Testing for reducing sugars
All monosaccharides and some disaccharides are
reducing sugars. They can donate electrons to
Benedict’s reagent (an alkaline solution of copper
(II) sulphate).
1. Grind up the food with water (if its not already a
liquid).
2. Add 2cm3 of the food sample to the same volume
of Benedict's reagent.
3. Heat in boiling water bath for 5 minutes.
If there is no reducing sugar present then the test
will be negative and the solution will remain blue.
If a reducing sugar is present then a positive test
will result in a precipitate the colour of which
depends on the concentration of reducing sugar….
Green (very low concentration)
Yellow (low concentration)
Orange - brown (medium concentration)
Brick red (high concentration)
The different colours mean that the Benedict’s
test is semi-quantitative and can be used to
estimate the approximate amount of reducing
sugar present in any sample.
Measuring the mass of the dried precipitate can
also be used to estimate the amount of reducing
sugar present in a sample.
Testing for non-reducing sugars
Some disaccharides are non-reducing sugars and they
do not give a positive test with Benedict’s reagent.
In order to detect a non-reducing sugar, first
demonstrate that a reducing sugar is not present by
conducting the Benedict’s test. If no colour change
occurs then:
1. Add 2cm3 of food sample to 2cm3 of dilute
hydrochloric acid. Heat in a boiling water bath for 5
minutes. (The acid will hydrolyse any disaccharide into
its monosaccharides).
2. Add drops of alkali (eg sodium hydrogen carbonate
solution) to neutralise the acid (check with pH paper).
3. Test the resulting solution by heating with
Benedict’s reagent in a boiling water bath. If a nonreducing sugar was originally present a positive test
should now result (eg orange-brown precipitate).
Identify if the following are reducing sugars and
whether they are monosaccharides or
dissacharides.
Sucrose (di-, non reducing)
Glucose (mono, reducing )
Fructose (mono, reducing)
Maltose (disacc, reducing)
Galactose (mono, reducing)
Lactose (disaccharide, reducing)
Carbohydrate digestion
Starch digestion
Chewing food creates a larger surface area
for salivary amylase (produced by the
salivary glands) to work.
amylase
Starch
maltose
In the stomach amylase is denatured by the
hydrochloric acid, so no further digestion
occurs.
In the small intestine starch digestion
continues with the arrival of pancreatic
amylase. It works in neutral pH conditions
provided by pancreatic juice and secretions
from the wall of the small intestine. Muscles
in the wall of the small intestine help to mix
the food with enzymes and move it along the
gut.
The epithelial cells lining the small intestine
produce the enzyme maltase..
maltase
Maltose
2 α glucose
Sucrose digestion
The disaccharide sucrose is a natural sugar found in
many plant cells. Chewing in the mouth helps to
break open the cell walls and release the sucrose.
Sucrose is digested by the enzyme sucrase which is
made by the epithelial cells lining the small
intestine.
sucrase
Sucrose
Glucose + fructose
Lactose digestion
Lactose is a disaccharide found in milk. It is
digested by the enzyme lactase which is produced
by the epithelial cells lining the small intestine.
lactase
Lactose
Glucose + galactose
Some people do not make enough/any lactase and
are lactose intolerant. Any lactose they consume
remains undigested and microbes in the large
intestine break it down releasing gas. Other
symptoms may include nausea and diarrhoea.
Absorption of carbohydrates in the small
intestine
Diffusion
As carbohydrates are digested there is
usually a high concentration of glucose in
the small intestine. The concentration of
glucose in the blood is usually low as it is
constantly being removed by cells for
respiration. In this situation glucose can
be absorbed by diffusion into the blood,
down its concentration gradient.
A favourable concentration gradient for
diffusion is maintained by….
1. The constant circulation of blood
removing glucose that has been absorbed
into the blood capillaries within the villi of
the small intestine.
2. Contraction of muscles in the small
intestine mixing of the contents of the gut
lumen so that food/glucose comes into
contact with the villi ready for
digestion/absorption.
Absorption of carbohydrates in the small intestine
Active transport
When the concentration of glucose in the small
intestine decreases, further glucose cannot be
absorbed by diffusion.
However, glucose can also be absorbed by active
transport. This mechanism utilises a sodium potassium ion pump.
1.
The sodium-potassium ion pump (a carrier
protein) uses energy from ATP to move sodium
ions out of the villi epithelial cells.
2. Sodium ions are now at a higher concentration
in the lumen of the small intestine, than in the
epithelial cells.
3. Sodium ions enter the
epithelial cells via a carrier
protein, moving down their
concentration gradient. A
molecule of glucose enters
with each sodium ion
(co-transport occurs).
4. Glucose is removed from
the epithelial cells through
a protein channel via
facilitated diffusion.
It then enters the blood.