Download Unit 2: Digestion

Unit 2: Digestion
IB Biology
Why do we digest food?
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The series of events in digestion include:
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Ingestion – you eat the food
Digestion – a series of chemical reactions where
food is converted into smaller and smaller
molecular forms
Absorption – small molecular forms are absorbed
through the cells of the digestive system and pass
into nearby blood vessels and lymphatic vessels
Transport – your circulatory system delivers the
small molecular nutrients to your body cells
Why do we digest food?
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Digestion solves a problem of molecular size
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Many of the foods that we eat have large molecules
that are too large to pass across any cell
membrane.
Molecules must pass through the cell membranes
of your intestines to get to your blood stream
Then they must pass through the cell membranes
of a capillary
Food we eat must therefore be chemically digested
to a suitable size
Molecule Type
Molecular form
ingested
Molecular form
after digestion
Protein
Protein
Amino acids
Lipids
Triglycerides
Glycerol and fatty
acids
Carbohydrates
Polysaccharides,
disaccharides,
monosaccharides
Monosaccharides
Nucleic acids
DNA, RNA
Nucleotides
Why do we digest food?
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Digestion allows you to turn molecules
into ‘your own’
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All foods are composed of either plant cells
or animal cells
Each contain molecules characteristic of a
living organism that is not a human being
Plant cells store excess carbohydrates in
the form of starch
Animals store excess carbohydrates as
glycogen
Why do we digest food?
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Each type of organism has its own set of
proteins
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Cow muscle (steak) is similar to human muscle but
the amino acid sequences are different
Nucleic acids (DNA and RNA) of other organisms
are also ingested
When molecules are digested they are
hydrolyzed (broken down) into their smallest
components
The components can then be reassembled
into the larger macromolecules that are useful
to you
The role of enzymes during
digestion
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As food move through the alimentary
canal, many digestive enzymes are
added.
Each enzyme is specific for specific
food types.
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Lipase: enzyme specific for lipid molecules
Amylase: enzyme specific for amylose (starch)
Enzymes are protein molecules which
act as catalysts for chemical reactions.
The role of enzymes during
digestion
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Enzymes lower the activation energy for
the reactions they catalyze.
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Reactions occurring with an enzyme will
require a lower input of energy than a
reaction without an enzyme.
Heat is the most common input of energy
Reactions catalyzed with enzymes occur at
a higher reaction rate and lower
temperature than one without an enzyme
The role of enzymes during
digestion
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Many digestive reactions need higher
temperatures than would be safe for a living
being.
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Stable body temperature for a human is 37°C
(98.6°F)
This temperature provides high enough activation
energy for metabolic reactions such as digestion to
occur with the aid of enzymes.
Digestive enzymes all help to catalyze
hydrolysis reactions.
The role of enzymes during
digestion
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The role of amylase is to hold starch in
its active site and put stress on the
covalent bonds that bind the glucose
molecule together
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With enough stress the surrounding thermal
energy will likely be enough to provide
molecular motion to break the bonds.
Enzymes do not necessarily cause a
chemical reaction; they just make them
more likely to occur at a given temperature.
Examples of digestive enzymes
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There are many enzymes specific to the
types of carbohydrates we ingest.
There are also many protease enzymes
that help our bodies digest proteins.
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Some protease enzymes work within a
protein by recognizing specific amino acid
pairs.
Some digest proteins from the outer ends
and work inward.
Salivary amylase
Pepsin (a
protease)
Pancreatic lipase
Source
Salivary glands
Stomach cells
Pancreas cells
Substrate
Amylose (starch)
Proteins
(polypeptides)
Lipids
Products
Maltose and glucose
Amino acids
Glycerol and fatty
acids
Optimum
pH
Neutral (pH 7)
Acidic (pH 3)
Neutral (pH 7)
The Human Digestive System
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Most of the human digestive system consists
of a tube called the alimentary canal.
The alimentary canal consists of:
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Mouth
Esophagus
Stomach
Small intestine
Large intestine (colon)
Rectum
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Food that you eat must either be digested and
absorbed by the body or remain undigested
and be eliminated as solid waste (feces).
Role of stomach, small intestine
and large intestine
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Stomach
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Food is brought to the stomach by the
esophagus (muscular tube)
After swallowing the food is forced down to
the stomach by a sequence of muscle
contractions called peristalsis.
Once food enters the stomach it is held for a
period of time to mix with gastric juices
Role of the stomach
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Gastric juice is a mixture of three secretions from the
cells of the stomach inner lining:
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Pepsin – a protease enzyme active in an acidic pH
Hydrochloric acid – helps degrade and break down foods and
creates the acidic environment necessary for pepsin to be
active
Mucus – lines the inside of the stomach wall to prevent
damage from the hydrochloric acid
The muscular wall of the stomach creates a churning
motion in order to mix food with gastric juices.
After a specific period of time the valve at the lower
end of the stomach opens to allow food to enter into
the small intestine
Role the small intestine
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Small Intestine
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The duodenum is the first portion of the
small intestine
In the duodenum three accessory organs
secrete juices into the small intestine to
continue digestion
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bile: from the liver and the gall bladder
trypsin (a protease), lipase, amylase, and
bicarbonate from the pancreas
Role of the small intestine
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In the small intestine molecules are produced that are
small enough to be absorbed
The inner wall of the small intestine is made of villi
(small finger-like extensions.
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Each villus contains a capillary bed (small vessel of the
circulatory system)
And a lacteal (a small vessel of the lymphatic system)
If the wall of the small intestine were smooth, there would be
very little surface area for absorption to take place
The function of the villi is to increase surface area for
absorption of molecules such as glucose, amino acids, and
fatty acids.
Most absorbed molecules are taken in through capillary beds
within each villus
Fatty acids however are taken in through the lacteal
Role of the small intestine
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All absorbed molecules are taken to a variety
of body cells by the circulatory system.
Within the body cells, molecules may be used
for energy (ex. Glucose) or may be used to
help build larger molecules within the cell (ex.
Amino acids)
Assimilation is the process of bringing the
nutrient molecule to the body cells when they
are used for building larger molecules.
Role of the Large Intestine
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Large intestine
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The majority of useful nutrients are absorbed while
food is inside the small intestine
The remnants of the original food at the end of the
small intestine is undigested (and unabsorbed)
Most of the water that we drink or that is part of the
food we eat is also still present
Water in the alimentary canal is beneficial because
it keeps the moving food in a fluid environment
Role of the Large Intestine
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A very large number of naturally
occurring Escherichia coli bacteria live
in the large intestine
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These bacteria are mutualistic organisms
We provide nutrients, water, and a warm
environment and the bacteria synthesize
vitamin K and maintain the healthy
environment
Any undigested food by the bacteria is
eliminated as feces
The process of digestion requires
“juice”
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Salivary glands
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Saliva is secreted into the mouth
Saliva includes the first of the many
digestive enzymes, salivary amylase.
Gastric glands
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Located in the inner lining of the stomach.
Secrete mucus, hydrochloric acid, and
pepsinogen (the precursor to pepsin – an
enzyme that begins protein digestion)
The process of digestion requires
“juice”
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The pancreas
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Pancreatic juice is sent to the small intestine
by the pancreas through a duct.
This juice contains another protease
enzyme, additional amylase, and lipase.
A form of hydrogen carbonate is present to
neutralize the acid fluid of the stomach
The process of digestion requires
“juice”
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The liver
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The liver secretes bile into the small
intestine
Bile can come straight from the liver or from
the gall bladder (where it is stored)
Bile emulsifies lipids and increases the
surface area of lipids for lipase to break
down
The process of digestion requires
“juice”
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Intestinal glandular cells
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Some of the cells that line the small intestine are
called glandular cells.
Glandular cells secrete a variety of digestive
enzymes
Some enzymes are added to the partially digested
fluid within the small intestine and some enzymes
remain attached to the villi cells.
The attached enzymes are also referred to as
“membrane-bound” enzymes and they catalyze
digestive reactions as the undigested food flow past
them in the lumen of the small intestine
The cells of the exocrine glands
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Exocrine glands are a collection of cells
that produce and secrete a product
which is sent to another part of the body
by way of ducts.
Often the secretion is a protein, which
many of the digestive enzymes are
proteins.
The cells of the exocrine glands
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Some of the major steps of protein synthesis and
secretion are outlined below:
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mRNA is transcribed from a gene of DNA
mRNA is translated at a ribosome in the cytoplasm of the cell
ribosomes are often attached to endoplasmic reticulum
making rough ER
synthesized proteins at ribosomes move through the channels
of the ER and reach a golgi body
Golgi bodies package protein into a vesicle
Vesicles fuse with the plasma membrane to release their
contents outside the cell in a process called exocytosis (or
secretion)
Several of the steps of protein synthesis and secretion require
ATP, therefore these cells typically contain a large number of
mitochondria
The cells of the exocrine glands
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Exocrine glands typically contain a large
number of ribosomes, a large amount of
endoplasmic reticulum, and many golgi
bodies, mitochondria, and vesicles.
The cells of the exocrine glands
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The arrangement of exocrine gland cells into
acini and ducts
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Exocrine glands secrete a product into a duct to be
transported to a specific location throughout the
body
Exocrine glands surround the end of every small
branch of the pancreatic duct
All the cells surround the small branches (ductule)
secrete digestive enzymes into it.
The ductule takes the secretion into increasingly
larger ducts until the pancreatic duct is reached
Components of saliva, gastric juice,
and pancreatic juice
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Saliva:
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Solvent is water
Amylase
Mucus
Gastric juice:
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Solvent is water
Mucus
Hydrochloric acid
Pepsin (secreted as
pepsinogen)
• Pancreatic juice:
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Solvent is water
Amylase
Bicarbonate
Trypsin (secreted as
trypsinogen)
– Lipase
Control of gastric juice secretion
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Digestive juices are not secreted at all
times, only as needed
Specific juices need to be secreted at the
right times to help hydrolyze the
molecule that is in need of digestion
Control of gastric juice secretion
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Ivan Pavlov experiments showed that
seeing and smelling food can begin the
process of digestion (salivation)
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Drops of saliva were measured from dogs
that were being prepared to be fed
The dogs nervous system influenced the
secretion from the salivary glands
The same applies to gastric juices from the
stomach, the sight or smell of food can
initiate gastric juice secretion
Control of gastric juice secretion
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After food enters the stomach, receptors within
the stomach lining are stimulated and will send
sensory signals to the brain.
The brain will respond by causing the stomach
to secrete more gastric juice
Distension (swelling) of the stomach results
from the production of the hormone gastrin
The hormone maintains the release of gastric
fluid, and the hydrochloric acid
Membrane-bound digestive
enzymes
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Some digestive enzymes do not mix with
the food to catalyze hydrolytic reactions
and digest the food
Instead these enzymes remain in the
membranes of the cells that line the
small intestine
Membrane-bound digestive
enzymes
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Maltase is an example of a membrane-bound
enzyme.
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Maltase hydrolyzes the disaccharide maltose into
two glucose molecules
Maltase remains embedded in the inner epithelial
cell membranes of villi and microvilli
Maltose floats to the active site on Maltase and the
reaction is catalyzed
The advantage of membrane-bound enzymes is
that these enzymes remain in the small intestine for
longer than free-floating enzymes, and the product
(glucose) is already in place for absorption
Humans cannot digest cellulose
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Although a large number of mammals
are herbivores, not a single mammal
produces an enzyme to digest cellulose
Cellulose is a polysaccharide
carbohydrate composed of thousands of
glucose monosaccharides
Humans cannot digest cellulose
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Herbivorous mammals contain a large
colony of mutualistic microorganisms
which produce cellulase (the enzyme
necessary to hydrolyze cellulose into
glucose
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Even with the help from bacteria the energy
yield from plant material is small
These animals must ingest a large amount
of plant material
Humans cannot digest cellulose
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Humans do not have a mutualistic
relationship with bacteria that produce
cellulase
The large majority of plant material that
humans eat makes its way to the
alimentary canal and exits the body in
feces
Why doesn’t the alimentary canal
digest itself?
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Protease enzymes such as pepsin and trypsin
hydrolyze peptide bonds within proteins
A fully active protease enzyme cannot
distinguish between the proteins in food and
the proteins that makeup the human body
In order to prevent the hydrolysis of human
proteins, pepsin and trypsin are initially
synthesized in a form that is not chemically
active
The inactive forms of these enzymes are
called zymogens
Why doesn’t the alimentary canal
digest itself?
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The inactive structure of pepsin contains 44
additional amino acids and is known as
pepsinogen
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Pepsinogen is produced in the lining of the stomach
wall and remains in the zymogen form until it
reaches the stomach cavity
When pepsinogen encounters the hydrochloric acid
of the stomach the additional 44 amino acids are
removed and the enzyme becomes pepsin
The inner lining of the stomach are protected from
the hydrochloric acid by mucus
Why doesn’t the alimentary canal
digest itself?
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Trypsinogen is the inactive structure of trypsin
and is synthesized in the pancreas and is one
of the components of pancreatic juice
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Pancreatic juice is brought to the duodenum of the
small intestine by the pancreatic duct
When partially digested food enters the duodenum,
an enzyme known as enterokinase (or
enteropeptidase) is produced.
This enzyme converts trypsinogen into trypsin
How do we digest lipids?
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Lipids have many functions for the human
body
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Ex. Glycerol and fatty acids help synthesize
phospholipids for cell membrane structure
Lipids are relatively insoluble in water which
makes up the aqueous liquid in the alimentary
canal
In the aqueous environment lipid molecules
“stick together” or coalesce
The coalesced globules have relatively little
surface area compared to their overall volume
How do we digest lipids?
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Lipase is the enzyme that hydrolyzes
triglyceride lipids
The partially digested lipids are added to the
rest of the partially digested molecules in the
duodenum of the small intestine
When lipase encounters the coalesced
globules of it catalyzes the hydrolysis of the
lipid molecules on the outside
The interior molecules will rarely encounter a
lipase molecule
How do we digest lipids?
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Bile produced by the liver is added to the
partially digested food in the duodenum
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Bile molecules contain both hydrophobic and
hydrophilic ends, and thus are partially soluble in
both lipids and in water
Bile molecules insert themselves between lipid
molecules and prevent them from coalescing
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This process is known as emulsification
Emulsification doesn’t change the molecular structure of
lipids but converts them into smaller pieces
The same volume of lipid now has a much greater surface
area for lipase to attach to
How do we digest lipids?
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The overall structure of lipase is hydrophilic
because many amino acids are polar
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This allows lipase to be soluble in the aqueous
environment of the alimentary canal
However, the amino acids at the active site of
lipase are predominately non-polar which accepts
the hydrophobic substrate (lipid) into the relatively
hydrophobic active site
This solves yet another problem of digestion of lipid
molecules
What causes stomach ulcers?
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Until fairly recently, scientists believed that
nothing could live in the highly acidic
environment of our stomachs (~pH 2)
In 1982-1983 Dr. Barry J. Marshall and Dr. J.
Robin Warren isolated living bacterial cells
from the stomach lining of patients suffering
from stomach ulcers and gastritis
(inflammation of the stomach lining)
At that time most scientists believed stomach
ulcers and gastritis were caused by excess
hydrochloric acid, possibly brought on by
stress
What causes stomach ulcers?
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Marshall and Warren have since shown the following
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Helicobacter pylori is a bacterium that survives when
introduced into the stomach, it then burrows itself beneath
the mucus layer and infects the stomach lining cells
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Helicobacter pylori employs the enzyme urease the create
ammonia to help neutralize the stomach acid
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Helicobacter pylori infection of the stomach lining can
cause ulcers and gastritis
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Patients with antibiotics tend to respond well to treatment
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Patients with gastritis (infected with H. pylori) for many
years (~20-30 years) are much more prone to stomach
cancer than the general population
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H. pylori may be the most common bacterial infection in the
world, over 3 billion people are estimated to be infected
Dr. Marshall and Dr. Warren were awarded the Nobel Prize in
medicine in 2005 for their work
Overview of small intestine
structure
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The small intestine is composed of three
sections
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The short duodenum where secretions from
the pancreas and liver are added to the
partially digested contents
The jejunum
The ileum
Overview of small intestine
structure
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Partially digested food passes through
the small intestine because of
contractions of two layers of smooth
muscle within the walls
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These layers are the longitudinal and
circular muscle layers
The contraction of these muscle layers is
known as peristalsis, this keeps the food
moving throughout the alimentary canal
Overview of small intestine
structure
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The innermost cellular lining of the small
intestine is known as the intestinal
mucosa
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This tissue is in direct contact with the
partially digested food and is responsible for
absorption
The mucosa forms the villi of the small
intestine to increase the surface area for
absorption
Adaptations of villi epithelial cells
for efficient absorption
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Digested molecules must pass through
the villi cells and are absorbed by either
a capillary bed or a lacteal
Microvilli
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The surface of each villus cell that faces into
the lumen (cavity) of the small intestine
contains many microscopic finger-like
projections known as microvilli
The function of microvilli is to further
increase the surface area for absorption
Adaptations of villi epithelial cells
for efficient absorption
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Mitochondria and pinocytotic vesicles
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Some molecules absorbed through the villi are
absorbed using an active transport mechanism
The epithelial villi cells require ATP for active
transport which explains why they contain
mitochondria
Pinocytotic vesicles are visible near the plasma
membrane of the villi cells, pinocytosis is another
active transport mechanism used to absorb
molecules
Adaptations of villi epithelial cells
for efficient absorption
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Tight junctions
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Most cells in the body are surrounded by interstitial
fluid which allows molecules to pass between them
Epithelial villi cells have to provide a selective
barrier to prevent pass of intercellular fluid and
dissolved molecules between adjoining cells
Epithelial villi cells are sealed to each other by
membrane-to-membrane ‘seals’ called tight
junctions
Transport mechanisms used to
absorb foods
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Facilitated diffusion
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Many digested molecules are small enough to fit
through the membrane, their concentration
gradients permit diffusion, but their polarity prevents
passage through the hydrophobic interior of the
plasma membrane
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Protein channels within the microvilli membranes solve this
problem
These channels have relatively non-polar amino acids that
make up the outer perimeter of the channel and polar
amino acids forming the interior of the channel
This interior channel allows the appropriately sized, polar
molecules to diffuse from the lumen into the cytoplasm of
the villi cells
Transport mechanisms used to
absorb foods
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Active transport
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The plasma membrane of the villi cells also contain
membrane proteins that require ATP to transport
molecules across the membrane
Many molecules being absorbed do not have a
concentration gradient across the plasma
membrane
Active transport allows transportation across the
membrane regardless of molecular concentrations
The mitochondria within the villi cells produces the
ATP for this process
Transport mechanisms used to
absorb foods
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Pinocytosis
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Microvilli membranes also perform
endocytosis (pinocytosis)
Small droplets of fluids within the lumen are
surrounded by membrane and form
pinocytotic vesicles
These vesicles are taken into the cytoplasm
of the villus and later the contents are
released into the cytoplasm
Substances remaining after
digestion
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Some substances cannot be digested and are
never absorbed, these substances continue to
the large intestine and are released as feces
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Cellulose – in the cell walls of food from plants
Lignin – another component of plant cells
Bile pigments – give characteristic color of feces
Bacteria – normal inhabitants of the digestive
system
Intestinal cells – break off as foods move through
the lumen
Functions of the Liver
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Circulation of blood to and from the liver
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The liver receives blood from major blood vessels
and is drained by one major blood vessel. The
hepatic artery is a branch of the aorta and carries
oxygenated blood to the liver
The two blood vessels carry blood into the
‘capillaries’ of the liver which are called sinusoids
Sinusoids are then drained by the hepatic vein (the
only blood vessel that takes blood away from the
liver)
Functions of the Liver
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The hepatic portal vein receives the blood
from the capillaries within the villi of the small
intestine
The blood within the hepatic portal vein varies
in two ways from the blood that enters into the
liver from the hepatic artery
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it is low pressure, deoxygenated blood because
it has been through a capillary bed
it varies in quantity of nutrients (i.e. glucose)
depending on the types of food ingested and the
timing of ingestion, digestion, and absorption of
foods within the small intestine
Functions of the Liver
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Blood within the hepatic vein is also low
pressure, deoxygenated blood but it
doesn’t vary in nutrients as much as the
hepatic portal vein
Stabilization of nutrients within the
hepatic vein is one of the major functions
of the liver especially storage of nutrients
Functions of the Liver
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Sinusoids are the capillaries of the liver
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The function of the liver is to remove some
substances from the blood and add others
Hepatocytes (liver cells) are responsible for the
removal or addition of substances
Oxygen-rich blood from the hepatic artery and
nutrient rich blood from the hepatic portal vein both
flow into the sinusoids of the liver
Exchanges occur between the blood and the
hepatocytes within the sinusoids
Functions of the Liver
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Sinusoids differ from typical capillary beds:
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sinusoids are wider than capillaries
sinusoids are lined by endothelial cells with gaps
between them
the gaps allow large molecules (proteins for
example) to be exchanged between hepatocytes
and the bloodstream
hepatocytes are in direct contact with blood
components making all exchanges more efficient
Functions of the Liver
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sinusoids contain Kupffer cells that help
break down older red blood cells for
recycling their components
sinusoids receive a mixture of
oxygenated blood (from hepatic artery)
and nutrient rich blood (from hepatic
portal vein). This mixture drains into
small branches of the hepatic vein
Functions of the Liver
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Regulation of nutrients in the blood
– Solutes that are dissolved in blood plasma
vary a little in concentration, but each type of
solute has a normal range they must
maintain for homeostasis.
– Any concentration below or above this range
creates physiological problems within the
body
Functions of the Liver
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Glucose for example:
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For most people, glucose levels in the blood are
lowest in the morning and highest after meals
When a meal is digested that is high in
carbohydrates (starch for example) the hepatic
portal vein contains blood with a very high
concentration of glucose
When this blood enters the sinusoids of the liver,
some excess glucose is taken in by the surrounding
hepatocytes and converted into the polysaccharide
glycogen
This keeps glucose levels within the normal range
Functions of the Liver
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If you haven’t eaten carbohydrates for a long time blood
glucose levels decrease as cells are using glucose for cellular
respiration
To keep glucose levels within the normal range, the stored
glycogen is reconverted into glucose and added into the
bloodstream of the sinusoids
The homeostatic mechanisms that work to maintain glucose
levels are regulated by the hormones insulin and glucagon
from the pancreas
When blood glucose levels are in the upper end of the normal
range, insulin is produced to stimulate hepatocytes to take
glucose in and convert it to glycogen
When blood glucose levels are in the lower end of the normal
range, the pancreas produces glucagon to stimulate
hepatocytes to convert glycogen back into glucose
Nutrient storage
Nutrient
Relevant Information
Glycogen
Polysaccharide carbohydrate
Iron
Iron is removed and stored following
breakdown of erythrocytes and
hemoglobin molecules
Vitamin A
Associated with good vision, one of the
first signs of deficiency is ‘night blindness’
Vitamin D
Vitamin D is often added to milk and milk
products in some countries
Synthesis of plasma proteins and
cholesterol
Molecule(s)
Relevant Information
Plasma proteins
Albumin – helps regulate osmotic pressure of
fluids in the body
Fibrinogen – soluble form of blood clotting protein
which is converted to fibrin when clot is needed
Globulins – widely diverse group of blood proteins
not all of which are produced in the liver
Cholesterol
Some cholesterol is ingested and absorbed in
foods, some is synthesized in the liver
Some cholesterol is used to produce bile, while
some is carried in the bloodstream to be used for
cell membranes
Detoxification
Molecule
Source
Ethanol
Alcoholic drinks
Food preservatives
Added to foods to retard
spoiling
Pesticides
Often used on produce
Herbicides
Also used on produce
Functions of the Liver
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Alcohol consumption damages liver cells
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Drinking often and heavily damages liver cells
The hepatic portal vein brings absorbed alcohol first
to the liver
Any alcohol not removed is eventually brought back
through the liver sinusoids many times by way of
the portal artery (within the bloodstream)
Each time the blood passes through the liver,
hepatocytes attempt to remove the alcohol from the
bloodstream
Functions of the Liver
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Alcohol has a magnified effect on liver tissue
compared to other tissues in the body
Long term alcohol abuse results in three
primary effects on the liver:
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Cirrhosis – This is scar tissue left when areas of
hepatocytes, blood vessels, and ducts have been
destroyed by exposure to alcohol. Areas of the
liver that show cirrhosis no longer function.
Fat accumulation – Damaged areas of the liver will
build up fat in place of normal liver tissue
Inflammation – This is the swelling of damaged liver
tissue due to alcohol exposure; sometimes referred
to as alcoholic hepatitis
http://www.tuhc.com/CPM/cirrhosis-liver.gif
http://www.herbalprovider.com/imgs/yahoo/fatty-liver-cirrhosis.jpg
Functions of the Liver
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Frequency and volume of alcohol consumption
are both positively correlated with liver
damage
Data shows that females are more susceptible
to liver damage than males
Liver damage that is not too severe is at least
partially reversible (the liver can regenerate
some of its damaged areas if the person
ceases to consume excessive amounts of
alcohol)
Liver damage can be fatal when the damage
becomes too severe
Functions of the Liver
•
The liver recycles components of
erythrocytes and hemoglobin
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Erythrocytes have a typical life span of
about 4 months
Every red blood cell needs to be replaced
every 120 days or so by the tissue of the
bone marrow
This is because erythrocytes are anucleate
(have no nucleus) and cannot undergo
mitosis to form new blood cells
Functions of the Liver
•
•
•
•
•
As erythrocytes approach their life span the cell
membrane becomes weak and eventually ruptures
More often this occurs in the spleen or bone marrow,
but it can also happen anywhere in the bloodstream
When the red blood cell ruptures it releases
hemoglobin molecules
As the blood circulates through the sinusoids of the
liver the circulating hemoglobin molecules are
ingested by Kupffer cells within the sinusoids
This ingestion is by phagocytosis because hemoglobin
molecules are very large proteins
Functions of the Liver
•
•
Hemoglobin consists of four
polypeptides (globin) and a non-protein
molecular component at the center of
each globin called a heme group (at the
center of each heme group is an iron
atom
Hemoglobin consists of four globins, four
heme groups, and four iron atoms
http://www.sciencecases.org/tazswana/hemoglobin.gif
Functions of the Liver
•
Within the Kupffer cells hemoglobin is
disassembled into its component parts
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–
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The four globin proteins are hydrolyzed into amino
acids. The amino acids are released back into the
bloodstream to become available for any cell for
protein synthesis
The iron atom is removed from each heme group.
Some iron is stored within the liver and some is
sent to the bone marrow to be used in production of
new erythrocytes
Once iron is removed from hemoglobin, the
remnants of the molecule are called bilirubin or bile
pigment. This is absorbed by the hepatocytes and
becomes a key component of bile.