Structure Of Cell Membrane And Functions PdfBy Georgia H. In and pdf 15.05.2021 at 07:44 5 min read
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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.
- Cell Membrane Function and Structure
- Cell Membrane: Structure and Physical Properties
- Cell membrane
- The Cell Membrane
Phospholipid—the dynamic structure between living and non-living world; a much obligatory supramolecule for present and future[J].
Cell Membrane Function and Structure
A biological membrane , biomembrane or cell membrane is a selectively permeable membrane that separates cell from the external environment or creates intracellular compartments.
Biological membranes, in the form of eukaryotic cell membranes , consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions.
The bulk of lipid in a cell membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell , consisting of lipid molecules bound tightly to surface of integral membrane proteins. The cell membranes are different from the isolating tissues formed by layers of cells, such as mucous membranes , basement membranes , and serous membranes.
The lipid bilayer consists of two layers- an outer leaflet and an inner leaflet. Certain proteins and lipids rest only on one surface of the membrane and not the other. In eucaryotic cells, new phospholipids are manufactured by enzymes bound to the part of the endoplasmic reticulum membrane that faces the cytosol. To enable the membrane as a whole to grow evenly, half of the new phospholipid molecules then have to be transferred to the opposite monolayer.
This transfer is catalyzed by enzymes called flippases. In the plasma membrane, flippases transfer specific phospholipids selectively, so that different types become concentrated in each monolayer. Using selective flippases is not the only way to produce asymmetry in lipid bilayers, however. In particular, a different mechanism operates for glycolipids—the lipids that show the most striking and consistent asymmetric distribution in animal cells.
The biological membrane is made up of lipids with hydrophobic tails and hydrophilic heads. These help organize membrane components into localized areas that are involved in specific processes, such as signal transduction.
Red blood cells, or erythrocytes, have a unique lipid composition. The bilayer of red blood cells is composed of cholesterol and phospholipids in equal proportions by weight. In the bilayer of red blood cells is phosphatidylserine.
However, it is flipped to the outer membrane to be used during blood clotting. Phospholipid bilayers contain different proteins. These membrane proteins have various functions and characteristics and catalyze different chemical reactions.
Integral proteins span the membranes with different domains on either side. Peripheral proteins are unlike integral proteins in that they hold weak interactions with the surface of the bilayer and can easily become dissociated from the membrane. Oligosaccharides are sugar containing polymers. In the membrane, they can be covalently bound to lipids to form glycolipids or covalently bound to proteins to form glycoproteins. Membranes contain sugar-containing lipid molecules known as glycolipids.
In the bilayer, the sugar groups of glycolipids are exposed at the cell surface, where they can form hydrogen bonds. Glycoproteins are integral proteins. The phospholipid bilayer is formed due to the aggregation of membrane lipids in aqueous solutions. Biological molecules are amphiphilic or amphipathic, i. The layers also contain hydrophobic tails, which meet with the hydrophobic tails of the complementary layer.
The hydrophobic tails are usually fatty acids that differ in lengths. Membranes in cells typically define enclosed spaces or compartments in which cells may maintain a chemical or biochemical environment that differs from the outside.
For example, the membrane around peroxisomes shields the rest of the cell from peroxides, chemicals that can be toxic to the cell, and the cell membrane separates a cell from its surrounding medium. Peroxisomes are one form of vacuole found in the cell that contain by-products of chemical reactions within the cell.
Most organelles are defined by such membranes, and are called "membrane-bound" organelles. Probably the most important feature of a biomembrane is that it is a selectively permeable structure. This means that the size, charge, and other chemical properties of the atoms and molecules attempting to cross it will determine whether they succeed in doing so. Selective permeability is essential for effective separation of a cell or organelle from its surroundings.
Biological membranes also have certain mechanical or elastic properties that allow them to change shape and move as required. Generally, small hydrophobic molecules can readily cross phospholipid bilayers by simple diffusion. Particles that are required for cellular function but are unable to diffuse freely across a membrane enter through a membrane transport protein or are taken in by means of endocytosis , where the membrane allows for a vacuole to join onto it and push its contents into the cell.
Many types of specialized plasma membranes can separate cell from external environment: apical, basolateral, presynaptic and postsynaptic ones, membranes of flagella, cilia, microvillus , filopodia and lamellipodia , the sarcolemma of muscle cells, as well as specialized myelin and dendritic spine membranes of neurons.
Plasma membranes can also form different types of "supramembrane" structures such as caveolae , postsynaptic density, podosome , invadopodium , desmosome, hemidesmosome , focal adhesion, and cell junctions. These types of membranes differ in lipid and protein composition. Distinct types of membranes also create intracellular organelles: endosome; smooth and rough endoplasmic reticulum; sarcoplasmic reticulum; Golgi apparatus; lysosome; mitochondrion inner and outer membranes ; nucleus inner and outer membranes ; peroxisome ; vacuole; cytoplasmic granules; cell vesicles phagosome, autophagosome , clathrin -coated vesicles, COPI -coated and COPII -coated vesicles and secretory vesicles including synaptosome , acrosomes , melanosomes, and chromaffin granules.
Different types of biological membranes have diverse lipid and protein compositions. The content of membranes defines their physical and biological properties. Some components of membranes play a key role in medicine, such as the efflux pumps that pump drugs out of a cell.
The hydrophobic core of the phospholipid bilayer is constantly in motion because of rotations around the bonds of lipid tails.
However, because of hydrogen bonding with water, the hydrophilic head groups exhibit less movement as their rotation and mobility are constrained. Below a transition temperature, a lipid bilayer loses fluidity when the highly mobile lipids exhibits less movement becoming a gel-like solid. Temperature-dependence fluidity constitutes an important physiological attribute for bacteria and cold-blooded organisms. These organisms maintain a constant fluidity by modifying membrane lipid fatty acid composition in accordance with differing temperatures.
In animal cells, membrane fluidity is modulated by the inclusion of the sterol cholesterol. Because cholesterol molecules are short and rigid, they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails. In this way, cholesterol tends to stiffen the bilayer, making it more rigid and less permeable. For all cells, membrane fluidity is important for many reasons. It enables membrane proteins to diffuse rapidly in the plane of the bilayer and to interact with one another, as is crucial, for example, in cell signaling.
It permits membrane lipids and proteins to diffuse from sites where they are inserted into the bilayer after their synthesis to other regions of the cell. It allows membranes to fuse with one another and mix their molecules, and it ensures that membrane molecules are distributed evenly between daughter cells when a cell divides.
If biological membranes were not fluid, it is hard to imagine how cells could live, grow, and reproduce. From Wikipedia, the free encyclopedia. Enclosing or separating membrane in organisms acting as selective semi-permeable barrier. This article is about various membranes in organisms. For the membranes surrounding cells, see cell membrane. Main articles: Cell membrane and Lipid bilayer. Chemistry and Physics of Lipids. Molecular Microbiology. Annual Review of Biophysics.
Essential Cell Biology third edition. Fundamentals of Biochemistry: Life at the Molecular Level 4 ed. The American Journal of Clinical Nutrition. Progress in Lipid Research. Bibcode : PLoSO.. Biological Membranes PDF. London, U. Archived from the original PDF on Retrieved Proceedings of the National Academy of Sciences. Bibcode : PNAS.. Accounts of Chemical Research. Membrane transport. Mechanisms for chemical transport through biological membranes.
Simple diffusion or non- mediated transport Facilitated diffusion Osmosis Channels Carriers. Uniporter Symporter Antiporter Primary active transport Secondary active transport. Efferocytosis Non-specific, adsorptive pinocytosis Phagocytosis Pinocytosis Potocytosis Receptor-mediated endocytosis Transcytosis.
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Download as PDF Printable version. Wikimedia Commons. Endocytosis Efferocytosis Non-specific, adsorptive pinocytosis Phagocytosis Pinocytosis Potocytosis Receptor-mediated endocytosis Transcytosis.
Cell Membrane: Structure and Physical Properties
Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. The plasma membrane must be very flexible to allow certain cells, such as red and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious plasma membrane functions. Among the most sophisticated plasma membrane functions is the ability for complex, integral proteins, receptors to transmit signals. These proteins act both as extracellular input receivers and as intracellular processing activators. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound.
Cell membranes are an essential component of the cell, providing separation between the intracellular and extracellular environment. They are composed of lipids, proteins and carbohydrates. In this article we shall consider the main functions of the cell membrane, the composition of membranes and clinical conditions in which a portion of the cell membrane is abnormal. The membrane bilayer contains many kinds of phospholipid molecules, with different sized head and tail molecules. Phospholipid molecules are therefore amphipathic — being both hydrophilic and hydrophobic. They spontaneously form bilayers in the water with the head groups facing out and the tail groups facing in.
Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane also known as the plasma membrane separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out. Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments.
Cell membrane function is thus an essential one for the health and survival of the cell. Membrane Composition. Membranes are complex structures composed of.
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, therefore, has two functions: first, to be a barrier keeping the constituents of the cell in and unwanted substances out and, second, to be a gate allowing transport into the cell of essential nutrients and movement from the cell of waste products. Cell membranes are composed primarily of fatty-acid-based lipids and proteins.
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.
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.
The Cell Membrane
The paucimolecular unit membrane model of the structure of the plasma membrane is critically reviewed in relation to current knowledge of the chemical and enzymatic composition of isolated plasma membranes, the properties of phospholipids, the chemistry of fixation for electron microscopy, the conformation of membrane proteins, the nature of the lipid-protein bonds in membranes, and possible mechanisms of transmembrane transport and membrane biosynthesis. It is concluded that the classical models, although not disproven, are not well supported by, and are difficult to reconcile with, the data now available. On the other hand, although a model based on lipoprotein subunits is, from a biochemical perspective, an attractive alternative, it too is far from proven. Many of the questions may be resolved by studies of membrane function and membrane biosynthesis rather than by a direct attack on membrane structure. National Center for Biotechnology Information , U. Journal List J Gen Physiol v. J Gen Physiol.
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Chapter 4: Membrane Structure and Function. Cell Membrane Proteins: 1) Transport Proteins: • Regulate movement of hydrophilic molecules through membrane.
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