FHB Lecture 1: Membrane Transport and RMP

Question 1

What is passive transport?

Answer 1

Movement of solutes across a membrane DOWNHILL/ down their concentration gradients, it requires no energy input

Question 2

What are the two things required for the movement (passive diffusion) of a solute?

Answer 2

1. presence of a premeable membrane
2. driving force (electrochemical gradient)

Question 3

What does the electrochemical gradient depend on ?

Answer 3

1. The concentration gradient of solute across a membrane
2. The eletrical potential difference (if the solute is a charged molecule).

Question 4

What is the equation to calculate the electrochemical gradient? How is calculating it for glucose different than for calcium?

Answer 4

1. Because glucose is uncharged, the electrochemical gradient is determined solely by the concentration gradient for glucose across the cell membrane.
2. Because K+ is charged, the electrochemical gradient is determined both by the concentration gradient and by the membrane voltage (Vm).

The electrochemical gradient is the difference between the Nernst potential (-90mV) and the resting membrane potential (-60mV)… so it’s 30.8 mV, which will drive K+ out of the cell

Question 5

What are the extracellular and intracellular concentrations of the following: Na+ , K+, Glucose, Ionized Calcium

Answer 5

Question 6

What is active transport? Difference between primary active transport and secondary active transport?

Answer 6

Active transport is when solutes travel uphill/up their concentration gradients (from low to high). It requires energy input.

In primary active transport, ATP hydrolysis is used as energy input (example: sodium potassium pump).

In secondary transport, the transport of one solute is coupled with the passive transport of another solute (example Na+/Ca2+ antiporter)

Question 7

How does the Na/K ATPase/pump work? What kind of current does it create?

Answer 7

Three Na+ are pumped outside of the cell for every 2K+ pumped inside. The Na/K pump maintains the concentration gradients of sodium and potassium. The 3out:2in stoichiometry creates a net outward/ net negative current.

The pump also keeps intracellular Na concentrations low, thereby maintaing cell volume.

Question 8

Explain the sodium calcium antiporter/exchanger

Answer 8

The Na+/CA2+ exchanger/antiporter uses secondary active transport to retain low intracellular calcium levels. For every 3 sodiums that enter the cell, it lets out 1 calcium ion. The movement of Na+ down its concentration gradient provides the energy for the counterexchange of Ca2+ out of the cell.

Question 9

What is flux? What is the equation for flux? What assumption does the equation hold?

Answer 9

Flux (J) = the number of moles of substance crossing an area of membrane (1 cm² ) per unit time (s)

Fick’s Law of Diffusion: J = -DA ((delta c)/(delta x))

This equation is for an UNCHARGED substance.

Question 10

Explain what the diffusion coeffecient is. Also explain how it pertains to solubility and size.

Answer 10

Diffusion coeffecient (D) is directly related to the solubility of a substance and inversely related to size. Very small water soluble substances like O2 and CO2 can pass through membranes more easily than predicted by their solubility. Larger molecules like glucose and proteins need specific transport mechanisms because they are too big.

Question 11

What is the rate of diffusion / “flux” proportional to?

Answer 11

Rate of diffusion is proportional to surface area x concentration gradient OVER membrane thickness

Question 12

Explain the effects of surface area on flux. How can the surface area increase and decrease in the lungs?

Answer 12

The bigger the surface area, the greater the rate of diffusion (“flux”).

In the lungs, surface area increases with exercise. It descreases with emphysema, pneumothorax, atelactis (lung collapse)

Question 13

Explain how concentration difference influences flux

Answer 13

The greater the concentraiton gradients, the higher the diffusion rate

Concentration gradients for O2 and CO2(pulmonary, muscles) increase with exercise.

They decrease with high altitudes.

Question 14

Write out the equation for the Nernst Eqn.

What does the nerst equation calculate?

What is the assumption that the equation uses?
Give the theorhetical nernst values for Ena, Ek, Eca

Answer 14

The Nernst equation describes an equilibrium where the force generated by an eletrical current is equal and opposite to the force created by the concentration gradient.

It is based on the assumption that the membrane is permeable to only one ion.

Ena = +60 mV

Ek = -90 mV

Eca= +120 mV

Question 15

Total water transport is dependent on ______.

Total energy difference = (______) + (______)

Answer 15

Total water transport is dependent on net driving force.

Total energy difference = chemical (aka concentraiton gradient) + pressure (hydrostatic)

Question 16

In a living cell, what do we normally care about when we think of water transport and what it is primarily driven by? What are a few exceptions to that assumption?

Answer 16

In a living animal cell, hydrostatic pressure differences across the cell membrane are essentially zero. Water transport into and out of a cell are therefore primarily driven by osmotic gradients only.

HOWEVER, in the regulation of fluid transport in the circulatory and renal microcirculation, hydrostatic pressure plays a big role in both normal and abnormal conditions.

Question 17

Define osmotic pressure. What does the osmotic pressure of a solution depend on?

Answer 17

Osmotic pressure is the pressure that moves water through a permeable membrane resulting from an unequal distribution of osmotically active particles.

Osmotic pressure of a solution depends on NUMBER of solutes in solution.

Question 18

Explain this figure using osmotic pressure and hydrostatic pressure.

Answer 18

Compartment A contains solute and compartment B contains distilled water (no solutes). The compartments are separated by a semi-permeable membrane (permeable to water). Due to osmosis, water will move down its concentration gradient from B to A, raising the water level A and decreasing the water level B. At equilibrium, the hydrostatic pressure exerted by the column of water (h) in compartment A will stop the movement of water from B to A. This hydrostatic pressure will be equal and opposite to the osmotic pressure exerted by the solutes in compartment A.

Question 19

What is the difference in calculating equivalents of something in solution vs osmoles of something in solution? (You can use Ca2+ as an example)

Answer 19

Equivalency cares about CHARGE

Osmoles care about how many particles are in solution

For example CA2+ has two equivalents per mol, but only one osmol per mole

Question 20

Define tonicity.

What is the steady state volume of a cell determined by?

Answer 20

Tonicity is defined as the effect a solution has on the volume of a cell.

Steady state volume is determined only by the impermeant solutes in the extracellular fluid.

Question 21

Using tonicity, describe the effects that an NaCl solution has on RBCs. Use this figure to help guide your understanding.

Answer 21

• The perfect osmolarity is 300-310 osmol
• Isotonic solution of NaCl = 154 mM (308 osmols, and also 0.9% NaCl)
• A hypotonic solution has a concentration less than 154 mM of NaCl. Hypotonic extracellular solutions will cause the cell to swell and lyse (water enters into the cell).
• Hypertonic solution of NaCl has a concentration greater than 154 mM. With a hypertonic extracellular solution, water will exit the RBC, causing it to shrink

Question 22

Explain what oncotic pressure is.

What is it important for physiologically speaking?

Answer 22

• Oncotic pressure is the osmotic pressure of plasma proteins
• It is important in the regulation of fluid balance/blood volume
• Dehydration: hypertonic extracellular solution, neuronal cell shrinkage
• Overhydration: hypotonic extracellular solution, neuronal cell swelling

Loss of plasma proteins can cause edema formation (possible causes are liver and renal failure)

Question 23

Explain how osmotic pressure relates to TBI/head trauma.

Answer 23

hyponatremia or head trauma can result in cerebral edema (brain swelling). - in head trauma, patients are given mannitol, an osmotically active particle, to enhance renal fluid excretion (osmotic diuresis).

Question 24

Explain how oncotic pressure relates to the Gibbs Donnan equilibrium? Use this graph

Answer 24

The magnitude of the osmotic pressure generated by a solution of protein is disproportionately larger than that predicted by van’t Hoff’s Law (linear relationship)

It deviates because the negatively charged albumin particles attract positively charged sodium ions. The extra sodium ions increase the oncotic pressure.

Question 25

What is the normal value of the oncotic pressure generated by proteins in human plasma?

Answer 25

26-28 mmHG

Question 26

Explain the following figure using Gibbs Donnan Equilibrium

Answer 26

Na is at equilibrium, and proteins are non-permeable, therefore they do not change; however Cl ions migrate from B  A down its concentration gradient. This results in increased negative changes in compartment A, this negative potential attracts Na ions towards compartment A. The accumulation of Na and Cl in compartment increases osmolarity in A and drives water movement towards A. The increase in water content in A results in decrease protein concentration in A

Question 27

Explain the RMP. What ion is is based off of?

Answer 27

• At rest, the cell membrane is primarily permeable to K+. - Because the [K+] is much higher (150 mM) inside the cell than outside (5 mM), K+ diffuses down its concentration gradient out of the cell. - As positive K+ ions leave the cell, fixed negative charges left within the cell result in a negative voltage or potential (Em) across the membrane. - Eventually, the negativity in the cell opposes further movement of K+ out of the cell, and the electrical and chemical gradients acting on K+ reach equilibrium, i.e. K+ equilibrium potential. - At the K+ equilibrium potential (Nernst potential) there is no NET movement of K+ across the membrane. - However, the membrane is not exclusively permeable to only K+. - There is a small permeability to Na+ leaking into the cell which makes the resting membrane potential slightly more positive than the K+ equilibrium potential.

Question 28

How does diabetes relate to osmotic pressure?

Answer 28

Increased plasma glucose concentration acts as an osmotically active particle to increase renal fluid loss (osmotic diuresis)

Question 29

How does the formation of concentrated urine relate to osmotic pressure?

Answer 29

Na+ gradients in the renal medullary create an osmotic gradient for reabsorption of water by ADH