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Biology 20
Digestive System
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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• Stomach• Small Intestine• Large Intestine• Rectum
Glands• Salivary Glands• Pancreas• Liver• Gall Bladder
Order of Events
Order of Events
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Digestive System
• Overview – Labeling of the Digestive system!
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Digestive System
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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….
Order of Events
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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 Mouth Cavity
• Trachea Esophagus Contains a flap of tissue called the
EPIGLOTTIS• This prevents food from entering the
trachea
Order of Events
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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
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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
Order of Events
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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. 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
stomach when standing on your head.
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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.
Absorption in the Small IntestineDigestedNutrient
Product Method ofAbsorption
Site ofAbsorption
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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?
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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
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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.
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
Biology 20
Control of Digestive Secretions
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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
Biology 20
FUNCTIONS OF THE LIVER
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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