Final answer:
The most important job of the cell membrane is to maintain the cell's integrity by selectively controlling the entry and exit of substances. This regulation is crucial for the cell's internal environment and for other functions like cell signaling, and the immune response.
Step-by-step explanation:
The cell membrane, also known as the plasma membrane, has several crucial roles, but its most important job is to maintain the integrity of the cell by managing what enters and leaves. The membrane is selectively permeable, a characteristic allowing it to control the internal environment by permitting some substances to pass freely while requiring others to use specialized structures or energy to cross. It also plays a significant part in other essential functions such as cell signaling, cell adhesion, and maintaining the cell's shape through its attachment to the cytoskeleton and extracellular matrix.
The ability of the cell membrane to flexibly adapt its shape is vital for cells like red and white blood cells, allowing them to navigate through the narrow passages in capillaries. Additionally, it carries markers that enable cells to recognize one another, imperative for tissue and organ formation and for the immune system's distinction between self and non-self.