Module 1 Lecture 6: Unassisted Membrane Transport Flashcards
Molecules that penetrate the membrane of their own accord use what force?
Passive forces
What are the two forces that allow molecules to passively cross the membrane?
Concentration gradient diffusion & movement along electrical gradient
What does passive diffusion of particles entail?
1) Molecules are always in constant random motion
2) Evenly distributed over time
What term is used to describe the distribution of molecules from one side to the next?
Diffusion
What does net diffusion mean?
The diffusion from side a to b - side b to a
When does net diffusion cease?
When dynamic equilibrium is reached on both sides
What is a concentration gradient?
Difference in concentration between two adjacent areas
From what level of concentration must molecules diffuse through for passive concentration?
High level of concentration to low level for passive diffusion to work
Example of passive diffusion in our body?
Oxygen transporting across the lung membrane
What does Fick’s law of diffusion explain?
There are many factors that collectively influence the rate of net diffusion of a substance across the membrane
What factors increase the rate of net diffusion
1) Greater concentration gradient
2) Increased surface area of a membrane
3) Lipid solubility of the substance
What factor(s) decrease rate of net diffusion?
1) Molecular weight (heavier molecules tend to cross slowly)
2) Increased distance (thickness of the membrane)
How are ions involved in passive diffusion?
Ions = electrically charged; movement of charged particles creates electrical gradient for ions to move through
What is an electrical gradient?
Similar to concentration gradient; difference in charge between two adjacent areas
How do ions make an electrical gradient?
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
What is an electrochemical gradient?
When an electrical and concentration gradient both act on an ion it results in an electrochemical gradient
How can water molecules penetrate the plasma membrane with their polarity?
Specific channels called aquaporins on the membrane that allow for the passage of water - billion/sec
If there were no aquaporins how would water typically get into the membrane?
Slipping through the PL molecules
Define osmosis
The movement of water when a selectively permeable membrane separates two unequal solutes concentrations (and water concentrations); net diffusion of water
Why is concentration used differently with osmosis?
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’
Ratio of solute to water molecule?
1 solute molecule : 1 water molecule
Where do water molecules passively move from?
High water concentration (low solute concentration) to low water concentration (high solute concentration)
How does osmosis work when you have a membrane separating water and a penetrating solute?
High water concentration moves towards the side with low water concentration and high solute concentration moves towards the side with low solute concentration
How osmosis works when a membrane separates unequal solutions of a non-penetrating solute
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.
What happens to the concentration of water and solute in the non-penetrating solute example?
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
What is osmotic pressure?
The tendency of water to move to the side with non-penetrating solutes and water
What is hydrostatic pressure?
Pressure that is exerted by something in place in the opposite direction
How does osmosis work when one side is pure water and the other side is a non-penetrable solute
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.
When equilibrium cannot be reached with one side being pure water and the other side being impenetrable solute does net diffusion of water occur?
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
If there is more solute concentration on one side and less water concentration what does that make water on the other side do?
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
