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What is a Carbohydrate?

  • Most abundant organic molecule on the planet!
  • Simple carbohydrates: (C.H20)n where n>=3
  • Can also contain N, S or P
  • Monosaccharide = 1 sugar unit
  • Can link with other sugar units with glycosidic bonds
  • Disaccharide = 2 units
  • Oligosaccharide = app. 2-10 units
  • Polysaccharides = >=10 units
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Function of Carbohydrates - Metabolism


Energy is trapped in photosynthesis as carbohydrates

Carbohydrate is main metabolic fuel

Polysaccharides such as starch and glycogen store energy


Carbon skeleton for macromolecules.

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Function of Carbohydrates - Structure


Plant cell walls - cellulose

Bacterial cell walls - murein

Insect exoskeletons - chitin

Extracellular matrix - mucopolysaccharides


Glycoproteins and glycoplipids


Key for recognition as foreign

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Asymmetric carbon: has 4 different groups bound

Chrial: 4 different groups that can be organised to create mirror images

Enantiomers: stereoisomers that are mirror images of each other

Diastereoisomers: stereoisomers that are not mirror images (more than one chiral centre) and have different physical and chemical properties

Epimer: diastereoisomers that differ in configuration around 1 carbon only

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Oligo- and Poly- saccharides

Oligosaccharide: 2-10 monosaccharides joined by glycosidic bonds

Polysaccharide: More than 10 monosaccharides

Homo-polysaccharide: longer chains of all the SAME type of monosaccharide units

Hetero-polysaccharide: longer chains built up from MORE THAN ONE TYPE of monosaccharide unit

Storage polysaccharide: storage of chemical energy, e.g. starch and glycogen

Structural polysaccharide: role in structure of the cell, e.g. cellulose in plant cell walls

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Storage polysaccharides

Glucose cannot be stored as a monomer due to osmotic pressure.

Properties of storage molecules:

  • easily made and broken down
  • compact

Animals use glycogen

Plants use starch. Starch consists of two glucose polymers:

  • Amylopectin
  • Amylose
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Homopolymer of glucose with a glycosidic link between alpha(1-4) and alpha(1-6)

Branches every 24-30 glucose molecules

Very large molecule - 10^6 glucose molecules

Helix and branched structures - compact

Branches - lots of sites for degradation

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Homopolymer of glycose with glycosidic link between alpha(1-4) and alpha(1-6) 

Branches every 8-12 glucose molecules

Highly branched - easily broken down by glycogen phosphorylase to give glucose-1-PO4

Debranching enzyme for alpha(1-6) branch points

Liver glycogen maintains blood glucose levels (~100g)

Muscle glycogen used in strenuous activity (~400g)

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Structural polysaccharides


  • Insoluble
  • Strong
  • Rigid


  • Homopolymer of glucose with beta(1-4) links
  • Up to 15000 glucose units
  • Found in all plants and fungi
  • Forms long straight chains
  • Every other glucose residue flipped 180 degrees
  • Extended chain
  • Parallel chains closely packed
  • Intra- and intechain H-bonding gives strength
  • Bundles of chains stack to form large rigid fibres
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Chitin - homopolysaccharide

Major structural component of insect and crustacea exoskeletons.

Cell walls of fungi and algae

Linear polymer of N-acetyl glucasamine with beta(1-4) links

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Glycosaminoglycans (mucopolysaccharides)

Long unbranched chains of repeating disaccharide units.

One sugar is N-acetyl glucosamine or N-acetyl galactosamine

Many carboxyl and sulphate groups - many negative charges

Chain extended, highly hydrated - very viscous, slimy mucous like consistency, elastic

Found in:

  • Ground substance of extracellular spaces
  • Synovial fluid of joints
  • Vitreous humor of the eye
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Hyaluronic acid

Ground substance of connective tissue and synovial fluid

Repeating disaccharide unit

D-glucuronate B(1-4) n-acetyl-glycosamine

High negative charge

Highly hydrated

Viscosity changes with shear force - shock absorber

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  • On the cell surface
  • In exported proteins e.g. extracellular matrix and blood

Distinguish between N-linked (N-X-S/T) and O-linked (S/T)

Involved in folding, signalling, recognition, increasing solubility

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Proteoglycan: Bottlebrush Model

Cartilage network of collagen fibres linked to mucopolysaccharide

Collagen provides strength and proteoglycans give resistance to compression

As cartilage compressed:

  • Water squeezed out
  • Negative charges exposed
  • Repel each other and resist further compression
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Key in folding exported proteins

Possible outcomes of glycosylations:

  • Protein is ready for use
  • Protein needs folding
  • Protein is terminally misfolded
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