Cell Biology - Carbohydrates

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Carbohydrates ­ Cell Biology
What are Carbohydrates?
The most abundant biomolecules on earth
Photosynthesis: >1 x 1011 tons (100 billion tons) of CO2 and H2O into cellulose and other
plant products
Carbohydrate polymers: structural and protective elements in cell walls of bacteria, plants
and animals
Sugars and starch: fuels for the central energy generating pathway of organisms
Complex carbohydrates attach to proteins to determine cellular localisation
Carbohydrates as informational molecules
Communicate between cells and their extracellular environments
Label proteins for cellular localization and retention in organelles
Label proteins for destruction
Serve as recognition sites for extracellular signalling molecules
Proteoglycans: found in extracellular matrix where one or more glucosaminoglycan chains
are covalently attached to a protein
Glycoproteins: one or several oligosaccharides of varying complexity covalently attached to
a protein
Glycolipids: membrane lipids in which the head groups are oligosaccharides
3 main classes of carbohydrates
Monosaccharides (simple sugars): a single polyhydroxy aldehyde or ketone unit
Oligosaccharides: short chains of monosaccharide units joined by glycosidic bonds.
Can bind to cell membrane and used in signaling
e.g. dissaccharides such as sucrose.

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Polysaccharides: sugar polymers containing 20 or more monosaccharide units.
e.g. starch, glycogen, cellulose
Carbohydrates Chemically are polyhydroxy aldehydes or ketones
Aldehyde functional group = R ­ COH
Ketone functional group = R ­ CO ­ R', where R/R' = C.
The simplest carbohydrates (only one sugar unit) either aldehydes (aldose) or ketones
(ketose). Most have a sweet taste.…read more

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Number of carbons
triose: 3 carbons
tetrose: 4 carbons
Pentose: 5 carbons
hexose: 6 carbons
heptose:7 carbons
Monosaccharides with 5 or more carbon atoms tend to have cyclic structures
The most common naturally occurring sugars are aldohexoses and aldopentoses
Monosaccharide Structure
The 3D structure of the sugar is very important
These are 2 molecules of glyceraldehyde, they have the same molecular formula and are
bonded to the same atoms (connectivity) but they are mirror images of one another.…read more

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The chiral carbon most distant from the carbonyl carbon (C=O) determines whether the sugar
is D or L. All D sugars have the OH group on the right; L sugars have the OH group on the left.
Most naturally occuring sugars (such as glucose) are D.…read more

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Hydroxyl group (­OH) of one cyclic monosaccharide can link with the hydroxyl group of
another to make a chain:
The a1 - 4 linkage shows which carbon atoms in each monosaccharide are linked together.
A C-O-C bond is known as an ETHER bond.…read more

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PolysaccharPolysaccharides
Most carbohydrates found in nature are polysaccharides (also called glycans)
They differ in their recurring monosaccharide units, in the types of bonding and the degree
of branching
Homopolysaccharides: contain only a single monomeric species, e.g. starch, glycogen,
cellulose
Heteropolysaccharidses: contain different monomeric species, e.g. peptidoglycans
Storage polysaccharides consists of two types of glucose polymer: amylose and amylopectin
Amylose: unbranched chains of D-glucose connected by a1- 4 linkages
Amylopectin: branched chains of D-glucose with a1-6 linkages
Types of Polysaccharides
1.…read more

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Structural Polysaccharides
Cellulose, chitin, and peptidoglycan form long strands with bonds between adjacent
strands
These strands may then be organized into fibres or layered in sheets to give cells and
organisms great strength and elasticity
Glycoproteins: Cell Identity
Although polysaccharides are unable to store information, they do display information on
the outer surface of cells in the form of glycoproteins ­ proteins joined to carbohydrates
by covalent bonds.…read more

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Breaking the C ­ C bonds and the C ­ OH bonds releases energy that can be "harvested"
into Adenosine TriPhosphate (ATP), the energy currency of the cell.…read more

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