Lecture 6. Cell Membrane Structure PDF

General Cell And Developmental Biology [BIO 102]  Cell Membrane Structure By Dr. Maha M Salah Assistant Professor of Medical Biochemistry and Molecular Biology  02 The plasma...

General Cell And Developmental Biology [BIO 102]  Cell Membrane Structure By Dr. Maha M Salah Assistant Professor of Medical Biochemistry and Molecular Biology  02 The plasma membrane that surrounds the cell is the boundary that separates a living cell from its surroundings and controls the passage of all inbound and outbound substances. Biology  ▪ The plasma membrane exhibits Selective Permeability; that is, it allows some substances to cross it more easily than others. ▪ The ability of the cell to discriminate in its chemical exchanges is fundamental to life, and it is the plasma membrane and its component molecules that make this selectivity possible. Biology Biology  ❑ Cellular membranes are fluid mosaics of lipids and proteins 1- Lipids and proteins are the main components of membranes, although carbohydrates are also important. 2- The most abundant lipids in most membranes are phospholipids. 3- A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region. Biology  4. Other types of membrane lipids are also amphipathic. 5. A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water. Biology  6. The membrane is often called a fluid mosaic (because many of the molecules (the pieces of the “mosaic”) drift laterally within the bilayer). 7. Steroid molecules maintain the membrane’s fluidity as the temperature fluctuates. Both animal and plant cell membranes contain steroids; the cholesterol in animal membranes is the most familiar example. Biology  8. Whereas phospholipids and steroids provide the membrane’s structure, proteins are especially important to its function. Some Membrane proteins are Integral, while others are Peripheral. On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the cytoskeleton; this maintain cell shape and stabilizes the location of certain membrane proteins. Biology  On the extracellular side, certain membrane proteins may attach to materials outside the cell. For example, in animal cells, membrane proteins may be attached to fibers of the extracellular matrix. Thus, they can coordinate extracellular and intracellular changes. These attachments combine to give animal cells a stronger framework than the plasma membrane alone could provide. A single cell may have cell-surface membrane proteins that carry out several different functions, such as: i. Transport through the cell membrane, ii. Enzymatic activity, iii. Attaching a cell to either a neighboring cell or the extracellular matrix. Biology  Furthermore, a single membrane protein may itself carry out multiple functions. ❑ Thus, the membrane is not only a structural mosaic, with many proteins embedded in the membrane, but also a functional mosaic carrying out a range of functions. Biology ❑ Cells have multiple types of membrane proteins: 1. Transport proteins: Transport proteins embedded in the phospholipid bilayer create passageways through which ions, glucose, and other polar substances pass into or out of the cell. 2. Enzymes: These proteins facilitate chemical reactions that otherwise would proceed too slowly to sustain life. 3. Recognition proteins: Carbohydrates attached to cell surface proteins serve as “name tags” that help the body’s immune system recognize its own cells. 4. Adhesion proteins: Enable cells to stick to one another. 5. Receptor proteins: Bind to molecules outside the cell and trigger a response inside the cell. Biology Biology  ❖ Understanding membrane proteins is a vital part of medicine, in part because at least half of all drugs bind to them. ✓ One example is omeprazole (Prilosec). ✓ Another is the antidepressant drug fluoxetine (Prozac). This drug relieves heartburn by blocking It prevents receptors on brain cell surfaces some of the transport proteins that from re-uptaking a mood-altering pump hydrogen ions into the stomach. biochemical called serotonin. Biology  The Fluidity of Membranes ▪ A membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies. ▪ The temperature at which a membrane solidifies depends on the types of lipids it is made of. ▪ As the temperature decreases, 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 more fluid. Biology ▪ Cholesterol acts as a bidirectional regulator of membrane fluidity because a) At relatively high temperatures—at 37°C, the body temperature of humans, for example—cholesterol makes the membrane less fluid by restraining phospholipid movement. b) Whereas at low temperatures, it intercalates between the phospholipids and prevents them from clustering together and stiffening. Thus, cholesterol can be thought of as a “fluidity buffer” for the membrane i.e., resisting changes in membrane fluidity that can be caused by changes in temperature. ▪ Compared to animals, plants have very low levels of cholesterol; rather, related steroid lipids buffer membrane fluidity in plant cells. ▪ 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. ❖ Evolution led to Differences in Membrane Lipid Composition: ✓ Variations in the cell membrane lipid compositions of many species appear to be evolutionary adaptations that maintain the appropriate membrane fluidity under specific environmental conditions. ✓ For instance, fishes that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails, enabling their membranes to remain fluid. ✓ The ability to change the lipid composition of cell membranes in response to changing temperatures has evolved in organisms that live where temperatures vary…. Examples? a) In many plants that tolerate extreme cold, such as winter wheat, the percentage of unsaturated phospholipids increases in autumn, an adjustment that keeps the membranes from solidifying during winter. b) Certain bacteria and archaea can also change the proportion of unsaturated phospholipids in their cell membranes, depending on the temperature at which they are growing.  Overall, natural selection has apparently favored organisms whose mix of membrane lipids ensures an appropriate level of membrane fluidity for their environment.  The Role of Membrane Carbohydrates in Cell-Cell Recognition o Cell- cell recognition, a cell’s ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. ✓ It is important, for example, in the sorting of cells into tissues and organs in an animal embryo. ✓ It is also the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrate animals. Biology  ▪ Cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane. ▪ Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. i. Some are covalently bonded to lipids, forming molecules called glycolipids. ii. However, most are covalently bonded to proteins, which are thereby glycoproteins. Biology  ▪ The carbohydrates on the extracellular side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual. The diversity of the molecules and their location on the cell’s surface enable membrane carbohydrates to function as markers that distinguish one cell from another. Biology  Membrane Structure results in Selective Permeability ▪ The selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. A] The Permeability of the Lipid Bilayer: Nonpolar molecules, such as hydrocarbons, CO2, and O2, are hydrophobic, as are lipids. They can all therefore dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of membrane proteins. However, the hydrophobic interior of the membrane disturbs direct passage, through the membrane, of ions and polar molecules, which are hydrophilic. Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water, a very small polar molecule, does not cross rapidly relative to nonpolar molecules.  B] Transport Proteins: ▪ Specific ions and a variety of polar molecules can’t move through cell membranes on their own. ▪ However, these hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane. Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. Biology  ✓ The passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins. ✓ Without aquaporins, only a tiny fraction of these water molecules would pass through the same area of the cell membrane in a second, so the channel protein brings about a tremendous increase in rate Biology  Other transport proteins called carrier proteins, hold onto their passengers and change shape in a way that shuttles them across the membrane. ▪ A transport protein is specific for the substance it translocate, allowing only a certain substance (or a small group of related substances) to cross the membrane. ▪ For example, a specific carrier protein in the plasma membrane of red blood cells transports glucose across the membrane 50,000 times faster than glucose can pass through on its own. This “glucose transporter” is so selective that it even rejects fructose, a structural isomer of glucose. Biology THANK YOU