Lecture 13: Membrane transport 1

Question 1

Two functions of membranes:

Answer 1

1. Must retain dissolved cellular material and prevent leakage.
2. Must allow for material to be exchanged into and out of cell.

Question 2

Explain why cells need to exchange materials across their membranes

Answer 2

1. Cells acquire the molecules and ions they need from their surrounding extra-cellular fluid (ECF) – an unceasing traffic of molecules and ions across the plasma membrane!!
   • Within eukaryotic cells, molecules and ions must be transported in and out of compartments too!
2. Cells make waste products, which need to be dumped either a.) outside the cell, or b.) into the vacuole (if a plant cell)

Question 3

How do Cells actively control what and when molecules pass through membranes?

Answer 3

Cells actively control what and when molecules pass through membranes by expressing (and controlling) integral membrane proteins

Question 4

What can enter the lipid bilayer?

Answer 4

Lipid bilayers are impermeable to ions and most polar molecules

Small nonpolar molecules can readily get through on their own

• Really small uncharged polar molecules can somewhat get through, but are really slowed down (water-aquaporins, ethanol, glycerol)

Larger uncharged polar molecules don’t really get through on their own

Charged ions? Not at all!

Macromolecules get across membranes through completely different mechanisms

Question 5

membrane potential

Answer 5

right at the membrane the charge is slightly different on each side, more –ive on inside cell and more +ive outside (-20 mV to -200 mV)

it can be used to help move other things across the plasma membrane

The electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell.

Question 6

List which ions are at higher concentrations inside or outside the cell

Answer 6

Na- high outside low inside

K+- high inside low outside

Mg- low inside high outside

Ca- high outside low inside

Cl- high outside low inside

Question 7

List the 4 main ways that molecules get across the membrane

Answer 7

A) Simple diffusion
Movement from higher to lower concentrations straight through the lipid bilayer.

B) Simple diffusion through a channel
Movement from higher to lower concentrations through the pore of a membrane channel protein

C) Facilitated diffusion
Movement from higher to lower concentrations via a membrane protein carrier (aka ‘facilitative transporter’)… still diffusion.

D) Active transport
Movement from low to high concentrations via a protein transporter (or ‘pump’). Requires energy, which often comes from ATP hydrolysis.

Question 8

Simple diffusion

Answer 8

Molecules and ions move spontaneously down their concentration gradient (i.e., from a region of higher to a region of lower concentration) by diffusion.

diffusion: the spontaneous movement of solute from regions of high concentration to low concentration - follows the 2nd law of thermodynamics (more order to less order)

Question 9

active transport

Answer 9

Molecules and ions can be moved against their concentration gradient
requires the expenditure of energy (usually from ATP).

Question 10

Explain how preventing diffusion with a biological membrane is a store of energy

Answer 10

Maintaining differences in concentration stores energy

When molecules diffuse, the entropy of the system increases. (remember, entropy = disorder)
Going from higher to lower order dissipates free-energy (ΔG).
Preventing this dissipation of energy with a semipermeable membrane thus stores this energy.
This energy can be dissipated later to do stuff.

Question 11

Osmosis

Answer 11

is just water moving down its concentration

gradient

Question 12

Tonicity

Answer 12

ability of a surrounding solution to cause a cell to gain or loose water

The tonicity of a solution that you drop a cell into is for non-penetrating solutes only! Thus osmolarity doesn’t always equal tonicity. [x] = concentration of x

Question 13

Hypotonic solution:

Answer 13

low [non-penetrating solute]
High [free water] Cell solution:
High [non-penetrating solute]
Low [free water]
Water will enter the cell until the cell lyses.

Question 14

Isotonic solution:

Answer 14

Equal [non-penetrating solute]
Equal [free water]
So the water entering and leaving the cell is balanced.

Question 15

Hypertonic

Answer 15

High [solute]
Low [free water]
So the water leaves the cell, and the cell will shrink.
Cell solution:
Low [non-penetrating solute]
High [free water] Water leaves the cell.

Question 16

Describe mechanism through which different cell types cope with fluids of different tonicitites

Answer 16

Animal cells keep themselves in an isotonic solution
- Animal cells spend energy pumping ions (mostly Na+ ions) outside the cell into the extracellular fluid to maintain an isotonic solution

Plant cells don’t have to ‘care’ about tonicity because they have that rigid plant cell wall that prevents them from bursting….

Question 17

Describe how a plant cell generates turgor pressure in relation to tonicity

Answer 17

The fluids outside of plant cells is hypotonic. The inside of the vacuole is hypertonic. Thus water flows into the plant cell and vacuole. This causes the cells to swell, but there’s a rigid plant cell wall in the way.
So the cell pushes against the cell wall, creating ‘turgor pressure’. The sum of all of the cells doing this allows a non- woody plant to stand upright.

Question 18

why do non-woody plants wilt when you don’t water them?

Answer 18

When you forget to water plants… The loss of water in the whole plant causes the fluid outside of the cells to become hypertonic.
Water flows out of the cells, the cells shrink in a process called plasmolysis. The turgor pressure goes to zero, and my plant wilts and looks sad

Question 19

Describe where protozoa live

Answer 19

Most free living protozoa live in dilute aqueous (non salty water) solutions, where osmotic effects are severe.
To deal with the constant in-flux of water they continuously actively pump water back out of the cell using an organelle known as the contractile vacuole.
Water accumulates within the contractile vacuole, a membrane-bound structure, which inflates. To expel the water, the vacuole uses exocytosis to remove the water

Question 20

How does the cell force water into the contractile vacuole?

Answer 20

It expends a lot of energy actively pumping ions into the vacuole to make a hypertonic solution in it, and then expends even more energy recovering ions from the external environment.(in dilute solution like lake water there are not enough ions to be found so it has to look for them

Question 21

Describe the function of aquaporins

Answer 21

Aquaporins mediate the flow of water through some membranes
- in some cell types the movement of water is much greater than it would be if it flowed through the lipid bilayer unassisted.

Faster flow can occur in the presence of trans-membrane aquaporin proteins (movement is still passive, no energy needed)
e.g. In the gut, several liters of water per day are moved into the lumen of stomach, intestine, then re-absorbed in the lower gut.

allows water molecules to move through in single file

Question 22

describe how aquaporins are selective for water molecules and not protons

Answer 22

Water flows single file through the pore… one billion per second!

Hydrogen bonding from the amino acids and peptide backbone coordinates the water molecules as they flow through.

The amino acid residues N203 and N68 occur at the narrowest point in the channel. The positive dipole on these residues interrupt the intra-water H-bonds and flip the water around.

We think this prevents H+ ions that can ‘hitch a ride’ (via H-bonding with the water column) from getting through the pore.

Thus, aquaporins are selective for transporting water across a membrane

Question 23

describe how kidney epithelial cells regulate whether aquaporins are active and transporting water on their plasma membrane

Answer 23

in the kidney, water is reabsorbed (i.e. less is excreted) in response to the hormone vasopressin (antidiuretic hormone)

Cells can’t open and close aquaporins to regulate flow, instead the flow is regulated by the location of aquaporin proteins! Cells can’t open and close aquaporins

When you’re hydrated, they’re kept in an internal membrane so water is mostly passed into the bladder.

The presence of vasopressin promotes reabsorption of water. Aquaporins move from internal membranes to cell membrane… thus more water is reabsorbed through the cell and into the bloodstream.