Carbohydrates
• Major source of energy from our diet
• Most abundant organic compounds in nature
Biology 12
Allen
• Serve both as structural compounds and as
energy reserves to fuel life processes
Biological Macromolecules:
• Composed of the elements C, H and O in a
1:2:1 ratio. Empirical formula is (CH2O)n
Carbohydrates
See Chap 1.2 in your text: Biology 12
• Alpha (α) glucose, a six carbon sugar (C6H12O6)
is the immediate energy source to cells.
• Millions of tons produced by plants & algae
every year by via photosynthesis
• saccharide and the suffix ose refer to sugar. i.e.
glucose or monosaccharide.
Types of Carbohydrates
• Monosaccharides
• Oligosaccharides
Contain 2 or 3 monosaccharide units
Monosaccharides
• Carbohydrate monomers are called monosaccharides.
• distinguished by the carbonyl group they posses and the
number of atoms in their carbon backbone.
• carbonyl groups: aldehyde or ketone
aldehyde
• Polysaccharides
Contain many monosaccharide units
ketone
1
…Monosaccharides
• Distinguished by the number of atoms in their carbon
backbone.
• Three carbons = Triose
• Four carbons
= Tetrose
• Five carbons
= Pentose
• Six carbons
= Hexose
aldehyde
Aldoses are
monosaccharides that
contain an aldehyde
group
ketone
Ketoses are
monosaccharide
s that contain a
ketone group
…Monosaccharides
• When fats are catabolized (broken down)
for energy, they produce ketones.
Excessive amounts of ketones in the blood
is called ketoacidosis which lowers pH.
• Make a connection between Diabetes
ketoacidosis.
• The two simplest monosaccharides
are…
– dihydroxyacetone (a triose with a
ketone group) and…
– glyceraldehyde (a triose with an
aldehyde group)
2
…Monosaccharides
• Aldoses are monosaccharides with an aldehyde
group and many hydroxyl (-OH) groups.
Learning Check
Identify each as tetrose, pentose or
hexose, and as aldose or ketose
H
• Ketoses are monosaccharides with a ketone group
and many hydroxyl (-OH) groups.
C O
H C OH
H C OH
H C OH
H C OH
CH2OH
A
CH 2OH
C O
HO C H
H C OH
CH 2OH
B
…Monosaccharides:
…Monosaccharides: linear vs. ring structures
linear vs. ring structures
• When glucose dissolves in water, the hydroxyl group on carbon 5
reacts with the aldehyde group at carbon 1 to form a closed, sixmembered ring.
• When this occurs, there is a 50% chance that the hydroxyl group at
carbon 1 will end up below the plane of the ring.
– Glucose with hydroxyl group below ring Æ α-glucose (α = alpha).
– Glucose with hydroxyl group above ring Æ β-glucose is formed (β =
Beta).
• Monosaccharides with five or more carbons are linear
molecules in the dry state, but readily form ring
structures when dissolved in water.
3
Learning Check
Oligosaccharides
• sugars containing two or three simple sugars
attached to one another by covalent bonds called
glycosidic linkages.
• Oligosaccharides include disaccharides, a sugar
made of two monosaccharides.
– Some disaccharides include..
Is this α-glucose or β-Glucose?
Glycosidic linkages
• Glycosidic linkages are bonds formed by condensation
reactions, in which the H atom comes from a hydroxyl group
on one sugar and the –OH group comes from a hydroxyl
group on the other. Both the examples below are 1,4
glycosidic linkages because the bond occurs between the 1st
and 4th carbons.
• Sucrose Æ glucose + fructose
• Lactose Æ glucose + galactose
• Maltose Æ glucose + glucose
these
monosaccharides
are monomers
Polysaccharides
• AKA complex carbohydrates
• Several hundred to several thousand
monosaccharide subunits held together by
glycosidic linkages.
• Some are chains, some are branched
4
…Polysaccharides
… Polysaccharides
• Amylose is an unbranched α-glucose polymer held
together by α-glucose 1,4 glycosidic linkages
• Amylopectin is a branched α-glucose polymer
composed of a main chain with glucose molecules
attached by α 1–4 glycosidic bonds and branch points
formed by α 1–6 glycosidic linkages
• Starch is a polysaccharide made of 20% amylose and
80% amylopectin by mass.
• Plants produce more carbohydrates than they need.
They use enzymes to link excess glucose molecules
and store it as insoluble starch granules.
… Polysaccharides
• Humans and other heterotrophs
break down starch into glucose and
use it for energy in cellular
respiration.
• A small amount of the glucose can
be converted to glycogen, another
polysaccharide, and stored in the
liver and muscles.
• Glycogen is similar to amylopectin
but it has more branches.
… Polysaccharides
• Cellulose is a straight chain polymer of β-glucose
held together by β 1–4 glycosidic linkages.
• Hydroxyl groups at the 1 and 4 positions in β-glucose
cause every other monomer to be inverted for the
glycosidic linkage to form.
5
Learning Check
• Why is every other monomer inverted in
cellulose?
… Polysaccharides
• Cellulose has a straight
shape. This allows
hydrogen bonds to form
between the hydroxyl
groups of parallel
molecules. Why does
this make cellulose
suitable for cell walls?
6