<P> The cell membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment . The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to hold them together to form tissues . Fungi, bacteria, most archaea, and plants also have a cell wall, which provides a mechanical support to the cell and precludes the passage of larger molecules . </P> <P> The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival . The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or "active", requiring the cell to expend energy in transporting it . The membrane also maintains the cell potential . The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell . The cell employs a number of transport mechanisms that involve biological membranes: </P> <P> 1 . Passive osmosis and diffusion: Some substances (small molecules, ions) such as carbon dioxide (CO) and oxygen (O), can move across the plasma membrane by diffusion, which is a passive transport process . Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane . Diffusion occurs when small molecules and ions move freely from high concentration to low concentration in order to equilibrate the membrane . It is considered a passive transport process because it does not require energy and is propelled by the concentration gradient created by each side of the membrane . Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water . Osmosis, in biological systems involves a solvent, moving through a semipermeable membrane similarly to passive diffusion as the solvent still moves with the concentration gradient and requires no energy . While water is the most common solvent in cell, it can also be other liquids as well as supercritical liquids and gases . </P> <P> 2 . Transmembrane protein channels and transporters: Transmembrane proteins extend through the lipid bilayer of the membranes; they function on both sides of the membrane to transport molecules across it . Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell . Such molecules can diffuse passively through protein channels such as aquaporins in facilitated diffusion or are pumped across the membrane by transmembrane transporters . Protein channel proteins, also called permeases, are usually quite specific, and they only recognize and transport a limited variety of chemical substances, often limited to a single substance . Another example of a transmembrane protein is a cell - surface receptor, which allow cell signaling molecules to communicate between cells . </P>

What is the role of the cell membrane in diffusion