CMB Lecture 2 PDF

LECTURE 1 also able to transport specific ions, thereby The Structure and Function of the Plasma establishing ionic gradients across itself. Membrane The plasma memb...

LECTURE 1 also able to transport specific ions, thereby The Structure and Function of the Plasma establishing ionic gradients across itself. Membrane The plasma membrane contains the machinery for Each part or organelle of the cell is an important physically transporting substances from one side of component of the cell as each of these organelles the membrane to another, often from a region have specialized functions that allow the cell to live where the solute is present at low concentration into or perform its role in an organism. Imagine a house a region where the solute is present at much higher without a wall. The outer walls of a house provide a concentration. strong, durable barrier that protects the humans The membrane’s transport machinery ………ionic living in it from harsh events such as typhoons and gradients across itself. even extreme heat. You might expect the outer ‫ ﭻ‬Membrane transport is essential for cellular life. boundary of a living cell to be constructed of an ‫ ﭻ‬As cells proceed through their life cycle, a vast equally tough and impenetrable barrier because it amount of exchange is necessary to maintain must also protect its delicate internal contents from function. a nonliving and often inhospitable environment. ‫ ﭻ‬Transport may involve the incorporation of biological molecules and the discharge of waste Plasma membrane products that are necessary for normal function. ✓ Biological membranes are fluid layers of lipid. This capability is especially critical for nerve and ✓ The membranes that encase all living cells are muscle cells. sheets of lipid only two molecules thick; more than 10,000 of these sheets piled on one another would 5. Responding to external signals equal the thickness of this sheet of paper. membranes possess receptors that combine with ✓ The lipid layer that forms the foundation of a cell specific molecules having a complementary membrane is composed of molecules called structure. Different types of cells have membranes phospholipids. with different receptors and are capable of recognizing and responding to different ligands in Functions of the membrane their environment. 1. Compartmentalization The plasma membrane plays a critical role in the Membrane compartmentalization allows specialized response of a cell to external stimuli, a process activities to proceed without external interference known as signal transduction. Membranes possess and enables cellular activities to be regulated receptors that combine with specific molecules independently. (called ligands) having a complementary structure. Different types of cells have membranes with 2. Scaffold for biochemical activities different receptors and are, therefore, capable of Membranes provide the cell with an extensive recognizing and responding to different ligands in framework or scaffolding within which components their environment. The interaction of a plasma can be ordered for effective interaction. membrane receptor with an external ligand may Scaffold for biochemical activities. Membranes not cause the membrane to generate a signal that only enclose compartments but are also distinct stimulates or inhibits internal activities. For compartments themselves. As long as reactants are example, signals generated at the plasma present in solution, their relative positions cannot be membrane may tell a cell to manufacture more stabilized, and their interactions are dependent on glycogen; to prepare for cell division; to move random collisions. Because of their construction, toward a higher concentration of a particular membranes provide the cell with an extensive compound; to release calcium from internal framework or scaffolding within which components storage; or possible to commit suicide. can be ordered for effective interaction. 6. Intercellular interaction 3. Providing a selectively permeable membrane allows cells to recognize and signal one another, to membranes prevent the unrestricted exchange of adhere when appropriate, and to exchange molecules from one side to the other. At the same materials and information. time, membranes provide the means of Situated at the outer edge of every living cell, the communication between the compartments they plasma membrane of multicellular organisms separate. mediates the interactions between a cell and its The plasma membrane, which encircles a cell, can neighbors. The plasma membrane allows cells to be compared to a moat around a castle: both serve recognize and signal one another, to adhere when as a general barrier, yet both have gated “bridges” appropriate, and to exchange materials and that promote the movement of selected elements information. into and out of the enclosed living space. 7. Energy transduction 4. Transporting solutes involved in the transfer of chemical energy from the membrane’s transport machinery allows a cell to carbohydrates and fats to ATP. accumulate substances, such as sugars and amino The series of steps by which electrons flow to acids, that are necessary to fuel its metabolism and oxygen permits a gradual lowering of the energy of build its macromolecules. The plasma membrane is the electrons. This part of the oxidative  phosphorylation stage is sometimes called the ✓ A current representation of the plasma membrane electron transport chain. showing the basic organization as that proposed by Membranes are intimately involved in the processes Singer and Nicolson. The external surface of most by which one type of energy is converted to another membrane proteins, as well as a small percentage type (energy transduction). Like for example, the of the phospholipids, contain short chains of sugars, most fundamental energy in sunlight is absorbed by making them glycoproteins and glycolipids. Those membrane-bound pigments, converted into portions of the polypeptide chains that extend chemical energy, and stored carbohydrates. through the lipid bilayer typically occur as alpha Membranes are also involved in the transfer of helices composed of hydrophobic amino acids. The chemical energy from carbohydrates and fats to two leaflets of the bilayer contain different types of ATP. In eukaryotes, the machinery for these energy lipids as indicated by the differently colored head conversions is contained within membranes of groups. The outer leaflet may contain microdomains chloroplasts and mitochondria. (as rafts) consisting of clusters of specific lipid species. History of the Plasma Membrane ✓ Ernest Overton 1890’s was a prominent British physiologist and pharmacologist known for his significant contributions to the understanding of cell membranes. His research focused on the permeability of cell membranes to various substances, particularly anesthetics and salts. ✓ Charles Ernest Overton 1890’s. ✓ Worked with cell root hairs of the plant. ✓ Discovered that nonpolar, lipids-soluble substances penetrate cells and polar, water soluble substances do not. ✓ Proposed that cells coats are likely a combination of phospholipids and cholesterol. ✓ The first insights into the chemical nature of the outer boundary layer of a cell were obtained by ✓ A phospholipid contains two fatty acids bound to the Ernst Overton of the University of Zurich during the glycerol backbone; a phosphorylated alcohol 1890s. To test the permeability of the outer occupies the third position on the backbone. boundary layer, Overton placed plant root hairs into ✓ are often diagrammed as a polar head with two hundreds of different solutions containing a diverse nonpolar hydrophobic tails array of solutes. He discovered that the more lipid- soluble the solute, the more rapidly it would enter Chemical composition of membranes the root hair cells. He concluded that the dissolving ✓ Membranes are lipid–protein assemblies in which power of the outer boundary layer of the cell the components are held together in a thin sheet by matched that of a fatty oil. noncovalent bonds. ✓ The core of the membrane consists of a The Fluid Mosaic Model ❖ sheet of lipids arranged in ✓ Plasma membrane is composed of both lipids and ❖ a bimolecular layer. globular proteins. ✓ The lipid bilayer serves primarily as a structural ✓ In 1972, S. Singer and G. Nicolson proposed that backbone of the membrane and provides the barrier the globular proteins are inserted into the lipid that prevents random movements of water-soluble bilayer, with their nonpolar segments in contact with materials into and out of the cell. the nonpolar interior of the bilayer and their polar ✓ The proteins of the membrane carry out most of the portions protruding out from the membrane surface. specific functions. ✓ In 1972, Seymour Jonathan Singer and Garth L. Nicolson proposed that the globular proteins are inserted into the lipid bilayer, with their nonpolar segments in contact with the nonpolar interior of the bilayer and their polar portions protruding out from the membrane surface. ✓ Each type of differentiated cell contains a unique complement of membrane proteins, which contributes to the specialized activities of that cell type Membrane Lipids ✓ Membrane contains a wide variety of lipids. All of which are amphiphatic, that is they contain both hydrophilic and hydrophobic regions. ✓ There are 3 main types of membrane lipid is: plasma membrane, or of fertilization, where two phosphoglycerides, sphingolipids, and cholesterol cells fuse to form a single cell, involve processes in ✓ Phosphoglycerides – most membrane lipids which two separate membranes come together to contain a phosphate group, which makes them become one continuous sheet. phospholipids. Because most membrane phospholipids are built on a glycerol backbone, they Membrane Carbohydrates are called phosphoglycerides ✓ Plays an important role in mediating the interactions ✓ Sphingolipids – a less abundant class of of a cell with its environment and sorting of sphingosine, an amino alcohol that contains a long membrane proteins to different cellular hydrocarbon chain. compartments. ✓ Another lipid component of certain membranes is Glycoproteins sterol cholesterol, which in certain animal cells may Glycolipids constitute up to 50% of the lipid molecules in the ✓ The plasma membranes of eukaryotic cells also plasma membrane. Cholesterol is absent from the contain carbohydrates. Carbohydrates play an plasma membrane of most plants and all bacterial important role in mediating the interactions of a cell cells. Cholesterol molecules are oriented with their with its environment and sorting membrane small hydrophilic hydroxyl group toward the proteins to different cellular compartments. membrane surface and the remainder of the ✓ Depending on the species and cell type, the molecule embedded in the lipid bilayer. carbohydrate content of the plasma membrane ranges between 2 and 10 percent by weight. More than 90% of the membrane’s carbohydrate is covalently linked to proteins to form glycoproteins; the remaining carbohydrate is covalently linked to lipids to form glycolipids. Membrane Proteins Integral proteins ✓ Receptors ✓ Channels or transporters ✓ Agents that transfer electrons ✓ Integral proteins are those that penetrate the lipid bilayer. Also called transmembrane proteins, that is, they pass entirely through the lipid bilayer and thus have domains that protrude from both the extracellular and cytoplasmic sides of the membrane. ✓ Most integral membrane proteins function as receptors that bind specific substances at the membrane surface as channels or transporters involved in the movement of ions and solutes across the membrane or as agents that transfer electrons during the processes of photosynthesis and Nature and Importance of Lipid bilayer respiration. ✓ Differs for every type of membrane ✓ Can have important effects on biological properties Membrane Lipids and Fluidity of membrane ✓ The physical state of the lipid of a membrane is ✓ Determine physical state of membrane described by its fluidity (or viscosity). Fluidity is a ✓ Influence the activity of particular proteins measure of the ease of flow, and viscosity is a ✓ provide the precursors for highly active chemical measure of the resistance to flow. If the messengers that regulate cellular function temperature of the bilayer is kept relatively warm ✓ The presence in membranes of this thin film of (37 degrees Celsius), the lipid exists in a relatively amphiphatic lipid molecules has remarkable fluid state. At this temperature, the lipid bilayer is consequences for cell structure and function. best described as a two-dimensional liquid crystal. Because of thermodynamic considerations, the That is, the lipid molecules and their hydrophobic hydrocarbon chains of the lipid bilayer are never tails are free to move in certain directions, even exposed to the surrounding aqueous solution. though they retain a considerable degree of order, ✓ Consequently, membranes are never seen to have molecules still retain a specified orientation. a free edge, they are always continuous, unbroken ✓ If the temperature is slowly lowered, a point is structures. As a result, membranes form extensive reached where the bilayer distinctly changes. The interconnected networks within the cell. Because of lipid is converted from a liquid crystalline phase to a the flexibility of the lipid bilayer, membranes are frozen crystalline gel in which the movement of the deformable and their overall shape can change phospholipid fatty acid chains is greatly restricted. during locomotion or cell division. The lipid bilayer ✓ The temperature at which this change occurs is is thought to facilitate the regulated fusion or called the transition temperature, which depends on budding of membranes. For example, the events of secretion, in which cytoplasmic vesicles fuse to the  the ability of the lipid molecules to be packed Maintaining Membrane Fluidity together. ✓ Internal temperature of most organisms (other than birds and mammals) fluctuates with the Factors affecting the membrane fluidity temperature of the external environment, aling ✓ Presence of saturated chains - Phospholipids animals ito → mga exothermic o yong mga cold with saturated chains pack together more tightly blooded animals. since it is essential for many that those containing unsaturated chains. The activities that the membranes of a cell remain in a greater the degree of unsaturation of the fatty acids fluid state, cells respond to changing conditions by of the bilayer, the lower the temperature before the altering the types of phospholipids of which they are bilayer gels. We can observe the effect of fatty acid made. This is an example of homeostasis at the saturation on meting temperature from some cellular level. And can be demonstrated in different familiar food products. Vegetable oils remain a liquid ways. in the refrigerator, whereas margarine is a solid. ✓ For example, if the temperature of a culture of cells Ano bang meron sa vegetable oil na wala sa is lowered, the cells respond metabolically. The margarine? Vegetable oils contain polyunsaturated initial response is mediated by enzymes that fatty acids, whereas the fatty acids of margarine remodel membranes, making the cell more cold have been saturated by chemical process that resistant. hydrogenates the double bonds of the vegetable oils used as the starting material. Remodeling is accomplished by ✓ Fatty acid chain length - The shorter the fatty acyl 1. Desaturation of single bonds in fatty acyl chains to chains of a phospholipid, the lower its melting form double bonds. Desaturation of single bonds to temperature. Shorter fatty acyl – lower melting form double bonds is catalyzed by enzymes called temp – less viscous. desaturases. ✓ Cholesterol - Because of their orientation within 2. Second is reshuffling the chains between the bilayer, cholesterol molecules disrupt the close phospholipid molecules to produce ones that contain packing of fatty acyl chains and interfere with their two unsaturated fatty acids, which greatly lowers mobility. The presence of cholesterol tends to the melting temperature of the bilayer. Reshuffling abolish sharp transition temperatures and creates a is accomplished by phospholipases, which split the condition to intermediate fluidity. In physiologic fatty acid from the glycerol backbone, and terms, cholesterol tends to increase the durability acyltransferases, which transfer fatty acids between while decreasing the permeability of a membrane. phospholipids. Importance of Membrane Fluidity The Dynamic Nature of the Plasma Membrane ✓ Membrane fluidity provides a perfect compromise Movement of substances across CM between a rigid, ordered structure in which mobility Several different processes are known by which would be absent and a completely fluid, non-viscous substances move across membranes: liquid in which the components of the membrane - Simple diffusion through the lipid bilayer could not be oriented and structural organization - Simple diffusion through an aqueous, protein-lined and mechanical support would be lacking. Allows channel for interactions to take place within the membrane - Diffusion that is facilitated by a protein transporter. ✓ In addition, fluidity allows for interactions to take - And active transport, which requires an energy- place within the membrane. For example, driven protein pump capable of moving substances membrane fluidity makes it possible for clusters of against a concentration gradient. membrane proteins to assemble at sites within the membrane and form specialized structures, such as Diffusion of Substances through Membranes intercellular junctions, light-capturing ✓ Substance must be present at higher concentration photosynthetic complexes, and synapses. Because on one side of the membrane than the other of membrane fluidity, molecules that interact can ✓ The membrane must be permeable to the come together, carry out necessary reactions, and substance. move apart. o Solute can pass through the lipid bilayer ✓ Fluidity also plays a key role in membrane o Solute can traverse an aqueous pore that assembly. Membranes arise only from preexisting spans the membrane membranes, and their growth is accomplished by the insertion of lipids and proteins into the fluid Passive transport matrix of the membranous sheet. ‫ ﭻ‬In passive transport, substances move from an area ✓ So, many of the most basic cellular processes, of higher concentration to an area of lower including cell movement, cell growth, cell division, concentration in a process called diffusion. formation of intercellular junctions, secretions and Simple Diffusion endocytosis, depend on the movement of ‫ ﭻ‬is the movement of particles from an area of higher membrane components and would probably not be concentration to an area of lower concentration. possible if membranes were rigid, nonfluid structures.  ‫ﭻ‬ Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. ‫ﭻ‬ To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion. ‫ﭻ‬ There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. ‫ﭻ‬ Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell. Active Transport ‫ ﭻ‬During active transport, ATP is required to move a substance across a membrane, often with the help Osmosis of protein carriers, and usually against its ‫ ﭻ‬is the diffusion of water through a semipermeable concentration gradient. membrane. ‫ ﭻ‬Like facilitated diffusion, active transport depends on integral membrane proteins that selectively bind a particular solute and move it across the membrane in a process driven by changes in the protein’s conformation. Unlike facilitated diffusion, however, movement of a solute against a gradient requires input of energy. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge. Carrier proteins are more selective, often only allowing one molecule to cross. A solution that has a higher concentration of solutes than the cell itself is said to be hypertonic. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic. In an isotonic solution, an environment in which two solutions have the same concentration of solutes (equal osmotic pressure). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells Facilitated diffusion ‫ ﭻ‬is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size and/or polarity. ‫ ﭻ‬A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP.