Table of Contents
plural: cell membranes
The cell’s outer membrane made up of a two layers of phospholipids with embedded proteins, and separates the contents of the cell from its outside environment, as well as regulates what enters and exits the cell
A cell is the structural, functional and biological unit of all organisms. It is a membrane-bound structure containing a cytoplasm and cytoplasmic structures. The membrane that surrounds the cell and separates it from the outside environment is called cell membrane. In animals the plasma membrane is the outermost covering of the cell whereas in plants, fungi, and some bacteria it is located beneath the cell wall. Although some cells form another layer above the cell membrane (called cell wall), other cells have the cell membrane as the only protective barrier between the cytoplasm and the outside of the cell.
The fluid mosaic model is the theorized model of certain biological membranes, including the cell membrane. Based on this model, the cell membrane is a lipid bilayer of phospholipids with embedded proteins and is selectively permeable. It means that some substances may pass through it (e.g. via passive transport). Other substances would not be able to pass through it without using transport mechanisms such as carrier proteins. This is vital to the function of biological membranes, which is to regulate what comes in and out through it. Because of this feature of the cell membrane, it is able to regulate the influx and efflux of substances, and thereby it helps maintain homeostasis. Moreover, the fluid mosaic model depicts a cell membrane that is fluid. The fluidity is due to the hydrophobic integral components such as lipids and membrane proteins that move laterally or sideways throughout the membrane. That means the membrane is not solid, but more like a ‘fluid’.
The major constituents of the cell membrane are: lipids, proteins, and carbohydrates. The lipids are of three types: phospholipids, glycolipids, and sterols. Phospholipids are the major type of lipid in the cell membrane. They are amphipathic compounds; the ‘head’ is hydrophilic whereas the lipophilic ‘tail’ is hydrophobic.
Being amphipathic, the phospholipid tails tend to avoid interacting with water. In contrast, the phospholipid heads may interact with water. Thus, when placed in water or an aqueous solution, the phospholipids tend to aggregate by orienting their tails towards each other. Thus, the ‘heads’ tend to face the water or the aqueous solution. As a result, the phospholipids in the cell membrane forms the distinctive “lipid bilayer”.
The two layers of phospholipids are arranged in a way that their hydrophobic tails are projecting to the interior whereas their hydrophilic heads are projecting the exterior. This organization of phospholipids in the cell membranes makes the latter selectively permeable to ions and molecules.
A glycolipid is a carbohydrate that is covalently linked to a lipid. Glycolipids are biomolecular structures in the phospholipid bilayer of the cell membrane whose carbohydrate component extends to the outside of the cell. Other lipids that are relatively in low proportions are glycolipids and sterols. Glycolipids (e.g. glycosphingolipid) are essential in providing stability and in cell to cell interactions, e.g. cell adhesion to form a tissue. They also facilitate cellular recognition, which is important in immunologic functions. Sterol (e.g. cholesterol) is a type of lipid in the cell membrane that provides structural integrity and fluidity.
Cholesterol in animal cell membranes enables the animal cells to change shape and therefore are rather flexible than plant cells (which are less flexible in shape due to the presence of the cell wall). Because of cholesterol, animal cells need not to have cell walls such as those in bacterial and plant cells.
Proteins are another major component of the cell membrane. They typically make up about half of the membrane volume. Membrane proteins are of three major types: integral proteins, peripheral membrane proteins, and lipid-anchored proteins. Integral proteins are membrane proteins firmly attached in the cell membrane.
Examples are transmembrane proteins (i.e. proteins that span the lipid bilayer of the membrane) and integral monotopic proteins (i.e. proteins that are permanently attached to the membrane from one side). Peripheral membrane proteins are proteins that temporarily adhere to the membrane, either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Lipid-anchored proteins are proteins on the cell surface that are covalently attached to lipids in the cell membrane.
Carbohydrates in the cell membrane are predominantly glycoproteins. A glycoprotein pertains to any protein covalently attached to a carbohydrate unit through the process of glycosylation. Glycoproteins are essential in cell-cell recognition in eukaryotes.
Common biological reactions
Cell recognition is one of the ways by which cells communicate with one another. It is possible through specific cellular adhesion molecules on the surface of the cell. Example of cell recognition is integrin (LFA-1) of T cell binding to ICAM of endothelial cell. Another is selectin (L) of lymphocyte binding to addressin (CD34) of endothelial cell.
One of the major functions of the cell membrane is transport. The cell membrane is involved in both the passive and active type of transport. In passive transport, substances move along the concentration gradient. This is in contrast to an active transport, which is a type of transport characterized by an uphill movement of substances (i.e. from lower to higher) and therefore requires a chemical energy, e.g. ATP. In moving substances across a biological membrane, a passive transport may or may not need the assistance of a membrane protein.
There are four major types of passive transport are (1) simple diffusion, (2) facilitated diffusion, (3) filtration, and (4) osmosis. Simple and facilitated diffusions refer to the net movement of molecules from higher to lower concentrations. Osmosis refers to the diffusion of a solvent (usually water molecules) through a semipermeable membrane from lower to higher solute concentrations. Filtration is the movement of water and solute molecules across the cell membrane driven by hydrostatic pressure that is generated by the cardiovascular system.
Endocytosis is the process in which cell takes in materials (e.g. proteins and hormones) from the outside by engulfing and fusing them with its plasma membrane. There are two types of endocytosis: phagocytosis, which literally means cell-eating, and pinocytosis, which literally means cell-drinking.
The cell engulfs by creating a small deformation inward (invagination) containing the substance to be transported inside the cell. The invagination is then pinched off from the cell membrane, resulting in a vesicle containing the substance. Since endocytosis requires ATP, it is considered as a form of Active transport.
Exocytosis is the process in which the cell seems to spit out materials from the cell. Thus, exocytosis seems the opposite process of endocytosis. The vesicle containing the material fuses with the cell membrane and then the contents are extruded outside the cell into the surrounding medium.
The structure and composition of the cell membrane makes it selectively permeable (or semipermeable), which means not every substance is allowed to enter or leave the cell. The cell membrane controls which substances can go in and out of the cell. It can allow a particular substance to pass through at a certain time, and then reject the same substance at a later time. The presence of surface molecules (e.g. glycoproteins, glycolipids, etc.) serves as the ‘signature’ of a cell. Every cell has a different ‘signature’ or ‘marker’ that is thought to function in cell recognition, or in a sort of cellular identification system. Its other main functions include cell adhesion, ion channel conductance, cell signaling, and attachment point for cytoskeleton (which is important in keeping the shape of the cell).
- plasma membrane
- cellular membrane
- cytoplasmic membrane
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