Carbohydrates

Carbohydrates fall into two groups: sugars and non-sugars, depending on their physical properties

1. sugars (which can be further sub-divided into:)

        a.    monosaccharides

• sweet, crystalline, soluble

        b.    disaccharides

• sweet, crystalline, soluble (but less so than monosaccharides)

2. non-sugars

polysaccharides

• not sweet, not crystalline, insoluble

monosaccharides

• Components are in the ratio = 1C:2H:1O 
  • so have a general formula of (CH₂O)_n where n is from 3 to 9 e.g. C₆H₁₂O₆

• All monosaccharides contain contain at least 2 hydroxyl (OH) groups and a carbonyl (C=O) group
  • The type of carbonyl group present results in a further sub-division into
• aldoses 
  • Aldoses contain an aldehyde group (H-C=O) e.g. Glucose

   

• ketoses 
  • Ketoses contain a ketone group (C=O) e.g. Fructose

• Monosaccharides can also be classified according to the number of carbon atoms they contain:
  
  • trioses (3C) e.g. glyceraldehyde and dihydroxyacetone
  • These have the general formula C₃H₆O₃
   
  • pentoses (5C) e.g. ribose
  • These have the general formula C₅H₁₀O₅
  • Note that deoxyribose (an extremely important sugar biologically - see DNA structure) has the formula  C₅H₁₀O₄ but is still classed as a pentose because it has 5 carbons and is considered to be a derivative of ribose

• hexoses (6C) e.g. glucose and fructose
• These have the general formula C₆H₁₂O₆

• isomers where two molecules have the same molecular formula but different structures, e.g. glucose and fructose, they are said to be isomers of each other.
 
• Structurally many monosaccharides can exist in more than one form, specifically as straight chain or as rings. 
• In general the ring forms predominate in biological systems but molecules can alternate between forms
• Unfortunately this gives us yet another way of describing the monosaccharides. This time describing the shape of the ring based on how many atoms are involved: 
  • a furanose ring has 5 members. This is characteristic of ketoses. The diagram below illustrates this

• a pyranose ring has 6 members. This is characteristic of aldoses. The diagram below shows glucose which can adopt one of two ring forms

• The two ring forms of glucose are called alpha-glucose and beta-glucose.
• These are stereoisomers - they have the same molecular AND structural formulae but the atoms are arranged differently in space

• all monosaccharides are reducing sugars because they have a carbonyl group which donates an electron to a recipient
• some disaccharides e.g. maltose can free a carbonyl group so are reducing sugars
• some e.g. sucrose can not free a carbonyl group so are non-reducing sugars

disaccharides

• formed when 2 monosaccharides are joined by condensation reaction
• involves the removal of water
• the bond formed is called a glycosidic bond 
• This an be either a 1,2 glycosidic bond or a 1,4 glycosidic bond depending on which carbon atoms are involved in the bond

• sucrose = is formed when glucose + fructose join together with a 1,2 glycosidic bond

• sucrose is the form in which sugars are transported around plants
• is the storage form in some plants (e.g. onion)

• maltose = is formed when glucose + glucose join together with a 1,4 glycosidic bond

• maltose is released when starch is digested. This involves numerous hydrolysis reactions (water is added) catalyzed by the enzyme amylase during 
  • digestion in animals
  • germination of seeds

   

• lactose = is formed from glucose + galactose and involves a 1,4 bond
• lactose is found in mammalian milk - so is an important part of an infant's diet

polysaccharides

• are polymers made up of monosaccharide monomers
• the monomers are joined together by multiple condensation reactions each resulting in a  glycosidic bond
• all (of the ones we are interested in) are polymers of glucose

starch is a mixture of 2 polysasccharides

• amylose: which is a polymer of alpha-glucose residues joined by 1,4 bonds
• amylopectin: which is a polymer of alpha-glucose residues mostly joined by 1,4 bonds but with branches provided by 1,6 bonds



• amylose
• makes up about 30% of starch
• is a helical structure
• the iodine test for starch works because the iodine molecules fit inside the helix of amylose and form a complex with it. The resultant complex has a different colour to that of iodine
• amylopectin
• makes up about 70% of starch
• is highly branched with a branch point every 20-30 residues. This results in a compact shape meaning that starch is an efficient storage molecule



• functions of starch
• is a carbohydrate store in plants because:
  • it is compact
  • it is insoluble
  • it has no osmotic effects
  • it is fairly unreactive so doesn't get involved in reactions
  • it is easily hydrolysed to soluble sugars as required
• starch is found present as starch grains inside amyloplasts in the cell cytoplasm

glycogen

• is a polymer of alpha-glucose monomers joined by 1,4 bonds 
• has branches provided by 1,6 bonds every 8-12 residues
• so is even more compact than amylopectin

• functions of glycogen
• is the energy store in animals
• is found present in liver cells and muscle
• is compact, insoluble, readily hydrolysed

cellulose

• is a polymer of beta-glucose joined by 1,4 bonds

• the molecules comprises straight, unbranched chains
• the chains have hydroxyl groups sticking out - this allows hydrogen bonding between chains holding the chains together into microfibrils



• functions of cellulose 
• as a structural component of plant cell walls providing tensile strength (i.e resisting pulling forces)
• in a plant cell wall the microfibrils are held together by matrix (of pectins and hemicelluloses) which cements them together



• cellulose is degraded by the enzyme cellulase 
• only fungi and bacteria produce cellulase