physiology-biochemistry questions..

[nisreen]

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hi.. i had doubts about the answers to the following questions, can someone please help me out with the answers..

1) Under normal conditions,the resting membrane potential of a neuron is not equal to the equilibrium potential for potassium because
a) sodium permeability equals potassium permeability
b) Na permeability exceeds K permeability
c) the membrane is impermeable to Na ions
d) the membrane has a low permeability to Cl ions
e) the membrane has a low permeability to Na ions (ans)

2) At a constant dietary intake, the concentration of fluoride in the body fluids is regulated by which of the following?
a) hormonal factors
b) soft tissue uptake and release
c) equilibrium with bone and urinary excretion (ans)
d) the factor regulating intestinal absorption of ingested fluoride

3) Which of the following mechanisms enables epinephrine to increase the breakdown of muscle glycogen?
a) stimulation of glucokinase
b) activation of glycogen synthetase
c) conversion of glycogen phosphorylase from a to b form
d) stimulation of cyclic AMP formation from ATP

4)I nternal hemiacetal linkages such as those found in glucose are reaction products of
a) aldehydes and amino groups
b) aldehydes and hydroxyl groups
c) amino and carboxyl groups
d) hydroxy and caboxyl groups
e) carbonyl groups and amide nitrogen

5) Which of the following pairs of amino acids both have a branched hydrocarbon side chain
A) arginine andvaline
b) isoleucine and threonine
c) tyrosine and serine
d) valine and leucine
e) valine and aspartic acid

6) Which of the following best describes visceral smooth muscles?
a) it requires neural input
b) it does not have a stable resting membrane potential
c) it requires a precontractile stretch to initiate contraction
d) it uses norepinephrine as the primary neurotransmitter

7) During quiet exhalation, which of the following forces air out of the lungs?
a) elastic recoil fo the lungs
b) contraction of the diaphragm
c) shortening of the intercostal muscles
d) positive pressure in the intrapleural space

8) If the substrate concentration in a simple enzymatic system were to be much less than the Km, the reaction rate would be
a) maximal
b) reduced by the addition of more substrate
c) proportional to the substrate concentration
d) uninfluenced by the addition of more substrate
e) inversely proportional to the substrate concentration

9) Which of the following best describes the conduction rate of a given neuron? the rate
a) remains relatively constant
b) varies with the type of stimulus
c) changes with the strength of the stimulus

10) Which of the following increase the glomerular filtration rate?
a) decreased number of glomeruli
b) decreased plasma oncotic pressure
c) dilation of efferent arterioles
d) increased pressure in bowman's capsule
e) constriction of afferent arterioles

11) Which of the following causes denervation hypersensitivity after sympathectomy?
a) enhanced innervatiom of the effector organ by collateral nerves
b) release of dopamine from preganglionic nerve terminal
c) increased number of receptors to the circulating transmitter
d) degeneration of postganglionic nerve terminals following preganglionic nerve section

12) Which part of the nephron uses most metabolic energy?
a) glomerulus
b) loop of henle
c) proximal tubule
d) collecting duct
e) distal convoluted tubule

13) Which of the folllowing spontaneous events causes miniature end plate potentials?
a) pacemaker activity of endplates
b) releasse of acetycholine in sub-threshold amonuts
c) secretion or norepinephrine
d) oscillation in Na and K conduction of endplate membranes

 

[Ride]

Here are my thoughts on a few of the Qs – please someone correct me if I’m wrong.

1) Under normal conditions,the resting membrane potential of a neuron is not equal to the equilibrium potential for potassium because
a) sodium permeability equals potassium permeability
b) Na permeability exceeds K permeability
c) the membrane is impermeable to Na ions
d) the membrane has a low permeability to Cl ions
e) the membrane has a low permeability to Na ions (ans)

Potassium equilibrium potential in nerves is about = -85mV.
Sodium equilibrium potential in nerves is about= +65mV.
Resting membrane potential is about = -70mV.

‘Resting membrane potential’ refers to the potential difference (in mV) between the intracellular extracellular compartments. If the cell was completely permeable to potassium and impermeable to everything else (other ions and charged particles), then the resting membrane potential of the cell would equal the potassium equilibrium potential (-85mV).

In general, the more permeable a cell is to a particular ion, the closer the resting membrane potential of the cell will be to the equilibrium potential of that ion.
(I think of it as if the ions try to pull the resting membrane potential of the cell closer to their own equilibrium potential; and the more permeable the cell membrane is to the ion, the better the ion is at achieving this goal).

Since neurons have a high resting conductance to K, it makes sense that the resting membrane potential (-70mV) of the neuron is really close to the K equilibrium potential (-85mV). However, due to the low conductance, or permeability, of the cell to Na, the resting membrane potential is ‘pulled’ a little towards the Na equilibrium potential (+65mV), such that the resting membrane potential is -70mV.


7) During quiet exhalation, which of the following forces air out of the lungs?
a) elastic recoil of the lungs
b) contraction of the diaphragm
c) shortening of the intercostal muscles
d) positive pressure in the intrapleural space

Intrapleural pressure returns to its resting value during a quiet exhalation (ie. it goes from about -6cmH2O at the beginning of expiration to about -3cmH2O at the end of expiration – these numbers come from BRS Path – my prof gave the values -10 and -5cmH2O). I think the only time intrapleural pressure becomes positive is during forced exhalation.
This eliminates (d).

The external intercostals are used for inhalation during exercise; the internal intercostals are used for exhalation during exercise.
This eliminates (c).

Inhalation is results from the contraction of the diaphragm (This eliminates (b)); this increases the volume of the thoracic cavity, causing the alveoli to be stretched apart, and thus creating a negative pressure in the alveoli. This results in a pressure gradient between the atmospheric air (zero pressure) and the alveoli (neg pressure) so that air is drawn into the alveoli. The elastic recoil of the lungs causes the intra-alveolar pressure to increase during inspiration (compressing the gas in the alveoli) until the alveolar pressure becomes greater than the atmospheric pressure. Then the pressure gradient is reversed, and exhalation begins. Thus, the passive exhalation of the lungs occurs because of the elastic recoil of the lungs (a).

8) If the substrate concentration in a simple enzymatic system were to be much less than the Km, the reaction rate would be
a) maximal
b) reduced by the addition of more substrate
c) proportional to the substrate concentration
d) uninfluenced by the addition of more substrate
e) inversely proportional to the substrate concentration

If you look up at the Michaelis-Menten (sp?) saturation curve when you read this explanation it will probably make more sense:
Km is the concentration of substrate at which the rate of the reaction is half the maximum reaction rate. So, if the substrate concentration is less than Km, the reaction would occur at less than half the maximal rate (eliminates (a)). When looking at the M-M curve, from zero substrate to a substrate concentration equal to Km, the curve is approximately linear with a positive slope, thus the reaction rate would be increased by the addition of more substrate (eliminates (b) and (d)). Because the linear portion of the curve has a positive slope, reaction rate would be proportional to the substrate concentration, rather than inversely proportional.

9) Which of the following best describes the conduction rate of a given neuron? the rate
a) remains relatively constant
b) varies with the type of stimulus
c) changes with the strength of the stimulus

(b) and (c) don’t make sense to me because when the appropriate stimulus results in the membrane potential exceeding threshold, an action potential is fired. Then, the conduction rate of the neuron is determined by the diameter of the neuron (bigger diameter -> faster conduction; smaller diameter -> slower conduction), and its myelination (more myelin -> faster conduction; less myelin -> slower conduction). So, I would pick (a), but I’m not positive. Other thoughts?

10) Which of the following increase the glomerular filtration rate?
a) decreased number of glomeruli
b) decreased plasma oncotic pressure
c) dilation of efferent arterioles
d) increased pressure in bowman's capsule
e) constriction of afferent arterioles

Decreased plasma oncotic pressure would increase GFR because the oncotic pressure in the glomerular capillary opposes filtration. -> answer is b

Note:
1. Glomerular capillary hydrostatic pressure increases filtration; bowman’s capsule (space) hydrostatic pressure decreases filtration
2. Glomerular capillary (plasma) oncotic pressure decreases filtration; bowman’s capsule (space) oncotic pressure increases filtration (but this is usually negligible)
(These points relate to the Starling equation – a good thing to memorize)

Decreased number of glomeruli would decrease GFR because there are less glomerular capillaries and GFR is filtration across the glomerular capillaries (eliminate a)

Increased pressure (ie. hydrostatic pressure) in Bowman’s capsule would oppose filtration and decrease filtration rate. (eliminate d)

Constriction of afferent arterioles would decrease the amount of blood flow into the glomeruli, thus decreasing capillary hydrostatic pressure, and decreasing GFR. (eliminate e)

I don’t see how dilation of efferent arterioles would affect GFR. (eliminate c, but I don’t have a good explanation of why)

 

[nisreen]

hi, thanks a lot for posting the answers and explanations.. the explanations were very helpful