As we have learned in our previous lectures, the plasma membrane acts as a selectively permeable barrier, allowing only certain molecules to pass through. This means it functions as a gatekeeper, controlling what enters and leaves the cell in order to maintain balance (homeostasis). 

The next question is: how are molecules transported through the plasma membrane? To move substances, cells use two main methods: passive transport, which requires no energy (such as diffusion and osmosis), and active transport, which uses energy (ATP) to move molecules against their natural flow. These transport processes keep the cell supplied with nutrients, remove wastes, and support proper functioning.