Osmosis, Tonicity and the Resting Membrane Potential

Question 1

osmosis

Answer 1

the net movement of water down it’s concentration gradient

when a solution has a high concentration of a solute (ex, glucose), it will also have a lower water concentration, while pure water will have a high concentration

so if a cell with high solutes were placed in a solution of pure water, the water would move down its concentration gradient into the cell and cause it to swell

Question 2

describe the 3 major factors that affect osmosis

Answer 2

• permeability of the membrane
• concentration gradients of the solutes
• osmotic pressure of the solution

Question 3

osmoles

Answer 3

the number of particles after dissolving is what matters for predicting osmosis hence the concentration is expressed in number of particles

ex) NaCl -> Na + Cl dissociated in water = 2 osmoles

Question 4

average osmolarity of body cells

Answer 4

300 mOsm/L

Question 5

what do we need to know to calculate osmolarity?

Answer 5

• osmoles
• molarity
  ex) 2 moles of KCl -> 2 x 2 (dissociated particles) = 4 osmoles

Question 6

which compartment has a higher concentration of water?

Answer 6

compartment 2 has a higher concentration of water because the osmolarity of 200 mM of CaCl₂ is higher = 600 mOsm/L

than 200 mM of glucose which doesn’t dissociate = 200 mOsm/L

therefore the water will diffuse into compartment 2 to 1

Question 7

calculate the osmolarity of the solution

200 mM CaCl₂

Answer 7

200 mM CaCl₂ = 200 x 3 = 600 mOsm/L

Question 8

isotonic, hypertonic, hypotonic

Answer 8

isotonic: same osmolarity as body fluids
hypertonic: more osmolarity than body fluids (water moves out of cell into solution)
hypotonic: less osmolarity than body fluids (water moves into cell into solution)

Question 9

tonicity

Answer 9

describes the ability of a solution to cause osmosis across the plasma membrane

Question 10

chemical gradient

Answer 10

molecules moving high to low concentration (concentration gradient)

Question 11

electrical gradients

Answer 11

charged molecules like ions move towards area of opposite charge until an electrochemical equilibrium is established (electrochemical gradient)

Question 12

resting membrane potential

Answer 12

electrical charge at rest across the membrane of the cell is -70 mV

more positive ions outside the cell membrane than inside the cell, the negative ions accumulate inside the surface of the plasma membrane and an equal amount of positive (anions) accumulate outside

RMP is an electrical potential

Question 13

list 2 factors that affect the cell’s membrane potential

Answer 13

1) the concentration gradients of different ions across the membrane
2) the permeability of the membrane of those ions

Question 14

electrogenic pump

Answer 14

a pump that maintains the difference in charge across the membrane

ex) Na⁺ K⁺ ATPase pumps the ions against their concentration gradients

Question 15

Na+ K+ ATPase in the role of maintenace of the RMP (resting membrane potential)

Answer 15

• 3 Na+ gets pumped out of the cell
• 2 K+ gets pumped into the cell
• makes a net negative charge inside the cell using ATP