Membrane Transport Note

MEMBRANE TRANSPORT MEMBRANE TRANSPORT Movement of materials across a membrane will depend on both the size and solubility of the material. Large and charged molecules cannot freely cross the plasma membrane without addiGonal assistance. • Small, lipophilic molecules can freely pass across the bilayer (e.g. O2 , CO2 , water, steroids) • Larger molecules or polar / charged molecules require membrane proteins to cross (e.g. glucose, ions) TYPES OF TRANSPORT Movement of materials across a membrane may occur via: Passive Transport: • Movement from high concentration to low concentration (i.e. movement is along or down a concentration gradient) • Does not involve the expenditure of energy (no ATP used) AcAve Transport: • Movement from low concentration to high concentration (i.e. movement occurs against a concentration gradient) • Does involve the expenditure of energy (ATP is hydrolysed) NEED ENERGY NO ENERGY HIGH LOW LOW LOW LEVELS ACTIVE PASSIVE HIGH LEVELS Passive transport mechanisms include either simple diffusion, facilitated diffusion or osmosis. The type of passive transport is determined by the permeability of the molecule involved. AcGve transport always involves impermeable molecules moving against a gradient (otherwise, they would simply move back passively along the gradient). AddiGonally, molecules may enter or exit the cell without crossing the membrane via cytosis. The membrane physically breaks and reforms around the material to facilitate its passage – this is an energy-dependent process and requires the use of ATP (but it is not acGve transport). SIMPLE DIFFUSION Diffusion is the net movement of molecules from a region of higher concentraGon to lower concentraGon. This movement along the gradient will conGnue unGl the molecules are evenly dispersed (i.e. equilibrium). Simple diffusion occurs when small or non-polar molecules can freely cross the plasma membrane without impediment (e.g. O2 , CO2 , ethanol, glycerol, fa]y acids). The rate of diffusion will be influenced by the size of the gradient and the kineGc energy of the parGcles. FACILITATED DIFFUSION CHANNEL CARRIER Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein. It is uGlised by molecules that cannot cross a phospholipid bilayer unassisted (i.e. large, polar molecules or ions). The movement is facilitated by transport proteins (either carrier proteins or channel proteins). • Channel proteins contain a hydrophilic pore to enable passage • Carrier proteins undergo a conformational change in order to translocate molecules across a bilayer (slower than channels) Channel proteins will typically contain a selecAvity filter that enables the protein to adopt an ‘open’ or a ‘closed’ conformaGon – this allows the protein to regulate the Gming and quanGty of material being moved. OSMOSIS Osmosis is the diffusion of free water molecules. While water is a polar molecule, it is sGll small enough to move across the phospholipid bilayer without assistance. The polarity of water will cause it to move towards charged or polar solutes that cannot freely cross the membrane (water is the universal solvent). Hence, osmosis involves the net movement of water molecules across a semipermeable membrane from a region of low solute concentraGon to a region of higher solute concentraGon (unGl equilibrium is reached). A high solute soluGon will draw water and is called hypertonic, while a low solute soluGon will lose water and is called hypotonic (if two soluGons have equal solute concentraGons, they are described as isotonic). Hypotonic: low solute levels = high free water Hypertonic: high solute level = low free water AQUAPORINS While water is small enough to pass freely across the phospholipid bilayer, certain cell membranes may also include integral proteins called aquaporins that funcGon as selecGve water channels. Aquaporins facilitate a much faster rate of water transport in response to solute concentraGons and their levels can be regulated to help control osmoGc condiGons in cells. Aquaporins are oben found in kidney cells (for osmoregulaGon). OSMOTIC EFFECTS An influx of water into a cell (due to an increase in solute concentraGon) will increase the osmoGc pressure within a cell and cause it to swell. If the cell lacks a rigid cell wall, it will eventually burst (lysis) – otherwise the membrane will press up against the cell wall, making the cell turgid. An efflux of water out of the cell (due to decrease in solute concentraGon inside the cell) will lower the osmoGc pressure within the cell and cause it to shrivel (crenaAon). If the cell has a rigid cell wall, the membrane will move away from this outer boundary and create a region of empty space (plasmolysis) – this is why plants will wilt in dry condiGons. Hypertonic Solu/on Isotonic Solu/on Hypotonic Solu/on Due to the effects of osmosis on cell morphology, it is important that cells lacking a cell wall are generally maintained in isotonic condiGons. This is why organs for transplant must be bathed in isotonic soluGons prior to surgery, and also why any intravenous fluids administered as part of medical treatments (such as dehydraGon or excessive blood loss) must be relaGvely isotonic. Any prolonged exposure to hypertonic or hypotonic soluGons would eventually lead to Gssue damage and threaten the survival of the individual. CONTRACTILE VACUOLES Unicellular organisms (such as proGsts) are especially suscepGble to changes in solute concentraGon, as the single cell must carry out all the funcGons of life. These cells will oben possess contracAle vacuoles to help regulate water levels. RadiaGng canals draw water into contracGle vacuoles, causing it to swell (diastole). The vacuole then fuses with the plasma membrane and the excess water is expelled (systole). The amount of water expelled and the frequency of contracGons will be determined by external solute concentraGons. ACTIVE TRANSPORT AcGve transport uses energy to move molecules against a gradient. This involves the use of protein pumps, which use ATP hydrolysis to trigger a conformaGonal change that translocates molecules to the regions of higher concentraGon. Only molecules that cannot freely cross the membrane can be acGvely transported against a gradient. ATP ADP BULK TRANSPORT (CYTOSIS) The fluidity of membranes allows materials to be taken in or released by cells without crossing the bilayer. The membrane is principally held together by weak hydrophobic associaGons between the fa]y acid tails of the phospholipids. These weak interacGons can be spontaneously broken and reformed in a process that is energy dependent and requires ATP hydrolysis. SecGons of a membrane can be excised to form internal vesicles, while membrane segments can be added as a result of fusion between the membrane and vesicle. • Exocytosis is the process by which intracellular materials exit a cell without crossing the membrane – the materials to be exported must first be packaged into a membranous vesicle by the Golgi apparatus. • Endocytosis is the process by which extracellular materials enter a cell without crossing the membrane.