Lecture 4 Biological Membranes PDF

LECTURE 4 BIOLOGICAL MEMBRANES Key concepts Cellular membranes are fluid mosaics of lipids and proteins. Membrane structure results in selective permeability. Passive transport is diffusion of a substance across a membrane with no energy investment. Active transport uses energy to move so...

LECTURE 4 BIOLOGICAL MEMBRANES Key concepts Cellular membranes are fluid mosaics of lipids and proteins. Membrane structure results in selective permeability. Passive transport is diffusion of a substance across a membrane with no energy investment. Active transport uses energy to move solutes against their gradients. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis. 4.1. A fluid mosaic Phospholipid bilayers The plasma membrane is the boundary that separates a living cell from its surroundings. Phospholipids are the most abundant lipid in the plasma membrane. Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions. Membranes are therefore held together by weak hydrophobic interactions. Lipid movement Fluid mosaic model: a membrane is a fluid structure with a “mosaic” of proteins embedded in it. Phospholipids in the plasma membrane can move within bilayer. Rarely a molecule flip-flops transversely. Protein movement Proteins can move through the membrane. Larry Frye and Michael Edidin differentially labelled membrane proteins of mouse and human cells. They fused the cells and observed the hybrid under a microscope. Fluidity of membranes As temperatures cool, membranes go from a fluid state to a solid state. Membranes must be fluid to work properly; they are usually about as fluid as salad oil. Temperature at which a membrane solidifies depends on types of lipids. Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids. Also, the membrane becomes more difficult to freeze if the hydrocarbon chains are short, as a shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another. Role of steroids in membrane fluidity The steroid cholesterol has different effects on membrane fluidity at different temperatures. At warm temperatures, it restrains movement of phospholipids, thereby preventing the membrane from becoming too fluid. At cool temperatures, it maintains fluidity by preventing tight packing. Variations in lipid composition of cell membranes of many species are likely due to adaptations to specific environmental conditions. Ability to change lipid compositions as temperature changes has evolved in organisms that live where temperatures vary. Peripheral and integral proteins A membrane is a collage of different proteins (like a tile mosaic), often grouped together, embedded in fluid matrix of lipid bilayer. Proteins determine most of membrane’s specific functions. Peripheral proteins are bound to the surface of the membrane. Integral proteins penetrate the hydrophobic core, and are embedded in the membrane. Integral proteins that span the membrane are called transmembrane proteins. The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices. Functions of membrane proteins Membrane proteins and HIV resistance Cell surface proteins play an important role in the medical field. HIV must bind to the immune cell surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell. HIV cannot enter the cells of resistant individuals that lack CCR5. Drugs have been developed to mask CCR5 co-receptor protein to block HIV entry, as a treatment for HIV infection. The role of membrane carbohydrates Cells recognize each other by binding to extracellular surface molecules, often containing carbohydrates. Membrane carbohydrates may be covalently bonded to lipids (glycolipids) or more commonly to proteins (glycoproteins). Do not forget proteoglycans (mainly sugars, with small peptides attached). Carbohydrates of plasma membranes vary among species, individuals, and even cell types in an individual. Synthesis and orientation Membranes have distinct inside and outside faces. The asymmetrical distribution of plasma membrane components is determined when membrane is built by ER and Golgi apparatus. 4.2. Passive transport Occurs through diffusion The cell does not have to expend energy Done by carrier proteins and channel proteins Selective permeability Cells must exchange materials with surroundings, a process controlled by the plasma membrane. Plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others. Hydrophobic (nonpolar) molecules can dissolve in lipid bilayer and pass through membrane rapidly. Polar (hydrophilic) molecules do not cross membrane easily. Diffusion Diffusion is the tendency for molecules to spread out evenly into available space. Each molecule moves randomly, but diffusion of a population of molecules may be directional. Dynamic equilibrium, as many molecules cross membrane in one direction as in the other. Osmosis Osmosis is the diffusion of water across a selectively permeable membrane. Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides. Water balance of animal cells Tonicity is the ability of a surrounding solution to cause cells to gain or lose water. Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane. Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water. Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water. Water balance of plant cells Cell walls help maintain water balance in plant cells. Plant cell in hypotonic solution swells until wall opposes uptake; it is turgid (firm). Plant cell in isotonic environment, no net movement of water; it is flaccid (limp). In a hypertonic environment, plant cells lose water, and plasma membrane pulls away from the cell wall causing plant to wilt; plasmolysis (usually lethal). Osmoregulation Hypertonic or hypotonic environments create osmotic problems for organisms. Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments. The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump. Facilitated diffusion Transport proteins allow hydrophilic substances to pass across membrane. In facilitated diffusion, transport proteins aid the passive movement of molecules across the plasma membrane. A transport protein is specific for the substance it moves. Transport proteins are classified as either channel proteins or carrier proteins. Carriers and channels Carrier proteins undergo a subtle change in shape (conformation) that translocates the solute-binding site across the membrane. Channel proteins provide corridors that allow a specific molecule or ion to cross. Channel proteins include: Aquaporins, facilitate the diffusion of water. Ion channels facilitate the diffusion of ions. Some ion channels are gated channels, which open or close in response to a stimulus.  4.3. Active transport Moves solutes against their concentration gradient Requires energy. Examples: Ø Sodium-potassium pump Ø Electrogenic pump Ø Proton pump Ø Cotransport Active transport Facilitated diffusion: the solute moves down its concentration gradient, and transport requires no energy. Some transport proteins, however, can move solutes against their concentration gradients. Active transport moves substances against their concentration gradients. Active transport requires energy, usually in the form of ATP. Active transport allows cells to maintain concentration gradients that differ from their surroundings. Sodium-potassium pump The sodium-potassium pump is one type of active transport system. Electrochemical gradient Membrane potential is the voltage difference across a membrane. Voltage is created by differences in the distribution of positively and negatively charged ions across a membrane. Two combined forces, collectively called electrochemical gradient, drive diffusion of ions across a membrane: A chemical force (the ion’s concentration gradient). An electrical force (effect of membrane potential on ion’s movement). An electrogenic pump is a transport protein that generates voltage across a membrane. Proton pump The sodium-potassium pump is the major electrogenic pump of animal cells. The main electrogenic pump of plants, fungi, and bacteria is a proton pump. Electrogenic pumps help store energy that can be used for cellular work. See how this proton pump maintains the membrane potential: Cotransport Cotransport occurs when active transport of a solute indirectly drives transport of other solutes. Plants commonly use a hydrogen ion gradient to drive active transport of nutrients into the cell. 4.4. Bulk transport Requires energy For transport of large molecules Exocytosis Endocytosis Bulk transport Small molecules and water enter or leave cell through lipid bilayer or via transport proteins. Large molecules, such as polysaccharides and proteins, cross membrane via vesicles (bulk transport). Bulk transport requires energy. Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents to the outside of the cell. Many secretory cells use exocytosis to export their products. Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane. In phagocytosis cell engulfs a particle in vacuole. Vacuole fuses with lysosome to digest particle. In pinocytosis, extracellular fluid is “gulped” into tiny vesicles. In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation.