Download The Digestive System

The Digestive System
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Cloning in the media
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Systems in the body
The more complex an organism is, the more organized
its cells are.
Similar cells are often grouped together
to form a tissue, and several of these
tissues may form an organ.
Organ systems are groups of organs
which perform a specific function.
In mammals, several systems help to
exchange substances with the
environment. This includes the
exchange of gases in the respiratory
system and the exchange of food
molecules in the digestive system.
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The digestive system
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Digestive functions
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Peristalsis
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Diffusion
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Diffusion and digestion
Carbohydrates, proteins and fats are made up of large
molecules that cannot be readily absorbed by the body.
Digestion breaks down large food molecules into smaller
molecules such as glucose, amino acids and fatty
acids that can be easily absorbed.
In which part of the digestive system is most food absorbed?
Small food molecules are usually absorbed in the
small intestine, diffusing across the intestine wall
and into the bloodstream.
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How is the small intestine adapted?
The wall of the small intestine is lined with many tiny fingerlike projections called villi. These are very thin (as little as
one cell thick) and increase the surface area of the small
intestine, both of which increase the speed of diffusion.
villus
capillary network
small intestine
blood vessels
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Diffusion and the small intestine
The villi have a good network of capillaries. Digested food
molecules are absorbed into the blood to be taken
elsewhere in the body.
How does this happen?
The concentration of dissolved food molecules is higher in
the small intestine than in the blood entering the villus.
This means that the
dissolved food
molecules diffuse from
the small intestine into
the blood, moving from
higher to lower
concentration.
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glucose
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Friendly bacteria
Some people believe that having high numbers of certain
‘friendly’ bacteria (or probiotics) in the intestines can improve
health. The numbers of ‘friendly’ bacteria can be increased by:
 taking probiotic tablets containing bacteria, such as
Bifidobacteria and lactic acid bacteria (Lactobacillus)
 eating the sugars that these bacteria feed on. These
are called prebiotic oligosaccharides and are found
in oats, chicory, bananas, garlic and onions.
Prebiotics and probiotics are claimed to improve
digestion, prevent colon cancer, reduce cholesterol
and blood pressure, and improve immune function.
What is the evidence for these claims?
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Lowering cholesterol
Plant stanol esters are claimed to reduce levels of lowdensity cholesterol in the blood. High levels of cholesterol
can lead to cardiovascular diseases.
In 1999, a study compared the decrease in cholesterol levels
between people on a low-cholesterol diet only, and people
who ate the same diet and also ate plant sterol esters.
patient diet
reduction in cholesterol
special diet alone
7.7%
special diet and plant stanol esters
18.3%
What do these results suggest?
What more would you need to know about the study to draw a
firm conclusion?
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Glossary
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Multiple-choice quiz
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