Life Science: Cells

A cell is often called the basic unit of life. To explain the main characteristics of cells, scientists have developed the cell theory.

The plasma membrane or cell membrane is the outer portion of a cell. It's important for separating and protecting internal cellular structures from the surrounding environment. This allows a cell to have internal conditions that are very different from its surroundings.

This membrane is also important for allowing only certain substances to enter or leave the cell. This is called selective permeability. For example, nutrients have to be able to get into a cell so that it can survive. Waste substances must also be able to leave the cell.

In some cells, the plasma membrane is not the very outermost cellular structure. It is the cell wall instead. Cell walls are external to the plasma membrane and provide additional support and protection. They can be found in plants, fungi, and bacteria.

DNA is short for deoxyribonucleic acid. It is the molecule that makes up genes, which carry important instructions for keeping a cell alive. A gene is the unit of heredity which is transferred from parents to offspring.

Everything else inside a cell is called the cytoplasm. It's made up of a jelly-like water-based fluid called cytosol. There are other non-fluid substances found in the cytoplasm, but they vary between types of cells.