ChemIONS

Whats the reason for the abundance of co-valent bonds in biological molecules(for ex. like protiens, steroids, carbohydrates). ?

Carbon! Carbon is unique because it so readily hybridizes. Its electron config, 1s2 2s2 2p2 very readily forms a 2sp3 hybrid that let's it form 4 bonds. All of the compounds you indicated are carbon to carbon bonds (for the greatest part...). C-C is a perfect cov bond. C-N is also a cov bond. Do a quick search on the 'Net for a ton of more information on carbon's hydrization and unique ability to bond to itself.

Yes. Its mainly because of a process called 'Catenation'. The ability of carbon (or any element) to catenate is primarily based on the bond energy of the element to itself. For example, carbon has the ability to form both sigma and pi bonds to itself. This is due to an overlap between pi-electron orbitals, allowing electron density to be shared and thus stabilising the bond. Silicon, on the other hand, has negligible overlap between pi-orbitals, and thus tends to not form pi-bonds by preference. As a result, silicon has a relatively poor capacity for catenation.