They can be regarded as the complexesof carbon and water.

Carbohydrate is a type of organic compounds

Containing carbon, hydrogen and oxygen.

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 atomsThey contain.

The triose 丙醣 C3H6O3 …….C3(H2O)3

The tetrose 丁醣 C4H8O4 …….C4(H2O)4

The pentose 戊醣 C5H10O5 ……C5(H2O)5

The hexose 己醣 C6H12O6 ……C6(H2O)6

The heptose 更醣 C7H14O7 ……C7(H2O)7

The most commonly described types are :Trioses, pentoses and hexoses.

Trioses are important metabolites. They are formed inthe processes of respiration and photosynthesis.

Its phosphate is calledPhosphoglyceraldehyde,Or PGAL.

Pentose contains 5 carbon atoms (C5H10O5),As ribose and ribulose.

I

It is used inMaking RNA,Ribonucleic acid.

Its phosphate,Ribulose biphosphate, RuBP,Is important in photosynthesis.

Structural isomers of hexose (C6H12O6).

Hexoses are always the compounds that provideImmediate 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 compoundCan 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 atomThat 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 solutionOf chemical with asymmetric carbon.

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 mirror image of a chemical with Asymmetric carbon cannot superimpose each other.

The D-amino acid and the L-amino acids are mirrorimage to each other.

D-amino acid L-amino acid

Chemical Angle of optical rotation

D-glucose +52.7

D-fructose -92.4

The angle of optical rotation of some sugars.

Chemical property of monosaccharides.

The most important chemical property Of monosaccharides is theReducing power.

The standard of the reducing power of carbohydrateIs 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 testsApplying this concept.

So, they are said to be the Tests for reducing sugars.

Disaccharides

Two monosaccharides may condensed togetherTo form a single molecule.

This dimer molecule is called a

In the process, one molecule of water is removed.So, the reaction is called a condensation reaction.

The chemical bond formed is called aGlycosidic 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 beWritten as …

There are three important disaccharides, maltose,Sucrose and lactose.

2 glucose 1 maltose + watercondensation

1 glucose + 1 fructose 1 sucrose + watercondensation

1 glucose + 1 galactose 1 lactose + watercondensation

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 monosaccharidesIs called hydrolysis,Because one molecule of water should be added.

The hydrolysis of disaccharides can be achievedBy the addition of strong acids,Or the corresponding enzymes.

1 Maltose 2 glucoses

1 sucrose 1 glucose + 1 fructose

1 lactose 1 glucose + 1 galactose

maltase

Sucrase (invertase)

lactase

The polymers of monosaccharidesAre 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 differentappearance.

The chemical bond linking glucoses in cellulose is called1-4glycoside 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 theEnergy 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.