Chapter 7 Membrane Structure and Function Handout PDF

Chapter7 Membrane Structure and Function Lecture Outline function and importance of the cell membrane? Overview: Life at the Edge The plasma membrane separates the living cell from its surroundings. This thin barrier, 8 nm thick, controls traffic...

Chapter7 Membrane Structure and Function Lecture Outline function and importance of the cell membrane? Overview: Life at the Edge The plasma membrane separates the living cell from its surroundings. This thin barrier, 8 nm thick, controls traffic into and out of the cell. The plasma membrane is selectively permeable, allowing some substances to cross more easily than others. Encloses a solution different from the surrounding solution while still permitting the uptake of nutrients and the elimination of waste products. Concept 7.1 Cellular membranes are fluid mosaics of lipids and proteins. The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important. The most abundant lipids are phospholipids. Phospholipids and most other membrane constituents are amphipathic molecules, which have both hydrophobic and hydrophilic regions. The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model. In this model, the membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) of phospholipids. The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water. A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholipid bilayer. When a freeze-fracture preparation is viewed with an electron microscope, protein particles are interspersed in a smooth matrix, thus supporting the fluid mosaic model. 1  Membranes are fluid Membrane molecules are held in place by relatively weak hydrophobic interactions. Most of the lipids and some proteins drift laterally in the plane of the membrane but rarely flip- flop from one phospholipid layer to the other, other large membrane proteins drift within the phospholipid bilayer. Other proteins never move and are anchored to the cytoskeleton. Membrane fluidity is influenced by temperature. As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids pack more closely. Membrane fluidity is also influenced by the components of the membrane. The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells. At warm temperatures (such as 37°C), cholesterol restrains the movement of phospholipids and reduces fluidity. At cool temperatures, cholesterol maintains fluidity by preventing tight packing. Thus, cholesterol acts as a “fluidity buffer” for the membrane, resisting changes in what makes the cell membrane fluidity as temperature changes. membrane not turning into which Proteins determine most of the membrane’s specific functions. fluid components There are two major populations of membrane proteins: integral and peripheral. determine Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely the spanning the membrane (as transmembrane proteins). function of Other integral proteins extend to the hydrophobic core. the cell The hydrophobic regions embedded in the membrane’s core consist of stretches of membrane? nonpolar amino acids, usually coiled into helices. The hydrophilic regions of integral proteins are in contact with the aqueous environment. Some integral proteins have a hydrophilic channel through their center that allows passage of hydrophilic substances. the function of the chanel? Peripheral proteins are not embedded in the lipid bilayer at all. Instead, they are loosely bound to the surface of the membrane,often to integral proteins. 2  membrane has two sides: cytoplasmic side( cytoskeleton proteins) and exterior side ( ECM) On the cytoplasmic side of the membrane, some membrane proteins are attached to the cytoskeleton, and on the exterior side of the membrane, some membrane proteins attach to the fibers of the extracellular matrix. These attachments combine to give animal cells a stronger framework than the plasma membrane itself could provide. The proteins of the plasma membrane have six major functions: 1. Transport of specific solutes into or out of cells 2. Enzymatic activity, sometimes catalyzing one of a number of steps of a metabolic pathway. 3. Signal transduction, relaying hormonal messages to the cell 4. Cell-cell recognition, allowing other proteins to attach two adjacent cells together 5. Intercellular joining of adjacent cells with gap or tigh tjunctions 6. Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape and stabilizing the location of certain membrane proteins what type of component is essential in cell- to cell recognition? Membrane carbohydrates are important for cell-cell recognition. Cell-cell recognition, the ability of a cell to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. Cell-cellrecognition is important in the sorting and organizing of cells into tissues and organs during development. Recognition is also the basis for the rejection of foreign cells by the immune system. Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units, may be covalently bonded to lipids, forming glycolipids, or more commonly to proteins, forming glycoproteins. Variation in oligosaccharide distinguishes each cell type, The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells. Carbohydrates are added to proteins in the ER, and the resulting glycoproteins are further modified in the Golgi apparatus. Glycolipids are also produced in the Golgi apparatus. Transmembrane proteins, membrane glycolipids, and secretory proteins are transported in vesicles to the plasma membrane. Concept 7.2 Membrane structure results in selective permeability. The fluid mosaic model helps explain how membranes regulate the cell’s molecular traffic. A steady traffic of small molecules and ions moves across the plasma membrane in both directionsn (trients enter, metabolic waste products leave). Substances do not move across the barrier randomly; membranes are selectively permeable. carbs attach to RER which contain ribosomes and this is the factory of 3 protein then the protein to result in glycoprotein  The cell is able to take up many varieties of small molecules and ions and exclude others. Movement of a molecule through a membrane depends on the interaction of the molecule with the hydrophobic core of the membrane. Nonpolarmolecules, such as hydrocarbons, CO2, and O2, are hydrophobic and can dissolve in the lipid bilayer and cross easily, without the assistance of membrane proteins. The hydrophobic core of the membrane restricts the direct passage of ions and polar molecules, which are hydrophilic. Proteins assist and regulate the transport of ions and polar molecules. Cell membranes are permeable to specific ions and a variety of polar molecules, which can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane. because Some transport proteins called channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane. The passage of water through the membrane can be greatly facilitated by channel proteins known as aquaporins. Without aquaporins, only a tiny fraction of these water molecules would diffuse through the same area of the cell membrane in a second, so the channel protein greatly increases the rate of water movement. Some transport proteins called carrier proteins bind to molecules and change shape to shuttle them across the membrane. Each transport protein is specific for the substance that it translocates. Forexample,the glucos etransport protein in the liver carries glucose into the cell but does not transport fructose, its structural isomer. The glucose transporter causes glucose to pass through the membrane50,000 times as fast as it would diffuse through on its own. what makes channel Transport proteins or transport proteins unique? -- Carrier-mediated T Non-carrier mediated T - 8 Co non polar small molecule Sceased , , , on energy consumption diffusion Active T - facilitated ex : Aquaporins simple diffusion diff. between ATP Passive T Require Channels -. - Low > - High - NO ATP carrier vs. protein High-Low Carrier Channels - - - Protein bind to Certain molecules or molecules of ions pass through it. change shape (small molecules or ions) 4 Concept 7.3 Passive transport is diffusion of a substance across a membrane with no energy investment. Diffusion is the tendency of the molecules of any substance to spread out in the available space. In the absence of other forces, a substance diffuses from where it is more concentrated to where it is less concentrated, down its concentration gradient. Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances. No work must be done to move substances The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen. Because membranes are selectively permeable, the interactions of the molecules with the membrane play a role in the diffusion rate. In the case of water,aquaporins allow water to diffuse very rapidly across the membranes of certain cells. Osmosis is the passive transport of water. The diffusion of water across a selectively permeable membrane is called osmosis. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms. Both solute concentration and membrane permeability affect tonicity, the ability of a solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane relative to the concentration of solutes in the cell itself. If a cell without a cell wall, such as an animal cell, is immersed in an environment that is isotonic to the cell, there is no net movement of water across the plasma membrane. Water flows across the membrane, but at the same rate in both directions. If the cell is immersed in a solution that is hypertonic to the cell, the cell loses water to its environment, shrivels, and probably dies. If the cell is immersed in a solution that is hypotonic to the cell, water enters the cell faster than it leaves, and the cell swells and lyses (bursts) like an overfilled water balloon. 5  Cell survival depends on the balance between water uptake and loss. Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance. The protist Paramecium is hypertonic to the pond water in which it lives. To solve this problem, Paramecium cells have a specialized organelle, the contractile vacuole, that functions as a bilge pump to force water out of the cell. The cells of plants, prokaryotes, fungi, and some protists have walls. A plant cell in a solution hypotonic to the cell contents swells due to osmosis until the elastic cell wall exerts a back-pressure on the cell that opposes further uptake. At this point the cell is turgid (very firm), a healthy state for most plant cells. If a plant cell and its surroundings are isotonic, there is no movement of water into the cell. The cell becomes flaccid (limp), and the plant may wilt. The cell wall provides no advantages when a plant cell is immersed in a hypertonic solution, as the plant cell loses water, its volume shrinks, the plasma membrane pulls away from the wall. This plasmolysis is usually lethal. The walled cells of bacteria and fungi also plasmolyze in hypertonic environments. Specific proteins facilitate the passive transport of water and selected solutes. Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. The passive movement of molecules down their concentration gradient via transport proteins is called facilitated diffusion. o Most transport proteins are very specific:They transport only particular substances but not others. Two types of transport proteins facilitate the movement of molecules or ions across membranes: channel proteins and carrier proteins. Many ion channels function as gated channels. soduim potassium pump These channels open or close depending on the presence or absence of a chemical or physical stimulus. Some transport proteins do not provide channels but appear to actually translocate the solute- binding site and the solute across the membrane as the transport protein changes shape. * channels are found in the cell membrane = Voltage-gated channels - Ligand-gated channels 6 channel to work ligand has to be available for this  Concept 7.4 Active transport uses energy to move solutes against their gradients. Some transport proteins can move solutes across membranes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated. Carrier mediated This active transport requires the cell to expend metabolic energy and enables a cell to maintain internal concentrations of small molecules that would otherwise diffuse across the membrane. Compared with its surroundings, an animal cell has a much higher concentration of potassium ions and a much lower concentration of sodium ions. The plasma membrane helps maintain these steep gradients by pumping sodium out of the cell and potassium into the cell. ATP supplies the energy for most active transport by transferring its terminal phosphate group directly to the transport protein. This process may induce a conformational change in the transport protein,translocating the bound solute across the membrane. The sodium-potassium pump works this way in exchanging 3 sodium ions (Na+) for Sodium-potassium ATPase. 2 potassium ions (K+) across the plasma membrane of animal cells. In cotransport, a membrane protein couples the transport of two solutes. Secondary Active Transport A single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport. As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration gradient. Plants use the mechanism of sucrose-proton cotransport to load sucrose into specialized cells in the veins of leaves for distribution to nonphotosynthetic organs such as roots. Understanding of cotransport proteins, osmosis, and water balance in animal cells has helped scientists develop effective treatments for the dehydration that results from diarrhea. & Patients are given a solution to drink that contains a high concentration of glucose and salt. The solutes are taken up by transport proteins on the intestinal cell surface and passed through the cells into the blood. The resulting increase in the solute concentration of the blood causes a flow of water from the intestine through the intestinal cells into the blood, rehydrating the patient. conformational changes ↓ change in shape 7  Bulk transport (tsan tiet Dals it's a non-carrier mediated exocytosis : vesicle from golgi , to the outside of the cell through cytoskeleton then the Exocytosis Endocytosis - Phagocytosis (Cellular eating) pinocytosis (cellular drinking) ECM - - Receptor-mediated endocytosis Concept 7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis. Small solutes and water enter or leave the cell through the lipid bilayer or by transport proteins. Particles and large molecules, such as polysaccharides and proteins, cross the membrane via packaging in vesicles. Like active transport, these processes require energy. In exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane. When the two membranes come in contact, the bilayers fuse and spill the contents to the outside. Pancreatic cells secrete insulin into the blood by exocytosis. Neurons use exocytosis to release neurotransmitters that signal other neurons or muscle cells. When plant cells are making walls, exocytosis delivers proteins and certain carbohydrates from Golgi vesicles to the outside of the cell. In contrast endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane. Endocytosis is a reversal of exocytosis, although different proteins are involved in the two processes. In endocytosis, a small area of the plasma membrane sinks inward to form a pocket. As the pocket deepens, it pinches in to form a vesicle containing the material that had been outside the cell. There are three types of endocytosis: phagocytosis (“cellular eating”), pinocytosis (“cellular drinking”), and receptor-mediated endocytosis. Receptor-mediated endocytosis enables a cell to acquire bulk quantities of specific materials that may be in low concentrations in the environment. Human cells use endocytosis to take in cholesterol for use in the synthesis of membranes and as a precursor for the synthesis of steroids. Cholesterol travels in the blood in low-densitylipoproteins(LDL), complexes of protein and lipid. These lipoproteins act as ligands by binding to LDL receptors on membranes and entering the cell by endocytosis. 8  Pseudo : false - 9