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EXCHANGE OF SUBSTANCES BETWEEN ISF AND ICF Cell membrane RBC Plasma ISF Capillary Wall ICF Mechanisms of Transport Across Cell Membrane • According to the means of transport there are two categories: 1- Non-carrier-mediated transport - Simple diffusion Passive a- Through bilayer b- Through channels 2- Carrier-mediated transport - Facilitated diffusion Passive Use carrier proteins - Active transport Active a- Primary b- Secondary c- Tertiary Simple Diffusion - Simple diffusion: molecules cross the cell membrane through openings or through intermolecular spaces without binding to protein carriers in the membrane. - Simple diffusion of: a) lipid-soluble substances through the lipid bilayer. The more lipid soluble the faster its rate of diffusion across the cell membrane. b) water and lipid-insoluble substances: water passes through channels in protein molecules. Other water soluble molecules if small enough can also go though channels. Some channels have gates which regulate movement of substances across the cell membrane. Factors affecting the Net Rate of Diffusion Rate of diffusion: number of diffusing particles passing through the membrane per unit time. Factors effecting net rate of diffusion: 1- Concentration difference across the membrane 2- Permeability of the membrane to diffusing molecule 3- Surface area of the membrane. 4- Molecular weight of the diffusing molecule. 5- Temperature. 6- Membrane electrical potential (ions, Nernst Equation) 7- Pressure difference across the membrane. 8- Distance. Concentration gradient x Surface area x DP x Temperature Rate of diffusion a MW x distance Factors affecting the Net Rate of Diffusion Concentration Difference Electrical Potential Pressure Effect of Electrical Potential on Rate of Diffusion - - - - - - _ + - - - - - _ - + - - - - Membrane Concentration equilibrium Electrochemical equilibrium - There will balance between electrical difference and concentration difference at some point. In the body at 37 oC the electrical difference that balance the concentration difference of univalent ions such as Na+ can be determined by Nernst Equation: EMF (mV) = + 61 log C1 C2 EMF = Electromotive force between side 1 and 2 of a membrane, C1 = concentration on side 1 C2 = concentration on side 2. Nernst Equation: EMF (mV) = +- 61 log EMF (mV) = - 61 log EMF (mV) = - 61 mV Inside C2 Outside 140 14 EMF (mV) = - 61 log 10 EMF (mV) = - 61 x 1 C1 - 61 mV 140 mM K+ 14 mM K+ Facilitated Diffusion Passive - Is a carrier-mediated diffusion. The diffusion of a substance through the cell membrane carrier-mediated is helped by a carrier protein. e.g : facilitated transport of glucose into cells. Carrier-mediate Transport - Facilitated diffusion can be regarded as a “passive” transport which does not require energy. All factors that affect the “rate” of simple diffusion also cause similar effects in facilitated diffusion. However, facilitated diffusion shows some additional characteristics which is seen only in carrier-mediated transport and not seen in simple diffusion: Characteristics of carrier-mediated diffusion: 1- Saturation 2- Specificity 3- Competition Active Transport Active transport is a type of carrier-mediated transport in which the energy is used to transport molecules. There are three types of active transport: 1- Primary active transport 2- Secondary active transport (co-transport and counter-transport) 3- Tertiary active transport Primary Active Transport The energy which drives the transporter to carry the solute across the membrane comes directly from ATP molecule hydrolysis at the transporter protein. An important function of Na/K pump is to control the volume of the cells. Na/K pump is electrogenic. Other important primary active transports are: transport of H+, Ca++, amino acid and some sugars. Secondary Active Transport Transport of a solute across the membrane helped by concentration difference of another solute (e.g Na+) across the membrane. I- Co-transport: Secondary active transport ECF ICF Na Na K K Primary Active Transport Na Glucose Na Glucose Secondary Active Transport II- Counter-transport: Tertiary Active Transport ECF ICF Na Na Na Na K K H+ H+ H+ Peptide Peptide Facilitated Diffusion Primary Active Transport Cotransport Primary Active Transports
