Final answer:
Phospholipids in the cell membrane allow for the diffusion of small molecules like water, while larger or charged molecules require facilitated diffusion through transport proteins. This selective permeability is essential for cellular function and homeostasis.
Step-by-step explanation:
Phospholipids can move peripherally and allow water and other smaller molecules to pass through into and out of the cell, which is a process known as diffusion. This movement across the cell membrane occurs because the phospholipid bilayer is selectively permeable. While very small polar molecules like water can slip through, usually through protein channels or between the lipid tails, large polar molecules, and ions are repelled by the hydrophobic interior of the bilayer and cannot pass through via simple diffusion. Instead, substances like glucose utilize facilitated diffusion with the help of transport proteins to move across the membrane. This process is a form of passive transport, as it does not require cellular energy.
Passive transport includes the osmosis of water through a semipermeable membrane, and simple diffusion of small, nonpolar molecules like oxygen and carbon dioxide. The membrane's selective permeability is crucial for the cell to maintain homeostasis, and while some materials can move freely, others require specific transport mechanisms to facilitate their movement across the cell membrane.