juice
• Pepsin
–an enzyme which begins protein digestion
• (proteins broken down into smaller chains)
But aren’t the stomach cells made of protein?
–Pepsin is produced and secreted in an
inactive form called pepsinogen
• which is converted into the active form by the
low pH.
• This protects the cell from self-digestion
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Stomach

stomach lined with mucus coating
• prevents acid and protein digesting
enzymes from damaging wall

when food is liquefied it is called
chyme.

squirted into small intestine through
the pyloric sphincter
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Stomach
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Small Intestine

About 6 meters long
• Three sections
1) Duodenum
• first 25 cm
• digestion
2) Jejunum
• next 2 meters
• absorption
3) Ileum
• next 4 or 5 meters
• absorption
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Small Intestine

FUNCTION
• complete the digestion of food
• absorb the nutrients into the circulatory
system (blood)
• important secretions from the pancreas
and the liver.
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Secretions -- Pancreatic Fluid

Contains
• NaHCO3 – sodium bicarbonate
– raises pH to 8
• Pancreatic Amylase
– an enzyme which continues the digestion of
starch into maltose units
• Lipase
– enzyme digesting lipids into fatty acids & glycerol
• Trypsin and Chymotrypsin
– enzymes which continue protein digestion
– also secreted in inactive forms
– activated by alkaline pH
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Small Intestine

LIVER and GALLBLADDER
• The liver produces bile
– brown fluid
• bile is stored in the gallbladder
• when fats enter the small intestine, the
gallbladder contracts and squirts bile into
the duodenum
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Small Intestine
• bile emulsifies fats
–physically breaks them down into
smaller drops
–Kaboom!!! Blows it up!!!
–greater surface area
for action by lipase
• bile is NOT an enzyme because it
does not cause a chemical change
in fat.
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Small Intestine

All types of foods are acted upon.
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Small Intestine

digestion of disaccharides and
dipeptides is completed by enzymes
produced by the small intestine
• sucrase
– digests sucrose
• maltase
– digests maltose
• lactase
– digests lactose
• aminopeptidase
– breaks down dipeptides into amino acids
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Small Intestine
once all food molecules are at
monomer stage, they are ready to be
absorbed into the circulatory system
 the inside of the S.I. is covered with
millions of tiny finger-like projections
called villi

• each villus is actually covered with
millions of microvilli
• the villi and microvilli increase surface
area for absorbing food molecules into
blood.
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Some notes on this diagram:
-
Add chymotrypsin to #3 with
trypsin
1 is amylose
2 is maltose
3 is protein
4 is dipeptides
5 is lipids
6 is glycerol and fatty acids
7 is amino acids
8 is glucose
9 is lactose
10 (which should be with 8) is galactose
11 is sucrose
12 (which should be with 8) is fructose
13 is vitamins and minerals
14 is water
The solid lines show enzyme
digestion
The dotted lines show
monomer absorption
•capillary
•Villi
lacteal
Absorption in the Small Intestine
Digested
Nutrient
Product
Method of
Absorption
Site of
Absorption
Small Intestine
notice that glycerol recombines with 3
fatty acids to form a fat molecule
which enters the lacteal instead of the
capillary
 Why is fat completely digested and
then recombined to form a fat
molecule again?

56
Large Intestine
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Large Intestine
Material NOT absorbed by this point
will enter the large intestine through
another sphincter – the ileal-caecal
sphincter
 The caecum is a small pouch at the
beginning of the large intestine.

• contains a small projection called the
appendix
– this is a vestigial organ which functions in
cellulose digestion in some herbivorous
mammals
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Functions of the Large Intestine

absorb water into blood

absorb vitamins and minerals into blood

eliminate undigested material from
digestive tract
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Large Intestine

Some bacteria live in the large
intestine of mammals
• they digest material that we are not able
to digest

a byproduct of this activity results in
the synthesis of vitamins
• Vitamin K
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Rectum & Anus
Rectum & Anus
last section of digestive tract
 rectum is a holding pouch for feces
 feces exits the digestive tract through
a sphincter muscle called the anus.
 rectal veins are found near opening

• if they get inflamed…
– the inside diameter of anus decreases
– passage of feces is difficult and painful
– called hemorrhoids or piles.
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What About Fiber???
 fiber
in the diet serves to retain
water throughout the digestive
tract resulting in soft feces.
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1 = Esophagus
2 = Stomach
3 = Small Intestine
4 = Pancreas
5 = Liver
6 = Large Intestine
7 =Large Intestine
8 =Large Intestine
9 = Large Intestine
10 = Rectum
Digestion Enzymes
Enzyme Review Video
 Crash Course Video
 Terrific Detailed Video

• This video talks about lipase being in saliva and
gastric juice as well as pancreatic fluid. This is true,
but most lipase (lipid digestion) happens in the small
intestine, so we generally ignore the other 2
sources.
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Biology 20
Control of Digestive Secretions
Saliva

Nervous
• Pavlov
• Sight, smell, presence, or even thought
of food stimulates vagus nerve
• results in the production of saliva
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Gastric Juice
 Nervous
• Stimulation of vagus nerve also
stimulates cells of stomach to begin
producing gastrin
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Gastric Juice

Hormonal
• Gastrin (a hormone) is released from
certain upper stomach cells into the
bloodstream.
• gastrin circulates until it reaches the
lower stomach cells resulting in the
release of gastric juices.
• Once pH falls below 2, gastrin stops
being secreted.
• If pH rises above 2.5 gastrin secretion
begins again.
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Pancreatic Juice

Nervous
• presence of food in mouth and stomach
stimulates pancreatic secretions

Hormonal
• presence of chyme in the S.I. causes the cells of
the duodenum to secrete the hormone secretin
into the blood
• Secretin (a hormone) causes the pancreas to
begin producing pancreatic juice
• Pancreatic Juice will enter the duodenum
through the pancreatic duct.
• when acidic chyme becomes alkaline,
production of secretin stops
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Liver & Gall Bladder
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Bile

Hormonal
• presence of chyme in the S.I. causes the
cells of the duodenum to secrete the
hormone cholesystekinin into the blood.
• This causes the gall bladder to contract,
propelling bile into the duodenum
through the bile duct
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Biology 20
FUNCTIONS OF THE LIVER
Liver
largest organ in the body
 receives blood from 2 supplies

• hepatic artery – oxygen rich blood from
heart
• hepatic portal vein – nutrient rich blood
from intestinal walls

liver acts as a “gatekeeper” between
blood leaving intestines and blood
entering general circulation
• the liver removes excess nutrients
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Liver & Gall Bladder
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Functions of the Liver
1) Digestive Function
• Production of bile which emulsifies fats
2) Regulation of Blood Glucose
• removes excess glucose from blood after meal
and converts it to glycogen
• once glycogen stores are full, it converts extra
glucose into fat which is released from liver into
the blood and stored in adipose tissue
• when blood sugar levels fall, liver converts
glycogen back into glucose and releases it into
blood
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Functions of the Liver
3) Breakdown of Red Blood Cells
• destroys old RBC’s
• the pigment is excreted in bile
• the iron is stored for future RBC’s
4) Synthesis of Plasma Proteins
• manufactures important blood proteins
– fibrinogen
– albumin
– globulin
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Functions of the Liver

Storage
• vitamin B12
• fat-soluble vitamins
• iron

Detoxification
• liver breaks down poisons and non-food
substances in blood
• e.g.: alcohol, caffeine, nicotine, drugs, excess
hormones

Excretory Function
• deamination (breakdown) of excess amino acids
• production of urea
– excreted in urine
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