1. Water: Water is the most abundant substance in cells, making up about 70% of their mass. It acts as a solvent for many cellular processes, transports nutrients and waste products, and helps regulate temperature.

2. Lipids (fats): Lipids are essential for cell structure and function. They form cell membranes, store energy, and provide insulation.

3. Carbohydrates (sugars): Carbohydrates are the primary source of energy for cells. They also play a role in cell structure and recognition.

4. Proteins: Proteins are the most diverse group of molecules in cells. They have a wide range of functions, including enzymes, structural components, transport molecules, and antibodies.

While these are the four basic substances, it's important to remember that cells contain many other important components as well, such as nucleic acids (DNA and RNA), vitamins, and minerals.