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Carbohydrate is a type of organic compounds
Containing carbon, hydrogen and oxygen.
They can be regarded as the complexes
of carbon and water.
Because the ratio of atoms number
of hydrogen and oxygen
Is the same as that in water molecule (2:1).
Some simple carbohydrate molecules
Can be the building blocks of the
More complicated ones.
Then, these building blocks are called
Monosaccharides
As other organic compounds,
All monosaccharides have a carbon backbone.
Also, they are poly-alochols
(one molecule containing many alcohol groups.)
There is one alcohol group on each carbon atom
But
one.
That exceptional carbon may carry
An aldehyde group (if it is a terminal carbon)
Or a ketone group (if it is a central carbon).
If it carries an aldehyde group,
It is called an aldose.
If it carries a ketone group,
It is called a ketose.
Aldoses as glucose, ribose etc.
Ketoses as fructose and ribulose etc.
Monosaccharides can also be classified
According to the amount of carbon atoms
They contain.
The triose
丙醣
The tetrose 丁醣
The pentose 戊醣
The hexose 己醣
The heptose 更醣
C3H6O3 …….C3(H2O)3
C4H8O4 …….C4(H2O)4
C5H10O5 ……C5(H2O)5
C6H12O6 ……C6(H2O)6
C7H14O7 ……C7(H2O)7
The most commonly described types are :
Trioses, pentoses and hexoses.
Trioses are important metabolites. They are formed in
the processes of respiration and photosynthesis.
Its phosphate is called
Phosphoglyceraldehyde,
Or PGAL.
Pentose contains 5 carbon atoms (C5H10O5),
As ribose and ribulose.
It is used in
I Making RNA,
Ribonucleic acid.
Its phosphate,
Ribulose biphosphate, RuBP,
Is important in photosynthesis.
Structural isomers of hexose (C6H12O6).
Hexoses are always the compounds that provide
Immediate energy to the cells.
Physical property of monosaccharides.
(1) Monosaccharides are easily soluble in water
and give a sweet taste.
(2) After dissolving in water,
monosaccharides can move across the
plasma membrane of the cell.
(Or it it said that there is carrier enzyme on
plasma membrane that can carry monosaccharides
across the plasma membrane.)
(3) After dissolving in water, monosaccharide
would lower the water potential of the
water body.
(4) After dropped into water,
monosaccharides would coil up to form
pentagon rings or hexagon rings.
If it is a pentagonal ring,
It is said to be resemble the structure of a furan.
Then, the sugar is called a furanose.
If it is a hexagonal ring,
It is said to be resemble the structure of a pyran.
Then, the sugar is called a pyranose.
Glucose, when dissolved in water, would form
a hexagonal structure called glucopyranose.
This is the change.
If the alcohol group detached from
The 5th carbon is added to the 1st carbon
From the same side,
It is said to be the α-glucopyranose.
Otherwise, it is the β-glucopyanose.
Fructose will form a furanose when dissolved in water.
Α and β would be named as glucose.
(5) Optical activity (an optional property)
Optical activity describes
Whether the water solution of a compound
Can rotate the plane of polarized light.
If the water solution of a compound
Can rotate the plane of polarized light,
It is said to be optical active.
Otherwise, it is optical inactive.
An optical active compound
Must contain at least one asymmetric carbon.
An asymmetric carbon is the carbon atom
That is connected with 4 different groups, as glyceraldehyde.
The determination of optical rotation of a chemical.
Rotation of the plane of polarized light by a water solution
Of chemical with asymmetric carbon.
The mirror image of a chemical with
Asymmetric carbon cannot superimpose each other.
If one rotate the plane of polarized light clockwise,
It is said to be dextrorotatory and denoted with a “D”.
Otherwise, it is levorotatory and denoted with an “L”.
The D-amino acid and the L-amino acids are mirror
image to each other.
D-amino acid
L-amino acid
The angle of optical rotation of some sugars.
Chemical
D-glucose
Angle of optical rotation
+52.7
D-fructose
-92.4
Chemical property of monosaccharides.
The most important chemical property
Of monosaccharides is the
Reducing power.
The standard of the reducing power of carbohydrate
Is the power to reduce copper (II) ion (Cu2+)
In alkaline solution into the Cu2O brick-red precipitate.
If a sugar can do this,
It is said to be a reducing sugar.
If it cannot,
It is said to be a non-reducing sugar.
All monosaccharides are reducing sugars.
Benedict’s Test and Fehling’s Test are tests
Applying this concept.
So, they are said to be the
Tests for reducing sugars.
Two monosaccharides may condensed together
To form a single molecule.
This dimer molecule is called a
Disaccharides
In the process, one molecule of water is removed.
So, the reaction is called a condensation reaction.
The chemical bond formed is called a
Glycosidic bond (linkage).
Maltose is formed from the condensation of 2 glucose molecules.
Formation of -glycoside
Linkage in sucrose.
-glycoside bond can be
formed with  glucose.
Or, it can be
Written as …
There are three important disaccharides, maltose,
Sucrose and lactose.
condensation
2 glucose
1 maltose + water
condensation
1 glucose + 1 fructose
1 sucrose + water
condensation
1 glucose + 1 galactose
1 lactose + water
Physical property of disaccharides.
(1) Disaccharides are easily soluble in water
and give a sweet taste.
(2) After dissolving in water,
disaccharides cannot move across the
plasma membrane of the cell.
(Or it it said that there is no carrier enzyme on
plasma membrane that can carry disaccharides
across the plasma membrane.)
(3) After dissolving in water, disaccharide
would lower the water potential of the
water body.
(4) Also, all disaccharides are optical active.
Chemical property of disaccharides.
In maltose and lactose, there is still
One alcohol group attaching the 1st carbon
Not engaged in the formation of glycosidic bond.
It is reductable.
So, they are still reducing sugars.
In sucrose, both reductable alcohol groups
Are involved in the formation of glycosidic bond.
So, it is a non-reducing sugar.
The decomposition of disaccharides into monosaccharides
Is called hydrolysis,
Because one molecule of water should be added.
The hydrolysis of disaccharides can be achieved
By the addition of strong acids,
Or the corresponding enzymes.
1 Maltose
1 sucrose
maltase
Sucrase (invertase)
2 glucoses
1 glucose + 1 fructose
lactase
1 lactose
1 glucose + 1 galactose
The polymers of monosaccharides
Are polysaccharides,
As starch, glycogen, cellulose and amylose
Are polymers of glucose.
Inulin is the polymer of fructoses.
They all are
polysaccharides.
The branching structure of the amylopectin.
The starch molecule is a long chain,
while an amylopectin is a branched one.
The starch granules from different sources have different
appearance.
The chemical bond linking glucoses in cellulose is called
1-4glycoside linkage.
Amylose is the unbranched molecule. Its structure is the
same as starch, but just smaller.
Physical property of polysaccharides.
(1) Polysaccharides are insoluble in water
and cannot give any taste.
(2) As it cannot be dissolved in water,
polysaccharides cannot move across the
plasma membrane of the cell and
cannot change the water potential of the
water body.
(3) No optical activity can be observed.
Disaccharides and polysaccharides always serve as the
Energy stores.
Few organisms would store disaccharides inside bodies,
Because disaccharides would cause water problems.
Only sugar cane and sugar beet would store sucrose.
Starch is stored in plants,
while glycogen is stored in animals.