Carbohydrates
Carbohydrates (literally hydrates of carbon) are chemical compounds which act as the primary
biological means of storing or consuming energy; other forms being via fat and protein. Relatively
complex carboyhydrates are known as polysaccharides.The simplest carbohydrates are
monosaccharides, which are small straight-chain aldehydes and ketones with many hydroxyl groups
added, usually one on each carbon except the functional group. Other carbohydrates are composed
of monosaccharide units, and break down under hydrolysis. These may be classified as
disaccharides, oligosaccharides, or polysaccharides, depending on whether they have two, several,
or many monosaccharide units.
Glucose
Fructose
Lactose
Sucrose
Directions: Choose one of the models below to represent its molecular structure by making a
visual representation. Make up a key that will help us understand what represents- C= Carbon,
H= Hydrogen, O= Oxygen . For instance White marshmallows might = Oxygen
Write up: Answer and be prepared to share these questions about your model. Refer to your text
for the following questions.
 How does the body need or use this sugar or carbohydrate? ( Synthesized)
 What is the food source for this carbohydrate?
 Is your sugar a monosaccharide or a disaccharide? Explain
 Using this handout create one other question related to your carbohydrate.
Fructose
Fructose, or levulose, is the form of sugar found in fruit and honey. It is a laevorotatory
monosaccharide with the same empirical formula as glucose but with a different structure. Although
fructose is a hexose (6 carbon atoms), it generally exists as a 5-membered hemiketal ring (a
furanose).
All fruit naturally contains a certain amount of fructose (often together with glucose), and it can be
extracted and concentrated to make an alternative sugar.
Fructose is often used in food products designed for people with diabetes mellitus or who have
problems with hypoglycaemia, because it is metabolised more slowly than cane sugar (sucrose) and
is sweeter, so it has a smaller effect on blood-sugar levels. However, some people can react badly
to fructose so it is not an option for those who need to restrict sucrose intake
Lactose is the sugar making up around 2-8% of the solids in milk. The name comes from the Latin
for milk, plus the -ose ending used to name sugars. Lactose is a disaccharide consisting of two
subunits, a galactose and a glucose linked together. Its empirical formula is C12H22O11 and its
molecular weight is 342.3. In the young of mammals, an enzyme called lactase is secreted by the
intestinal villi, and this enzyme cleaves the molecule into its two subunits for absorption.
Normally, as the young grow up, production of lactase gradually ceases, and they are then unable to
metabolise lactose. This is perhaps an evolutionary mechanism to enforce weaning of the young.
This loss of lactase on maturation is also the default pattern in most of the human race. However, in
some humans (mostly those with ancestry in the approximate geographic region of Europe, the
Middle East and India), the enzyme is retained in adulthood, and dairy products form a substantial
part of the adult diet. It would appear that millennia of animal husbandry in those regions, with the
milking of sheep, cattle, goats and water buffalo, has caused an evolutionary adaptation to an adult
diet containing milk. (This process of retaining infant characteristics into adulthood is one of the
simplest routes of evolutionary adaptation, and is known as neoteny) The fact that at least some
humans have made adaptations to lactose in the adult diet would, incidentally, appear to cast doubt
on some arguments by proponents of the so-called 'Stone-age diet', who argue that human
metabolic needs have not changed since the last ice age.
Sucrose Molecule
Sucrose (C12H22O11) is the chemical name of table sugar. Sucrose is a disaccharide; each molecule consists of two
"simple" sugars (a glucose and a fructose), called monosaccharides.
Composition
Sucrose is a disaccharide composed of one molecule of glucose connected via an α(1-2) glycosidic bond to one
molecule of fructose.
Production
Sucrose is generally extracted from sugar cane or sugar beet and then purified and crystallized. Other (minor)
commercial sources are sorghum and sugar maples.
Usage
Pure sucrose is the most common sweetener in the modern, industrialized world. People, and in fact most other
mammals except members of the cat family, will gladly accept a food sweetened with sucrose, even if they aren't
hungry. Processed food and junk food often have sucrose added.
Health effects
Sucrose has several adverse health effects. The most common is tooth decay, in which bacteria in the mouth turn
sucrose into acid that attacks tooth enamel. Sucrose has a high calorie content and is also believed to cause obesity.
People with diabetes mellitus need to control their intake of sucrose.
Sugar substitutes Because of the health effects of sucrose, several substitutes have been developed,
although none appear to be as versatile as sugar in cooking and they may have other health consequences.
Glucose
Glucose a simple monosaccharide sugar, is one of the most important carbohydrates and is used as
a source of energy in animals and plants. Glucose is one of the main products of photosynthesis and
starts respiration. The natural form (D-glucose) is also referred to as dextrose, especially in the food
industry.
A Haworth projection representation of the structure of glucose
Glucose (C6H12O6) is a hexose -- a monosaccharide containing six carbon atoms. Glucose is an
aldehyde (contains a -CHO group). Five of the carbons plus an oxygen atom form a loop called a
"pyranose ring", the most stable form for six-carbon aldoses. In this ring, each carbon is linked to
hydroxyl and hydrogen side groups with the exception of the fifth atom, which links to a 6th carbon
atom outside the ring, forming a CH2OH group. This ring structure exists in equilibrium with a more
reactive acyclic form, which makes up 0.0026% at pH 7.
Glucose is a ubiquitous fuel in biology. We can speculate on the reasons why glucose, and not
another monosaccharide such as fructose, is so widely used. Glucose can form from formaldehyde
under abiotic conditions, so it may well have been available to primitive biochemical systems.
Probably more important to advanced life is the low tendency of glucose, by comparison to other
hexose sugars, to nonspecifically react with the amino groups of proteins. This reaction
(glycosylation) reduces or destroys the function of many enzymes. The low rate of glycosylation is
due to glucose's preference for the less reactive cyclic isomer. In respiration, through a series of
enzyme-catalysed reactions, glucose is oxidized to eventually to form carbon dioxide and water,
yielding energy, mostly in the form of ATP.
Chemically joined together, glucose and fructose form sucrose. Starch, cellulose, and glycogen are
common glucose polymers (polysaccharides).
The older name dextrose arose because a solution of D-glucose rotates polarised light towards the
right. In the same vein D-fructose was called "levulose" because a solution of levulose rotates
polarised light to the left.
Isomerism
There are two enantiomers (mirror-image isomers) of the sugar -- D-glucose and L-glucose, but in
living organisms only the D-isomer is found. The ring structure may form in two different ways,
yielding ± (alpha) glucose and ² (beta) glucose. Structurally, they differ in the orientation of the
hydroxyl group linked to the first carbon in the ring. The ± form has the hydroxyl group "below" the
hydrogen (as the molecule is conventionally drawn, as in the figure above), while the ² form has the
hydroxyl group "above" the hydrogen. These two forms interconvert on a timescale of hours in
aqueous solution, to a ratio of ±: ² 36:64, in a process called mutarotation.
D-Glucose has the same configuration at its penultimate carbon as D-glyceraldehyde.
Synthesis
1. The product of photosynthesis in plants and some prokaryotes.
2. Formed in the liver by the breakdown of glycogen stores (Glucose polymers).
3. Synthesized in liver and kidneys from intermediates