The cell membrane, also called the plasma membrane, is like the "gatekeeper" of the cell. It plays a big role in controlling what goes in and out of the cell. This ability to let some things in while keeping others out is super important for keeping everything running smoothly inside the cell.

Key Functions of the Cell Membrane

1. Selective Permeability:

   • The cell membrane is picky about which molecules can enter. About 90% of the small molecules that get inside a cell do so through different ways of getting through the membrane.

2. Transport Mechanisms:

   • Passive Transport: This is a process that doesn't need energy. For example, small molecules like oxygen and carbon dioxide can just pass through easily.

   • Active Transport: This process does need energy (called ATP) to move substances against what’s called their concentration gradient. This means they are moving from a place where there’s a lot of them to a place where there are fewer. About 40% of the cell's energy is used for active transport.

3. Signal Transduction:

   • The cell membrane has special receptors that help the cell respond to signals from the outside. About 50% of all known medicines target these receptors.

Membrane Composition

• The cell membrane is mainly made of a phospholipid bilayer, which is about 50% fat and 50% protein. This structure makes the membrane strong and also holds proteins that help with transporting substances and sending signals.

• Cholesterol molecules are also found in the membrane. They help keep the membrane flexible, which is important when the temperature changes. In fact, temperature changes can affect how fluid the membrane is by about 20%.

In short, the cell membrane is super important as the "gatekeeper" of the cell. Its ability to control what enters and exits, use energy for transport, and respond to signals is vital for the cell's survival and function.