Cell Membrane: Definition, Structure, and Examples
Cell Membrane Definition
Cell Membrane is the outer membrane of the cell, which is made up of two layers of phospholipids with embedded proteins, separates the cell’s contents from its surroundings and regulates what enters and leaves the cell.
What is Cell Membrane?
Cells are the structural, functional, and biological units that make up all living things. It’s a membrane-bound cytoplasmic structure with cytoplasmic structures. The cell membrane is the membrane that surrounds the cell and isolates it from the outside world. The plasma membrane is the cell’s outermost layer in mammals, although it is found behind the cell wall in plants, fungi, and some bacteria. Although some cells produce a cell wall above the cell membrane, the cell membrane is the only protective barrier between the cytoplasm and the exterior of the cell in other cells.
Cell Membrane Structure
The fluid mosaic model, which includes the cell membrane, is a theoretical model of some biological membranes. The cell membrane, according to this hypothesis, is a phospholipid bilayer with embedded proteins that is selectively permeable. Some chemicals may be able to travel through it (e.g. via passive transport).
Other molecules, such as carrier proteins, would not be able to move through them without employing transport processes. This is critical to the function of biological membranes, which is to control what passes through them. This property of the cell membrane allows it to control the inflow and outflow of chemicals, assisting in the maintenance of homeostasis. Furthermore, the fluid mosaic model represents a fluid cell membrane. The hydrophobic integral components of the membrane, such as lipids and membrane proteins, move laterally or sideways, resulting in fluidity. That indicates the membrane is more like a fluid than a solid.
Cell Membrane Composition
Lipids, proteins, and carbohydrates are the main components of the cell membrane. Phospholipids, glycolipids, and sterols are the three kinds of lipids. The main kind of lipid in the cell membrane is phospholipids. They are amphipathic compounds with a hydrophilic ‘head’ and a hydrophobic lipophilic ‘tail.’
The phospholipid tails are amphipathic, which means they resist interacting with water. The phospholipid heads, on the other hand, may interact with water. When phospholipids are put in water or an aqueous solution, their tails prefer to orient towards one another, causing them to aggregate. As a result, the ‘heads’ are inclined to face the water or aqueous solution. As a result, the cell membrane’s phospholipids create the characteristic “lipid bilayer.”
The hydrophobic tails of the two layers of phospholipids are oriented toward the interior, while the hydrophilic heads are oriented toward the outside. The arrangement of phospholipids in cell membranes allows ions and molecules to pass through them selectively.
A glycolipid is a lipid that is covalently bonded to a carbohydrate. Glycolipids are biomolecular entities that exist in the phospholipid bilayer of the cell membrane and include a carbohydrate component that extends outside the cell. Glycolipids and sterols are two more lipids that have modest amounts. Glycolipids (e.g. glycosphingolipid) are necessary for cell stability and cell-to-cell interactions, such as cell adhesion in tissue formation. They also make cellular recognition easier, which is crucial for immunologic activities. Sterol (e.g. cholesterol) is a lipid that gives structural integrity and fluidity to the cell membrane.
Cholesterol in the membranes of animal cells allows them to alter their form, making them more flexible than plant cells (which are less flexible in shape due to the presence of the cell wall). Animal cells do not require cell walls like those found in bacteria and plants because of cholesterol.
Another important component of the cell membrane is proteins. They make up around half of the membrane’s volume. Integral proteins, peripheral membrane proteins, and lipid-anchored proteins are the three kinds of membrane proteins. Membrane proteins that are firmly linked to the cell membrane are known as integral proteins.
Transmembrane proteins (proteins that cross the membrane’s lipid bilayer) and integral monotopic proteins are two examples (i.e, From one side, proteins that are firmly linked to the membrane). Peripheral membrane proteins are proteins that use a mix of hydrophobic, electrostatic, and other non-covalent interactions to bind to the membrane, either to the lipid bilayer or to integral proteins. Lipid-anchored proteins are proteins that are covalently linked to lipids in the cell membrane and are found on the cell surface.
Glycoproteins make up the majority of the carbohydrates in the cell membrane. A glycoprotein is a protein that has been glycosylated to form a covalent bond with a carbohydrate unit. In eukaryotes, glycoproteins are required for cell-cell recognition.
Importance of Cell Membrane
One of the ways that cells interact with one another is through cell recognition. Specific cellular adhesion molecules on the cell’s surface make this possible. T cell integrin (LFA-1) binding to endothelial cell ICAM is an example of cell recognition. Another example is lymphocyte selectin (L) binding to endothelial cell addressin (CD34).
Transport is one of the most important activities of the cell membrane. In both passive and active transport, the cell membrane is involved. Substances travel along a concentration gradient in passive transport. This is in contrast to active transport, which is defined by the movement of material upward (i.e. from lower to higher) and hence necessitates the use of chemical energy, such as ATP. Passive transport may or may not require the aid of a membrane protein when transporting molecules across a biological membrane.
(1) simple diffusion, (2) assisted diffusion, (3) filtration, and (4) osmosis are the four primary forms of passive transport. The net migration of molecules from higher to lower concentrations is referred to as simple and assisted diffusion. The migration of a solvent (typically water molecules) from lower to higher solute concentrations via a semipermeable membrane is referred to as osmosis. Filtration is the process of moving water and solute molecules across a cell membrane, which is aided by the circulatory system’s hydrostatic pressure.
Endocytosis is the process by which a cell absorbs elements from the environment (such as proteins and hormones) by engulfing and fusing them with its plasma membrane. phagocytosis, which literally means “eating of cells,” and pinocytosis, which literally means “drinking of cells,” are the two kinds of phagocytosis.
The cell engulfs the material to be carried into the cell by forming a tiny distortion inward (invagination). The invagination is subsequently squeezed away from the cell membrane, forming a vesicle with the material within. Endocytosis is a type of active transport since it necessitates the use of ATP.
Exocytosis is the process through which a cell seems to spit out its contents. Exocytosis appears to be the polar opposite of endocytosis. The contents of the vesicle carrying the substance are ejected outside the cell into the surrounding media when the vesicle containing the material fuses with the cell membrane.
Cell Membrane Function
The cell membrane’s shape and composition make it selectively permeable (or semipermeable), which implies that not all substances are permitted to enter or exit the cell. The cell membrane regulates which chemicals are allowed to enter and exit the cell. It has the ability to allow a material to pass through at one moment and then reject the same substance at another.
The presence of surface molecules (such as glycoproteins, glycolipids, and others) acts as a cell’s “signature.” Every cell has its own unique‘signature’ or‘marker,’ which is considered to aid in cell recognition or a cellular identification system. Cell adhesion, ion channel conductance, cell communication, and cytoskeleton attachment points are among its other major roles (which are important in keeping the shape of the cell).
Cell Membrane Citations
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