Movement Across Membranes - Biology PDF

Biology FX3003 Movement across Membranes  Transport across the Plasma Membrane Achieved by four basic methods; Transport mechanisms are either passive or active a). Diffusion  a). Diffusion Movement of substances from a high concentration to a low concentrat...

Biology FX3003 Movement across Membranes  Transport across the Plasma Membrane Achieved by four basic methods; Transport mechanisms are either passive or active a). Diffusion  a). Diffusion Movement of substances from a high concentration to a low concentration, down a concentration gradient Passive process using kinetic energy.. and depends upon concentration gradient, temperature etc.. Either directly through the lipid bilayer, or through the protein channels. Dissolved oxygen, carbon dioxide, small ions Na K Cl etc…. + + -  At the subcellular level, where distances are very short, diffusion is extremely rapid e.g. at the synapse.  Materials move from one end of an organelle to another  Materials move from one end of an organelle to another in a fraction of a millisecond, or from the centre of a cell to its surface in a millisecond.  Diffusion over a centimetre may take an hour or more; over meters it takes years. Diffusion would be useless to distribute materials throughout the human body, but within our cells or across layers of one or two cells it can be adequately rapid.  Note that each type of diffusible molecule will diffuse down its own concentration gradient so that it is possible to have two or more types of molecules diffusing in opposite directions. The effect of the plasma membrane on the rate of diffusion depends upon the nature of the diffusing substance. Hydrophobic (non-polar) groups - diffusion not impeded by phospholipids e.g oxygen, lipids and lipid solvents Small hydrophilic (polar) groups diffusion impeded by phosopholipids, Small hydrophilic (polar) groups diffusion impeded by phosopholipids, therefore more restricted to hydrophilic protein pores. Large hydrophilic (polar) groups diffusion impeded by phospholipids, and by the size of the protein pores.  Because the membrane is permeable to some molecules and ions and not others, it is said to be selectively (partially) permeable or semi-permeable. Diffusion rate may be expressed with Fick’s Law, where; Diffusion rate is proportional to: Surface area * difference in concentration Thickness of the membrane Facilitated diffusion  Facilitated diffusion uses channel proteins (gated channels) or carrier proteins. Passive process using kinetic energy, but speeded up with the use of dedicated channels for specific ions. The system is saturated at this concentration. cotransport2.swf b) OSMOSIS  Osmosis is the diffusion of water molecules from a region of high water concentration to a region of low water concentration through a partially permeable membrane.  Water molecules in pure water are all free to move about at random and thus diffuse across the partially permeable membrane. In which direction would you expect the water molecules to move? A to B ? or B to A? In fact there will be movement of water molecules in both directions due to the kinetic energy of the molecules but there will be no net movement of water. This is an example of a dynamic equilibrium.  In a concentrated solute solution most water molecules are associated with the solute molecules.  Which side has the greatest concentration of water molecules? In which direction would you expect water molecules to move? A to B ? B to A? What would happen to the level of water in A?  Water molecules in the solute solution move much more slowly than in pure water, and most of them are retained. The water potential of a solution is the tendency for water molecules to enter or leave that solution by osmosis. Pure water has the highest water potential, which by definition is set as zero.  Dissolving solute molecules into pure water will reduce the movement of water molecules and thus lower the water potential. Solutions (at atmospheric pressure) have negative values of water potential.  Water diffuses from a region of high water potential (less negative or zero value) to a region of lower water potential (more negative value). Two important factors which determine the water potential of solutions are the presence of dissolved solutes (solute potential) and mechanical pressure acting on water (pressure potential).  The following equation (in which water potential is represented as the Greek letter , psi) is used: water = solute + pressure potential potential potential  = s + p Solute potential s  The solute potential is the negative component of water potential due to the presence of solute molecules. Solutes lower the water potential of the solution, and give it a negative value. Solute potential was also known as 'osmotic pressure' or 'osmotic potential'. The solute potential of a concentrated solution can be demonstrated with apparatus known as an osmometer. The pressure that is applied to stop water entering the solution by osmosis is the solute potential of that solution. Pressure potential p If a pressure is applied to a solution in a partially permeable bag, it increases the water potential. Hydrostatic pressure to which water is subjected is called pressure potential.  Pressure potential is usually positive. In most cells (such as a turgid plant cell), the pressure potential is positive. Note: In xylem of a transpiring plant the water column is under tension and the pressure potential is negative.  Mechanical pressure in cell water relations was previously known as 'turgor pressure' or 'wall pressure'. Cells and Water Potential If the external water potential is the same (isotonic) If the external water potential is less negative (hypotonic) If the external water potential is more negative (hypertonic)  Terms: isotonic, hypotonic, hypertonic Refer to the concentration of the solute molecules in a solution not the concentration of the solvent molecules. Although the terms are still used, it is preferable to think in terms of water potential c) Active Transport This allows molecules or ions to move against a concentration gradient. It requires energy derived from cellular respiration, in the form of ATP. It would also seem to use some form of carrier protein, possibly ATPase. Allows the accumulation of nutrients, and the removal of toxic wastes, even if concentration is low. Movement of ions is not only a function of concentration, but also electrical charge. Ions are attracted towards regions of opposite charge …. Electrochemical gradient. Most cells are negatively charged with respect to the outside medium, cations may be attracted into the cells, and anions repulsed. Concentration will help determine the final direction of movement.  Two major ions of extracellular and intracellular fluids are Na and K. + + Plasma Cell 144 ------------------------ 15 mM Na+ 5 ------------------------ 150 mM K+ Sodium is actively pumped out Potassium actively pumped in If respiration is inhibited (cyanide), then ions will diffuse passively through the plasma membrane until equilibrium is reached. The active transport of Na+ and K+ would seem to be coupled together, though not always. This combined system is called the sodium-potassium pump. This combined system is called the sodium-potassium pump. If K+ ions are removed from the surrounding medium, then Na+ will influx into the cell and K outflux. + The mechanism of active pumping stops with the lack of K ions, + and passive diffusion occurs to restore equilibrium. The pump is a protein (ATPase) that spans the membrane.  Sodium and ATP are collected on the inside, energy is released from the ATP, and sodium is expelled. Potassium is collected on the outside, and closes the protein gate, forcing the potassium into the cell.  For every 2K+ taken into the cell 3Na are removed, + thus a negative potential is built up inside the cell, (potential difference across the membrane). For red blood cells this potential difference is about –10mV.  Membranes are 100 times more permeable to K than Na. + + A 3rd of all ATP consumed by a resting animal is used to pump Na+ and K+.  The Na-K pump is used to control cell volume (osmoregulation), maintaining electrical activity in nerve and muscle cells, and other active transport mechanisms for sugars and amino acids. secondaryactivetransport.swf  Active transport is critical in the functioning of : the intestine, nerve / muscle cells, and in the kidney. High concentrations of potassium are also required for protein synthesis, glycolysis etc… d) Endocytosis and Exocytosis Bulk transport of materials through membranes. Endocytosis = into cell Exocytosis = out of cell Endocytosis involves the infolding or extension of the plasma membrane to form vesicles or vacuoles. (Fluid-filled membrane bound sac). Phagocytosis (Cell eating)  Phagocytosis (Cell eating) Tends to be associated with the uptake of solid material. The sac formed during the process is known as the phagocytic vacuole. Eg. White blood cells.  Pinocytosis (Cell drinking) uptake of solutions, colloids or fine suspensions. The vesicles formed are small … may be so small that they are formed by micropinocytosis, …..micropinocytotic vesicles. …………