UNIT-2-MEMBRANES.pdf

MEMBRANES Mrs. Christine C. Coriento Content ▣ The Structure of Membranes ▣ Phospholipids: The Membranes Foundation ▣ Proteins: Multifunctional Components ▣ Passive Transport Across Membranes ▣ Active Transport Across Membranes ▣ Bulk Transport...

MEMBRANES Mrs. Christine C. Coriento Content ▣ The Structure of Membranes ▣ Phospholipids: The Membranes Foundation ▣ Proteins: Multifunctional Components ▣ Passive Transport Across Membranes ▣ Active Transport Across Membranes ▣ Bulk Transport 2 ‘’ A cell’s interactions with the environment are critical, a give-and-take that never ceases. Without it, life could not exist. 3  ▣ Describe the 1. components of biological membranes; The Structure ▣ Explain the fluid mosaic model of of Membranes membrane structure; 4 The fluid mosaic model shows proteins embedded in a fluid lipid bilayer. ▣ The lipid layer that forms the foundation of a cell’s membrane is a bilayer formed of phospholipids. ▣ These phospholipids include primarily the glycerol phospholipids and the sphingolipids such as sphingomyelin. ▣ An early model portrayed the plasma membrane as a sandwich: a phospholipid bilayer between two layers of globular protein. 5 Phospholipid Sphingomyelin 6 The fluid mosaic model shows proteins embedded in a fluid lipid bilayer. ▣ In 1972, S. Jonathan Singer and Garth J. Nicolson revised the model in a simple but profound way: □ They 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. ▣ In this model, called the fluid mosaic model, a mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond. 7 The fluid mosaic model shows proteins embedded in a fluid lipid bilayer. ▣ Two categories of membrane proteins based on their association with the membrane: □ Integral membrane proteins are embedded in the membrane, and □ peripheral proteins are associated with the surface of the membrane. 8 The Fluid Mosaic Model of Cell Membranes 9 Cellular membranes consist of four component groups. 1. Phospholipid bilayer. Every cell membrane is composed of phospholipids in a bilayer. ▣ The other components of the membrane are embedded within the bilayer, which provides a flexible matrix and, at the same time, imposes a barrier to permeability. ▣ Animal cell membranes also contain cholesterol, a steroid with a polar hydroxyl group (–OH). ▣ Plant cells have other sterols, but little or no cholesterol. 10 Cellular membranes consist of four component groups. 2. Transmembrane proteins. A major component of every membrane is a collection of proteins that float in the lipid bilayer. ❑ These proteins have a variety of functions, including transport and communication across the membrane. ❑ Many integral membrane proteins are not fixed in position. ❑ They can move about, just as the phospholipid molecules do. ❑ Some membranes are crowded with proteins, but in others, the proteins are more sparsely distributed. 11 Cellular membranes consist of four component groups. 3. Interior protein network. Membranes are structurally supported by intracellular proteins that reinforce the membrane’s shape. ❑ Ex. a red blood cell has a characteristic biconcave shape because a scaffold made of a protein called spectrin links proteins in the plasma membrane with actin filaments in the cell’s cytoskeleton. ❑ Membranes use networks of other proteins to control the lateral movements of some key membrane proteins, anchoring them to specific sites. 12 Cellular membranes consist of four component groups. 4. Cell-surface markers. Different cell types exhibit different varieties of these glycoproteins and glycolipids on their surfaces, which act as cell identity markers. 13 14  2. ▣ List the different components of phospholipids; ▣ Explain how Phospholipids: membranes form spontaneously; The Membrane’s ▣ Describe the Foundation factors involved in membrane fluidity; Phospholipids spontaneously form bilayers. ❑ because of their amphipathic structure. ❑ The polar head groups are hydrophilic, whereas the nonpolar hydrocarbon tails are hydrophobic. ❑ The two nonpolar fatty acids extend in one direction, roughly parallel to each other, and the polar phosphate group points in the other direction. 16 Phospholipids spontaneously form bilayers. ▣ The nonpolar interior of a lipid bilayer impedes the passage of any water-soluble substances through the bilayer, just as a layer of oil impedes the passage of a drop of water. ▣ This barrier to water-soluble substances is the key biological property of the lipid bilayer. 18 The phospholipid bilayer is fluid. ▣ A lipid bilayer is stable because water’s affinity for hydrogen bonding never stops. ▣ Although water drives phospholipids into a bilayer configuration, it does not have any effect on the mobility of phospholipids and their nonlipid neighbors in the bilayer. ▣ Because phospholipids interact relatively weakly with one another, individual phospholipids and unanchored proteins are comparatively free to move about within the membrane. 19 Membrane fluidity varies with lipid composition. ▣ Membrane fluidity can be altered by changing the membrane’s lipid composition. □ Glycerol phospholipids that are saturated, or mono cis-unsaturated, tend to make the membrane less fluid, as they pack well. □ Similarly, the sphingolipids, which are usually unsaturated, also make the membrane less fluid. ▣ Changes in the environment can have drastic effects on the plasma membrane of single-celled organisms such as bacteria. 21 Membrane fluidity varies with lipid composition. ▣ Increasing temperature makes a membrane more fluid, and decreasing temperature makes it less fluid. □ Some bacteria contain enzymes called fatty acid desaturases that can introduce double bonds into fatty acids in membranes. □ At colder temperatures, the double bonds make the membrane more fluid, counteracting the environmental effect of reduced temperature. 22 Phospholipid composition affects membrane structure. 23  ▣ Illustrate the 3. functions of membrane proteins; ▣ Illustrate how proteins can Proteins: associate with the Multifunctional membrane; ▣ Identify a Components transmembrane domain;  ▣ Cell membranes contain a complex assembly of proteins ‘’ enmeshed in the fluid soup of phospholipid molecules. This very flexible organization permits a broad range of interactions with the environment, some directly involving membrane proteins. 25 Proteins and protein complexes perform key functions. 1. Transporters. Membranes are very selective, allowing only certain solutes to enter or leave the cell, through either channels or carriers composed of proteins. 2. Enzymes. Cells carry out many chemical reactions on the interior surface of the plasma membrane, using enzymes attached to the membrane. 3. Cell-surface receptors. Membranes are exquisitely sensitive to chemical messages, which are detected by receptor proteins on their surfaces. 26 Proteins and protein complexes perform key functions. 27 Proteins and protein complexes perform key functions. 4. Cell-surface identity markers. Most cell types carry their own ID tags, specific combinations of cell-surface proteins and protein complexes such as glycoproteins that are characteristic of that cell type. 5. Cell-to-cell adhesion proteins. Cells use specific proteins to glue themselves to one another by forming temporary interactions, and others form a more permanent bond. 6. Attachments to the cytoskeleton. Surface proteins that interact with other cells are often anchored to the cytoskeleton by linking proteins. 28 Proteins and protein complexes perform key functions. 29  ▣ Compare simple 4. diffusion and facilitated diffusion. ▣ Differentiate between channel Passive proteins and carrier Transport Across proteins. ▣ Predict the direction Membranes of water movement by osmosis. Passive Transport ▣ Many substances can move in and out of the cell without the cell’s having to expend energy. ▣ Some ions and molecules can pass through the membrane fairly easily and do so because of a concentration gradient—a difference in concentration inside the membrane versus outside. ▣ Some substances also move in response to a concentration gradient, but do so through specific protein channels in the membrane. 31 Transport can occur by simple diffusion. ▣ The random motion of particles causes net movement of these substances from regions of high concentration to regions of lower concentration, a process called diffusion. ▣ Net movement by diffusion will continue until the concentration is the same in all regions. ▣ This includes molecules like O2 and nonpolar organic molecules such as steroid hormones. 32 Proteins allow membrane diffusion to be selective. ▣ Some molecules enter the cell by diffusion through specific channel proteins or carrier proteins embedded in the plasma membrane, provided there is a higher concentration of the molecule outside the cell than inside. ▣ This process of diffusion mediated by a membrane protein is called facilitated diffusion. ▣ These channels and carriers are usually selective for one type of molecule, and thus the cell membrane is said to be selectively permeable. 33 Proteins allow membrane diffusion to be selective. ▣ Channel proteins have a hydrophilic interior that provides an aqueous channel through which polar molecules can pass when the channel is open. ▣ Carrier proteins, in contrast to channels, bind specifically to the molecule they assist, much as an enzyme binds to its substrate. 34 Proteins allow membrane diffusion to be selective. 35 Facilitated diffusion of ions through channels ▣ Because of their charge, ions interact well with polar molecules such as water, but are repelled by nonpolar molecules such as the interior of the plasma membrane. ▣ Therefore, ions cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of membrane transport proteins. ▣ Ion channels possess a hydrated interior that spans the membrane. Ions can diffuse through the channel in either direction, depending on their relative concentration across the membrane. 36 Facilitated diffusion of ions through channels ▣ Three conditions determine the direction of net movement of the ions: (1) their relative concentrations on either side of the membrane, (2) the voltage difference across the membrane and for the gated channels, and (3) the state of the gate (open or closed). ❑ A voltage difference is an electrical potential difference across the membrane called a membrane potential. 37 Facilitated diffusion by carrier proteins ▣ Carrier proteins can help transport both ions and other solutes such as some sugars and amino acids, across the membrane. ▣ Transport through a carrier is still a form of diffusion and therefore requires a concentration difference across the membrane. ▣ This situation is somewhat like that of a stadium (the cell) where a crowd must pass through turnstiles to enter. 38 Osmosis is the movement of water across membranes. ▣ When a membrane separates two solutions with different concentrations of solutes, the concentrations of free water molecules on the two sides of the membrane also differ. ▣ The side with higher solute concentration has tied up more water molecules in hydration shells and thus has fewer free water molecules. ▣ As a consequence of this difference, free water molecules move down their concentration gradient, toward the higher solute concentration. ▣ This net diffusion of water across a membrane toward a higher solute concentration is called osmosis. 40 Osmosis is the movement of water across membranes. ▣ The concentration of all solutes in a solution determines the osmotic concentration of the solution. ▣ If two solutions have unequal osmotic concentrations, the solution with the higher concentration is hypertonic (Greek hyper, “more than”), and the solution with the lower concentration is hypotonic (Greek hypo, “less than”). ▣ When two solutions have the same osmotic concentration, the solutions are isotonic (Greek iso, “equal”). ‘’ 43 Maintaining Osmotic Balance ▣ Extrusion. Some single-celled eukaryotes, such as the protist Paramecium, use organelles called contractile vacuoles to remove water. The vacuole possesses a small pore that opens to the outside of the cell. By contracting rhythmically, the vacuole pumps out (extrudes) through this pore the water that is continuously drawn into the cell by osmotic forces. Maintaining Osmotic Balance ▣ Isosmotic Regulation. Some organisms that live in the ocean adjust their internal concentration of solutes to match that of the surrounding seawater. Because they are isosmotic with respect to their environment, no net flow of water occurs into or out of these cells. ▣ Many terrestrial animals solve the problem in a similar way, by circulating a fluid through their bodies that bathes cells in an isotonic solution. The blood in your body, for example, contains a high concentration of the protein albumin, which elevates the solute concentration of the blood to match that of your cells’ cytoplasm.  Maintaining Osmotic Balance ▣ Turgor. Most plant cells are hypertonic to their immediate environment, containing a high concentration of solutes in their central vacuoles. The resulting internal hydrostatic pressure, known as turgor pressure, presses the plasma membrane firmly against the interior of the cell wall, making the cell rigid. Most green plants depend on turgor pressure to maintain their shape, and thus they wilt when they lack sufficient water.  ▣ Differentiate 5. between active transport and diffusion; ▣ Describe the Active Transport function of the Na Across +/K + pump; ▣ Explain the Membranes energetics of coupled transport; Active Transport ▣ Cells can also actively move substances across a cell membrane up their concentration gradients. ▣ This process requires the expenditure of energy, typically from ATP, and is therefore called active transport. ▣ It enables a cell to take up additional molecules of a substance that is already present in its cytoplasm in concentrations higher than in the extracellular fluid. 49 Active transport uses energy to move materials against a concentration gradient. ▣ Active transport involves highly selective protein carriers within the membrane that bind to the transported substance, which could be an ion or a simple molecule, such as a sugar, an amino acid, or a nucleotide. □ uniporters if they transport a single type of molecule; □ symporters transport two molecules in the same direction, and □ antiporters transport two molecules in opposite directions. 50 The sodium–potassium pump runs directly on ATP. ▣ More than 1/3 of all of the energy expended by an animal cell that is not actively dividing is used in the active transport of sodium (Na+) and potassium (K+) ions. ▣ Most animal cells have a low internal concentration of Na+, relative to their surroundings, and a high internal concentration of K+. ▣ They maintain these concentration differences by actively pumping Na+ out of the cell and K+ in. 51 The sodium–potassium pump runs directly on ATP. ▣ The remarkable protein that transports these two ions across the cell membrane is known as the sodium–potassium pump (Na+/K+ pump) ▣ This carrier protein uses the energy stored in ATP to move these two ions. ▣ In this case, the energy is used to change the conformation of the carrier protein, which changes its affinity for either Na+ ions or K+ ions. 52 53 Coupled transport uses ATP indirectly. ▣ Some molecules are moved against their concentration gradient by using the energy stored in a gradient of a different molecule. ▣ In this process, called coupled transport, the energy released as one molecule moves down its concentration gradient is captured and used to move a different molecule against its gradient. ▣ The energy stored in ATP molecules can be used to create a gradient of Na+ and K+ across the membrane. ▣ These gradients can then be used to power the transport of other molecules across the membrane 55 Coupled transport uses ATP indirectly. ▣ The active glucose transporter uses the Na+ gradient produced by the Na+/K+ pump as a source of energy to power the movement of glucose into the cell. ▣ In this system, both glucose and Na+ bind to the transport protein, which allows Na+ to pass into the cell down its concentration gradient, capturing the energy and using it to move glucose into the cell. ▣ In this kind of cotransport, both molecules are moving in the same direction across the membrane; therefore the transporter is a symporter. 56 57  6. ▣ Distinguish between endocytosis and exocytosis; and Bulk Transport by Endocytosis ▣ Illustrate how endocytosis can be and Exocytosis specific. The lipid nature of cell plasma membranes raises a second problem. The substances cells require for growth are mostly large, polar molecules that cannot cross the hydrophobic barrier a lipid bilayer creates. How do these substances get into cells? 59 Bulk material enters the cell in vesicles. ▣ In endocytosis, the plasma membrane envelops food particles and fluids. ▣ Cells use three major types of endocytosis: □ phagocytosis, □ pinocytosis, □ receptor-mediated endocytosis ▣ Like active transport, these processes also require energy expenditure. 60 Phagocytosis and pinocytosis ▣ If the material the cell takes in is particulate (made up of discrete particles), such as an organism or some other fragment of organic matter, the process is called phagocytosis (Greek phagein, “to eat,” + cytos, “cell”). ▣ If the material the cell takes in is liquid, the process is called pinocytosis (Greek pinein, “to drink”). □ Pinocytosis is common among animal cells. □ Ex. Mammalian egg cells “nurse” from surrounding cells; the nearby cells secrete nutrients that the maturing egg cell takes up by pinocytosis. 61 Phagocytosis and pinocytosis 62 Receptor-mediated endocystosis ▣ Molecules are often transported into eukaryotic cells through receptor-mediated endocytosis. ▣ These molecules first bind to specific receptors in the plasma membrane—they have a conformation that fits snugly into the receptor. ▣ Different cell types contain a characteristic battery of receptor types, each for a different kind of molecule in their membranes. 65 Receptor-mediated endocytosis ▣ The portion of the receptor molecule that lies inside the membrane is trapped in an indented pit coated on the cytoplasmic side with the protein clathrin. ▣ Each pit acts like a molecular mousetrap, closing over to form an internal vesicle when the right molecule enters the pit. ▣ The trigger that releases the trap is the binding of the properly fitted target molecule to the embedded receptor. ▣ When binding occurs, the cell reacts by initiating endocytosis; the process is highly specific and very fast. 66 Receptor-mediated endocytosis 67 Material can leave the cell by exocytosis. ▣ The reverse of endocytosis is exocytosis, the discharge of material from vesicles at the cell surface. □ In plant cells, it is an important means of exporting the materials needed to construct the cell wall through the plasma membrane. □ Among protists, contractile vacuole discharge is considered a form of it. □ In animal cells, it provides a mechanism for secreting many hormones, neurotransmitters, digestive enzymes, and other substances. 69 Exocytosis 70 72 73 The End 74