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Chapter 1. General Principles & Energy Production in Medical Physiology

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In a patch clamp experiment on a neuron, a student is able to accurately measure membrane potential. At the start of the measurements, the student finds that the resting membrane potential is normal for a neuron, measuring –70 mV. The student manipulates the preparation to allow for membrane potential to stabilize at the K+ equilibrium (EK). In this preparation, the intracellular concentration for K+ is 150 mmol/L and the extracellular concentration for K+ is 5.5 mmol/L. Assuming the temperature of the experiment is 37°C, what is the equilibrium potential of the neuron in the experiment?

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A. –70 mV

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B. –90 mV

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C. +70 mV

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D. +90 mV

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The correct answer is B. The EK can be calculated using the Nernst equation: EK = (RT/FZK) ln [K+o]/[K+i]. Converting to log and substituting for RT/FZK yields: 61.5 log [K+o]/[K+i]. Using the numbers above yields: 61.5 log (150/5.5) = –90 mV. A represents a typical resting potential for a neuron, close to, but slightly different than the EK; C and D represent values for EK if valence of K+ were (–), or, if the intracellular and extracellular concentrations were reversed.

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Parents of a child being treated for diabetes notice changes in their child that include a flushed face, a smell of acetone on his breath and a change in his breathing pattern that includes consistent sighs and deep breaths. They visit the family physician for tests and find excess sugars and ketones in the boy’s urine, serum bicarbonate levels less than 15 mEq/L and blood pH less than 7.3. The doctor determines that the child is suffering from diabetic ketoacidosis and takes necessary steps to alter insulin, blood glucose and hydration. Note that a diagnosis of metabolic acidosis is determined even though the pH is 0.2 units less than normal. The difference in concentration of H+ in a solution of pH 2.0 compared with one of pH 7.0 is

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A. 5-fold

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B. 1/5 as much

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C. 105-fold

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D. 10–5 as much

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The correct answer is C. The biological H+ concentration ([H+]), critical for the proper function of proteins and enzymes in the body, is measured on a logarithmic scale. Thus, a difference of 5 units of pH is equivalent to a 5-log difference, or ...

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