Digestive System
Obtaining nutrients
Nutrition
Energy
• Body cells need energy to run cell
processes.
• Animals obtain chemical energy from
food. Energy is derived from breaking
chemical bonds.
• Food energy is measured in units called
calories.
Food labeling
• A typical label for
packaged food lists
calories and major
nutrients.
• While the
information is
incomplete, it does
help people make
some important
choices.
Lipids
• Lipids are fats, oils, and
waxes.
• Lipids are energy
dense, containing 9
calories per gram.
• Lipids are used for
storing energy, making
cell membranes, and
synthesizing steroid
hormones.
Many lipids are made up of
fatty acids and glycerine.
Carbohydrates
• Sugars and starch are
the carbohydrates that
humans can digest.
“Fiber” is indigestible
carbohydrates, such as
cellulose and inulin.
• Glucose is needed by
all body cells as energy.
Nerve cells must have
glucose to operate.
Starch is made up of glucose.
• Sometimes my students describe sugars
or starch as “fatty,” or tell me that sugar
has lots of fat in it. Look at the diagrams
of starch, sugar, and fat in your notes.
Are they right or not?
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Proteins
• Amino acids from
digested proteins are
used by cells to build all
the proteins that our
body needs.
• Humans need a balance
of the 20 amino acids,
which can be obtained
from animal proteins, or
by blending plant
protein sources.
Proteins are made up of
amino acids.
• Since there are 20 different amino acids,
and hundreds or thousands of amino
acids in a single protein, there are
billions of possible combinations and
therefore many, many different,
distinctive proteins. We’ve learned about
some already. Name some proteins
we’ve learned about. (Hint: “Meat” is a
source, not a protein.)
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Macromolecule summary
Polymers
Complex
Carbohydrates
(i.e. starch)
Monomers
Roles
Glucose and other Broken apart to get energy to
simple sugars
make ATP.
Proteins
Amino acids
Used to make our own
enzymes and other body
proteins.
Lipids (Fats,
waxes, oils, and
steroids)
Fatty acid chains,
glycerine (except
steroids)
Used for cellular energy and
energy storage; used to make
cell membranes, steroid
hormones.
Minerals
• Sodium, potassium,
zinc, iron, calcium,
copper, and selenium
are among the
minerals that humans
need.
• Most minerals can be
found in whole grains,
fruits, vegetables, nuts,
and meats. Highly
processed foods may
be deficient.
Major Minerals
Mineral
Major roles
Natural sources
Calcium
Bone and tooth formation; muscle
and nerve function.
Dairy products, leafy greens, dry
beans.
Iron
Used to make hemoglobin and
myoglobin.
Red meats, eggs, nuts, whole
grains, leafy greens.
Zinc
Component of certain enzymes,
required for growth.
Meats, whole grains, nuts, legumes.
Phosphorous
Bone and tooth formation; pH of body Dairy products, grains.
fluids, phospholipids.
Potassium
Maintains pH of body fluids; used in
action potentials.
Many fruits and vegetables, meats,
milk.
Sodium
Maintains pH of body fluids; used in
action potentials.
Table salt, meats.
Selenium
Used by the immune system.
Nuts, esp. Brazil nuts; many fruits
and vegetables.
Vitamins
• Vitamins play many
different roles in
metabolism.
• We do not obtain
energy from vitamins;
however, some
vitamins are necessary
to run energy-related
processes in cells.
Major Vitamins
Vitamins
Major roles
Natural sources
Vitamin A (fat
soluble
Used to make visual pigments;
maintains epithelial tissues; needed
for normal growth.
Orange and yellow fruits and
vegetables, egg yolk, dairy
products.
B complex vitamins
(water soluble)
Used in cellular respiration to
metabolize sugars and other carbon
compounds.
Whole grains, legumes, many
fruits and vegetables. B12 comes
from animal sources.
Vitamin C (water
soluble)
Used in collagen synthesis, possible
role in immune function.
Fresh fruits and vegetables.
Vitamin D (fat
soluble)
Bone growth, calcium absorption,
possible role in immune function.
Eggs, dairy products. Sunlight on
skin oils creates Vitamin D.
Vitamin E (fat
soluble)
Antioxidant, reduces cellular
damage.
Nuts, whole grains, leafy
vegetables.
Vitamin K
Plays a role in blood clotting.
Produced by intestinal bacteria.
Antioxidants
• Free-radicals are
released by cell
damage and many cell
processes.
• Anti-oxidants, such as
Vitamins C and E, and
many plant pigments,
combine with free
radicals and reduce
their effects, which
slows cell aging.
Moral of the story: eat colorful
food
No, Skittles don’t count as
“colorful food!”
The monomers of proteins:
25%
1.
2.
3.
4.
25%
25%
25%
Simple sugars
Lipids
Amino acids
Depends on the
protein
1
2
3
4
Humans obtain energy from:
20%
1.
2.
3.
4.
5.
20%
20%
2
3
20%
20%
Carbohydrates
Vitamins
Minerals
Antioxidants
All of these
1
4
5
Mom says carrots make you see better.
Any reason why this might be true?
1. No, but they do
contain antioxidants.
2. Yes. Beta carotenes
in carrots are
converted to Vitamin
A.
3. No, it’s just an old
folk belief.
33%
1
33%
2
33%
3
Which nutrients are “anti-aging” because
they prevent free-radical damage?
25%
1.
2.
3.
4.
25%
25%
25%
Lipids
Minerals
Antioxidants
B-vitamins
1
2
3
4
Which vitamin can you overdose
on?
25%
1.
2.
3.
4.
25%
25%
25%
B vitamins
Vitamin C
Vitamin A
None – you can’t
overdose on
vitamins.
1
2
3
4
• Trick question: what ingredient in
“energy drinks” actually gives you
energy?
• What would be incorrect about a label
that said, “calorie-free energy drink”?
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Digestion
Two-way digestion
• Simple animals have a
single digestive pouch
with a single opening.
• Food enters through
the opening, waste
leaves through the
same opening.
• These organisms must
finish digesting before
eating again.
One-way digestion
• More complex animals
have one-way
digestion.
• Food enters one
opening and waste
leaves from another.
• Animals with one-way
systems can eat any
time, which is an
advantage.
Mechanical digestion
• In humans, mechanical
digestion takes place in the
mouth.
• Human incisors and
canines are adapted for
tearing food, while molars
are adapted for grinding
food.
• Saliva, which contains
enzymes, mixes with food.
Stomach
• Acid digestion occurs in
the stomach.
• Gastrin, a hormone,
stimulates acid release.
• Pepsin, an enzyme that
breaks up proteins,
requires an acidic
environment to become
active.
Stomach problems
• Stomach ulcers cause
pain when the stomach
is empty. These are
bacterial infections that
create ulcerations.
• Acid reflux is often due
to weakness in the
sphincter muscle at the
top of the stomach.
Duodenum
• Alkaline digestion takes
place in the upper small
intestine, the duodenum.
• Enzymes from the
pancreas require an
alkaline environment to
be active.
Pancreas and Liver
• Pancreas releases
pancreatic juice,
containing bicarbonate,
lipases, proteases, and
amylase.
• The liver makes bile,
which emulsifies fats.
Bile is made from
cholesterol, which is
made in the liver.
Small intestine
• The walls of the small
intestine are lined with
millions of microvilli.
This is the site of
nutrient absorption.
• Small intestines also
produce many digestive
enzymes to break large
polymers completely
down into monomers.
Villi and microvilli
Digestion and pH
Location
pH
Enzymes
Molecules digested
Mouth
neutral
Amylase
Starch
Stomach
acidic
Pepsin (a peptidase)
Initial protein
digestion
Small intestine
Basic to neutral
Mixture of amylase,
peptidases, lipases
Digestion of
starches, final
breakdown of
proteins, digestion of
lipids.
Large intestine
• Water from digested food
is absorbed in the large
intestine.
• Bacteria present in the
large intestine feed on
unabsorbed nutrients, and
produce several vitamins.
• Fecal material is formed
from fiber and other
undigested material.
Most digestion occurs in:
25%
1.
2.
3.
4.
25%
25%
25%
The mouth
The stomach
The small intestines
The large intestines
1
2
3
4
Digestion of starches begins in the:
25%
1.
2.
3.
4.
25%
25%
25%
Mouth
Stomach
Small intestine
Large intestine
1
2
3
4
Why do we have microvilli?
1. To protect the
intestinal lining.
2. To vastly increase
the intestine’s
surface area.
3. To block acids
coming from the
stomach.
33%
1
33%
2
33%
3
Does a colon need cleaned?
• Products like these raise
worry in people who
believe their colon is
“dirty.” These are laxatives
that are unnecessary, or
even harmful.
• “Cleansing” products can
disrupt bowel function and
do not cure any real
illnesses. A diet high in
fiber is a healthier option.
Hormones and digestion
• Many hormones produced by the digestive system
itself are responsible for appetite and digestion.
• Gastrin in the stomach, produced when food is present,
signals release of acid.
• Secretin signals the release of sodium bicarbonate by
the pancreas.
• Ghrelin and Leptin, recently discovered, control
appetite. Changes in these hormones cause
overeating, because people with too little of these
hormones don’t realize they’re full.
Hormones
• NOVA Science Now video: Hormones and
Obesity