Module 1 Lecture 6: Unassisted Membrane Transport

Question 1

Molecules that penetrate the membrane of their own accord use what force?

Answer 1

Passive forces

Question 2

What are the two forces that allow molecules to passively cross the membrane?

Answer 2

Concentration gradient diffusion & movement along electrical gradient

Question 3

What does passive diffusion of particles entail?

Answer 3

1) Molecules are always in constant random motion
2) Evenly distributed over time

Question 4

What term is used to describe the distribution of molecules from one side to the next?

Answer 4

Diffusion

Question 5

What does net diffusion mean?

Answer 5

The diffusion from side a to b - side b to a

Question 6

When does net diffusion cease?

Answer 6

When dynamic equilibrium is reached on both sides

Question 7

What is a concentration gradient?

Answer 7

Difference in concentration between two adjacent areas

Question 8

From what level of concentration must molecules diffuse through for passive concentration?

Answer 8

High level of concentration to low level for passive diffusion to work

Question 9

Example of passive diffusion in our body?

Answer 9

Oxygen transporting across the lung membrane

Question 10

What does Fick’s law of diffusion explain?

Answer 10

There are many factors that collectively influence the rate of net diffusion of a substance across the membrane

Question 11

What factors increase the rate of net diffusion

Answer 11

1) Greater concentration gradient
2) Increased surface area of a membrane
3) Lipid solubility of the substance

Question 12

What factor(s) decrease rate of net diffusion?

Answer 12

1) Molecular weight (heavier molecules tend to cross slowly)
2) Increased distance (thickness of the membrane)

Question 13

How are ions involved in passive diffusion?

Answer 13

Ions = electrically charged; movement of charged particles creates electrical gradient for ions to move through

Question 14

What is an electrical gradient?

Answer 14

Similar to concentration gradient; difference in charge between two adjacent areas

Question 15

How do ions make an electrical gradient?

Answer 15

Cations will move towards the more negative side while anions move towards a more positive side; difference in charge creates an electrical gradient for only ions to pass through

Question 16

What is an electrochemical gradient?

Answer 16

When an electrical and concentration gradient both act on an ion it results in an electrochemical gradient

Question 17

How can water molecules penetrate the plasma membrane with their polarity?

Answer 17

Specific channels called aquaporins on the membrane that allow for the passage of water - billion/sec

Question 18

If there were no aquaporins how would water typically get into the membrane?

Answer 18

Slipping through the PL molecules

Question 19

Define osmosis

Answer 19

The movement of water when a selectively permeable membrane separates two unequal solutes concentrations (and water concentrations); net diffusion of water

Question 20

Why is concentration used differently with osmosis?

Answer 20

Concentration typically refers to how much solute in a given volume of water. In this case we talk about the solute and water both being ‘density of solutes’

Question 21

Ratio of solute to water molecule?

Answer 21

1 solute molecule : 1 water molecule

Question 22

Where do water molecules passively move from?

Answer 22

High water concentration (low solute concentration) to low water concentration (high solute concentration)

Question 23

How does osmosis work when you have a membrane separating water and a penetrating solute?

Answer 23

High water concentration moves towards the side with low water concentration and high solute concentration moves towards the side with low solute concentration

Question 24

How osmosis works when a membrane separates unequal solutions of a non-penetrating solute

Answer 24

Solute can’t cross the membrane but the water can. Water will continue to cross the membrane to join side 2 that has less water concentration (high solute concentration) but that side cannot let its solute into side 1. Volume change occurs.

Question 25

What happens to the concentration of water and solute in the non-penetrating solute example?

Answer 25

Side 1 volume of water decreases but concentration of solute increases and side 2 volume of water increases and concentration of solute on side 2 decreases. Equal concentration of solute and water is achieved

Question 26

What is osmotic pressure?

Answer 26

The tendency of water to move to the side with non-penetrating solutes and water

Question 27

What is hydrostatic pressure?

Answer 27

Pressure that is exerted by something in place in the opposite direction

Question 28

How does osmosis work when one side is pure water and the other side is a non-penetrable solute

Answer 28

Osmosis from side 1 to side 2 occurs but never equal concentrations. Water can diffuse and dilute the solute in side 2 as much as it wants but it’ll never become water and side 2 will never cross the membrane.

Question 29

When equilibrium cannot be reached with one side being pure water and the other side being impenetrable solute does net diffusion of water occur?

Answer 29

No because when side 2 gains a large amount of volume, hydrostatic (water stopping) pressure pushes the osmotic flow back. Osmotic pressure still occurs but pushed back by hydrostatic pressure until there is a counterbalance

Question 30

If there is more solute concentration on one side and less water concentration what does that make water on the other side do?

Answer 30

It makes water from the other side want to move into the side with high solute concentration (osmotic pressure) and hence more opposing pressure to stop the osmotic flow