File Name: cell membrane structure and function .zip
Cell membrane , also called plasma membrane , thin membrane that surrounds every living cell , delimiting the cell from the environment around it. Outside the cell, in the surrounding water-based environment, are ions , acids , and alkalis that are toxic to the cell, as well as nutrients that the cell must absorb in order to live and grow.
The cell membrane plasma membrane is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell while keeping other substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane also serves to help support the cell and help maintain its shape.
The cell membrane also known as the plasma membrane PM or cytoplasmic membrane, and historically referred to as the plasmalemma is a biological membrane that separates the interior of all cells from the outside environment the extracellular space which protects the cell from its environment.
The membrane also contains membrane proteins , including integral proteins that go across the membrane serving as membrane transporters , and peripheral proteins that loosely attach to the outer peripheral side of the cell membrane, acting as enzymes shaping the cell.
In this way, it is selectively permeable to ions and organic molecules. In the field of synthetic biology, cell membranes can be artificially reassembled. In the early 19th century, cells were recognized as being separate entities, unconnected, and bound by individual cell walls after it was found that plant cells could be separated.
This theory extended to include animal cells to suggest a universal mechanism for cell protection and development. By the second half of the 19th century, microscopy was still not advanced enough to make a distinction between cell membranes and cell walls.
However, some microscopists correctly identified at this time that while invisible, it could be inferred that cell membranes existed in animal cells due to intracellular movement of components internally but not externally and that membranes were not the equivalent of a cell wall to plant cell.
It was also inferred that cell membranes were not vital components to all cells. Many refuted the existence of a cell membrane still towards the end of the 19th century. In , an update to the Cell Theory stated that cell membranes existed, but were merely secondary structures. It was not until later studies with osmosis and permeability that cell membranes gained more recognition.
The lipid bilayer hypothesis, proposed in by Gorter and Grendel,  created speculation to the description of the cell membrane bilayer structure based on crystallographic studies and soap bubble observations. In an attempt to accept or reject the hypothesis, researchers measured membrane thickness. The choice of the dielectric constant used in these studies was called into question but future tests could not disprove the results of the initial experiment.
Independently, the leptoscope was invented in order to measure very thin membranes by comparing the intensity of light reflected from a sample to the intensity of a membrane standard of known thickness. The instrument could resolve thicknesses that depended on pH measurements and the presence of membrane proteins that ranged from 8. Later in the s, the membrane structure model developed in general agreement to be the paucimolecular model of Davson and Danielli This model was based on studies of surface tension between oils and echinoderm eggs.
Since the surface tension values appeared to be much lower than would be expected for an oil—water interface, it was assumed that some substance was responsible for lowering the interfacial tensions in the surface of cells. It was suggested that a lipid bilayer was in between two thin protein layers.
The paucimolecular model immediately became popular and it dominated cell membrane studies for the following 30 years, until it became rivaled by the fluid mosaic model of Singer and Nicolson Despite the numerous models of the cell membrane proposed prior to the fluid mosaic model , it remains the primary archetype for the cell membrane long after its inception in the s.
The fluid mosaic model not only provided an accurate representation of membrane mechanics, it enhanced the study of hydrophobic forces, which would later develop into an essential descriptive limitation to describe biological macromolecules. For many centuries, the scientists cited disagreed with the significance of the structure they were seeing as the cell membrane. For almost two centuries, the membranes were seen but mostly disregarded this as an important structure with cellular function.
It was not until the 20th century that the significance of the cell membrane as it was acknowledged. From this, they furthered the idea that this structure would have to be in a formation that mimicked layers.
Once studied further, it was found by comparing the sum of the cell surfaces and the surfaces of the lipids, a ratio was estimated; thus, providing the first basis of the bilayer structure known today. This discovery initiated many new studies that arose globally within various fields of scientific studies, confirming that the structure and functions of the cell membrane are widely accepted.
The structure has been variously referred to by different writers as the ectoplast de Vries , ,  Plasmahaut plasma skin, Pfeffer , , ,  Hautschicht skin layer, Pfeffer, ; used with a different meaning by Hofmeister , , plasmatic membrane Pfeffer, ,  plasma membrane, cytoplasmic membrane, cell envelope and cell membrane.
Cell membranes contain a variety of biological molecules , notably lipids and proteins. Composition is not set, but constantly changing for fluidity and changes in the environment, even fluctuating during different stages of cell development. Specifically, the amount of cholesterol in human primary neuron cell membrane changes, and this change in composition affects fluidity throughout development stages. The cell membrane consists of three classes of amphipathic lipids: phospholipids , glycolipids , and sterols.
However, for the majority of eukaryotic cells, the composition of plasma membranes is about half lipids and half proteins by weight. The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and The and carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always "cis". The length and the degree of unsaturation of fatty acid chains have a profound effect on membrane fluidity as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature increasing the fluidity of the membrane.
The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Under physiological conditions phospholipid molecules in the cell membrane are in the liquid crystalline state. It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. Lipid rafts and caveolae are examples of cholesterol -enriched microdomains in the cell membrane.
In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane. Cholesterol, a major component of animal plasma membranes, regulates the fluidity of the overall membrane, meaning that cholesterol controls the amount of movement of the various cell membrane components based on its concentrations.
The opposite is true for the role of cholesterol in cooler temperatures. Cholesterol production, and thus concentration, is up-regulated increased in response to cold temperature. At cold temperatures, cholesterol interferes with fatty acid chain interactions. Acting as antifreeze, cholesterol maintains the fluidity of the membrane. Cholesterol is more abundant in cold-weather animals than warm-weather animals. In plants, which lack cholesterol, related compounds called sterols perform the same function as cholesterol.
Lipid vesicles or liposomes are approximately spherical pockets that are enclosed by a lipid bilayer. Lipid vesicles and liposomes are formed by first suspending a lipid in an aqueous solution then agitating the mixture through sonication , resulting in a vesicle. By measuring the rate of efflux from that of the inside of the vesicle to the ambient solution, allows researcher to better understand membrane permeability. Vesicles can be formed with molecules and ions inside the vesicle by forming the vesicle with the desired molecule or ion present in the solution.
Proteins can also be embedded into the membrane through solubilizing the desired proteins in the presence of detergents and attaching them to the phospholipids in which the liposome is formed. These provide researchers with a tool to examine various membrane protein functions. Plasma membranes also contain carbohydrates , predominantly glycoproteins , but with some glycolipids cerebrosides and gangliosides.
Carbohydrates are important in the role of cell-cell recognition in eukaryotes; they are located on the surface of the cell where they recognize host cells and share information, viruses that bind to cells using these receptors cause an infection  For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane. The glycocalyx is an important feature in all cells, especially epithelia with microvilli.
Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing ,  and many others. The penultimate sugar is galactose and the terminal sugar is sialic acid , as the sugar backbone is modified in the Golgi apparatus.
Sialic acid carries a negative charge, providing an external barrier to charged particles. Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms. As shown in the adjacent table, integral proteins are amphipathic transmembrane proteins. Examples of integral proteins include ion channels, proton pumps, and g-protein coupled receptors.
Ion channels allow inorganic ions such as sodium, potassium, calcium, or chlorine to diffuse down their electrochemical gradient across the lipid bilayer through hydrophilic pores across the membrane. The electrical behavior of cells i. Processes such as electron transport and generating ATP use proton pumps. G-protein coupled receptors are used in processes such as cell to cell signaling, the regulation of the production of cAMP, and the regulation of ion channels. The cell membrane, being exposed to the outside environment, is an important site of cell—cell communication.
As such, a large variety of protein receptors and identification proteins, such as antigens , are present on the surface of the membrane. Functions of membrane proteins can also include cell—cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane.
Most membrane proteins must be inserted in some way into the membrane. Once inserted, the proteins are then transported to their final destination in vesicles, where the vesicle fuses with the target membrane. 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.
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:.
Passive osmosis and diffusion : Some substances small molecules, ions such as carbon dioxide CO 2 and oxygen O 2 , 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. 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.
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. 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. Endocytosis : Endocytosis is the process in which cells absorb molecules by engulfing them.
The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. This invagination is caused by proteins on the outside on the cell membrane, acting as receptors and clustering into depressions that eventually promote accumulation of more proteins and lipids on the cytosolic side of the membrane. Endocytosis is a pathway for internalizing solid particles "cell eating" or phagocytosis , small molecules and ions "cell drinking" or pinocytosis , and macromolecules.
Endocytosis requires energy and is thus a form of active transport.
Biological membranes are key elements for the maintenance of cell architecture and physiology. Beyond a pure barrier separating the inner space of the cell from the outer, the plasma membrane is a scaffold and player in cell-to-cell communication and the initiation of intracellular signals among other functions. Critical to this function is the plasma membrane compartmentalization in lipid microdomains that control the localization and productive interactions of proteins involved in cell signal propagation. In addition, cells are divided into compartments limited by other membranes whose integrity and homeostasis are finely controlled, and which determine the identity and function of the different organelles. Here, we review current knowledge on membrane lipid composition in the plasma membrane and endomembrane compartments, emphasizing its role in sustaining organelle structure and function. The correct composition and structure of cell membranes define key pathophysiological aspects of cells. Therefore, we explore the therapeutic potential of manipulating membrane lipid composition with approaches like membrane lipid therapy, aiming to normalize cell functions through the modification of membrane lipid bilayers.
The cell membrane also known as the plasma membrane PM or cytoplasmic membrane, and historically referred to as the plasmalemma is a biological membrane that separates the interior of all cells from the outside environment the extracellular space which protects the cell from its environment. The membrane also contains membrane proteins , including integral proteins that go across the membrane serving as membrane transporters , and peripheral proteins that loosely attach to the outer peripheral side of the cell membrane, acting as enzymes shaping the cell. In this way, it is selectively permeable to ions and organic molecules. In the field of synthetic biology, cell membranes can be artificially reassembled. In the early 19th century, cells were recognized as being separate entities, unconnected, and bound by individual cell walls after it was found that plant cells could be separated. This theory extended to include animal cells to suggest a universal mechanism for cell protection and development.
The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals. The plasma membrane also known as the cell membrane or cytoplasmic membrane is a biological membrane that separates the interior of a cell from its outside environment. The primary function of the plasma membrane is to protect the cell from its surroundings. Composed of a phospholipid bilayer with embedded proteins, the plasma membrane is selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells. Plasma membranes must be very flexible in order to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. The plasma 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 help group cells together to form tissues. The membrane also maintains the cell potential.
Constructing Supported Cell Membranes with Controllable Orientation
Integrated Molecular and Cellular Biophysics pp Cite as. The cell membrane or plasma membrane is a thin closed sheet that fulfils a double role: a morphological — delimitates the cell from its external microenvironment and confines all of its subcellular organelles; b functional — regulates the exchange of substance between internal and external media, maintains actively the ionic asymmetry between its sides, and intermediates internalization or externalization of physical and chemical signals important for cell functions. The plasma membrane undergoes continual changes both in its molecular composition and its structure i. It plays an important role in the economy of the cell, exerting a selective control on the entire traffic of ions, water, and molecules. The membrane is involved also in intake endocytosis and secretion exocytosis of large particles.