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Chapter 5 Notes - Membrane Dynamics

# Chapter 5 Notes - Membrane Dynamics - L aw of mass balance...

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Law of mass balance – amount of substance in the body is to remain constant - Gain must be offset by an equal loss Substances that are maintained through mass balance include oxygen, carbon dioxide, salts and hydrogen ions. Excretion – takes place through the lungs, urine, feces or skin - Substances can also be metabolized in order to sustain a mass balance in the body Clearance – the rate at which a molecule disappears from the body - Basis for breathalyzer tests, since ethanol is cleared by the lungs, and exhaled alcohol is the basis of this test. Mass flow (amount x/min)= concentration (amount x/vol) x volume flow (vol/min) Example: person given an intravenous infusion of glucose solution that has a concentration of 50 g glucose/L. The infusion is given at a rate of 2 mL/min 50 g glucose/1000 mL solution x 2 mL solution/min = 0.1 g glucose/min Osmotic equilibrium – the total amount of solute per volume of fluid is equal on the two sides of a given membrane Na, Cl, HCO3- are more concentrated in the ECF than in the ICF, where K ions usually are more concentrated. Sodium that leaks into the cell and potassium that leaks out of the cell is restored by the sodium-potassium ATPase pump The inside of the cell is slightly negative relative to the extracellular fluid . - Because a few extra negative ions are found in the intracellular fluid. Their matching positive ions are found in the ECF Cell membranes are selectively permeable The lipid and protein composition of a given cell determines which molecules will enter the cell and which will leave. Small molecules and those that are lipid soluble can pass through the phospholipid bilayer fairly easily Large or less lipid-soluble molecules are excluded unless there is a specific mechanism for transporting them across.

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- Usually, membrane proteins are the usual mediators cells use to transport molecules across their membranes. If molecules must enter the cell via a transport/membrane proteins or through vesicles, then they are doing so via facilitated diffusion . Along with simple diffusion, these two mechanisms are types of passive transport . Active transport requires energy, but passive transport does not. Diffusion – the process of a molecule moving from an area of high concentration to an area of low concentration, down its concentration gradient . - Diffusion is a passive process. - The rate of diffusion depends on the magnitude of the concentration gradient (i.e. the larger the concentration gradient is, the faster diffusion takes place. - When you first open a bottle of cologne, the rate of diffusion is high because there is a large concentration gradient. Over time, the rate of diffusion decreases because there is not as large of a concentration gradient.
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