# Biological Molecules

The majority of biological molecules are **carbon**-based. Each carbon atom has the ability to form four **bonds**, typically with other carbon, hydrogen, oxygen, nitrogen, sulfur or phosphorus atoms.

When carbon atoms bond together, they create the **carbon skeleton**. The shape of this carbon skeleton, along with the positioning of other **chemical groups** defines the unique properties of a molecule.

The picture below provides an example of a biological molecule – serotonin. Serotonin is a signalling molecule used by neurons to communicate with each other.

<figure><img src="https://3514673221-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdDE3NSiCXcu5YQDQmgdU%2Fuploads%2FGPzDVtM3DVaLLyoRodPA%2FFig_7_serotonin.png?alt=media&#x26;token=3ea361b9-b3e8-458f-81ae-e6992a8957cb" alt="" width="375"><figcaption><p>Serotonin is a biological molecule. Note that in this molecule carbon atoms form single or double bonds with other atoms.<br>Image source: Molview.org</p></figcaption></figure>

## What Are Polymers? <a href="#what-are-macromolecules" id="what-are-macromolecules"></a>

Proteins, nucleic acids and many of the carbohydrates are **polymers** – long molecules made of multiple repeating units called **monomers**. Monomers can be either identical (e.g. in cellulose) or similar (e.g. in DNA).

<figure><img src="https://3514673221-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdDE3NSiCXcu5YQDQmgdU%2Fuploads%2Fj6B9VMGrOFWLqGWnoAt4%2FFig_8_polymer.png?alt=media&#x26;token=28890894-101d-4981-9f5f-dd41f2a6d57e" alt="" width="188"><figcaption><p>Formation and degradation of polymers<br>Image source:<br>Christinelmiller - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=98001922</p></figcaption></figure>

## Major Classes of Biological Molecules  <a href="#the-bare-minimum" id="the-bare-minimum"></a>

If you already have a biological background or do not want to indulge in the details of biological molecules' structure and functions, then consider the below information just enough to move on to the next chapter.

The four major classes of biological molecules are carbohydrates, proteins, nucleic acids (DNA and RNA) and lipids.

| Class                       | Function                                                                                              |
| --------------------------- | ----------------------------------------------------------------------------------------------------- |
| Carbohydrates               | Serve as an energy source and as structural components                                                |
| Proteins                    | Catalyse biochemical reactions, transport molecules, have structural, signalling and other structures |
| Nucleic acids (DNA and RNA) | Store and carry genetic information                                                                   |
| Lipids                      | Store energy, make up cell membranes and serve as hormones                                            |