Biological molecules: The sugars
The sugars
http://www.elmhurst.edu/~chm/vchembook/545polycarbo.html
Carbohydrates
General names for carbohydrates include sugars, starches, saccharides, and
polysachharides. The term saccharide is derived from the Latin word "sacchararum"
from the sweet taste of sugars.
The name "carbohydrate" means a "hydrate of carbon." The name derives from the
general formula of carbohydrate, Cx(H2O)y - x and y may or not be equal and range in
value from 3 to 12 or more. For example, glucose is C6(H2O)6 (written as C6H12O6).
1
Biological molecules: The sugars
Functions of carbohydrates
Carbohydrate are initially synthesised in plants from a complex series of reactions involving
photosynthesis
•
•
•
•
Store energy in the form of starch (photosynthesis in plants) or glycogen in animals (including humans)
Provide energy through metabolism pathways and cycles
supply carbon for synthesis of other compounds
Form structural components in cells and tissues
Photosynthesis basically uses water and carbon dioxide to form simple sugars. These
simple sugars can then be converted into other molecules such as starch, fat, protein,
DNA, and RNA - in other words all living matter!
Metabolism
Metabolism occurs in animals after the ingestion of plant or animal foods. Respiration in cells uses the
glucose to produce energy.
Combustion does much the same thing
2
Biological molecules: The sugars
Common saccharides
Monosaccharides
Glucose
from the Greek word for sweet wine; grape sugar, blood sugar, dextrose
Galactose
Greek word for milk - "galact", found as a component of lactose in milk
Fructose
Latin word for fruit - "fructus", also known as levulose, found in fruits and honey;
sweetest sugar
Ribose
Ribose and Deoxyribose are found in the backbone structure of RNA and DNA.
Disaccharides
Sucrose
French word for sugar - "sucre", a disaccharide containing glucose and
fructose; table sugar, cane sugar, beet sugar
Lactose
Latin word for milk "lact"; a dissaccharide found in milk containing glucose and
galactose.
Maltose
French word for "malt"; a disaccharide containing two units of glucose;
found in germinating grains, used to make beer.
Common polysaccharides
Starch
Plants store glucose as the polysaccharide starch.
Cellulose
The major component in the rigid cell walls in plants is cellulose
that is linear.
Glycogen
This is the animal version of starch in plants. It is mainly stored in the
liver and muscle
3
Biological molecules: The sugars
Sugar classification
Sugar classification can very, but the most useful is the following:
The carbohydrates are classified into groups according to the number of individual simple sugar units.
Monosaccharides contain a single unit, disaccharides contain two sugar units; and polysaccharides
contain many sugar units as polymers (usually of glucose)
Disaccharides
Monosaccharides
Polysaccharides
Glucose
Sucrose
Starch
Galactose
Maltose
Glycogen
Fructose
Lactose
Cellulose
Ribose
Glyceraldehyde
Monosaccharides can be further classified by the number of carbon atoms present.
6 - Hexose
5 - Pentose
Glucose
Ribose
3 - Triose
Glyceraldehyde
Galactose
Fructose
Functional groups:
Aldoses contain the aldehyde group
Ketoses contain the ketone group
Reducing contain a hemiacetal or hemiketal group (glucose, galactose, fructose, maltose, lactose)
Non-reducing contain no hemiacetal groups (sucrose and all polysaccharides)
4
Biological molecules: The sugars
Carbohydrates - isomers
Glyceraldehyde is the simplest carbohydrate and contains a chiral carbon. This forms the basis for naming
other isomers. It can exist in two forms that are mirror images of each other.
The absolute configuration is defined by D-glyceraldehyde. with the aldehyde group in the "up" direction,
the -OH group must project to the right side of the molecule for the D-isomer.
aldehyde group
chiral carbon
Monosaccharides are placed in the D-family if the -OH group nearest the chiral carbon projects
to the right.
5
Biological molecules: The sugars
Here is an example
Glucose
Glucose is the most common carbohydrate that is classified as a monosaccharide, an aldose, a
hexose, and a reducing sugar. It is also known as dextrose due to the way it rotates light to
the right.
It is also called blood sugar as it circulates at a concentration of 65 - 110 mg/mL blood.
Glucose is also the primary product from photosynthesis.
6
Biological molecules: The sugars
Glucose - a cyclic form
In aqueous solutions, only 0.02% of glucose exists in the chain form.
As carbohydrates contain alcohol and aldehyde or ketone groups, cyclic structures can easily be
formed.
The result is a 6 member ring consisting of 5 carbons and 1 oxygen.
Chair and boat structures
Alpha and Beta Glucose in the chair structure
The position of the -OH group on carbon # 1 is important.
When the cyclic structure is drawn, it looks like a chair or boat. It is the chair position that is used.
The Beta position is defined as the -OH being on the same side as carbon # 6. The Alpha position is
on the other side.
7
Biological molecules: The sugars
Galactose
Galactose exists more commonly as the disaccharide, lactose (milk sugar). It is found as the
monosaccharide in peas.
It is classified as a monosaccharide, an aldose, a hexose, and is a reducing sugar.
Galactosemia - Genetic Enzyme Deficiency:
1:18,000 babies do not have an enzyme that metabolises galactose from lactose. This builds
up in the blood and urine, leading to mental retardation, failure to grow, formation of
cataracts, and in severe cases, death by liver damage.
The treatment is to use formula milk that uses sucrose. Another enzyme that can metabolise
galactose is developed during maturation, so the treatment is only for infants.
Dextro
8
Biological molecules: The sugars
The pattern for galactose is the same as for glucose
The only difference between glucose and galactose is the position of the -OH group on
carbon # 4
9
Biological molecules: The sugars
Fructose
Fructose is often found together with glucose and sucrose in honey and fruit juices.
Fructose will join with glucose to form the disaccharide, sucrose
It used to be called levulose because it rotated light to the left (levo-)
It is classified as a monosaccharide, a ketose sugar, a hexose and is a reducing sugar
Bees gather nectar, which contains sucrose. They then use an enzyme to hydrolyse the
sucrose into glucose and fructose.
Ring structure of fructose
The chair form of fructose has a similar pattern as for glucose, but as there
is the ketone group on carbon 2, the ring closes here.
This results in a ring with 4 carbons and 1 oxygen.
Fructose forms a 5 ring structure because of the ketone group on carbon #2
10
Biological molecules: The sugars
The alpha and beta designation follows as with glucose
Ribose
Ribose and deoxyribose are the building blocks of the backbone chains in nucleic acids, RNA and DNA.
Ribose and deoxyribose are classified as monosachharides, aldoses, pentoses, and are reducing sugars
11
Biological molecules: The sugars
Ring structure for ribose
The chair form follows a similar pattern to glucose, with the ring closing at carbon 1. Because
ribose is a pentose, a 5 member ring is formed with 4 carbons and 1 oxygen
Ribose and deoxyribose
The -OH group on carbon 2 results in ribose. If the oxygen is missing, then deoxyribose is
formed
12
Biological molecules: The sugars
Maltose
Maltose is made from two glucose units and is the least common in nature. It is a reducing sugar.
Hydrolysis of starch
In the hydrolysis of a di- or poly saccharide, a water molecule helps break the acetal bond
shown.
break
13
Biological molecules: The sugars
Lactose
Lactose is made from galactose and glucose units. 4 -6 % in cow´s milk, and 5 -8 % in human
milk. It is also a by product from the manufacture of cheese.
Sucrose
Sucrose is made from the six member ring, glucose and the five member ring, fructose units. It is
commonly called table sugar, and can be obtained from sugar cane or sugar beet.
14
Biological molecules: The sugars
Invert sugar
When sucrose is hydrolysed, it forms a 1:1 mixture of glucose and fructose. This is the main
ingredient in honey. The mixture contains isomers which cause the light to interact differently
(inverted).
Starch
Plants store glucose as starch. This is made of 10-20% amylose, and 80-90% amylopectin.
Amylose forms a colloidal dispersion in hot water, whereas amylopectin does not dissolve.
Amylose exists of glucose units joined at carbon 1 and 4
15
Biological molecules: The sugars
Amylopectin
This has branches that join at carbons 1 and 6.
Amylopectin has branches every 12-20 units, whereas glycogen (in animals) occurs every 8-10 units.
Cellulose
It is made by plants and uses β- glucose to form straight chains that can bond to form layers.
16
Biological molecules: The sugars
remember, cellulose, O´s
together
17