Plasma Membrane Rap The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Phospholipids Cholesterol Embedded Proteins Glycoproteins Glycolipids CARBOHYDRATES most abundant lipid in the plasma membrane amphipathic molecules, containing hydrophobic and hydrophilic regions J. Singer & G. Nicolson (1972) proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water (fluid mosaic model) Plasma Membrane Video Freeze-fracture studies of the plasma membrane supported the fluid mosaic model Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer Inside of cytoplasmic layer TECHNIQUE Extracellular layer Knife Proteins Inside of extracellular layer RESULTS Cytoplasmic layer Plasma membrane Phospholipids Contd Phospholipids can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane As temperatures cool, membranes become more solid The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil Ring structure reduces membrane fluidity at moderate temperatures by reducing phospholipid movement low temperatures hinders solidification by disrupting the regular packing of phospholipids. warm temperatures (such as 37C), cholesterol restrains movement of phospholipids Cholesterol is known as temperature buffer. Cholesterol (c) Cholesterol within the animal cell membrane 2) Cholesterol collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membranes specific functions The two sides of a membrane have different protein and lipid compositions. Peripheral proteins are bound to the surface of the membrane Not embedded Loosely bound to surface of membrane Integral proteins penetrate the hydrophobic core transmembrane proteins span the membrane hydrophobic regions consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices Hydrophilic regions exposed to inside/outside of cell Transport proteins allow passage of large hydrophilic (polar) substances and ions across the membrane (FACILITATED DIFFUSION) 1) channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel Channel proteins called aquaporins facilitate the passage of water 2) carrier proteins bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves Small, uncharged polar and nonpolar molecules can freely pass through a cell membrane Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Hydrocarbons, CO 2 and O 2 Polar molecules, such as sugars, do not cross the membrane easily C 6 H 12 O 6, or charged molecules Six major functions Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) Proteins Contd Cells recognize each other by binding to surface molecules, usually carbohydrates Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Carbohydrates are the third major component of plasma membranes found on the exterior surface of cells bound either to proteins (forming glycoproteins) or bound to lipids (forming glycolipids) Chains can be either straight or branched Form specialized sites on the cell surface that allow cells to recognize each other. unique patterns that allow the cell to be recognized allows the immune system to differentiate between body cells (called self) and foreign cells or tissues (called non-self). Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them. Membranes have distinct inside and outside faces The two sides of a membrane have different protein and lipid compositions. The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus Molecules that start out on the inside face of the ER end up on the outside face of the plasma membrane Import of resources and export of wastes Substances diffuse down their concentration gradient ( the difference in concentration of a substance from one area to another) From an area of high concentration to low concentration No work must be done to move substances down the concentration gradient O 2 gets into cells this way for cellular respiration Diffusion is the tendency for molecules to spread out evenly into the available space At dynamic equilibrium, as many molecules cross one way as cross in the other direction This is not static even at equilibrium molecules are still moving. Molecules of dyeMembrane (cross section) WATER Net diffusion (a) Diffusion of one solute Equilibrium (b) Diffusion of two solutes Fig. 7-11b Net diffusion Equilibrium The diffusion of one solute is unaffected by the diffusion of another solute. Osmosis is the diffusion of water across a selectively permeable membrane The direction of osmosis is determined only by a difference in total solute concentration. Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration or you can think [high water] to [low water] Lower concentration of solute (sugar) Fig H2OH2O Higher concentration of sugar Selectively permeable membrane Same concentration of sugar Osmosis Tonicity is the ability of a solution to cause a cell 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 Fig Hypotonic solution (a ) Animal cell (b ) Plant cell H2OH2O Lysed H2OH2O Turgid (normal) H2OH2O H2OH2O H2OH2O H2OH2O Normal Isotonic solution Flaccid H2OH2O H2OH2O Shriveled Plasmolyzed Hypertonic solution Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments Ex) The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Cell walls of plants, bacteria, fungi, algae, and some archaea help maintain water balance plant cell in a hypotonic solution cell swells turgid (firm) plant cell and its surroundings are isotonic there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt Plant cell in hypertonic environment cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Most transport proteins are very specific 2 types of transport proteins Channel proteins and carrier proteins Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Aquaporins, for facilitated diffusion of water speed up osmosis Ion channels that open or close in response to a stimulus (gated channels) Carrier proteins transport substances out of or into the cell by facilitated diffusion and active transport. Each designed to recognize only ONE substance or ONE group of very similar substances. Di ffusion of sugars, amino acids, nucleoside. Uptake of glucose. Transportation of salts, glucose, and amino acids undergo a subtle change in shape that translocates the solute-binding site across the membrane Substances diffuse against their concentration gradient From an area of low concentration to high concentration Work must be done to move substances from low to high performed by specific proteins embedded in the membranes Allows cells to maintain concentration gradients that differ from their surroundings Ex) sodium-potassium pump in nerve conduction Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions Cells have a net negative internal charge Favors passive transport of cations into the cell and anions out Two combined forces, collectively called the electrochemical gradient, drive the diffusion (PASSIVELY) of ions across a membrane: A chemical force (the ions concentration gradient) An electrical force (the effect of the membrane potential on the ions movement) VIDEO Concentration gradient Electrical gradient (c) An electrogenic pump is a transport protein that generates voltage across a membrane (increasing membrane potential) EX: Sodium-Potassium pump major electrogenic pump of animals 3Na+out 2K+ in = overall positive charge to the extracellular fluid The main electrogenic pump of plants, fungi, and bacteria is a proton pump These pumps generate voltage across membranes which stores energy for use for the cell (mitochondria and chloroplasts) ADP ATP ATPase is phosphorylated with P i from ATP. Protein changes conformation. ICF ECF Protein changes conformation. 3 Na + from ICF bind 3 Na + released into ECF 2 K + from ECF bind 2 K + released into ICF K+ concentration less outside, more inside Na+ concentration is more outside, less inside Na+/K+ maintains these concentration gradients using ATP to pump 3Na+ out and 2K+ Nerve cells depend on this pump for responding to stimuli and transmitting impulses A single ATP-powered pump that transports one solute can indirectly drive the active transport of other solutes Actively transported substance diffuses back passively through protein, its movement can be actively paired with another substance to move it against the concentration gradient Large molecules cross the membrane in bulk via vesicles polysaccharides and proteins Bulk transport requires energy Transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Pancreatic beta cells releasing insulin Neurons releasing neurotransmiters Plants making cell walls Cell takes in macromolecules by forming vesicles from the plasma membrane reversal of exocytosis, involving different proteins There are three types of endocytosis: Phagocytosis (cellular eating) Pinocytosis (cellular drinking) Receptor-mediated endocytosis phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by ligand signal In phagocytosis (cellular eating) a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle In pinocytosis,( cellular drinking) molecules are taken up when extracellular fluid is gulped into tiny vesicles In receptor-mediated endocytosis, (picky eater) binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by ligand signal Bacterium Lysosome Phagocyte The phagocytic white blood cell encounters a bacterium that binds to the cell membrane. The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome. The phagosome containing the bacterium separates from the cell membrane and moves into the cytoplasm. The phagosome fuses with lysosomes containing digestive enzymes. The bacterium is killed and digested within the vesicle Ligand binds to membrane receptor. Clathrin-coated pit Receptor Extracellular fluid Intracellular fluid To lysosome or Golgi complex Receptor-ligand migrates to clathrin-coated pit. Endocytosis Vesicle loses clathrin coat. Ligands go to lysosomes or Golgi for processing. Transport vesicle with receptors moves to the cell membrane. Transport vesicle and cell membrane fuse (membrane recycling). Exocytosis Clathrin Endosome Receptors and ligands separate. SUMMARY OF TRANSPORT Molecules move from a region of high concentration, spontaneously. In other words, this happens because it results in a release of energy. Molecules in this situation are described as going down their concentration gradient. The direction of the electrical driving force results in the ion moving toward a region where the opposite charge exists. The magnitude of the electrical force depends upon the size of the membrane potential and the charge of the ion. The greater the membrane potential or the charge of the ion the greater the electrical driving force. The total forces acting upon ions across a membrane is a combination of both chemical and electrical forces and is referred to as the electrochemical driving force. The net direction of this force is equal to the sum of both forces. If the chemical and electrical driving forces are in opposite directions, the point at which each equals the other is the equilibrium potential. 1. Define the following terms: amphipathic molecules, aquaporins, diffusion 2. Explain how membrane fluidity is influenced by temperature and membrane composition 3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions 4. Explain how transport proteins facilitate diffusion 5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps 6. Explain how large molecules are transported across a cell membrane