L2.1 Diffusion & Osmosis

Question 1

what does this mean: “the plasma membrane is selectively permeable”

Answer 1

it means only some molecules can pass freely across the membrane

Question 2

what is the passage of molecules across the membrane dependent on?

Answer 2

particle size and lipid solubility of the particle (charge)

Question 3

what molecules can pass across the plasma membrane?

Answer 3

permeable: gases (O2 and CO2), water, ethanol
impermeable: ions, glucose and proteins

Question 4

what are the types of membrane transport? what is the difference? and types of each?

Answer 4

passive transport (doesn’t require any energy-ATP): simple diffusion, osmosis, facilitated diffusion (channels and carriers)

active transport (requires energy- ATP): primary active transport, secondary active transport, vesicular transport (endocytosis and exocytosis)

Question 5

what is diffusion?

Answer 5

the movement of molecules from an area of high concentration to an area of low concentration

Question 6

what is concentration gradients?

Answer 6

differences in concentration across two area is called a concentration gradient. the greater the difference, the greater the concentration gradient

Question 7

“the molecules will eventually reach an equilibrium”: what does this mean?

Answer 7

evenly distributed. molecules diffuse down the concentration gradient

Question 8

what is the rate of diffusion controlled by? (5)

Answer 8

1. the size of the concentration gradient (bigger gradient = faster diffusion)
2. membrane surface area (bigger surface area= faster diffusion)
3. size of the molecule (small molecules diffuse more quickly than large molecules)
4. diffusion distance (decreasing diffusion distance = increasing diffusion rate)
5. lipid solubility of the molecule
   - hydrophobic/lipophilic: enters membrane easily
   - hydrophilic/lipophobic: repelled by membrane- most substances in the body

Question 9

what is osmosis?

Answer 9

the diffusion of WATER across a selectively permeable membrane= the movement of water from an area of high water concentration/low solute concentration to an area of low water concentration/high solute concentration

Question 10

what is ‘solution’?

Answer 10

combination of solute and solvent

Question 11

what is ‘solvent’?

Answer 11

liquid that substances dissolve into

Question 12

what is ‘solute’?

Answer 12

substances that dissolve in liquids

Question 13

what are ‘penetrating solutes’?

Answer 13

can cross the plasma membrane

Question 14

what are ‘non-penetrating’ solutes?

Answer 14

cannot cross the membrane

Question 15

what happens if the membrane that separates the solutions is permeable to both water and solutes?

Answer 15

then both can move until they reach an equilibrium- the amount of water and solutes is balanced across the membrane

Question 16

what happens if the membrane that separates the solutions is permeable to water but impermeable to solutes?

Answer 16

the water will diffuse across the membrane (osmosis), but the solute won’t. the amount of water becomes unbalanced across the membrane

Question 17

what is osmolarity? and what is it measured in?

Answer 17

osmolarity= number of solute particles per litre of solution
measured in mOsm/L (milliosmotes/litre)- most body fluids are 300mOsm/L

Question 18

what is tonicity? and what is it dependent on?

Answer 18

the ability of a solution to change the shape (tone) of a cell by changing the cells internal water volume (via osmosis). depends on the concentration of non-penetrating solutes on either side of the cell membrane

Question 19

what is the difference between an isotonic, hypertonic and hypotonic solution?

Answer 19

isotonic solution- same concentration as ICF
hypertonic solution- higher concentration than ICF
hypotonic solution- lower concentration than ICF

Question 20

In an isotonic solution (ICF- 300mOsm/L; ECF- 300mOsm/L):

• what is the concentration of non-penetrating solutes in the solution compared to the cell?
• will there be a net movement of water?
• what is the effect on cell shape?

Answer 20

In an isotonic solution (ICF- 300mOsm/L; ECF- 300mOsm/L):

• what is the concentration of non-penetrating solutes in the solution compared to the cell? SAME AS THE CELL (300mOsm/L)
• will there be a net movement of water? NO
• what is the effect on cell shape? NONE

Question 21

In an hypertonic solution (ICF- 300mOsm/L; ECF- 400mOsm/L):

• what is the concentration of non-penetrating solutes in the solution compared to the cell?
• will there be a net movement of water?
• what is the effect on cell shape?

Answer 21

In an hypertonic solution (ICF- 300mOsm/L; ECF- 400mOsm/L):

• what is the concentration of non-penetrating solutes in the solution compared to the cell? HIGHER
• will there be a net movement of water? YES (OUT OF THE CELL)
• what is the effect on cell shape? SHRINK

Question 22

In an hypotonic solution (ICF- 300mOsm/L; ECF- 200mOsm/L):

• what is the concentration of non-penetrating solutes in the solution compared to the cell?
• will there be a net movement of water?
• what is the effect on cell shape?

Answer 22

In an hypotonic solution (ICF- 300mOsm/L; ECF- 200mOsm/L):

• what is the concentration of non-penetrating solutes in the solution compared to the cell? LOWER
• will there be a net movement of water? YES (INTO THE CELL)
• what is the effect on cell shape? EXPAND

Question 23

see challenge question in notes

Answer 23

see challenge question in notes

Question 24

in determining tonicity, you must take into account the _______ of the solution and whether the plasma membrane is _______ to the molecules

Answer 24

in determining tonicity, you must take into account the OSMOLARITY of the solution and whether the plasma membrane is PERMEABLE to the molecules