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The Digestive System
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6.1.1 Why digest food?
Food consists of:
– Carbohydrates
– Lipids
– Proteins
– Nucleic acids
– Minerals
– Vitamins
– Water
These are large, insoluble polymer
molecules. They must be digested into
monomers small enough to pass
through the cell membrane of the cells
lining the digestive tract so they can be
absorbed in to the blood stream
Can be absorbed directly (because of
small size), so do not need to be
“digested”
6.1.2 Enzymes and digestion
• Digestion involves hydrolysis of food
molecules
breaking
apart
molecules by
adding water
Hydrolysis of lactose (a
disaccharide) into glucose and
galactose (both monosaccharides)
with the addition of water
Increase the rate
• Digestive enzymes catalyze the hydrolysis of
insoluble food molecules to soluble end
products
The enzyme sucrase
catalyzes the hydrolysis of
sucrose (a disaccharide)
into glucose and fructose
(both monosaccharides)
with the addition of water
6.1.2 Enzymes and digestion
Digestive Enzymes
Digestive Enzymes
Break down complex substances into simpler substances that can be
absorbed by the body
Complex proteins
Complex sugars
Fat molecules
Amino acids
Glucose
Fatty acids
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Macromolecules need to be digested
– Carbohydrates: main source of energy
(short term)
Complex sugars → Simple sugars
– Proteins: growth, antibodies, hormones,
pumps, enzymes, carriers
Polypeptides → Amino acids
– Fats: source of energy (long term)/
storage, absorption, help produce
hormones, steroids, part of cell membrane
Triglyceride → Fatty acids + Glycerol
Enzymes:
protease
amylase
lactase
nuclease
lipase
Speed up reactions
by lowering activation
energy (chemical
reaction requires less
energy to occur)
Fig. 5.6 How enzymes work
The enzyme remains
unchanged!
Important Digestive Enzymes
•
•
•
•
•
•
Salivary Amylase
Pepsin
Trypsin
Dipeptidase
Pancreatic Amylase
Lipase
6.1.4 Digestive System
Parts of the Alimentary Canal
The Digestive System
Consists of the alimentary canal (the long tube)
and several accessory organs.
anal canal
mouth
pharynx
Parts of the
Alimentary Canal
esophagus
stomach
large intestine
small intestine
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Organs of the
Digestive System
Parotid
salivary
gland
Pharynx
Organs of the Digestive System
Tongue
Mouth
Tooth
Sublingual
salivary gland
Submandibular
salivary gland
Esophagus
Liver
Stomach
Gall bladder
Duodenum
Pancreas
Large intestine
Small
intestine
Anal canal
Rectum
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toast
mouth
Tracing Toast
pharynx
epiglottis
esophagus
stomach
anus
feces
rectum
duodenum
Tracing a piece of
toast through the
alimentary canal
jejunum
sigmoid
colon
ileum
descending
colon
transverse
colon
ascending
colon
cecum
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Animation of digestive system
Types of Digestion:
• Mechanical: chewing, mashing, cutting, pounding (mouth /
stomach)
• Chemical: enzymes (speed up chemical reactions)
The Digestive Tract
• Parts of the Digestive
Tract
– Mouth
– Esophagus
– Stomach
– Small Intestine
– Large Intestine
– Rectum & Anus
Accessory Parts
• Organs that are not in the digestive tract
but help in digestion
– Teeth
– Tongue
– Salivary glands
– Liver
– Gall bladder
– Pancreas
The Digestive System
Mouth
• Functions:
– Food enters the mouth or oral cavity
– Tasting
– Mechanical breakdown of food
– Secretion of saliva (by the salivary
glands)
Anatomy of the Mouth
Mouth
• Tongue
– Mixes and rolls food into tiny
mashed up bits (Bolus)
– Pushes the bolus toward the
esophagus when swallowing.
Salivary Glands
• Salivary glands – produce and secrete saliva into
the oral cavity.
– Parotid (beneath
the cheeks)
– Submaxillary (below
the jaw bone)
– Sublingual (below
the tongue)
Saliva
Saliva moistens the food and contains
enzymes that begin digestion of
starch into smaller polysaccharides.
• From wikipedia:
– Salivareferred to in various contexts as spit,
spittle, drivel, drool, or slobber, is the
watery substance produced in the mouths of
humans and most animals.
Important Digestive Enzymes
• Salivary Amylase is the
1st enzyme and begins
working on starches to
break them down into
simpler sugars.
• They are secreted by the
salivary glands and active
in the mouth.
• It’s optimal pH range is
about 7.0.
Anatomy of the Mouth and Throat
Esophagus
• A straight muscular tube that is about 10 inches
(25 cm) long which connects the mouth to the
stomach
• Food takes about 4 to 8 seconds as it passes
through to the stomach.
• Its walls contain smooth muscles that contracts
in wavy motion (Peristalsis).
• Peristalsis propels food and liquid slowly
down the esophagus into the stomach.
• Cardiac Sphincter (ring-like valve) relaxes to
allow food into the stomach.
Peristalsis in the Esophagus
Peristalsis Video
Stomach
• J-shaped muscular sac, where most digestion
occurs.
• Has inner folds (rugae) that increase the surface
area of the stomach.
Stomach
Gastric Juices
•Food is mixed with gastric juices secreted by
the stomach wall
•Hydrochloric acid (HCl) activates
enzymes, breaks down food molecules and
kills bacteria
•Enzymes (proteases) break down proteins
•Mucus lubricates food and protects the
stomach wall from HCl.
Stomach
• Churns and grinds together the bolus into
smaller pieces.
• Converts the bolus into a liquid
(chyme) after 4 hrs of mechanical and
chemical digestion
• Chyme passes through the pyloric
sphincter into the small intestine.
AKA Cardiac Sphincter
Movements in Stomach
Important Digestive Enzymes
• Proteases are a group
of enzymes that
hydrolyze proteins into
amino acids (or smaller
pieces of proteins).
• Pepsin
• Trypsin
• Dipeptidase
Important Digestive Enzymes
• Pepsin is the major
stomach enzyme and
begins working on
proteins to break them
down into smaller
polypeptides.
• It is secreted by the
stomach cells and is
active in the stomach.
• It’s optimal pH range is
about 2.0.
The Stomach
Label the Stomach
(a) Lumen of the stomach which stores the food from a meal
(b) Gastric pits from which mucus, enzymes and acid are
secreted
(c) Mucus secreting cells. Mucus protects the surface of the
stomach from auto-digestion
(d) Parietal cells that produce HCL which kills microorganisms
that enter the digestive system (food & tracheal mucus). This
also converts inactive pepsinogen to active pepsin
(e) Chief cells: produces pepsinogen, a protease enzyme
Small Intestine
• Long (20 ft), coiled tube beneath the stomach.
• This is where digestion is completed and
products are absorbed into the bloodstream.
• Has three parts:
• Duodenum – upper part; about 10 in; connected to
the stomach. Where the digestive juices from the
pancreas and the liver combine with chyme.
Sodium bicarbonate (pH 9) is secreted to
neutralize the acid.
• Jejunum – about 8 ft; completion of digestion
• Ileum – about 12 ft; absorption of nutrients
Small Intestine
• Site of greatest amount of digestion and absorption
Important Digestive Enzymes
• Trypsin breaks down
polypeptides into
smaller peptides.
• They are secreted by
the pancreas and active
in the duodenum of the
small intestine.
• It’s optimal pH range is
about 8.0.
Important Digestive Enzymes
• Dipeptidase breaks
down peptides into
amino acids.
• They are secreted by
the small intestinal cells
and active in the small
intestine.
• It’s optimal pH range is
about 8.0.
Important Digestive Enzymes
• Pancreatic Amylase
finishes carbohydrate
digestion by breaking
down starch into the
disaccharide maltose.
• They are secreted by the
pancreas and active in
the small intestine.
• It’s optimal pH range is
about 7.0.
Important Digestive Enzymes
• Lipase enzymes work
on lipids to break them
down into glycerol and
fatty acid chains.
• Pancreatic lipase is
secreted by the
pancreas.
• It’s optimal pH is 7.2.
Small Intestine
• Takes about 4 – 8 hrs to complete its journey.
• Mucosa (inner wall) – secretes several enzymes
that acts on the food.
• Where the pancreatic enzymes are emptied into.
• Digested nutrients are absorbed through
intestinal walls into the blood supply.
• Peristalsis moves the undigested food along.
Small Intestine
• Has folded inner walls covered
with fingerlike projections (villi;
sing. – villus)
• Each villus has tinier projections
called microvilli that absorb
digested food.
• Villi and microvilli increase the
surface area of the small intestine
for greater absorption.
The Small Intestine
Label the Small Intestine
(a) Villus which increases the surface area for absorption of
the products of digestion
(b) Microvilli border of the epithelial cell increases the
surface area for absorption.
(c) Lacteals are vessels that transport lipids out of the small
intestine into the lymphatic system.
(d) In the wall of the small intestine are the blood vessels to
transport absorbed products to the general circulation.
There are also muscles to maintain peristalsis
Absorption vs. Assimilation
• Soluble products are
taken up into the
epithelial cells of the
small intestine.
• The products then
diffuse into the
bloodstream.
• Soluble products are
transported through
the blood to body
tissues.
• The cells of these
tissues absorb them
for their own use or
storage.
Absorption vs. Assimilation
How absorption happens
• The structure of the villi increases the surface
area for the absorption of digested food
molecules.
(a) folds increase SA:VOL ration by X 3
(b) Villi project into the lumen of the gut
increasing the surface area by X 10
(c) Microvilli are outward folds of the
plasma membrane increasing the surface
area another X10
Diagram a villus
• Using the red
textbook (page 860),
draw and label a villus
from the human small
intestine.
• Include the following
labels: villi, microvilli,
capillaries, lacteals,
epithelial cells, lymph
vessel
Large Intestine
• a.k.a. Colon
• larger diameter, but shorter (5 ft)
• Water is absorbed from the
undigested food making the waste
harder until it becomes solid.
• Waste stays for 10 – 12 hours.
Large Intestine
Large Intestine
• Waste is pushed into the expanded
portion (rectum) of the large intestine.
• Solid waste stays in the rectum until it
is excreted through the anus as
feces.
• Appendix hangs on the right side of
the large intestine.
The Large Intestine
The “Colon”
• (a) The lumen of the colon
• (b) The mucus producing goblet cells
• (c) Muscular walls to maintain peristalsis
Accessory Organs
Produce or store enzymes that helps in digestion.
Accessory Organs
• Liver
– Largest solid organ of the body
– It weighs 1.6 kg. and is 20 cm.
across.
– Produces bile (watery,
greenish substance), which
aids in the absorption of fats.
– Secretes bile to the gall
bladder
The Liver
– Stores vitamins A,D,E,K
– Stores sugar and glycogen
– Synthesizes proteins
– Eliminates biochemical products (RBCs)
– Detoxifies drugs, alcohol, and environmental
toxins.
Accessory Organs
• Gall bladder
– Stores bile in between meals
– Secretes bile to the duodenum through
the bile duct during mealtime.
• Bile contains bile salts, pigments,
cholesterol and phospholipids.
• Bile is an emulsifier NOT an enzyme.
• Emulsifier – dissolves fat into the
watery contents of the intestine.
Gall Bladder
Accessory Organs
• Pancreas
– Produces a juice that contains
enzymes to break down
carbohydrates, fats and protein.
– Secretes the juice into the
duodenum through the pancreatic
duct.
–Also secretes hormones, like
insulin.
The Pancreas
Path of Digestion
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•
•
Mouth
Esophagus
Stomach
Small Intestine
Large Intestine
Anus
Let’s see what happens!