Membrane transport

• Cell are surrounded by fluid called extracellular fluid (ECF) and the cell also contains intracellular fluid (ICF)
• The difference between ICF and ECF fluids are caused by transport mechanism
• Extracellular fluid is high in sodium concentration, high in chloride concentration low in potassium
• Intracellular fluid: phosphate and proteins are greater than ECF
• The lipid bilayer constitutes a barrier for the movement of most water soluble. however, most lipid bilayer substances can pass directly through the lipid bilayer
• Protein molecules in the lipid bilayer constitute an alternate transport pathway
• Channel proteins provide a watery pathway for the molecules to move through the membrane
• Carrier proteins binds with specific molecules and then undergo conformational changes that move molecules through the molecules
• Relative permeability (from high membrane permeability to low membrane permeability)
• Lipids can easily pass the membrane since it is also made of a lipid bilayer
• Passive transport has no ATP required , the substance moves downhill, from higher to lower concentration. Also it may not require a carrier protein
• Types of Passive transport

  1. Filtration
  2. Diffusion
• Filtration movement of both solute and solvent through a porous membrane, drive by hydrostatic pressure
• Diffusion random movement of the molecules either through intermolecular spaces in the cell membrane or combination with carrier protein
• Concentration gradient exits diffusion occurs. The energy that causes diffusion is the energy of the normal kinetic motion
• Simple diffusion movement of molecules or ions via membrane openings or intermolecular spaces without utilizing any carrier protein
• simple diffusion can occur by way of two pathways 1 through the interstices of lipid bilayer and 2 is through watery channels in transport proteins that span the cell membrane
• Facilitated diffusion needs a carrier protein that will aid the passage of the molecule to the membrane via chemical bonds, providing a shuttle for the molecules through the membrane
• among the most important substances that cross cell membranes through facilitated diffusion are glucose and most of the amino acids
• permeability the permeability of a membrane for a given substance is expressed as the net rate of diffusion of the substance through each unit of the membrane for a unit concentration difference between two sides of the membrane
• Concentration difference the grater the difference in concentration of the substance between 2 regions the greater will be the rate diffusion
• electrical potential if an electrical potential is applied across a membrane, the ions move through the membrane because of their electrical charges
• the electrical difference that balance a given concentration difference can be determined with the Nernst equation
• Nernst potential a membrane potential that prevents net diffusion of an ion in the either direction through the membrane
• Cross sectional area the area of boundary across which diffusion is taking place. This factor is directly proportional to the rate of diffusion
• Temperature the higher the temperature the faster the rate
• distance the greater the distance to be travelled the slower the rate
• Size of the diffusing molecule the greater the molecule size, the slower the rate
• Osmosis process of net movement of water caused by concentration difference of water
• movement of water from less area of solute concentration to an area of greater solute concentration
• Osmotic pressure is the pressure difference required to stop osmosis. It prevents too much flux of water as it moves through the membrane
• solutions that have the same osmolality as plasma are said to be isotonic. (plasma osmolality is 290mOsm/L)
• Exosmosis and endosmosis. When two solutions have different osmotic pressure. the solution having the lower osmotic pressure is said to be hypoosmotic, while the solution having higher osmotic pressure is said to be hyperosmotic
• Cell placed in hypotonic solution will increase in volume due to endosmosis
• Cell placed in hypertonic solution will decrease in the volume due to exosmosis
• Active transport the movement of solute from lower concentration to greater concentration, or it moves in an uphill current. Requires ATP as a solute moves through the membrane
• Primary active transport the energy is derived directly from the breakdown