Suppose that the egg has a diameter of 200 mm. What fractional change in the internal Na+
Question:
Suppose that the egg has a diameter of 200 mm. What fractional change in the internal Na+ concentration results from the fertilization-induced change in Vm? Assume that Na+ ions are distributed throughout the cell volume. The concentration increases by
(a) 1 part in 104;
(b) 1 part in 105;
(c) 1 part in 106;
(d) 1 part in 107.
Upon fertilization, the eggs of many species undergo a rapid change in potential difference across their outer membrane. This change affects the physiological development of the eggs. The potential difference across the membrane is called the membrane potential, Vm , which is the potential inside the membrane minus the potential outside it. The membrane potential arises when enzymes use the energy available in ATP to expel three sodium ions (Na+) actively and accumulate two potassium ions (K+) inside the membrane—making the interior less positively charged than the exterior. For a sea urchin egg, Vm is about -70 mV; that is, the potential inside is 70 mV less than that outside. The egg membrane behaves as a capacitor with a capacitance of about 1 µF/cm2. The membrane of the unfertilized egg is selectively permeable to K+; that is, K+ can readily pass through certain channels in the membrane, but other ions cannot. When a sea urchin egg is fertilized, Na+ channels in the membrane open, Na+ enters the egg, and Vm rapidly increases to +30 mV, where it remains for several minutes. The concentration of Na+ is about 30 mmol/L in the egg’s interior but 450 mmol/L in the surrounding seawater. The K+ concentration is about 200 mmol/L inside but 10 mmol/L outside. A useful constant that connects electrical and chemical units is the Faraday number, which has a value of approximately 105 C/mol; that is, Avogadro’s number (a mole) of monovalent ions, such as Na+ or K+, carries a charge of 105 C.
Step by Step Answer:
University Physics with Modern Physics
ISBN: 978-0133977981
14th edition
Authors: Hugh D. Young, Roger A. Freedman