:-)
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oozing on the muggy shore of the gulf coast
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Morning wonderful CSers,
VILLI, con`t
Each villus is richly supplied with blood (via interstitial fluid)
which passes through a network of tiny tubes called capillaries. The
sugars and amino acids as well as vitamins, salts and some water pass
from the intestinal contents into the capillaries of the villus.
Although this transport may be accomplished by diffusion in some cases,
other mechanisms are involved also. Glucose, for example, continues to
be absorbed even when its concentration in the intestinal fluid becomes
less than its concentration in the blood (about 0.1%). This absorption
is by active transport. Pinocytosis may also play a significant role in
the absorption of digested foods. Remember that only one-third of the
ingested fats become hydrolyzed to fatty acids and glycerol. These
molecules, acting as bile salts, emulsify the remaining two-thirds into
tiny droplets. The droplets are then absorped, most likely by
pinocytosis, into the epithelial cells of the villus. Once inside, many
of the fatty acid and glycerol molecules become resynthesized into fat
in the smooth endoplasmic reticulum of these cells. These fat droplets
do not enter the blood capillary network. Instead they enter a duct, the
lacteal, whose walls are more porous. The lacteals of the villi are part
of the lymphatic system. Once inside the lacteals, the fat droplets are
carried slowly through the lymphatic system until it joins with the
blood circulory system. After a fat rich meal, the lacteals take on a
whitish, milky appearance because of the large number of fat droplets in
them.
Although the liver is not strictly an organ of digestion, its secretion
of bile plays an inportant role in the digestion of fats. When chyme
enters the duodenum, a hormone (cholecystokinin) is released to the
bloodstream from cells in the lining of the duodenum. This hormone
causes the gall bladder to contract and expel its contents into the
duodenum. Bile contains other materials besides bile salts. Amoung these
are the bile pigments, the products of the breakdown of the red blood
pigment,hemoglobin, removed by the liver from old red blood cells. The
brownish color of the bile pigments gives the feces their brown color.
Before the blood that leaves the villi reaches the general circulation
it passes through the liver. All blood from the intestines passes
through the liver for cleaning before getting back into the regular
bloodstream. Here extraordinary components,( nonnutritive molecules)
that are picked up by absorption from the intestine, or excess amounts
of ordinary components, are screened out. All monosacharides except
glocose (fructose,galactose) are removed by the liver and converted into
glucose. Any glucose in excess of the normal 0.1% concentration in the
blood is removed and converted into insoluble polysaccharide (glycogen).
Amino acids which are in excess of the body`s anabolic needs are
deaminated by the liver. In deamination, the nitrogen-containing amino
(-NH2) portion of the molecule is removed. This is then converted into a
nitrogenous waste,urea. The non-nitrogen-containing residue of the amino
acid can then enter the metabolic pathway of cellar respiration and be
oxidized for energy. Some fat-soluble vitamins, such as vitamins A and
D, that reach the liver in greater than normal amounts are removed and
stored by the liver. These vitamins can be released back into the blood
later as needed. The system of blood vessels that carries all the blood
from the intestines through the liver before passing it to the general
circulation is called the *hepatic portal* system. The portal tube
brings to the liver blood from the stomach,spleen, pancreas, and the
intestine. The blood brought to the liver by the portal tube is venous
blood. The liver screens all blood passing through it so that the
composition of the blood will be normal. For example, even after a rich
meal in strarches and sugars, the glucose level of the blood leaving the
liver will not be much higher than the normal 0.1%. If the liver
malfunctions in this reguard you have diabetes. If the concentration of
glucose in the blood should start to fall between meals, the liver will
hydrolyze enough of its glycogen reserves to maintain the level at
0.1%. NUCLEIC ACIDS
All organisms store the information specifying the structures of their
proteins in nucleic acids. Nucleic acids are long polymers of repeating
subunits called *nucleotides*. Each nucleotide is a composite molecule
of three smaller building blocks.
(1) A five-carbon sugar
(2) A phosphate group (PO4)
(3) An organic nitrogen-containing base
In the formation of a nucleic acid chain, the individual sugars are
linked together in a line by the phosphate groups. The phosphate group
of one sugar binds to the hydroxyl group of another. This bonding is
called a *phosphodiester* bond. A nucleic acid is simply a chain of
five-carbon sugars (called ribose sugars) linked by phosphodiester
bonds, with an organic base protruding from each sugar. This is called
DNA. The DNA of a steer (present in hamburger) contains all the building
blocks to build new genetic material in a growing child. But obviously
steer genes cannot be absorbed intact. They must be broken down into the
basic building blocks for reuse in our bodies. The process of breaking
the large macromolecules of proteins, carbohydrates,fats, and nucleic
acids is all part of digestion.
Three categories (groups) of enzymes take part in breaking down each
class of macromolecules.
The first are *endohydrolases* that attack the macromolecules at
internal positions in the polymer and break it into many pieces of
intermediate size. Second is the action of *exohydrolases* which remove
successive terminal building blocks (amino acids,sugars, or nucleotides)
from one or the other end of the pieces. The third group is the
dipeptidases and disaccharidases which hydrolyze dipeptides and
disaccharides into their component two amino acids or two sugar
molecules, respectively.
Bless you Bob Lee
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oozing on the muggy shore of the gulf coast
[email protected]
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