# The building blocks of life

The "Building Blocks of Life" refer to the fundamental molecules and processes that make up all living organisms. A key concept is that the variety of life, both past and present, is extensive, but the biochemical basis of life is remarkably similar for all living things, suggesting a common ancestor.

* **Essential Elements**
  * Life on Earth is **carbon-based**.
  * The four most abundant elements in living organisms are carbon, hydrogen, oxygen, and nitrogen, which together account for over 99% of the atoms found in all living things.
  * **Carbon** is particularly vital because its atoms can join together to form long chains or ring structures. These carbon structures act as the fundamental **skeletons of organic molecules**. All organic molecules inherently contain carbon and hydrogen.
* **Chemical Evolution and Biological Molecules**
  * It is believed that, before life evolved, there was a period of **chemical evolution** where simple carbon-based biological molecules originated from even simpler molecules that existed on the early Earth.
  * These relatively simple biological molecules serve as the **building blocks** for the larger, more complex biological molecules necessary for life.
  * The close relationship between the **structure of molecules and their functions** is a striking principle in molecular biology.
* **Monomers, Polymers, and Macromolecules**
  * **Monomers** are small, basic molecular units from which larger molecules are constructed. Many monomers are joined together by **covalent bonds**.
  * **Polymers** are large, complex molecules formed when a large number of monomers are joined together.
  * **Macromolecules** are giant molecules. Many biological polymers are macromolecules.
  * The three main types of macromolecules in living organisms are **polysaccharides**, **proteins** (also called polypeptides), and **nucleic acids** (also called polynucleotides).
  * **Lipids** are also key biological molecules but are not polymers.
* **Formation and Breakdown of Polymers**
  * Most biological polymers are formed from their monomers by **condensation reactions**. A condensation reaction joins two molecules together, forming a chemical bond and **eliminating a molecule of water**.
  * Biological polymers can be broken down into monomers by **hydrolysis reactions**. A hydrolysis reaction breaks the chemical bond between monomers by **using a water molecule**, effectively being the opposite of a condensation reaction.
* **Specific Examples of Building Blocks**
  * **Monosaccharides** (e.g., glucose) are the monomers of carbohydrates, which form **polysaccharides** like starch, glycogen, and cellulose.
  * **Amino acids** are the monomers of **proteins**. A condensation reaction between two amino acids forms a **peptide bond**. Proteins can be made of one or more polypeptide chains.
  * **Nucleotides** are the monomers that make up **DNA** and **RNA**. A condensation reaction between two nucleotides forms a **phosphodiester bond**.
  * **Lipids** include **triglycerides** (formed from glycerol and three fatty acids) and **phospholipids** (glycerol, two fatty acids, and a phosphate group). A condensation reaction between glycerol and a fatty acid forms an **ester bond**. Triglycerides are non-polar hydrophobic molecules.