Topic 6 - chapter 12 Membrane transport

Question 1

The difference between a transporter and a channel?

Answer 1

A transporter has moving parts and can shift small molecules (inorganic ions & small molecules) from one side to the other by changing its shape.
Channels, in contrast, form tiny hydrophilic pores thru which solutes (inorganic ions) can pass by diffusion

Question 2

Most plentiful inorganic ions?

Answer 2

Na+, K+, Ca2+, Cl-, H+

Question 3

Highest ion inside/outside the cell? Concentrations?

Answer 3

inside cell K+ 140mM, outside cell K+ 5mM

outside Na+ 145mM, outside cell 5-15mM

Question 4

What prevents the cell from being torn apart by electrical forces?

Answer 4

the quantity of +ve charge inside cell must be balanced by almost equal amount of -ve charge inside. Same applies to outside the cell

Question 5

High [Na+] outside the cell balanced with …?

Answer 5

Cl-

Question 6

High [K+] inside the cell balanced by …?

Answer 6

a variety of -vely charged ions (anions)

Question 7

The plasma membrane acts as … … membrane?

Answer 7

selectively permeable membrane

Question 8

List some small nonpolar molecules

Answer 8

O2, CO2, N2, benzene

Question 9

List some uncharged polar molecules

Answer 9

water, ethanol, glycerol

Question 10

Do uncharged polar molecules diffuse across the membrane rapidly?

Answer 10

yes, if they are small enough

Question 11

List some large uncharged polar molecules

Answer 11

AAs, glucose, nucleosides

Question 12

T or F - lipid membranes are impermeable to all ions and charged molecules, no matter how small

Answer 12

true

Question 13

On what basis do channels discriminate?

Answer 13

size & electric charge ie. if a channel is open, an ion or molecule small enough with the appropriate charge can freely pass thru

Question 14

Molecules or solutes that require no energy get into cells by …?

Answer 14

passive diffusion or facilitated diffusion

Question 15

Plasma membrane contains transporters that import which nutrients?

Answer 15

sugars, AA’s, nucleotides, Na+ (out of cell)

Question 16

Lysosome membrane contains a transporter for which molecule?

Answer 16

H+ to acidify the lysosome interior

Question 17

Mitochondrion membrane contains a transporter for which molecule?

Answer 17

pyruvate & ATP

Question 18

A transporter that mediates passive transport?

Answer 18

glucose transporter (at least 2 conformations)
binds only D-glucose

Question 19

The electrochemical gradient is best described as…?

Answer 19

the net driving force as a composite of 1. concentration gradient 2. voltage across membrane

Question 20

The charge inside the cell is …?

Answer 20

more -ve than the outside

Question 21

3 main ways cells carry out active transport…

Answer 21

1. Coupled transporters: one solute goes down [ ] gradient transferring other solute up [ ] gradient
2. ATP-driven pumps: ATP hydrolysis drives pump
3. Light-driven pumps: energy from light (bacterial cells eg. bacteriorhodopsin)

Question 22

The main role of the Na+-K+ ATPase is…?

Answer 22

to maintain high Na+ outside the cell (energy store) and high K+ inside the cell maintaining the concentration gradient and also accounts for 30% of total ATP consumption

Question 23

Describe the Na+-K+ ATPase cycle…

Answer 23

1. Na+ binds to pump (from inside cell) 2. ATP phosphorylates pump -> ADP + Pi 3. Pi causes pump to release Na+ into extracellular space 4. K+ binds to pump from outside cell 5. pump dephosphorylated 6. pump returns to normal conformation & K+ ejected into cell

Question 24

How do animal cells maintain osmotic equilibrium?

Answer 24

keeping intracellular solute [ ] low by pumping out ions

Question 25

How do protozoans maintain osmotic equilibrium?

Answer 25

by periodically ejecting water that moves into the cell via contractile vacuoles

Question 26

How do plant cells maintain osmotic equilibrium?

Answer 26

cell walls prevent swelling as osmosis & active transport into cell -> turgor pressure

Question 27

Is Ca2+ high or low inside the cell? Why?

Answer 27

low, because of Ca2+ pumps that move them from cytosol to SR

Question 28

A bit more about the Ca2+ pump...

Answer 28

polypeptide of protein crosses membrane as 10 alpha helices Ca2+ binds to binding cavity -> ATP phosphorylates aspartic acid -> conformational change (activator domain & phosphorylation domain move closer together) -> Ca2+ back into SR

Question 29

Can Ca2+ be used for any other reason?

Answer 29

yes - second messenger

Question 30

Example of glucose symports?

Answer 30

apical surface of gut lumen

Question 31

Example of glucose uniport?

Answer 31

basal & lateral surface of gut cells

Question 32

How is Na+ kept at low [ ] in the cytosol of the gut?

Answer 32

Na+/K+ pumps

Question 33

H+ pumps are found where?

Answer 33

Bacterial, plant & fungi (incl yeasts) cells and also in lysosome animal cells

Question 34

Give 4 examples of transporters. State whether they are active or passive, where they are found, energy source and function.

Answer 34

1. glucose transporter. plasma membrane most animal cells. no energy source. passive import of glucose 2. Na+ -driven glucose pump. apical plasma membrane of kidney and intestinal cells. Na+ gradient. active import of glucose 3. Na+/K+ pump. plasma membrane of most animals. ATP hydrolysis. pump Na+ out of cell, K+ into cell 4. Bacteriorhodopsin. plasma membrane of some bacteria. light. active export of H+

Question 35

Describe 2 differences between ion channels and simple pores in the membrane

Answer 35

- ion channels are very selective - depends on diameter and shape of the ion channel & distribution of charged amino acids lining the channel eg. carbonyl oxygen atoms in a K+ channel - gated (open & close to control movement of ions)

Question 36

Ion channels compared to transporters (speed of movement of solutes)...

Answer 36

ion channels are 1000 times faster than transporters

Question 37

What is the patch-clamp recording used for? What did the technique show us?

Answer 37

to measure electric current flowing thru single channel (ie. study ion movements & ion channels) technique showed us that ion channels open & close randomly

Question 38

Name 3 types of ion channel

Answer 38

1. voltage-gated (membrane potential) 2. ligand-gated (ligand binding) 3. stress-gated (applied mechanical force - EAR)

Question 39

What does the Nernst equation tell us?

Answer 39

That the force responsible for the ion transport across membranes has 2 components 1. electrical membrane potential 2. concentration gradient = electrochemical gradient

Question 40

How are electrical signals converted to chemical signals?

Answer 40

via voltage-gated Ca+ channels

Question 41

How are chemical signals converted back to electrical signals?

Answer 41

ligand-gated ion channels: aka. transmitter-gated ion channels (ion-channel-couple receptors)

Question 42

Example of transmitter-gated ion channel?

Answer 42

acetylcholine receptor (allows Na+ ions to flow into postsynaptic cell to regenerate AP)

Question 43

What does an inhibitory input do?

Answer 43

binds neurotransmitter -> Cl- into cell -> decreases likelihood of firing AP