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Chapter 5

Chapter 5.docx

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Biological Sciences
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Chapter 5 1 Mass balance and homeostasis Body is an open system that exchanges heat and materials with the outside environmetnt Law of mass balance - if the amount of a substance in the body is to remain constant, any gain must be offset by an equal loss o Amount of substance in the body = intake + production excretion metabolism Excretion the elimination of material from the body, and it usually takes place through the urine, feces, lungs, or skin. Xenobiotic any foreign substance in the body Excretion clears substances from the body Clearance the rate at which a molecule disappears from the body by excretion, metabolism, or both Mass flow used to determine the rate of intake, output, or production of a substance o Mass flow (x/min) = concentration (x/vol) x volume flow (vol/min) Homeostasis Does Not Mean Equilibrium Homeostasis stability of the extracellular fluid compartment Osmotic equilibrium the total amount of solute per volume of fluid is equal on the two sides of the cell membrane Chemical equilibrium certain solutes are more concentrated in one of the two body compartments than in the other Proteins and other large anions are concentrated in the plasma but almost absent from the interstitial fluid because they are unable to cross the endothelium + + + + Na -K -ATPase - energy- utilizing enzyme that returns K ions that leak out of the cell and Na ions that leak into the cell to their original positions Electrical disequilibrium inside of cell is slightly negative relative to the outside of the cell; changes in electrical disequilibrium create electrical signals Transport Occurs within and Between Compartments ICF can be distinguished from the ECF by the ICFs high concentration of K ions and low + - 2+ concentrations of Na , Cl, and Ca . Chapter 5 2 Diffusion Cell membranes are selectively permeable the lipid and protein composition of a given cell membrane determines which molecules will enter the cell and which will leave 2 properties of a molecule influence its movement across cell membranes o Size of the molecule o Molecules lipid solubility Very large lipophobic molecules cannot be transported on proteins and must enter and leave cells in vesicles 2 ways to categorize how molecules move across membranes o Movement according to physical requirements Whether it takes place through the phospholipid bilayer, with the aid of a membrane protein, or by using vesicles o Movement according to its energy requirement Passive transport does not require the input of energy Active transport requires the input of energy from some outside source Diffusion Uses Only the Energy of Molecular Movement Passive transport across membranes uses the kinetic energy inherent in molecules Diffusion the movement of molecules from an area of high concentration to an area of lower concentration Diffusion has 7 properties 1. Diffusion is a passive process Does not require input of energy 2. Molecules move from an area of higher concentration to an area of lower concentration Concentration (chemical) gradient diff in concentration of a substance between 2 places The larger the concentration gradient, the faster diffusion takes place 3. Net movement of molecules occur until the concentration is equal everywhere Once molecules of a given substance have distributed evenly, the system reaches equilibrium and diffusion stops Dynamic equilibrium concentration has equalized throughout the system but molecules continue to move 4. Diffusion is rapid over short distances, slow over long distances 5. Diffusion is directly related to temperature The higher the temp, the faster diffusion takes place 6. Diffusion rate is inversely related to molecular size The larger the molecule, the slower its diffusion through a given medium 7. Diffusion can take place in an open system or across a partition that separates 2 systemsChapter 5 3 IMPORTANT: ions do not move by diffusion!! o Ion movement is influenced by electrical gradients since they are attracted to opposite charges Lipophilic Molecules Can Diffuse Through the Phospholipid Bilayer Simple diffusion diffusion directly across the phospholipid bilayer of a membrane and has the following properties in addition to the properties of diffusion listed earlier o rate of diffusion depends on the ability of the diffusing molecule to dissolve in the lipid layer of the membrane aka diffusion rate depends on how permeable the membrane is to the diffusing molecules *****WATER IS AN EXCEPTION***** o rate of diffusion across a membrane is directly proportional to the surface area of the membrane larger the surface area, the more molecules can diffuse per unit time o rate of diffusion is inversely proportional to membrane thickness thicker the memb
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