Carbohydrates Flashcards
Functions of carbohydrates
1) Energy stores (glycogen, starch).
2) Components of structural material
(e.g. chitin for insect exoskeletons).
3) Parts of other important
macromolecules – DNA and RNA,
glycoproteins, and glycolipids.
4) Signalling function (recognition of cell surface molecules on other cells)
general formula for carbohydrates
C: H O:
1 : 2: 1
carbohydrates are hydrates of carbon with the
general formula Cx(H2O)x
Characteristics of monosaccharides
- Monosaccharides are colourless, crystalline solids that are
soluble in water and other polar solutions – in other words
monosaccharides are hydrophilic. - Monosaccharides have names ending with “ose”.
Glucose, Ribose, Deoxyribose…. - Most have a sweet taste
Continued
- The backbone of the monosaccharide is a carbon chain with
all carbons joined by single covalent bonds.
Glucose when solubilised, they form a ring structures
5- Monosaccharide chains contain a single carbonyl group
(C=O)
If it contains an aldehyde
carbonyl group.
aldose
D-Glyceraldehyde
If it contains a ketone
carbonyl group
ketose
Dihydroxyacetone
Common 6-carbon monosaccharides
Glucose
Most abundant sugar in nature; in many foods, e.g.
bread, pasta, fruit, cereals.
Most of the carbohydrates we eat are converted into
glucose for energy.
Fructose
Naturally occurring in foods such as honey & fruit.
Galactose
In nature primarily found as part of lactose (milk sugar).
Mannose A by-product of metabolism, also found in some fruitsand vegetables.
Monosaccharides have chirality
The chiral carbon which
determines whether a
carbohydrate is in the D
or L configuration is the
one furthest from the
carbonyl group
In glucose
Carbons 2-5 are chiral
C6 = Not a chiral carbon. Chiral
carbon should have 4
different atoms/groups
attached to it!
Which chiral carbon
determines D/L
configuration?
C5
D-glucose and L-glucose
are enantiomers.
Enantiomers
1- Do they have the same chemical formula? YES
2- Do they have chiral carbons? YES
3- Can they be superimposed? NO
4- Are they mirror images? NO
diastereomers.
1- Do they have the same chemical formula?
2- Do they have chiral carbons?
3- Can they be superimposed?
4- Are they mirror images?
YES
YES
NO
NO
These molecules are called diastereomers.
Hexoses…..
Same Chemical Forumula
How many diastereomers can a molecule have?
A molecule with n chiral carbons has (2n-2) diastereomers.
Example:
A molecule with 4 chiral carbons will have 24-2 = 14 diastereomers.
carbon derived from the carbonyl carbon is known as
the anomeric carbon
D-glucose
The ring has 6 members
Rings with 6 members are
called pyranose rings.
Fructose
Fructose is a ketose
The ring has 5 members
Rings with 5 members are
called furanose rings.
Common furanose monosaccharides
Deoxyribose is part of the structure of DNA
Ribose is part of RNA.
Forming carbohydrate polymers
Monosaccharides are joined together to form
disaccharides, oligosaccharides and polysaccharides by
condensation reactions.
Covalent bonds that form between a carbohydrate and
another molecule are called glycosidic bonds (this
other molecule does not need to be another
carbohydrate).
Maltose
Condensation reaction between α-D-glucose + either α or β-D-glucose
Covalent bond is α-1,4-glycosidic
Formula is: C12H22O11
Sucrose
Condensation reaction between α-D-glucose + b-D-fructose
Covalent bond is α-1,2-glycosidic
Formula is: C12H22O11
Lactose
Condensation reaction between b-D-galactose + a/b-D-glucose
Covalent bond is b-1,4-glycosidic
Formula is: C12H22O11
Cellobiose
Condensation reaction between b-D-glucose + b-D-glucose
Covalent bond is b-1,4-glycosidic
Formula is: C12H22O11
Why are some bonds named as a bonds and some b?
alpha glycosidic bonds are formed when the OH on the C1
(anomeric carbon) is below the ring
beta glycosidic bonds are formed when the OH on the C1
(anomeric carbon) is above the ring.
Starch
energy store of plants
Very large polymer of a-D-glucose.
Amylopectin: Branched polymer (every 24-30 glucose
molecules there is a branch).
Glucose monomers are linked through a-1,4-
glycosidic bonds.
The bond linking branches to the main chain is an
a-1,6-glycosidic bond.
Amylose: Linear polymer. Glucose monomers are
linked through a-1,4-glycosidic bonds.