Notes on absorption unit 1
updated 04/08/2008
Absorption of the products of digestion
Active transport
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Energy is used
Transport against a concentration gradient.
Protein pumps change their shape
Pumps are specific
May transport two molecules at once
Active transport includes:
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Ca2+ pumps in muscles
Active reabsorption in nephrons (the kidney)
Absorption of the products of digestion
Sugar loading into phloem
Read Jones and Jones section 3.27 page 78
METHOD
USES
ENERGY
USES PROTEINS
SPECIFIC
CONTROLLABLE
Simple Diffusion
Osmosis
Facilitated Diffusion
Active Transport
Absorption
The stomach is not adapted for absorption but a few substances can pass through
into the blood e.g. alcohol and aspirin.
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Notes on absorption unit 1
updated 04/08/2008
The ileum is well adapted to absorption:
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Glucose is absorbed by indirect active transport. Sodium ions are pumped out of the
cells into the lumen of the gut and then re enter the cells down their diffusion
gradient carrying glucose molecules with them.
There is a diagram of this p79 Jones and Jones. Copy it here:
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Notes on absorption unit 1
updated 04/08/2008
More on cholera
The cholera bacterium is an example of a prokaryotic organism. Draw and label a
prokaryotic cell in this box:
Vibrio cholerae produces several toxins.
One of these is an enterotoxin.
Definition =
The enterotoxin has two parts – A and B. Part B binds to a receptor on the epithelial
cells of the small intestine, then part A penetrates into the cell. It binds to a protein
inside the cell called G protein, resulting in the stimulation of an enzyme called
adenylate cyclase. This enzyme produces a molecule called cyclic AMP from ATP,
which causes the cell to excrete chloride ions into the lumen of the small intestine
through proteins called chloride channels.
The secretion of chloride ions into the small intestine causes water to follow, down
the water potential gradient by osmosis, resulting in a large amount of extra water
being lost to the lumen of the small intestine. The large intestine cannot absorb this
extra fluid quickly enough = diarrhoea.
See diagram.
If you are interested in some further research or reading:
The symptoms of cholera are due to the effect of the toxin on chloride channels.
Cystic Fibrosis is an inherited disease, people with CF have defective chloride
channels. There is a suggestion that being a carrier of the CF gene may give some
protection against cholera. How?
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Notes on absorption unit 1
updated 04/08/2008
Oral rehydration solutions (ORS)
 What is oral rehydration therapy (ORT) and how does it work?
If a person contracts cholera, it is imperative that they are treated as quickly as
possible, since untreated cholera can have a case fatality rate as high as 50%.
Fortunately, effective treatment can reduce this rate to less than 1%.
ORT is the giving of fluid by mouth to prevent and/or correct the dehydration that
is a result of diarrhoea.
ORT does not stop the diarrhoea, but it replaces the lost fluids and essential salts.
The glucose contained in ORS enables the intestine to absorb the fluid and the salts
more efficiently (think about the glucose sodium co-transporter).
Between 1980 and 2000, ORT decreased the number of children under five dying of
diarrhea from 4.6 million worldwide to 1.8 million — a 60% reduction.
 How have more effective rehydration solutions been developed?
For more than 25 years UNICEF and WHO have recommended a single formulation
of glucose-based ORS to prevent or treat dehydration from diarrhoea. However,
treatment with ORS does not reduce the amount or duration of diarrhoea. For this
reason the ORS had less acceptance by mothers and health workers, preferring a
treatment that causes diarrhoea to stop.
During the past 20 years numerous studies have been undertaken to develop an
'improved' ORS. The goal was a product that would be at least as safe and effective
as standard ORS for preventing or treating dehydration from all types of diarrhoea
but which, in addition, would reduce diarrhoea. One successful approach was reducing
the solution's glucose and salt (NaCl) concentrations.
Some ORS have starch (such as rice flour) added instead of glucose. The starch is
digested in the small intestine to give a steady supply of glucose for transport, but
reduces the water potential of the mixture. This allows more water to be
reabsorbed in the intestines, reducing the amount of diarrhoea.
 Drug testing
ORS, like any drug, need to be tested before use.
Use the green textbook and the internet to answer the following questions:
1. What are the stages in drug development?
2. How do drug trials follow a regulated set of ethical procedures?
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