Final answer:
Substances cross cellular membranes either via simple diffusion without assistance or facilitated diffusion with the aid of proteins. Simple diffusion is for very small, hydrophobic molecules, while facilitated diffusion is for larger, hydrophilic molecules and ions and utilizes specific proteins in the membrane. Osmosis, a special case of diffusion, is the movement of water across a semipermeable membrane.
Step-by-step explanation:
Substances cross cellular membranes by two main mechanisms: simple diffusion and facilitated diffusion. Simple diffusion is the process whereby very small, hydrophobic molecules, such as oxygen and carbon dioxide, pass through the cell membrane without assistance. Facilitated diffusion, on the other hand, involves larger or hydrophilic molecules and charged ions crossing the membrane with the aid of transport proteins, such as channel proteins or carrier proteins. Facilitated diffusion is a form of passive transport, meaning it does not require energy and moves substances down their concentration gradient, but it requires the assistance of membrane proteins.
Channel proteins are involved in facilitated diffusion and are less selective, usually discriminating their cargo based on size and charge, whereas carrier proteins are more selective, often allowing only one particular type of molecule to cross. Facilitated diffusion has a saturation point, similar to enzyme catalysis, beyond which increasing the concentration of the substance does not increase the rate of transport because all of the transport proteins are occupied.
In contrast to facilitated diffusion, osmosis specifically refers to the movement of water molecules across a membrane, driven by the concentration gradient of the water. It is a special case of diffusion where only water is transported through a semipermeable membrane. Diffusion, including facilitated diffusion, is often contrasted with active transport, where substances move against a concentration gradient and require energy.