Cell Membrane Structure.pdf

Cell Membrane Structure Interestingly enough, this fluidity means that if you insert a very fine needle into a cell (in analogy), the membrane will A cell's plasma membrane defines the boundary of the cel...

Cell Membrane Structure Interestingly enough, this fluidity means that if you insert a very fine needle into a cell (in analogy), the membrane will A cell's plasma membrane defines the boundary of the cell simply part to flow around the needle and will not burst; and determines the nature of its contact with the environment. once the needle is removed, the membrane will flow back together seamlessly without any damage and as if nothing ever It is the edge of life, the boundary that separates the living happened. cell from its surroundings and controls traffic into and out of the cell it surrounds. It enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux because it must be sufficiently flexible to allow certain cells, such as red blood cells and white blood cells to change shape as they pass through narrow capillaries.  Static – not being modified; it remain as it is  Dynamic – can be changed/modified This also cells also exclude some substances from entering while taking in others, as well as excreting some, all but in a controlled quantity. The plasma membrane is made up primarily of a bilayer (two- layer) of phospholipids peppered with embedded proteins, This kind of controlled molecular movement has something to glycolipids, carbohydrates, and glycoproteins, and, in animal do with the permeability of the cell membrane. cells, cholesterol.  Permeability – the property that allows certain  Phospholipid bilayer – Composed of two layers or substances to pass through. faces (outer face and inner face)  Permeable – having pores or openings that permit liquids or gasses to pass through. The amount of cholesterol in animal plasma membranes regulates the fluidity of the membrane and changes based on Like all biological membranes, cell membrane also exhibits the temperature of the cell's environment just like how selective permeability; that is, it allows some substances to cooking oil solidifies under cold environment. cross it more easily than others. In other words, cholesterol acts as antifreeze in the cell  Selective or semi-permeable membrane and is more abundant in animals that live in cold climates. To perform these roles, the cell membrane needs lipids, which makes a semipermeable barrier between the cell and its  Cholesterol – Helps maintain the fluidity of the environment. plasma membrane. If there is too little cholesterol the membrane will solidify. To perform these roles, the cell membrane needs lipids, which  Blubber – Found in animals; reserve of fat (adipose makes a semipermeable barrier between the cell and its tissue) that produces heat also known as brown fat environment. cells. It is used by animals in cold environments to survive. It also need proteins, which are involved in cross-membrane  The ocean strata are cold, animals living deep down transport and cell communication as well as carbohydrates, ocean environment can adapt to this. which decorates both proteins and lipids to help cells  Omega-3 fatty acids – abundant in fish that live deep recognize each other. down below the ocean  Lipids – substances that are immiscible to water; There are three major components of the cell membrane. they do not mix with water. It has no charge (non 1. Phospholipids polar), and it contributes to permeability. 2. Proteins, and Examples: Oil, waxes, fats, etc. 3. Carbohydrates  Polar – with charges (+) CATIONS (-) ANIONS  Non Polar – Has no charge and has difficulty Phospholipids interacting whit polar molecules The main fabric of the membrane is composed of two layers The currently accepted model for the structure of the of phospholipid molecules, and the polar ends of these membrane, called the fluid mosaic model, was first proposed molecules are in contact with aqueous fluid both inside and in 1972 by Seymour Jonathan Singer and Garth L. outside the cell. Nicolson. In contrast, the interior of the membrane, between its two  Fluid – Refers to its dynamic nature (constantly surfaces are nonpolar because of the fatty acid tails and this changing) region has no attraction to water or other polar molecules.  Mosaic – describes its patchwork nature, made up of various macromolecules  Water - Polar Base on the model, the membrane is a mosaic of components - Thus, the exterior surface is said to be hydrophilic, while the primarily, proteins, phospholipids, cholesterol, carbohydrates, interior region is said to be hydrophobic. and proteins - in which the components are able to flow and (hydro = water; philic = love; phobic - fear) change position, while maintaining the basic integrity of the membrane.  Head (exterior) – Attracted to water; polar, water- loving (hydrophilic), in contact with aqueous fluids   Fatty acid tails – Not attracted to water; non polar, Proteins immiscible to water (hydrophobic), they should not be in contact with aqueous fluid Proteins make up the 2nd major component of the membrane. Some proteins are embedded in the membrane and may A phospholipid is a lipid made of glycerol, two fatty acid extend partway into the membrane, cross the membrane tails, and a phosphate-linked head group. entirely, or be loosely attached to its inside or outside face. There are two main categories of membrane proteins; integral Biological membranes usually involve two layers of and peripheral. phospholipids with their "tails" pointing inward, an arrangement called a phospholipid bilayer.  Proteins – The body’s building block. Blocks that help grow and repair muscles and tissues Phospholipids are well suited for the role of plasma membrane as it is amphipathic, meaning that they have both hydrophilic and hydrophobic regions. Integral membrane proteins are integrated into the membrane: they have at least one hydrophobic core that  Head – Made of glycerol and phosphate-linked head anchors them to the hydrophobic core of the phospholipid group; hydrophilic and negatively charged (because bilayer. of the phosphate group)  Fatty acid tails – Lipids; non polar, not attracted to Some stick only partway into the membrane, while others water; amphipathic (having both hydrophobic and stretch from one side of the membrane to the other and are hydrophilic regions) exposed on either side. Proteins that extend all the way across the membrane are called transmembrane proteins. Some integral proteins may The serve as channels or pumps to move materials into or out of the cell.  Integral Proteins – Necessary for substances to pass through the membrane and have a specific hydrophobic region at the middle or outside  Transmembrane Proteins – Acts as channels or conduits (like tunnels) that allow polar molecules to pass through without interacting with the hydrophobic region of the membrane hydrophilic, or "water- loving", portion of a Peripheral membrane proteins are found on the exterior or phospholipid is its head, which contains a interior surfaces of membranes, attached either to integral negatively charged phosphate group as well as proteins or phospholipid molecules. Unlike integral an additional small group, which may also be membrane proteins, peripheral membrane proteins do not charged or polar stick in to the hydrophobic core of the membrane, and they tend to be more loosely attached. The hydrophilic heads of phospholipids face outward, facing the aqueous (watery) fluid both inside and outside the cell.  Peripheral Proteins – These proteins are not Since water is a polar molecule, it readily forms electrostatic embedded into the membrane and can be hydrophilic (charge-based) interactions with the phospholipid heads or hydrophobic since they do not interact with the hydrophobic core of the cell membrane. The hydrophobic, or "water-fearing", part of a phospholipid consists of its long, nonpolar fatty acid tails. The fatty acid Both integral and peripheral proteins may serve as enzymes, tails can easily interact with other nonpolar molecules, but as structural attachments for the fibers of the cytoskeleton, they interact poorly with water. or as part of the cell's recognition sites. Because of this, it's more energetically favorable for the Carbohydrates phospholipids to tuck their tatty acid tails away in the interior of the membrane, where they are shielded from the Carbohydrates are the 3rd major component of the cell surrounding water. membrane. They are always found on the exterior surfaces of cells and are bound either to proteins (forming glycoproteins) The phospholipid bilayer formed by these interactions makes a or to lipids (forming glycolipids). good barrier between the interior and exterior of the cell, because water and other polar or charged substances cannot These carbohydrate chains may consist of 2-60 easily cross the hydrophobic core of the membrane. monosaccharide units and may be either straight or branched. This molecular orientation maximizes contact of hydrophilic regions of proteins with water in the cytosol and extracellular  Monosaccharide (simple sugar) – Building block of fluid, while providing their hydrophobic parts with a non carbohydrates, they consist of a single molecule aqueous environment. mono – one saccharide – sugar  Ex: Glucose, Fructose, Galactose  Along with membrane proteins, these carbohydrates form distinctive cellular markers, sort of like molecular ID badges, that allow cells to recognize each other. These markers are very important in the immune system, allowing immune cells to differentiate between body cells, which they shouldn't attack, and foreign cells or tissues, which they should. Membrane Fluidity The structure of the fatty acid tails of the phospholipid is important in determining the properties of the membrane, and in particular, how fluid it is. According to scientists, the membrane has about the consistency of an olive or salad oil, that is, not to watery but at the same time, not to viscous. Saturated fatty acids have no double bonds (saturated with hydrogens), so they are relatively straight. Unsaturated fatty acids, on the other hand, contains one or more double bonds, often resulting in a bend known as kink. The saturated and unsaturated fatty acid tails of phospholipids behave differently as temperature changes.  Saturated – These fatty acids have no double bonds, so they are straight and compact. When the membrane contains only saturated fatty acids, it becomes rigid and solidifies at lower temperatures.  Unsaturated – These have double bonds, which cause “kinks” or “bends” in the chain, preventing tight packing. They maintain membrane fluidity even at lower temperatures, as the kinks create spaces between molecules. A membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies as it becomes dense and rigid, much as bacon grease forms lard when it cools. The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails. Because of kinks in the tails where double bonds are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, making the membrane stay at a fluid state. Most cell membranes contain a mixture of phospholipids, some with two saturated (straight) tails and others with one saturated and one unsaturated (bent) tail. Many organisms - fish for example - can adjust physiologically to cold environments by changing the proportion of unsaturated fatty acids in their membranes. In addition to phospholipids, animals have an additional membrane component that helps to maintain fluidity. Cholesterol, another type of lipid that is embedded among the phospholipids of the membrane, helps to minimize the effects of temperature on fluidity. At low temperatures, cholesterol increases fluidity by keeping phospholipids from packing tightly together, while at high temperatures, it actually reduces fluidity. In this way, cholesterol expands the range of temperatures at which a membrane maintains a functional, healthy fluidity. Membranes must be fluid to work properly; the fluidity of a membrane affects both its permeability and the ability of membrane proteins to move to where their function is needed. When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive if their activity requires movement within the membrane. However, membranes that are too fluid cannot support protein function either. Therefore, extreme environments pose a challenge for life, resulting in evolutionary adaptations that include differences in membrane lipid composition.