Glucose is by far the most
common carbohydrate and classified as a monosaccharide,
an aldose, a hexose, and is a reducing sugar. It is also known
as dextrose, because it is dextrorotatory (meaning that as
an optical isomer is rotates plane
polarized light to the right and also an origin for the D
designation. Glucose is also called blood sugar as it circulates in the blood at a
concentration of 65-110 mg/mL of blood.
Glucose is initially synthesized by chlorophyll in plants using carbon dioxide from the air and
sunlight as an energy source. Glucose is further converted to
starch for storage.

Ring Structure for Glucose:

Up until now we have been
presenting the structure of
glucose as a chain. In reality, an
aqueous sugar solution contains
only 0.02% of the glucose in the
chain form, the majority of the
structure is in the cyclic chair
form.
Since carbohydrates contain both
alcohol and aldehyde or ketone
functional groups, the straight-
chain form is easily converted
into the chair form - hemiacetal
ring structure. Due to the
tetrahedral geometry of carbons
that ultimately make a 6
membered stable ring , the -OH
on carbon #5 is converted into
the ether linkage to close the
ring with carbon #1. This makes
a 6 member ring - five carbons
and one oxygen.
Steps in the ring closure
(hemiacetal synthesis):
1. The electrons on the alcohol
oxygen are used to bond the
carbon #1 to make an ether (red
oxygen atom).
2. The hydrogen (green) is
transferred to the carbonyl
oxygen (green) to make a new
alcohol group (green).
The chair structures are always
written with the orientation
depicted on the left to avoid
confusion.
Hemiacetal Functional Group:
Carbon # 1 is now called the
anomeric carbon and is the
center of a hemiacetal functional
group. A carbon that has both an
ether oxygen and an alcohol
group is a hemiacetal.

Glucose in the Chair Structures:

The position of the -OH group on
the anomeric carbon (#1) is an important distinction for carbohydrate chemistry. The Beta position is defined as
the -OH being on the same side of the ring as the C # 6. In the
chair structure this results in a horizontal projection.
The Alpha position is defined as the -OH being on the opposite
side of the ring as the C # 6. In the chair structure this results in a downward projection.
The alpha and beta label is not
applied to any other carbon -
only the anomeric carbon, in this
case # 1.