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Monosaccharides
& Polysaccharides
The Differences
Between Monosaccharides & Polysaccharides
By J. Dianne Dotson
Carbohydrates provide energy
and structure to living things. They are made of carbon, oxygen and hydrogen.
Monosaccharides comprise the
simplest carbohydrates, the building block molecules, and contain single sugar
units.
Disaccharides are made of
two sugar units, and polysaccharides contain several such units.
Monosaccharides are rare in
nature while polysaccharides are prevalent.
TL ; DR (Too Long; Didn't Read)
Monosaccharides and
polysaccharides comprise carbohydrates.
Monosaccharides are simple
sugar unit molecules, whereas polysaccharides are enormous, linking thousands
of sugar units.
Monosaccharides provide
cells with short-term energy.
Polysaccharides provide
long-term energy storage and rigid structure to cell walls and exoskeletons of
animals.
Molecular Features of
Monosaccharides and Polysaccharides
Monosaccharides contain at
least three carbon atoms.
Hexoses, the most common
monosaccharides, contain six carbons. Examples of hexoses include glucose,
galactose and fructose.
Glucose represents the chief
source for energy in cellular respiration, its small size granting it the
ability to enter cell membranes.
Fructose serves as a storage
sugar.
Pentoses contain five
carbons (such as ribose and deoxyribose), and trioses contain three carbons
(such as glyceraldehyde).
Monosaccharides are quite
small and form either chain or ring structures.
Polysaccharides, however,
contain hundreds or even thousands of monosaccharides and a high molecular
weight.
Energy Availability and
Storage
While monosaccharides such
as glucose provide short-term energy, polysaccharides provide longer storage of
energy.
Cells use monosaccharides
quickly. The molecules can bond to cell membrane lipids and aid in signaling.
But for longer storage,
monosaccharides must be converted to either disaccharides or polysaccharides
via condensation polymerization.
The polysaccharides become
too large to cross a cell membrane, hence their storage capability.
Starches represent polysaccharides
used by plants and their seeds to store energy.
Starches are made of the
glucose polymers, amylose and amylopectin.
Polysaccharides can be
broken down or hydrolyzed in the cell, as energy is needed in the form of
monosaccharides. This is how animals use plant starches to make glucose for
metabolism.
Polysaccharide Structures
and Functions
Cellulose, the most abundant
polysaccharide and organic molecule, may contain 50 percent of the world’s
carbon.
The base monosaccharide of
cellulose is glucose.
The straight cellulose
molecules make up rows in a stable form via the weak but prevalent hydrogen
bonds between them.
Made by plants, fungi and
algae, cellulose provides the rigid structure of plant cell walls, which also
protect against diseases.
Many animals cannot digest
cellulose, but those that can use intestinal micro-organisms and enzymes for
the task.
Fermentation occurs in the
colon of other animals and humans that cannot digest cellulose.
Animals produce a similar
polysaccharide, chitin, made from a modified monosaccharide.
Chitin comprises
exoskeletons. Both cellulose and chitin make up compact energy storage units.
Another polysaccharide,
glycogen, can be broken down from its compact form quickly into its constituent
glucose monosaccharides.
Humans store glycogen as a
rapid energy source in the liver and muscles.
Pectins, arabinoxylans,
xyloglucans and glucomannans represent additional complex polysaccharides.
Monosaccharides are soluble
in water, but many polysaccharides have poor water solubility in water.
Polysaccharides can form
gels, depending on their solubility. This is why they are often used to thicken
foods.
The Importance of
Monosaccharides and Polysaccharides
Both monosaccharides and
polysaccharides provide energy.
The monosaccharides yield
energy quickly for cells, while polysaccharides provide longer energy storage
and structural stability.
Both are essential to all
living things as the largest source of food and food energy.
Polysaccharides from cell
walls make up the fiber humans eat, while monosaccharides provide the sweetness
in foods.
As humans eat, chewing
breaks down polysaccharides into smaller particles that eventually, through
digestion, yield up the simple monosaccharides that can pass into the
bloodstream.
J.
Dianne Dotson is a science writer with
a degree in zoology/ecology and evolutionary biology. She spent nine years
working in laboratory and clinical research. A lifelong writer, Dianne is also
a content manager and science fiction & fantasy novelist. Dianne features
science as well as writing topics on her website, jdiannedotson.com.
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