Chapter 4Membrane Function3. Membrane FunctionTransport of MoleculesSignal TransductionWorking cells must control the flow of materials to and from the environment. Membrane proteins help with this task, they are the custom agents of the cells.Here are some functions of protein membranesCELL 1CELL 2Permeability of the Plasma Membrane Some molecules freely cross membrane Water, small, noncharged molecules Water may also use aquaporins to cross membrane Other molecules cannot use Channel proteins Carrier proteins Vesicles Endocytosis or exocytosismacromoleculeH2Oprotein+-charged moleculesand ionsphospholipidmoleculenonchargedmoleculesCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Molecules can pass through the membrane through different mechanismsA. Passive Transport: Diffusion Across MembranesB. Osmosis and Water BalanceC. Active TransportD. Exocytosis and Endocytosis: Traffic of Large MoleculesPassage of Molecules Into and Out of the Cell Diffusion is one result of the movement of molecules. Molecules tend to spread into the available space. Diffusion is passive transport; NO energy is needed. Molecules contain heat energy. They vibrate and move around. A. Passive Transport:Diffusion Across Membranes Another type of passive transport is facilitated diffusion, the transport of some substances by specific transport proteins that act as selective corridors. Gases can diffuse through a membrane Oxygen and carbon dioxide enter and exit this wayCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.capillaryalveolusbronchioleoxygenO2O2O2O2O2O2O2O2O2O2O2O2Several factors influence the rate of diffusion Temperature As temperature increases, the rate of diffusion increases Pressure Electrical currents Molecular sizeB. Osmosis and Water Balance in CellsOsmosis is the passive transport of water across a selectively permeable membrane.Osmosis Diffusion of water across a differentially permeable membrane Diffusion always occurs from higher to lower concentration Osmotic pressure is the pressure that develops in a system due to osmosis The greater the possible osmotic pressure, the more likely it is that water will diffuse in that directionA hypertonic solution Has a higher concentration of solute.A hypotonic solution Has a lower concentration of solute.An isotonic solution Has an equal concentration of solute.Water Balance in Animal Cells The survival of a cell depends on its ability to balance water uptake and loss. Osmoregulation is the control of water balance in animals.Water Balance in Plant Cells Water balance in plant cells is different. They have rigid cell walls. They are at the mercy of the environment.C. Active Transport:The Pumping of Molecules Across MembranesActive transport requires energy to move molecules across a membrane.Transport by Carrier Proteins Carrier proteins are specific Combine with a molecule or ion to be transported across the membrane Carrier proteins are required for: Facilitated Transport Active TransportActive Transport: Facilitated transport Small molecules that are not lipid-soluble Molecules follow the concentration gradient Energy is NOT requiredCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.InsideplasmamembranecarrierproteinsoluteOutside Active Transport Molecules combine with carrier proteins Often called pumps Molecules move against the concentration gradient Entering or leaving cell Energy is requiredK+K+K+K+K+K+K+K+K +K+K+K+K+K+K+K+K+K+PPPPNa+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Inside6. Change in shape results and causes carrier to release 2 K+ inside the cell.carrierproteinOutsideK+K+K+Na+Na+Na+Na+ADPATPK+K+K+Na+Na+3. Change in shape results and causes carrier to release 3 Na+ outside the cell.2. ATP is split, and phosphate group attaches to carrier.4. Carrier has a shape that allows it to take up 2 K+.5. Phosphate group is released from carrier.1. Carrier has a shape that allows it to take up 3 Na+.Sodium-Potassium PumpDiffusionFacilitatedDiffusionOsmosisInside the cellCytoplasmPassiveNo energyActiveEnergyIN SUMMARYATPD. Exocytosis and Endocytosis:Traffic of Large MoleculesMechanisms that involve fusion with the plasma membrane and use of vesicles. Exocytosis (Exo Exit)Secretes substances outside of the cell. EndocytosisTakes substances inside of the cell.ExocytosisEndocytosis In phagocytosis (“cellular eating”), a cell engulfs a particle and packages it within a food vacuole. In pinocytosis (“cellular drinking”), a cell “gulps” droplets of fluid by forming tiny vesicles.Different endocytosis: Receptor-mediated endocytosis is triggered by the binding of external molecules to membrane proteins.Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.parameciumsolutesolutea. Phagocytosisb. Pinocytosisvacuolecoated vesicleplasma membranecoated pitc. Receptor-mediated endocytosis399.9 mvesicleva