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BB10a: Cells, Biomolecules & Genetics, 2003 Semester 1. Thursday 2 Oct. Lecture 4 Structures & functions of common carbohydrates Andrew Pearson. BB10A: Common carbohydrates. Basic units: called monosaccharides  are simple sugars such as glucose and fructose.

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bb10a cells biomolecules genetics 2003 semester 1
BB10a: Cells, Biomolecules & Genetics, 2003 Semester 1.
  • Thursday 2 Oct.
  • Lecture 4
  • Structures & functions of common carbohydrates
  • Andrew Pearson

BB10A: Common carbohydrates

Basic units: called monosaccharides

 aresimple sugars such as glucose and fructose.

The structures of monosaccharides can be drawn

in different ways, to highlight different aspects:

the Fischer projection is best for showing the

stereoisomerism of the various substituents;

but the Howarth projection is more useful

to biochemists because it approximates the 3-D

structures and helps explain stabilities.


BB10A: Common carbohydrates

Fischer projections


Properties of monosaccharides.

Containing many hydroxyl groups:

sugars interact easily with water:

they are hydrophilic & water-soluble.

Containing many hydroxyl groups:

sugars are highly oxidised and can be easily hydrolysed into fragments.

Containing many hydroxyl groups:

sugars are easily derivatised to give related molecules, including aminosugars, acetylated sugars, dimers, oligomers and polymers.

BB10A: Common carbohydrates


The relationship between the various isomers of sugars is not complicated to biochemists:

Biomolecules like sugars are selected because of their 3-D structure, and the wrong isomer simply does not fit into binding sites.

The distinction between D- and L- glucose is not something that many biochemists concern

themselves with nowadays: some bacterial products contain the odd L-sugar, all others are D-.

BB10A: Common carbohydrates


Precise 3dimensional fit between a substrate and enzyme.






Adapted From

Voet & Voet



two monosaccharides joined by

O-glycosidic bond.

Three occur naturally in stable form,

Lactose in breast milk (sugar transport)

Sucrose in plant sap (plant transport sugar)

Trehalose in fungi and insect heamolymph (for transport)

all others shown in books are digestion products of something bigger.

BB10A: Common carbohydrates


BB10A: Common carbohydrates








BB10A: Common carbohydrates

Glycogen, in animals, is similar but has a

greater degree of branching.


BB10A: Common carbohydrates

Cellulose is a linear homopolymer of glucose, but unlike amylose the O-glycosidic bond is beta in cellulose; alpha in amylose.


Cellulose: note the H-bonds.

A tetrasaccharide from heparan sulphate: note the substituted sugars.

Repeating unit of chitin: from insects and crustaceans


The raffinose series at the end of a number of “unusual” plant storage polysaccharides. Our intestinal digestive enzymes are unable to cleave stachyose into smaller units – but anaerobes in our lower intestines can..

BB10A: Common carbohydrates


The difference in size between proteoglycans

which are giant macromolecules

and glycoproteins,

which are small soluble protein macromolecules.

BB10A: Common carbohydrates


Functions of Monosaccharides

    • broken down for energy (by either fermentation or respiration)
    • interconverted/carbon skeleton used for biosyntheses
    • solvation of hydrophobes: in aqueous transport media such as serum
    • transport form of energy in animal serum, plant sap and intracellularly in general.  

BB10A: Common carbohydrates



  • short branched heteropolymeric chains of
  • monosaccharides, often derivatised,
  • with high informational content in their normal
  • hydrated state.
  • Functions:
  • markers for outer surface of cell membranes
  • protein targeting markers in endoplasmic reticulum system
  • prevention of membrane protein flip-flop
  • formation of glycocalyx

BB10A: Common carbohydrates



long chains of monosaccharides,

sometimes branched, either

Homopolymers (only one type of monosacch.)

(e.g. cellulose, glycogen, amylose, amylopectin)

or heteropolymers(more than one monosacch.)

(e.g. glycosaminoglycans, algal cell wall gels,

peptidoglycan, chitin)

BB10A: Common carbohydrates


BB10A: Common carbohydrates

  • Functions:
  • energy reserves in plants:
  • (starch and some of the flatulogenic polymers)
  • energy reserves in animals:
  • glycogen
  • structural in all except animals:
  • chitin, celluloses, peptidoglycan
  • lubrication and shock-absorbing
  • in synovial fluids, connective tissues

CHO function table

BB10A: Common carbohydrates