Living things rely on several important molecules, including water, carbon (organic) molecules, acids, and bases.
A molecule is a group of two or more atoms bonded together. Earth has specific molecules that make it habitable for life. Earth has large quantities of water, which most scientists agree is vital for life to develop. Life on Earth is carbon-based, meaning that many important molecules in living things contain carbon. Another term for a molecule that contains at least one carbon atom is organic molecule. Organic molecules describe many different kinds of molecules, many of which are vital for carrying out the functions of life in organisms. An organism is a living thing that grows, reproduces, responds to its environment, and uses energy.
Earth's organisms rely on many types of organic molecules in order to live. These fall into four categories: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates are organic molecules that provide energy to cells. Lipids are organic molecules that make up the cell membrane, which surrounds the cell, and provide energy storage and protection for organisms. Proteins are large molecules that speed up reactions, transport material in and out of cells, and provide much of the structure. Nucleic acids make up the instructions for each specific organism—how it looks, what it eats, how it reproduces, and so on.
Most living things require an environment that is pH neutral, meaning it is neither too acidic nor too basic. A solution's pH is its measurement of acidity or basicity. To maintain a neutral environment, organisms use buffers. A buffer is a solution that resists changes in pH.