2) Why are cell membranes said to be selectively permeable?
Cell membranes are said to be selectively permeable because they allow certain substances to pass through while preventing the passage of others. This selectivity is crucial for maintaining the internal environment of a cell and regulating the movement of molecules in and out of the cell.
Cell membranes are composed of a phospholipid bilayer, with hydrophilic heads facing outwards and hydrophobic tails facing inward. This structure creates a barrier that restricts the movement of large and charged molecules, which are hydrophilic, through the hydrophobic interior of the membrane.
However, smaller, non-polar molecules such as oxygen and carbon dioxide can easily diffuse across the lipid bilayer due to their hydrophobic nature. Additionally, the membrane contains various transport proteins that selectively facilitate the movement of specific molecules across the membrane by either passive diffusion, facilitated diffusion, or active transport.
This selective permeability allows the cell to control its internal environment by regulating the entry and exit of ions, nutrients, waste products, and signaling molecules. It helps the cell maintain homeostasis, which is essential for normal cell functioning and survival.