Biological Macromolecules

There are four major classes of macromolecules in living organisms: carbohydrates, lipids, proteins, and nucleic acids. Macromolecules are large molecules which composed of a large number of repeating subunits; you can call them polymers (except lipids).
Reactions of Polymers:
Dehydration reaction (condensation reaction, anabolic reaction):
monomer(-OH) + monomer(-H) + energy → polymer + H2O
Hydrolysis (catabolic reaction):
polymer +H2O → monomer(-OH) + monomer(-H) + energy
*Enzymes can act as catalysts in both reactions.


Carbohydrates:Carbohydrates are produced by plants through the process of photosynthesis, and act as the energy source, structural compounds and raw materials. Carbohydrates can be classified into three groups: monosaccharaides, disaccharides, and polysaccharides.

Monosaccharaide: monosaccharaides are simple sugars which contain a single chain of carbon (three to seven carbon atoms), multiple hydroxyl groups, and a carbonyl group(C=O). If the carbonyl group is attached to the end of the carbon chain, it must be an aldose; if it is located on the carbon chain, it is ketoses. Aldoses and ketoses are isomers if they contain same number of carbon atoms. Ex: glucose, galactose and fructose are isomers which have the same chemical formula C6H12O6.

Disaccharide: disaccharides are sugars contain two monosaccharaides which attached to one another by glycosidic linkages. There are three major disaccharides: maltose, sucrose, and lactose.
α-glucose + α-glucose → maltose (held by an α 1-4 glycosidic linkage)
Ex: Beer
α-glucose + α-fructose → sucrose (held by an α 1-2 glycosidic linkage)
Ex: Table sugar
α-glucose + α-galactose → lactose
Ex: Milk

Polysaccharide: polysaccharides are monosaccharide polymers and held by glycosidic linkages. They can either store energy or act as a structural compound.
  1)Energy storage: excess α-glucose molecules are linked by α 1-4 glycosidic linkage in the main chain and α 1-4 glycosidic linkage at the branched points. Starch and glycogen can both be digested into single glucose by enzymes.
Starch: (in plant, monomer: α glucose) starch is a mixture of two different polysaccharides- amylose (unbranched) and amylopectin (branched).
Glycogen: (in human and animals, monomer: α glucose) the structure of glycogen is similar to amylopectin (branched). Glycogen can be transferred to energy during physical exercise.


polysaccharides

 2)Structural compound: unlike starch and glycogen, cellulose and chitin cannot be digested by the enzymes in human body, thus they are used to build cell walls or hard exoskeleton.
Cellulose: (in plant, monomer: β glucose) the monomers in cellulose are held by β1-4 glycosidic linkage. Because cellulose is a straight chain polymer and neither coiled nor branched, also have the hydroxyl groups formed hydrogen bonds between parallel molecules, cellulose can produce micro fibrils to build cell walls.
Chitin: (in animals, monomer: N-acetylglucosamine) chitin can be found in the hard exoskeleton of insects or crustaceans, also in some medical applications.