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Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Carbohydrates

Passing the mouse cursor over the diagrams and italicised text below should bring up more or less detail

Oligosaccharides is a general term to cover simple sugars:
oligo means few, saccharide means sugar


Carbohydrates include sugars (monosaccharides and disaccharides) - which have fairly simple molecules - and more complicated molecules called polysaccharides, which are built up by condensation.

Monosaccharide molecules




alphaglucoseallin (4K)
glucose

fructoseallin (4K)
fructose

the galactose molecule
galactose

Clicking below changes graphics
glucose vs galactose . . . simpler

A little calculation

Using the values below, work out the RMM (relative molecular mass) for glucose C6H12O6.
RAM (relative atomic mass): C=12, H=1, O=16
What is the RMM of glucose?


Carbohydrate monomers are called monosaccharides - single sugars.

Three examples of monosaccharides are: These are all hexoses - six-carbon sugars - sharing the same formula C6H12O6, and their molecules are normally considered to be in the shape of a ring, although these are in equilibrium with straight chain "stick" structures which are more reactive. Different monosaccharides differ in the arrangement of their rings and the direction of their H and OH groups - above or below the ring. In most of the diagrans on this page a single vertical line is used to show 2 bonds at the edge of the ring, but they may also be shown as a 'V' shape.

Glucose and galactose have a six-membered ring, including oxygen at one point and a -CH2OH group projecting off the ring at the back. * In glucose the OH group on carbon 4 (on the left hand side in these diagrams) projects downwards, whereas in galactose it projects upwards.

Fructose is usually displayed as a 5-membered ring, with -CH2OH groups extending out on either side.
These molecules are probably better visualised in three dimensions (references below) but you are expected to be able to draw simple versions of these outline diagrams.
Several categories of organic compounds can exist in two forms ('D' and 'L') - each one a mirror image of the other - and this applies to carbohydrates. However natural carbohydrates are only found in the the D-form.

Alpha and beta forms

In glucose and galactose, variation in the direction of -H and -OH groups at carbon 1 (on the right hand side in the diagrams below) results in different isomers: alpha and beta forms - alpha has -OH below the ring.
In fructose this variation is at carbon 2.

These alpha and beta forms do not differ in their chemical properties, but there is a difference when in combination with other monosaccharides - basically changing the direction of elongation.

3alphas (14K)
alpha forms of glucose, galactose and fructose (mouseover diagram for beta)


In fact chemical groups (aldehyde or ketone, exposed when the ring opens) attached to these carbon atoms (1 in glucose and galactose, 2 in fructose) are responsible for reacting with the standard test reagent for reducing sugars, as used in Benedict's test (last reference below). All monosaccharides are reducing sugars.

Biological function of monosaccharides

Glucose is used for the release of energy by respiration. The energy is effectively in the bonds between the atoms, released when the molecule is broken down and oxidised.

Disaccharide molecules

maltose (7K)
maltose: (two glucoses joined with an α1→4 linkage)

sucrose (5K)
sucrose: (glucose joined to fructose with an α1→β2 linkage)

lactose (5K)
lactose: (galactose joined to glucose with a β1→4 linkage)

Another little calculation

Work out the RMM (relative molecular mass) for maltose C12H22O11.
You can work it out as above, but there is an easier way!
What is the RMM of maltose?

A condensation reaction between 2 monosaccharides results in a disaccharide - "double sugar" - a dimer.

The bond between the two monosaccharides is called a glycosidic bond.
Since this is formed by condensation between two -OH groups (one of which provides -OH and the other just -H), there is a remaining -O- (oxygen atom) which forms a bridge between the two monosaccharide residues.

These bonds are given (pairs of) numbers [and often an arrow] to signify the positions of the bonding groups in each monosaccharide residue: typically 1→4 or 1→6. Most of these bonds involve the groups responsible for alpha/beta forms of at least one of the molecules, and this can affect the orientation of the next monosaccharide residue, so the bond description may also include α or β.

Q Why do we refer to these as residues?
A >
Because they have lost something (-H or -OH) in the process of condensation.

Three examples of disaccharides are: These all share the same chemical formula: C12H22O11.

These dimers share several characteristics with their monomers:
They are small(ish) in size, soluble (in water), sweet and sticky - all of which can be explained by the projecting OH groups.
They are also called sugars as well.

Maltose and lactose can be described as reducing sugars - they react with Benedict's reagent - as they have an unattached C1 on a glucose residue (at the right end of these diagrams).

However sucrose is a non-reducing sugar. The C1 of glucose is bonded with C2 of fructose, so sucrose does not react with Benedict's reagent. However, this bond can be fairly easily hydrolysed: either using dilute acid or the enzyme sucrase (invertase). This releases glucose and fructose, which are both reducing sugars.


In situations where sugars are released - in photosynthesising tissue in plants and in the digestion process in animals - the accumulation of sugars may result in a concentrated solution which may have damaging osmotic effects on cells, so sugars are converted into polysaccharides.

Polysaccharide molecules

16branch (18K)
A branching point: (glucoses joined with α1→4 linkages, but an α1→6 linkage gives a different direction)

Continuing the condensation process to combine much larger numbers (ofter thousands!) of monosaccharides results in the formation of different polysaccharides.
Glycosidic bond formation is usually between carbon atoms at the edges of the molecules - carbon 1 and 4 of glucose, resulting in a linear structure, but if a bond is also made with another carbon - number 6 - then the strand will branch.
Polysaccharides can have different functions: Three examples of polysaccharides are:
glyco (79K)
A small section of glycogen - showing all the -H and -OH groups
The branching of the molecule in 3 dimensions can be seen

Glycogen and starch

Both glycogen and starch (amylose, amylopectin) have a similar basic molecular structure.
Large sections of these molecules are held together by α 1,4 glycosidic bonds, resulting in a structure which appears linear on the page but in 3 dimensions it coils into a loose helix.

RSstarch (4K)
A simplified diagram of a section of starch - amylose, or amylopectin between branches

In glycogen and amylopectin, there are also occasional branches at α 1,6 glycosidic bonds - producing a compact rounded, bushy molecule.
There are many ends for addition/removal of glucose, by the action of digestive enzymes.

Being polymers, these are quite large molecules, and they remain in the cell where they are are formed by condensation - underlining their biological function of energy storage.

Also being insoluble these polysaccharides have no osmotic effect (do not affect water potential).

All the glucose residues in starch and glycogen are effectively bound, so these do not react with Benedict's reagent.

However starch and glycogent both do react with iodine solution (iodine in potassium iodide, I/KI solution). This migrates into the tubular section of the molecule. As a result a blue-black colour is formed, as distinct from the orange-red of the test reagent.

Cellulose

cellulosesimple (21K)
A small section of cellulose - showing all the -H and -OH groups
The flatness of the molecule can be seen

Cellulose has a different structure: it forms only β 1,4 glycosidic bonds which cause every other glucose residue to be inverted, resulting in a flat strap-like structure - long and straight chains. This forms into fibrils - linked together by many hydrogen bonds - with considerable tensile strength, and which are difficult to break down using enzymes. This is the main component of plant cell walls - cellulose provides strength to the cell wall.

RScellulose (4K)
A simplified diagram of a section of cellulose

Explain the difference in the structure of the starch molecule and the cellulose molecule shown above.
Starch is formed from condensation of α-glucose but cellulose is composed of β-glucose.
This is due to the position of hydrogen and hydroxyl groups on carbon atom 1 being inverted: -OH points down in alpha, up in beta.

Glycoproteins and Glycolipids

A prefix associated with carbohydrates is glyco-.
Cell membranes freqently have short sections of carbohydrates projecting from proteins or lipids into the extracellular space.
These are called glycoproteins and glycolipids, and they may play a part in cell communication and recognition.
These may stabilise the membrane structure through formation of hydrogen bonds with surrounding water molecules, or form receptors for specific hormones, neurotransmitters etc.
Glycoproteins can also serve as cell surface antigens. For example the ABO series of antigens in red blood cells are made up of groups of 4 or 5 sugar residues, with only minor differences.
A side issue:
Find out what Glycyrrhiza glabra is: why it is so named, and what product is derived from it (and how that got its name).

Interactive 3-D molecular graphic models on this site

(also accessible from the drop-down menu above)

Glucose
Alpha and Beta Glucose molecules
Fructose
Galactose and Glucose comparison

Maltose
Sucrose
Lactose
Cellobiose (a dimer from cellulose)

Glycogen
Amylose
Amylopectin
Cellulose

Other relevant topics on this site

Introduction to Biochemistry
Molecular structure of glucose and other carbohydrates
Food Tests

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