<P> The cell membrane (or plasma membrane or plasmalemma) 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 . Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water . </P> <P> 2 . Transmembrane protein channels and transporters: Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell . Such molecules diffuse passively through protein channels such as aquaporins (in the case of water (H O)) in facilitated diffusion or are pumped across the membrane by transmembrane transporters . Protein channel proteins, also called permeases, are usually quite specific, recognizing and transporting only a limited food group of chemical substances, often even only a single substance . </P>

What is a cell membrane in a plant cell