The BIG Four The BIG Four

The BIG Four Molecules of Life: Biopolymers

Dr. Dale Hancock [email protected] Room 377 Biochemistry building

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Fat Carbohydrate Protein Nucleic Acid

The BIG Four

General Biopolymer properties:

• Fat • Carbohydrate

•All linear biopolymers have a defined beginning and end. •Biopolymer synthesis is an anabolic process (requires energy input)

• Protein • Nucleic Acid

General Biopolymer properties cont.: •All biopolymers are synthesized in one direction only. •Some of the monomer is lost in polymerization, leaving a “residue” incorporated in the growing chain.

What is required for information containing biopolymers? Different side chains residue

Defined start

Atom with at least 3 bonding options

Common Backbone

Defined end

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Elements of Life

The unique properties of Carbon

• They all come from the first 2 to 3 rows of the periodic table. • C, H, O, N, P, S • They all have small atomic radii • They can form strong covalent bonds.

• Life depends on carbon • All major biopolymers have a substantially carbon backbone • It is not the most abundant element on the earth’s crust; O: 47%, Si: 28%, Al: 7.9%, Fe: 4.5%

Carbon in all its glory!!

The unique properties of Carbon • It can directly bond to itself and form long chains. • This property is known as caternation. • Side chains can “hang off” the polymer backbone



• To form a polymer an element must have at least 3 valence or bonding electrons; after all a chain requires 2 bonds. • Carbon forms 4 bonds. • It must also form strong bonds with itself; stronger than its bond with oxygen.

• There are 3 types of reactions that are most likely to occur on this planet: oxidation, hydrolysis and dissociation. The oxygen and water vapour in the atmosphere and the ambient temperature of ~20o make oxidation and hydrolysis the most likely reactions.

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So why is life carbon based and not silicon based?

• Carbon is the only element with 4 bonding electrons which can form strong bonds with itself. BUT • Doesn’t Silicon also form 4 bonds?

• Some geeks may argue life is silicon based!! • There is after all more silicon in the earth

So why is life carbon based and not silicon based?

The answer is glass!!

This is life..but not as we know it!

Why is life not silicon based? • Despite the fact that there is more silicon in the earth’s crust than carbon… • Silicon readily forms strong bonds with oxygen; stronger than with itself. • Most of the silicon on earth exists as silicates (O-Si-O) or glass (and sand!)

Why is life not silicon based? • Silicon has a larger atomic radius…why would that be significant?

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Why is life not silicon based? • Silicon has a larger atomic radius…why would that be significant? • The larger the atomic radius the weaker the covalent bond.

The unique properties of Carbon • Carbon compounds are relatively inert or kinetically stable to hydrolysis and oxidation. • In general organic reactions tend to be under kinetic control rather than thermodynamic control.

The unique properties of Carbon • This means that although the reaction is thermodynamically favourable (the product has less energy than the reactant) the rate of reaction is so slow as to be almost nonexistent, due to a high activation energy. This makes these reactions attractive for enzyme control.

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