Complex Molecules

Complex Molecules

Carbon atoms have a really special property.

They can form the backbone of long, complex molecules, and it’s with these molecules that chemistry gets really interesting.

Carbon can do this because it has four outer electrons, so it can link up with other carbon atoms to form long chains, and still have room for atoms on either side.

A simple carbon chain, with hydrogen atoms on each side.
A simple carbon chain, with hydrogen atoms on each side.

Scientists say that life on Earth is ‘carbon-based’ because the molecules that make us, like proteins, carbs, fats, and DNA are all based on enormous, branching carbon chains.

The simplest type of carbon chains are called ‘hydrocarbons’. They are chains of carbon atoms with a hydrogen atom on each side. When the chains are short, we call them fossil fuels. When the chains are long, we call them plastics.

In other words, these chained molecules have a huge range of uses. They power our world, and make most of our products, which might make them the most significant class of chemicals in modern civilisation.

Some complex hydrocarbon chains. Black are carbon atoms and white are hydrogen atoms. 
Some complex hydrocarbon chains. Black are carbon atoms and white are hydrogen atoms. Photo by BBC Science

Small differences in the length of the chains give them very chemical different attributes.

Natural gas is just carbon atom long (C1), Gasoline is four to twelve long (C4-C12), Kerosene is C10-C16, and engine oil is C18-C34.

Each of them occur naturally in a jumbled mix called crude oil. It’s extracted from the ground and separated and categorised, a process known as oil refinement.

Short chain hydrocarbons burn easily. The reaction releases carbon dioxide and a ton of energy, and with the invention of the combustion engine they sparked the Industrial Revolution.

Natural Gas has just one carbon atom, C1
Natural Gas has just one carbon atom, C1

Plastics can be thousands of carbon atoms long. They almost never occur naturally, we make them synthetically by combining smaller hydrocarbons. Plastic’s main advantage is that they can be cheaply moulded into any shape from chairs to beach balls. They’ve been a breakthrough in manufacturing and they play a role in almost every advanced technology.

The two basic types of plastics are Low Densite Polyethylene (LDPE) and High Density Polyethylene (HDPE). LDPE is thin and flexible, and is used for plastic bags, cling wrap, and milk cartons. HDPE is hard and rigid, and is used for bins, pipes, and toys. The difference between them is that LDPE has many branches off its main carbon chain, which forces the molecules to spread out. HDPE has no branches, so its molecules are densely packed together.

But as plastics are a new type of material not seen before in nature, there are few living organisms that can digest and break them down. A single plastic bottle can linger in the environment for centuries to thousands of years until the sun disintegrates it.

Despite their totally different uses, people will sometimes burn plastics as a low quality substitite for fuels because of their chemical similarity.

But carbon can also form huge, complex molecules with many different branches.

A basic protein molecule is based on a long carbon chain
A basic protein molecule is based on a long carbon chain

Many of them get so complicated that we can’t synthetically create them like we can with plastic. They can only be made by one of the most complex structures in the known universe, the DNA of a living cell.

Part of the role of DNA is to transform nutrients into the gigantic, complex molecules that are the building blocks of its cells. The most important types for life are carbohydrates, fats, and proteins.

Each of them have a special range of functions that help the organism survive.

For an example, an important type of fat called a phospholipid. Phospholipids are a combination of two smaller types of molecules, each of which is made from a carbon chain.

One end, called the head, has a molecule that is polar (just like water). This means it has a slight charge, and the charge attracts water.

The other end, the tail, has a group of molecules that are non-polar, which has no attraction to water.

This is a simple but clever design, as if you drop a bunch of these phospholipids into water, they will automatically arrange themselves into a sphere, with walls like in the diagram below.

DNA manufactures these molecules, and they become the walls of its cell, protecting it from the outside world.

Phospholipids naturally form a barrier against water. Source:
Phospholipids naturally form a barrier against water. Source:

When you combine millions of clever tricks of chemistry like this, you get a functioning cell – and life. But underlying almost all of them is the basic carbon atom and it’s simple set of rules that leads to this incredible complexity.


Say hey to Owlbert Einstein

Curator of the Big Ideas Network

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