Carbohydrates
Carbohydrates
• “Hydrates of carbon” (C, H,
& O)
• Polyhydroxy aldehydes
(ALDOSES) or ketones
(KETOSES)
• Usually Cx(H2O)y
• “Sugars”
• Single unit:
Monosaccharide
• Two units: Disaccharide
• Three units: Trisaccharide,
etc.
• Many units: Polysaccharide
Carbohydrates
• Functions
– Energy stores, fuels, and metabolic
intermediates
– Ribose and deoxyribose serve as structural
framework to RNA and DNA
– Structural elements in the cell walls of
bacteria and plants
– Linked to lipids and proteins
• Mediates interactions between cells and other
elements in the cellular environment
Monosaccharides
• The simplest carbohydrates; aldehydes or
ketones that have two or more hydroxyl groups
Glucose Cyclizes
Fructose Cyclizes
Ribose and Deoxyribose
Glycosidic Bonds
Modified Monosaccharides
• Addition of substituents other than alcohols;
often found on cell surfaces
Disaccharides
Polysaccharides
• Energy storage and structural roles
• Are homopolymers if all of the
monosaccharides are the same,
heteropolymers if not.
Some Common Polysaccharides
•
Starch: glucose polymer (alpha)
– Potato, rice, wheat, corn
•
Glycogen: branched glucose
polymer
– Animal storage
•
Cellulose: glucose polymer (beta)
– Plant structures, paper, cotton,
wood
•
Chitin: Modified glucose (N-Acetyl
Glucose)
– Fungi cell wall, insect exoskeleton
Polysaccharides
Starch
A plant energy storage polysaccharide
• Two forms: amylose and amylopectin
• Most starch is 10-30% amylose and 7090% amylopectin
• Branches in amylopectin every 12-30
residues
• Amylose has alpha(1,4) links, one
reducing end
Starch
• Amylose-unbranched
Starch
• Amylopectin-branched
Starch
A plant storage polysaccharide
• Amylose is poorly soluble in water, but
forms micellar suspensions
• In these suspensions, amylose is helical
– iodine fits into the helices to produce a blue
color
Why branching in Starch?
Consider the phosphorylase reaction...
• Phosphorylase releases glucose-1-P products
from the amylose or amylopectin chains
• The more branches, the more sites for
phosphorylase attack
• Branches provide a mechanism for quickly
releasing (or storing) glucose units for (or from)
metabolism
Glycogen
•
•
•
•
•
The glucose storage device in animals
Glycogen constitutes up to 10% of liver mass and
1-2% of muscle mass
Glycogen is stored energy for the organism
Only difference from starch: number of branches
Alpha(1,6) branches every 8-12 residues
Like amylopectin, glycogen gives a red-violet color
with iodine
Dextrans
A small but significant difference from starch and glycogen
• If you change the main linkages between
glucose from alpha(1,4) to alpha(1,6), you
get a new family of polysaccharides dextrans
• Branches can be (1,2), (1,3), or (1,4)
Dextrans
A small but significant difference from starch and glycogen
• Dextrans formed by bacteria are
components of dental plaque
• Cross-linked dextrans are used as
"Sephadex" gels in column
chromatography
• These gels are up to 98% water!
Structural Polysaccharides
Composition similar to storage polysaccharides, but small
structural differences greatly influence properties
• Cellulose is the most abundant natural polymer on
earth
• Cellulose is the principal strength and support of
trees and plants
• Cellulose can also be soft and fuzzy - in cotton
Cellulose
Structural Polysaccharides
Composition similar to storage polysaccharides, but small
structural differences greatly influence properties
• Beta(1,4) linkages make all the difference!
• Strands of cellulose form extended ribbons
Other Structural
Polysaccharides
• Chitin - exoskeletons of crustaceans,
insects and spiders, and cell walls of fungi
– similar to cellulose, but C-2s are N-acetyl
– cellulose strands are parallel, chitins can be
parallell or antiparallel
Other Structural
Polysaccharides
• Alginates - Ca-binding polymers in algae
– Eg. CARRAGEENAN
• alternating 3-linked-α-D-galactopyranose and 4-linked-β-Dgalactopyranose units
• Sulfonated, uses cations to gel; Common types 
• Agarose and agaropectin - galactose polymers
Glycosaminoglycans
• Made of disaccharide repeating units containing
a derivative of an amino sugar, either
glucosamine or galactosamine
Glycosaminoglycans
• Present in the animal cell surface or the
extracellular matrix
• Usually attached to proteins to form
proteoglycans
• Proteoglycans resemble polysaccharides more
than proteins
• Function as lubricants and structural
components in connective tissue, mediate
adhesion of cells to the extracellular matrix, and
bind factors that stimulate cell proliferation
Glycoproteins
• Carbohydrates attached to proteins
• Carbohydrates are a much smaller percentage
of the weight of glycoproteins than of
proteoglycans
• Present in cell membranes
– Cell adhesion and the binding of sperm to
eggs
• Present in soluble proteins
CARBOHYDRATES
• Extra carbohydrates is stored in the liver
and muscle tissues as glycogen
• Carbohydrates supply 4 kcal of energy per
gram
• “Good” and “Bad” carbohydrates
• Glycemic index
On the “Sugarfree” Phenomenon
• Saccharin
– Oldest artificial
sweetener
• Aspartame and
Acesulfame-K
– Not carbohydrates
• Sucralose
– “Splenda”