A biological membrane is a thin, flexible barrier that surrounds and protects cells. It is primarily made up of a double layer of lipids, which are fat-like molecules, along with proteins that help transport substances in and out of the cell. This structure allows the cell to maintain a stable internal environment, known as homeostasis, while also communicating with its surroundings. These membranes are crucial for various cellular functions, including energy production and nutrient absorption. For example, the plasma membrane controls what enters and exits the cell, while the mitochondrial membrane plays a key role in generating energy. Overall, biological membranes are essential for life.