Download Digestion Topics 6,11,H

Digestion
Topic 6.1
Enzymatic breakdown of food 6.1.1
and 6.1.2
• In order for food to cross cell membranes
it must be hydrolyzed (broken down) into
small molecules.
• This is accomplished by enzymes specific
to each molecule. Often, more than one
enzyme is responsible for breaking down
macromolecules (more about this in 6.1.3)
What do you remember about
enzymes?
• Enzymes are biological catalysts.
• Substrate molecules fit at an active site on
the enzyme. The resulting induced fit
causes either the breaking of (in digestion)
or formation of (in biosynthesis) chemical
bonds.
• Enzymes work best under specific
conditions of temperature, pH, etc…
• Enzymes increase the rate of chemical
reactions by lowering the activation energy
of reactions.
• Enzymes are proteins.
• Enzymes may become denatured by salts,
pH changes and high temperatures. They
also may not work as well at low
temperatures.
What you eat!
• Proteins are ingested as proteins and are
broken into…...
• Lipids are ingested as triglycerides and
are broken into……
• Carbohydrates are ingested as poly, di
and monosaccharides are broken into….
• Nucleic acids are ingested as DNA and
RNA and are broken into…..
You are what you eat!
• The monomers produced by digestion are
then reassembled into macromolecules
(proteins, lipids, carbohydrates, and
nucleic acids) by your cells to make you!
6.1.3
• In order for “food” to be absorbed by your
cells it must be small and water soluble.
Salivary amylase
• Secreted by salivary gland
• Breaks down amylose into maltose and
glucose
• pH is neutral
Pepsin
• Secreted by cells lining stomach
• Breaks polypeptides into amino acids
• Works at a pH of 3 (in acidic environment
of stomach)
Pancreatic lipase
• Secreted by cells of pancreas
• Breaks down lipids into glycerol and fatty
acids
• Works best at a neutral pH
6.1.4
• Draw and label a diagram of the human
digestive system.
Nourishing your cells
• Ingestion - eating
• Digestion- enzymatic hydrolysis
• Absorption - small molecules are
absorbed through cells of the digestive
system and pass into blood or lymphatic
vessels
• Transport – circulatory system delivers
nutrients to your cells.
• Most animals have a complete digestive
tract AKA alimentary canal. 1 tube running
mouth to anus.
• Food moves in one direction
• Regions of canal are specialized for
certain functions.
• Food can be ingested while food eaten
previously is still being digested.
Oral Cavity
• Both physical (teeth) and chemical (saliva)
digestion,
• Saliva – mucin, buffer, salivary amylase
(hydrolyzes starch and glycogen 
maltose and small polysaccharides)
• Tongue – tastes and manipulates food,
shapes it into a bolus.
Pharynx
• Junction of esophagus and trachea
• Epiglottis – flap of cartilage that blocks
glottis (opening to trachea)
Esophagus
• After swallowing food is forced to the stomach
by peristaltic waves of smooth muscle lining the
esophagus.
• These waves travel in one direction.
• The lower end of the esophagus where contents
empty into the stomach has a circular muscle
sphincter known as the gastroesophageal
sphincter. This sphincter is constantly
constricted to prevent the gastric contents from
entering and damaging the esophagus due to
the HCl content of the stomach.
Stomach
• Stores food, preliminary digestion
• Produces gastric juice
• Gastric juice is secreted by epithelial cells
in pits in stomach wall.
• Contains
-HCl
-pepsinogen (secreted by chief cells)
which is activated by HCl (secreted by
parietal cells) to form pepsin.
• Mucus cells produce mucus (protects,
lubricates)
• Stomach churns every 20 seconds, mixes
food and gastric juice to form chyme
• Gastroesophageal sphincter and pyloric
sphincter trap food in stomach. Pyloric
regulates entrance of chyme to intestine.
• Empties in 2-6 hours
• Stomach has folds called rugae which
allow stomach to stretch as it fills
Small Intestine
• Nearly all chemical digestion occurs here.
• Pancreatic amylase: starch, glycogen
disaccharides
• Maltase: maltose  glucose
• Other disaccharidases (found in intestinal
epithelium) break down lactose and
sucrose and they are absorbed
• Trypsin and chymotrypsin (both secreted
by pancreas) break certain peptide bonds
• Dipeptidases
• Aminopeptidase (intestinal epithelium) –
removes aa from end of molecule w/ free
amino group
• Carboxpeptidase (pancreas) – removes aa
from end of molecule w/ free carboxyl
• Trypsin, chymotrypsin, carboxypeptidase are
inactive when secreted. Enteropeptidase
activates them in intestines
• Nucleases – hydrolyze DNA
• Bile salts (made in liver, stored in gall bladder,
transported to SI by bile duct). Emulsify fat into
small droplets than lipases break them down.
Absorption
• Villi – large folds of intestinal lining
• Microvilli – appendages of epithelial cells
of villi
• Increase SA for absorption of nutrients
• Each villus has a network of capillaries
and a lacteal (lymph vessel)
• Materials may be transported actively or
passively.
• Capillaries converge into hepatic portal
vessel which leads to liver. In liver
molecules are stored and/or processed.
6.1.7 Structure of Villus
• May be as many as 40 villi per square mm
of intestinal lining.
• Outer tissue is a single layer of epithelium,
permeable to digested food
• A network of capillaries lies under
epithelium so food passes quickly to
bloodstream
• Lacteal absorbs lipoproteins that cannot
pass into capillaries.
• Lipoproteins are glycerol and fatty acids
surrounded by protein to help them remain
suspended in lymphatic fluid
• Muscle fibers surround lacteal and
contract to squeeze fluid along lacteal.
Elimination and water absorption
• Large intestine AKA colon joins SI at a
junction which has a sphincter
• Cecum (pouch) and appendix are found
near the junction.
• 7.0 L of water are used each day for
digestion, 90% is reabsorbed.
• Feces remain as undigested food passes
through LI – takes 12-24 hours. Stored in
rectum until eliminated.
• E. coli live in human colon and live on
organic material. These generate gases
as well as vitamin K
Absorption vs Assimilation
6.1.6
• Absorption - soluble products of digestion are
taken up by various mechanisms into the
epithelial cells that line the gut. These epithelial
cells then load the various absorbed molecules
into the blood stream.
• Assimilation - soluble products of digestion are
transported to the various tissues by the
circulatory system. The cells of the tissues then
absorb the molecules for use within this tissues
H.3.1
• Be able to draw and label the:
– Villi
– Lumen
– Longitudinal muscle
– Circular muscle
– See page 611
Transverse section of ileum
H.3.1
Explain structural features of
epithelial cell of villus
H.3.2
• See page 612 for illustration
• Lining of intestine is known as mucosa
and is responsible for absorption of
digested food
• Recall that the villi increase the surface
area for absorption and that each villus
has a capillary bed and a lacteal
• Microvilli – each villus has microscopic
projections called microvilli which further
increase surface area for absorption
• Mitochondria – produce ATP for absorption
via active transport
• Pinocytotic vesicles – found near surface
of plasma membrane and are used for
active transport
• Tight junctions – membranes between
adjacent epithelial cells are bound tightly
together. Most molecules cannot pass
between these junctions.
• Tight junctions force digested food
molecules through epithelial cells and into
capillaries or lacteals.
H.3.3 Transport mechanisms
Facilitated diffusion
• Molecules that are polar have difficulty
travelling through the hydrophobic portion
of the cell membrane of the villi
• Protein channels in the membranes that
have polar interiors allow polar molecules
to diffuse from the lumen into the villi
Review membranes and transport
pg. 29-38
H.3.3 Transport mechanisms
Active transport
• Some membrane proteins of microvilli
require ATP to transport molecules across
the membrane
• This occurs when there is not a
concentration gradient to drive materials
into the epithelial cells
• ATP made by mitochondria in epithelial
cells provides the energy
H.3.3 Transport mechanisms
Pinocytosis
• Droplets of fluid from the lumen of the SI
are surrounded by plasma membrane and
engulfed.
• This process forms pinocytotic vesicles
• Once the vesicle is in the cytoplasm, the
contents are released.
• Requires ATP
Indigestible materials
Become part of feces
• Cellulose – plant cell walls
• Lignin – “woody” deposit in plant cell walls
• Bile pigments – “fun fact!” give color to
feces!
• Bacteria – E.coli are normally found in our
digestive tract
• Intestinal cells – slough off as food moves
through lumen
Digestive Secretions H.2.1 & H.2.3
• Salivary glands – salivary amylase, mucus
• Gastric glands – mucus, HCl, pepsinogen
• Pancreas – an exocrine and endocrine gland.
Secretes trypsinogen, amylase, lipase, hydrogen
carbonate (bicarbonate)
• Liver – bile
• Intestinal glandular cells – produce secretions
that may be added to lumen or stay attached to
villi membranes to react with undigested
substrate as it flows past in the lumen
H.2.2 Cells of exocrine glands
• Exocrine glands produce secretions that
travel to a specific location via ducts
• Most of these secretions are enzymes
which are forms of protein
• Review protein synthesis (page 61-65)
and operation of the Golgi apparatus
(page 37-38)
See EM of exocrine cell page 606
• Because exocrine glands are involved in
the production and release of
enzymes/proteins the cells of exocrine
glands can be expected to have:
– Rough ER
– Additional ribosomes
– Golgi bodies
– Vesicles
– mitochondria
• The cells of exocrine glands can be found
clustered around branches of ducts
(ductules) that lead to the target organ.
• This arrangement looks like a cul-de-sac
• The arrangement of cells and ductules is
called an acinus. Many acini drain into
larger ducts
H.2.4-H.2.7 Gastric Secretions
• Pavlov’s experiment lead to our
understanding of the autonomic nervous
system
• The sound of a bell caused dogs to
salivate even when food was not present
• In humans the sight or smell of food causes
salivation and the release of gastric juice.
• Food in the stomach stimulates the brain to
make the stomach produce more gastric juice
• As the stomach becomes distended a hormone
called gastrin is produced which leads to the
sustained release of gastric fluid
• Enzymes secreted by duct into the lumen
of the stomach and small intestine mix
with food, causing digestion.
• These enzymes are short-lived and may
be digested themselves
• Some enzymes remain attached to the
membrane lining the small intestine. Ex)
maltase. What are the benefits of this?
Cellulose digestion
• There isn’t any!
• Cellulose is a polymer of b glucose.
Starches are a polymer of a glucose. Our
enzymes can break down the alpha form
but not the beta form.
• Mammals who are grazers rely on
mutualistic bacteria which produce
cellulase.
Lipid digestion
• Because they are insoluble in water, lipids are
difficult to digest. They tend to clump together
(coalesce) which makes their surface
area/volume ratio unfavorable for enzymatic
breakdown.
• Lipase catalyzes the hydrolysis of lipid
molecules on just the outside of the globule
• Read page 610 to find out a cool fact about
lipase
• Bile breaks the globules (emulsifies) into smaller
globules so more lipase can act on the fats
Best of all….
•
•
•
•
How does HCl activate pepsinogen?
See page 609!
Word of the day…zymogen!
Mystery of the day…Why do the enzymes
trypsin and pepsin not end with –ase?
Undernourishment
• Glycogen and fat have been consumed
and body begins breaking down protein
(muscle, brain) for food.
• Causes irreversible damage or death
Overnourishment
•
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AKA obesity
Body stores excess food molecules as fat.
If diet is rich in fat it tends to store the fat.
If diet is rich in carbs, it steps up
carbohydrate metabolism.
• Many dieters quickly return to original
weight and many obese people maintain a
set weight. Feedback mechanisms
regulate this.
• Adipose cells make the hormone leptin. A
high leptin level tells brain to depress
appetite and increase muscle activity and
heat production.
• If body fat decreases, less leptin is made.
Appetite increases and activity decreases.