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Digestive System
Role of the digestive system:
ingest food
break food down into small molecules
that can cross plasma membranes
absorb nutrient molecules
eliminate nondigestible remains
Incomplete vs. Complete digestive system
incomplete digestive system has a single
opening – mouth (ex. planaria) - food and
waste enter and exit the mouth – nutrients
are distributed by diffusion
complete digestive system has a mouth
and an anus (ex. earthworm) – mouth
takes in food, digestive tract digests and
absorbs nutrients, undigested remains
leave the body through the anus
Human Digestive System – tube within a
tube body plan
1.Mouth – mechanical and chemical digestion
• food is chewed
• food is mixed with saliva made by three major
pairs of salivary glands – send saliva to mouth
through ducts
• saliva contains salivary amylase – enzyme
that begins digestion of starch
• food mixed with saliva forms a mass called
the bolus
2. Pharynx – back of throat with openings to
esophagus and trachea
• epiglottis covers glottis to prevent food
from entering trachea
• food passes into esophagus
• Esophagus – tube made of muscle through
which food passes to stomach
• peristalsis – contractions of muscles to
move food down esophagus (as well as all
muscular movement to move food through
entire digestive system)
Stomach – thick-walled, J-shaped organ that lies on the
left side of body under diaphragm – storage, chemical and
mechanical digestion of food
epithelial lining of stomach contains millions of gastric pits
which lead to gastric glands
gastric glands produce gastric juices containing:
– HCl (pH around 2) – kills bacteria and other microbes,
promotes the activation of pepsinogen into pepsin
– pepsinogen – inactive precursor to pepsin – enzyme
that breaks down protein
thick layer of mucus protects the stomach
stomach contents are called chyme
stomach is closed off by sphincters at both ends:
– cardiac sphincter prevents food from going back up the
– pyloric sphincter controls passage of chyme to small
Small Intestine – coiled tube about 3 meters long, does
chemical digestion and absorption of nutrient molecules
digestion in the small intestine is accomplished by
digestive secretions from: a) the liver, b) the pancreas, and
c) the cells of small intestine itself
The Liver and Gallbladder – provide bile to break down fat
liver has many functions – storage of fats and
carbohydrates for energy, regulation of blood glucose
levels, synthesis of blood proteins, storage of iron and
certain vitamins, conversion of ammonia to urea, and
detoxification of other harmful substances
in digestion, the liver produces bile (mixture of bile salts,
water, other salts, and cholesterol) – a liquid made by liver
and stored in gallbladder
gallbladder has a duct leading to small intestine (bile duct)
bile acts as an emulsifying agent, breaking large globs of
fat into microscopic particles
this increases the surface area available for lipases
(enzymes) to act on fats
• Pancreas – lies in loop between the
stomach and small intestine
• made of two types of cells – one produces
hormones to regulate blood sugar levels, the
other produces pancreatic juice released
into small intestine
• pancreatic juice contains sodium
bicarbonate (neutralize stomach acid),
pancreatic amylase (digests carbohydrates),
pancreatic lipase (digests lipids), and
pancreatic proteases including trypsin,
chymotrypsin, and carboxypeptidase (digest
small intestine wall is studded with cells
that are specialized to complete digestion
and absorb nutrient molecules
enzymes such as proteases, sucrase,
lactase, maltase and lipase are embedded
in the membranes of the cells
final phase of digestion occurs as
nutrients are being absorbed into the cell
most absorption of nutrients into blood
occurs in small intestine
Fats are absorbed into the lymphatic
system (lacteal)
small intestine wall is highly folded to
increase surface area for absorption
folded surface is covered by tiny fingerlike
projections called villi (0.5 to 1.5 mm in
each individual cell of the villi has further,
microscopic extension called microvilli –
further increases surface area for
villi are surrounded by capillaries to carry
off the absorbed nutrients
• Products of digestion are absorbed when
they make contact with epithelial cells of the
– Absorbed by active transport
– Products of digestion include:
Monosaccharide sugars
Amino acids
Fatty acids
Vitamins and minerals
• Assimilation occurs if absorbed nutrients are
used to build larger molecules in the cell (i.e.
amino acids used to build a protein)
Large Intestine – about 5 feet long, divided into the colon
(most of its length) and the rectum (last 6 inches)
leftovers of digestion flow into large intestine (mixture of
water, undigested fats and proteins, indigestible fibers)
contains large population of bacteria that live on
unabsorbed nutrients
synthesize vitamin B12, thiamin, riboflavin, and vitamin K
cells that line the large intestine absorb these vitamins,
leftover water, and salts
after absorption of water, waste becomes semisolid feces
(consists of indigestible waste and bacteria)
feces stored in rectum – expansion of rectum stimulates
desire to defecate
Examples of Digestive Enzymes
Gastric glands
Substrate Starch
Products Maltose and
amino acids
Glycerol and
fatty acids
Optimum Neutral (pH 7) Acidic (pH 3)
(pH 7)
animal nutrients fall into five categories: 1) lipids, 2)
carbohydrates, 3) proteins, 4) minerals, and 5) vitamins
1. Vitamins – organic compounds that the body is unable to
produce but uses for metabolic purposes
essential to cellular metabolism
Water-Soluble vitamins – cannot be stored, typically
function as coenzymes in metabolic reactions (ex. vitamin
C and vitamin B complex)
Fat-Soluble vitamins – can be stored
Vitamin A – used in production of pigments
necessary for proper functioning of eye (deficiency
causes night blindness – found in green and yellow
vegetables and milk products)
Vitamin D – used for Calcium absorption and metabolism
(egg yolk, milk, fish oils)
Vitamin E – reacts with and detoxifies oxygen radicals in
lipid metabolism
Vitamin K – essential for blood clotting
2. Minerals – required by body as constituents of
cells and body fluids, and as structural
components of tissue (Ca needed for bones and
teeth as well as for nerve conduction and muscle
3. Carbohydrates and Fats – primary source of
4. Proteins – serve as a source of amino acids to
make new molecules
 amino acids are used to make certain
hormones, other amino acids, some
neurotransmitters, and new proteins
 liver can synthesize 9 of the 20 amino acids
 essential amino acids – those that cannot be
synthesized, must be supplied by the diet