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Chapter 5

Chapter Notes for Chapter 5 of Human Physiology 5e

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Biological Sciences

Human Physiology (Fifth Edition) Silverthorn, D. U. Chapter 5 Membrane Dynamics Mass Balance and Homeostasis To maintain homeostasis, the body uses mass balance The law of mass balance: if the amount of a substance in the body has to stay constant, any gain must be offset by an equal loss; o Total amount of x in the body = intake + production excretion metabolism To maintain mass balance, the body can use excretion (elimination of material from the body through urine, feces, lungs, skin) or metabolize the substance into a different substance (which will create a new mass balance disturbance through addition of metabolite) Excretion Clears Substances from the Body o Clearance: rate at which a molecule leaves the body by excretion, metabolism, or both; usually expressed as a volume of blood plasma cleared of x per unit of time; o Kidney and liver major organs involved in clearing materials from the body o Saliva, sweat, breast milk, and hair contain solutes cleared from the body o Mass flow = concentration x volume flow; used to determine the rate of uptake, output, or production of x, as well as its movement from one compartment to another Homeostasis Does Not Mean Equilibrium o Homeostasis often refers to the stability of the extracellular fluid compartment (consisting of the plasma and interstitial fluid) o Dynamic disequilibrium (chemical disequilibrium): state of the two fluid compartments in which the different concentrations of solutes in the ECF and ICF create a concentration gradient; continual input of energy is needed to keep the body in this state o Free movement of water allows the ECF and ICF to reach a state called osmotic equilibrium where the total amount of solute per volume of fluid is equal on either side of the cell membrane o The body as a whole is electrically neutral, but a few anions are found in the ICF while their matching cations are in the ECF, making the inside of cells slightly negative (electrical disequilibrium) o ICF, ECF = osmotic equilibrium, chemical and electrical disequilibrium, which are dynamic steady states (meaning materials are constantly moving, but there is no net movement) o Homeostasis works to maintain the dynamic steady states Transport Occurs Within and Between Compartments www.notesolution.com o Some transport mechanisms need energy from ATP, while others are able to use kinetic/potential energy o Movement between compartments requires a molecule to cross one or more cell membranes, whereas movement within the compartment is less restricted o Bulk flow: a pressure gradient causes fluid from regions of higher pressure to regions of lower pressure Diffusion Cells membranes are selectively permeable Membrane permeability is variable and can be changed by altering the proteins or lipids of the membrane The size of the molecule and its lipid solubility influence its movement across membranes (large, less lipid-soluble molecules usually cannot pass the membrane unless the cell has specific membrane proteins to transport them or vesicles to move them) Passive transport: movement across the membrane not requiring energy input Active transport: movement across the membrane needing energy input Diffusion Uses Only the Energy of Molecular Movement o Diffusion: passive movement of uncharged molecules from an area of higher concentration to lower concentration due to random molecular movement o 7 properties: 1. A passive process: does not require outside energy; uses only kinetic energy from molecules 2. Moves down concentration gradient: high to low; rate of diffusion depends on concentration gradient (larger difference, faster diffusion) 3. Net movement of molecules occurs until equilibrium: molecules are now evenly distributed; dynamic equilibrium state 4. Fast over short distances, slow over long distances: time needed 2 for a molecule to diffuse = distance 5. Directly related to temperature: rate of diffusion increases as temperatures increase 6. Inversely related to molecular size: the larger the molecule, the slower its diffusion through a given medium 7. Takes place in an open system (where there are no boundaries) or across a partition that separates two systems (but only if the partition allows the molecules to cross) o Ions do not move by diffusion; diffusion is random molecular motion down a concentration gradient Ion movement depends on electrical gradient because of the attraction/repulsion of charges Lipophilic Molecules Can Diffuse Through the Phospholipid Bilayer o Simple diffusion: diffusion directly across the bilayer, only moves lipids, steroids, and small lipophilic molecules www.notesolution.com rate of diffusing depends on ability of diffusion molecules to dissolve in lipid layer rate of diffusion is directly proportional to the surface area of the membrane; the larger the surface area, the more molecules that can diffuse rate of diffusion is inversely proportional to membrane thickness; thicker membrane, slower rate Protein-Mediated Transport mediated transport: transport across membranes with the help of membrane proteins facilitated diffusion: movement of molecules across the membrane according to concentration gradient with the help of membrane proteins active transport: movement across membrane against concentration gradient, needing energy from ATP Membrane Proteins Function as Structural Proteins, Enzymes, Receptors, and Transporters o Structural Proteins: connect the membrane to the cytoskeleton to maintain shape, create cell junctions that hold tissue together, attach cells to the extracellular matrix o Enzymes: catalyze chemical reactions; some attach to intracellular surface for an important role in transferring signals o Receptors: part of the bodys chemical signaling system; when bound to its ligand another event at the membrane is often triggered
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