5 Membrane Dynamics

Membrane Dynamics 5 Homeostasis Does Not Mean Equilibrium Osmosis and Tonicity The Body Is Mostly Water The Body Is in Osmotic Equilibrium Osmolarity Describes the Number of Particles in Solution Tonicity Describes the Volume Change of a Cell Transport Processes Cell Membranes Are Selectively Permeable D i ff usion Lipophilic Molecules Cross Membranes by Simple Diff usion Protein-Mediated Transport Membrane Proteins Have Four Major Functions Channel Proteins Form Open, Water-Filled Passageways Carrier Proteins Change Conformation to Move Molecules Facilitated Diff usion Uses Carrier Proteins Active Transport Moves Substances Against Their Concentration Gradients Carrier-Mediated Transport Exhibits Specifi city, Competition, and Saturation Vesicular Transport Phagocytosis Creates Vesicles Using the Cytoskeleton Endocytosis Creates Smaller Vesicles Organisms could Exocytosis Releases Molecules Too Large for Transport Proteins not have evolved Epithelial Transport without relatively Epithelial Transport May Be Paracellular or Transcellular impermeable Transcellular Transport of Glucose Uses Membrane Proteins membranes to Transcytosis Uses Vesicles to Cross an Epithelium surround the cell constituents. The Resting Membrane Potential — E. N. Harvey, in H. Electricity Review Davson and The Cell Membrane Enables Separation of Electrical Charge in the Body J. F. Danielli’s The The Resting Membrane Potential Is Due Mostly to Potassium Permeability of Natural Changes in Ion Permeability Change the Membrane Potential Membranes (1952). Integrated Membrane Processes: Insulin Secretion Background Basics Polar and nonpolar molecules Protein and lipid structure Cell junctions Molarity and solutions Membrane structure Cytoskeleton Types of epithelia Enzymes Gastric secreting From Chapter 5 of Human Physiology: An Integrated Approach, Sixth Edition. Dee Unglaub Silverthorn. cells of a mouse Copyright © 2013 by Pearson Education, Inc. All rights reserved. 137 Membrane Dynamics n 1992 the medical personnel at isolated Atoifi Hospital in the and intracellular fl uid (ICF) are equal, some solutes are more Solomon Islands of the South Pacific were faced with a di- concentrated in one of the two body compartments than in the I lemma. A patient was vomiting and needed intravenous (IV) other ( Fig. 5.1 d). Th is means the body is in a state of chemical fl uids, but the hospital’s supply had run out, and it would be sev- disequilibrium. eral days before a plane could bring more. Th eir solution was to try Figure 5.1 d shows the uneven distribution of major sol- something they had only heard about—make an IV of coconut utes among the body fl uid compartments. For example, sodium, - water, the sterile solution that forms in the hollow center of devel- chloride, and bicarbonate (HCO3 ) ions are more concentrated oping coconuts. For two days the patient received a slow drip of in extracellular fl uid than in intracellular fl uid. Potassium ions fl uid into his veins directly from young coconuts suspended next are more concentrated inside the cell. Calcium (not shown in to his bed. He soon recovered and was well enough to go home. * the fi gure) is more concentrated in the extracellular fl uid than No one knows who fi rst tried coconut water as an IV solu- in the cytosol, although many cells store Ca2+ inside organelles tion, although stories have been passed down that both the Japa- such as the endoplasmic reticulum and mitochondria. nese and the British used it in the Pacifi c Th eater of Operations Even the extracellular fl uid is not at equilibrium between during World War II. Choosing the appropriate IV solution is its two subcompartments, the plasma and the interstitial fl uid more than a matter of luck, however. It requires a solid under- (IF) . Plasma is the liquid matrix of blood and is found inside standing of the body’s compartments and of the ways diff erent the circulatory system. Proteins and other large anions are con- solutes pass between them. centrated in the plasma but cannot cross the leaky exchange epithelium of blood vessels, so they are mostly absent from the interstitial fl uid. On the other hand, smaller molecules and ions Homeostasis Does Not Mean Equilibrium such as Na + a n d Cl- are small enough to pass freely between Th e body has two distinct fl uid compartments: the cells and the the endothelial cells and therefore have the same concentrations fl uid that surrounds the cells ( Fig. 5.1 ). Th e extracellular fl uid in plasma and interstitial fl uid. (ECF) outside the cells is the buff er between the cells and the Th e concentration diff erences of chemical disequilibrium environment outside the body. Everything that enters or leaves are a hallmark of a living organism, as only the continual in- most cells passes through the ECF. put of energy keeps the body in this state. If solutes leak across Water is essentially the only molecule that moves freely be- the cell membranes dividing the intracellular and extracellular tween cells and the extracellular fl uid. Because of this free move- compartments, energy is required to return them to the com- + ment of water, the extracellular and intracellular compartments partment they left . For example, K ions that leak out of the cell + reach a state of osmotic equilibrium { osmos, push or thrust}, in and Na ions that leak into the cell are returned to their original + which the fl uid concentrations are equal on the two sides of the compartments by an energy-utilizing enzyme known as the Na + cell membrane. (Concentration is expressed as amount of solute - K - ATPase , or the sodium-potassium pump. When cells die per volume.) Although the overall concentrations of the ECF and cannot use energy, they obey the second law of thermody- namics and return to a state of randomness that is marked by loss of chemical disequilibrium. RUNNING PROBLEM Many body solutes mentioned so far are ions, and for this reason we must also consider the distribution of electri- Cystic Fibrosis cal charge between the intracellular and extracellular compart- Over 100 years ago, midwives performed an unusual test on ments. Th e body as a whole is electrically neutral, but a few extra the infants they delivered: the midwife would lick the infant’s negative ions are found in the intracellular fluid, while their forehead. A salty taste meant that the child was destined matching positive ions are located in the extracellular fl uid. As a to die of a mysterious disease that withered the fl esh and result, the inside of cells is slightly negative relative to the extra- robbed the breath. Today, a similar “sweat test” will be cellular fl uid. Th is ionic imbalance results in a state of electrical performed in a major hospital—this time with state-of-the- disequilibrium. Changes in this disequilibrium create electrical art techniques—on Daniel Biller, a 2-year-old with a history signals. We discuss this topic in more detail later in this chapter. of weight loss and respiratory problems. The name of the In summary, note that homeostasis is not the same as equi- mysterious disease? Cystic fi brosis. librium. Th e intracellular and extracellular compartments of the body are in osmotic equilibrium, but in chemical and electrical disequilibrium. Furthermore, osmotic equilibrium and the two disequilibria are dynamic steady states . Th e goal of homeostasis is * D. Campbell-Falck et al. Th e intravenous use of coconut water. Am J to maintain the dynamic steady states of the body’s compartments. Emerg Med 18: 108–111, 2000. In the remainder of this chapter, we discuss these three steady states, and the role transport mechanisms and the selective permeability of cell membranes play in maintaining these states. 138 Fig. 5.1 ESSENTIALS Body Fluid Compartments (a) The body fluids are in two compartments: the extracellular fluid (ECF) and intracellular fluid (ICF). The ECF and ICF are in osmotic equilibrium but have very different chemical composition. Intracellular fluid is 2/3 of the KEY total body water volume. Material moving into and out of the ICF Intracellular fluid must cross the cell membrane. Interstitial fluid Plasma Extracellular fluid includes all fluid outside the cells. The ECF is 1/3 of the body fluid volume. The ECF consists of: • Interstitial fluid (IF), which lies between the circulatory system and the cells, is 75% of the ECF volume. • Plasma, the liquid matrix of blood, is 25% of the ECF volume. Substances moving between the plasma and interstitiial fluid must cross the leaky exchange epithelium of the capillary wall. (b) This figure shows the compartment volumes for the (c) Fluid compartments are often illustrated “standard” 70-kg man. with box diagrams like this one. 100% GRAPH QUESTIONS 1. Using the ECF volume shown 80% in (b), calculate the volumes of the plasma and interstitial 28 L fluid. 60% 2. What is this person's total body water volume? 3. Use your answers from the two Interstitial Intracellular questions above to calculate fluid fluid 40% Plasma 14 L the percentage of total body water in the plasma and Plasma interstitial fluid. (25% of ECF) 20% 4. A woman weighs 121 pounds. Percent of total body water Percent Interstitial Fluid Using the standard proportions (75% of ECF) for the fluid compartments, calculate her ECF, ICF, and Intracellular Extracellular plasma volumes. (2.2 lb = 1 kg. ECF ICF fluid (ICF) fluid (ECF) 1 kg water = 1 L) 1/3 2/3 Cell membrane (d) The body compartments are in a state of chemical disequilibrium. The cell membrane is a selectively permeable barrier between the ECF and ICF. 160 GRAPH QUESTIONS 140 KEY 5. How does the ion composition of 120 Na plasma differ from that of the IF? K 6. What ions are concentrated in the 100 Cl ECF? In the ICF? 80 HCO3 Proteins 60 40 Ion concentration (mmol/L) 20 Intracellular fluid Interstitial fluid Plasma 139 Membrane Dynamics Answers: End of Chapter Concept Check Water Content as Percentage of Table 5.1 1 .

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