Write function of plasma membrane.

The plasma membrane serves as a selective barrier that regulates the passage of substances into and out of the cell, maintaining the distinct internal environment necessary for cellular survival. This selective permeability allows essential nutrients like glucose, amino acids, and oxygen to enter while permitting waste products like carbon dioxide and urea to exit, all while preventing harmful substances from entering. The membrane accomplishes this through various transport mechanisms including simple diffusion for small nonpolar molecules, facilitated diffusion through protein channels for specific substances, and active transport using energy (ATP) to move molecules against their concentration gradient.

Beyond its gatekeeper role, the plasma membrane performs several other vital functions. It provides structural support and helps maintain cell shape through interaction with the internal cytoskeleton. The membrane's surface proteins act as receptors for hormones, growth factors, and neurotransmitters, enabling cell communication and signal transduction. Recognition proteins on the membrane surface identify the cell as "self" to the immune system and facilitate cell-to-cell recognition during tissue formation. In specialized cells, the membrane forms structures like microvilli to increase surface area for absorption or invaginates to form vesicles during endocytosis and exocytosis, processes crucial for nutrient uptake and secretion. The membrane's fluidity and dynamic nature allow it to self-repair minor damage and adjust to changing conditions, making it an active, responsive boundary rather than a passive wall.