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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; oTotal 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 oClearance: 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; oKidney and liver – major organs involved in clearing materials from the body oSaliva, sweat, breast milk, and hair – contain solutes cleared from the body oMass 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 oHomeostasis often refers to the stability of the extracellular fluid compartment (consisting of the plasma and interstitial fluid) oDynamic 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 oSome transport mechanisms need energy from ATP, while others are able to use kinetic/potential energy oMovement between compartments requires a molecule to cross one or more cell membranes, whereas movement within the compartment is less restricted oBulk 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 for a molecule to 2 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 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 oStructural Proteins: connect the membrane to the cytoskeleton to maintain shape, create cell junctions that hold tissue together, attach cells to the extracellular matrix oEnzymes: catalyze chemical reactions; some attach to intracellular surface for an important role in transferring signals oReceptors: part of the body’s chemical signaling system; when bound to its ligand another event at the membrane is often triggered o Transporters: move molecules across membranes; divided into channel and carrier proteins Channel proteins allow more rapid transport across the membrane, but limited to transporting small ions and water Carrier proteins are slower, but can move larger molecules Channel Proteins Form Open, Water-Filled Passageways o When water-filled channels are open, tens of millions of ions per second can get through unobstructed o More than 100 ion channels have been identified which may be specific for one or a few ions oSelectivity of a channel is determined by the diameter of its central pore and the charge of the amino acids lining the channel (opposites attract) oGated channels are most often closed, which allows them to regulate the movement of ions through them Gated channels may be chemically gated (messenger molecules), voltage-gated (electric state), or mechanically gated (physical forces) Carrier Proteins Change Conformation to Move Molecules oCarrier proteins bind to specific substrates and carry them across the membrane by changing conformation; large, complex proteins with multiple subunits oUniport carriers (move one kind of molecule), cotransporter (moves more than one kind at a time), symport carriers (molecules are moving in the same direction), antiport carriers (molecules are moving in opposite directions) oCarrier proteins never create a continuous passage between inside and outside of the cell (like a revolving door; never directly connecting) Facilitated Diffusion Uses Carrier Proteins oFacilitated diffusion = protein- mediated transport, no outside energy is needed oSame properties as simple diffusion; molecules move down concentration gradient, requires no energy, net movement stops at equilibrium Active Transport Moves Substances Against Their Concentration Gradients oMoves from low to high concentration, requires outside energy from ATP oCreates a disequilibrium state by making concentration differences more defined oPrimary (direct) active transport: energy to push molecules against the gradient comes directly from ATP Many primary active transporters are known as ATPases (enzyme acting upon ATP) Na-K Pump may be the most important transport protein in animals as it maintains [Na] and [K] across cell membranes o Secondary (indirect) active transport: uses potential energy stored in the concentration gradients to move molecules GLUT transporters are reversible and able to move glucose into or out of the cell, depending on the conc. Gradient o Energy used is imported from the environment in the chemical bonds of glucose, which is transferred to the energy bonds of ATP through oxidative phosphorylation in the
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