Biological molecules

Carbon based life forms are made form molecules using carbon atoms

A carbon atom has 6 protons6 neutrons and 6 electrons

This gives 4 electrons in the outer shell so a carbon atom will make 4 covalent bonds to have a stable 8 electrons

Here carbon (blue) has made a covalent bond with each of 4 hydrogens (red)This makes a molecule called methane with a chemical formula of CH4

Methane is a 3-dimensional structure like this:

Which can be drawn like this: Or more simply like this:

Carbon can share an electron with another carbon, forming a covalent bond

Notice that there are now only 3 hydrogen atoms on each carbon CH3CH3 or C2H6

This is ethene

By forming covalent bonds between carbon atoms long carbon chains can be built

Notice that each carbon makes 4 bonds so the terminal carbons have 3 hydrogens, the central ones have 2

Other atoms like oxygen and nitrogen can also form covalent bonds with carbon

Oxygen requires 2 more electrons to form a stable shell so often forms a double bond, sharing 2 electrons with carbon which also shares 2 electrons

Double BondsThese can form between 2 carbons or between other atoms

Each carbon still has 4 bonds, but there are 2 bonds between the carbonsSo there is only space for 2 hydrogen atoms to attach to each carbonThis is EthyleneCH2CH2 or C2H4

There can even be a triple bond between carbon molecules

Notice that sometimes we simply draw a line without showing Hydrogen on the end – it is implied

Double Bonds can form between carbon and other

moleculesOr as part of a ring structure

Functional groupsThe order in which atoms attach to a carbon chain will determine the properties

of the molecule

Aldehyde group - CHO

This attaches to a carbon chain

This is a carbonyl group

Aldehydes often smell aromaticAcetone and some vitamins are aldehydes

Keto group - CO

A central carbon bonds to oxygenThis is another carbonyl group

Hydroxyl group - OH

This attaches to a carbon

Molecules with this group are called alcohols

Hydroxyl groups are important in making bonds by condensation

Molecules are named according to the position of the hydroxyl group

Hydroxyl is terminal

Hydroxyl is in the middle

Carboxyl group - COOH

These are called acidsThe hydrogen will dissociate and become a proton (H+) in solution – making the solution acidic

The oxygen left becomes positively charged giving the molecule an overall charge

Charged molecules are called polar molecules

Saturated Carbon chains have no double bonds

Unsaturated carbon chains have one or more double bonds and therefore fewer hydrogen

molecules

OPTICAL ISOMERISMThis may occur when 2 molecules have the same

molecular formula but different arrangements in space, forming mirror images.

If a central carbon has FOUR DIFFERENT groups attaches it will be asymmetrical, and may exist in two different forms.

The central black carbon has 4 different groups attachedBy rearranging these we can form isomers which are mirror images

Like a left and a right hand

This is important in biology because molecules must fit into enzymesIf the shape is wrong it would be like trying to put your left hand into a right-hand glove

Isomers change the angle of plane-polarised light;Bending it either right or leftThey are named accordingly

Molecules are actually based on the properties of the isomers of glyceraldehyde

D = rightL = left