Lecture 4: Protein transporters

Question 1

Solutes diffuse from an area of _______ concentration to an area of _______ concentration. What is this called?

Answer 1

high
low
Flux

Question 2

What is osmosis?

Answer 2

When water flows of areas of where water is in high concentration to where water is in low concentration across a semipermeable membrane

Question 3

The cell membrane limits free movement, to varying degrees. This is because the membrane is a _______ __________

Answer 3

selective barrier

Question 4

Membrane spanning proteins provide an alternative, regulated mechanism to what?

Answer 4

passive diffusion

Question 5

What is the net flux?

Answer 5

the sum of the total number of opposing fluxes

Question 6

The larger the concentration difference, the bigger the what?

Answer 6

flux

Question 7

What do membrane proteins allow?

Answer 7

they allow more solutes to be moved through the membrane, even at low concentrations of solutes

Question 8

What is a limitation of membrane proteins?

Answer 8

The solute has to interact with the membrane protein which means that only one protein can go in at a time and therefore there is a saturation point

Question 9

What is upreulation?

Answer 9

Adding more proteins into the membrane which increases the flux

Question 10

Describe protein mediated membrane transport. What two types are there?

Answer 10

Proteins span the membrane. These proteins function as enzymes and they catalyse the transport of solutes across the membrane
The two types are:
1. passive transport
2. active transport

Question 11

What is involved in passive protein mediated membrane transport?

Answer 11

This involves channels and carriers (facilitated diffusion) it is dependent on concentration gradient

Question 12

What is involved in active protein mediated membrane transport?

Answer 12

Primary active transport which is ATP dependent or secondary active transport which uses carriers and transporters and use the downhill gradient for one solute to transport another solute uphill

Question 13

Channels and carriers are very _______. What do they require and what do they have?

Answer 13

selective
they require interaction between substrate and protein
they have a binding site/selectivity filter in the protein

Question 14

What is the main difference between channels and carriers?

Answer 14

The rate of transport

• channels are very fast
• carriers are much slower

Question 15

Channels have a ______ and when that opens, you have a _______

Answer 15

gate

pore

Question 16

Why are carriers much slower than channels?

Answer 16

because there has to be an interaction between and protein and the solute and it requires conformational change in the protein to move the solute across the membrane

Question 17

What is another name for a channel that participate in facilitated diffusion? Define this

Answer 17

gated pore or non-gated pore

this is a small, selective tunnel through which molecules can move rapidly

Question 18

What three things do channels move via facilitated diffusion?

Answer 18

ion
water
small solutes

Question 19

Is the movement of ions, water and small solutes a passive or active process?

Answer 19

this is a passive mechanism: the movement of a solute via a channel is determined by the gradient for the solute (both concentration and electrical gradients)

Question 20

Naming of channels can vary. What are some of the ways they are named after?

Answer 20

• they are named after the things that go through them
• they are named after a disease state (eg. cystic fibrosis transmembrane regulatory protein)
• they are named after an inhibitor (eg. Ryanodine receptor)

Question 21

What two states can gated channels exist in?

Answer 21

open or closed state

Question 22

When gated channels open, they open to the _________ extent every time. What does this mean?

Answer 22

same

therefore a channel is either nothing when it is closed or maximum when it is open

Question 23

Do gated channels open spontaneously?

Answer 23

yes

however propensity of opening can be modified by other agent

Question 24

Are channels prone to competition? How is this dealt with?

Answer 24

Yes

• If you (for eg.) wanted Cl- to go through the CFTR channel but CO32- can also go through, the concentration of Cl- would have to be really high on one side so that the chance of Cl- bumping into the protein is higher (has a greater flux).
• you can also have selectivity eg. have a positively charge in the pore so only negatively charged ions can go through

Question 25

As well as selectivity and competition, how else can a pore have reduced activity?

Answer 25

- inhibition: sometimes an inhibitor will block the channel, sometimes it will lock the channel open

Question 26

Chanel open to the same extent every time. Because of this, the permeability of membrane to a solute that crosses the membrane can only be determines by two things. What are these?

Answer 26

1. the number of channels (N) for that solute that are in the membrane 2. the open probability (Po) of the channels that are in the membrane

Question 27

What is open probablilty?

Answer 27

this is the fraction of time the channel stays open

Question 28

Proteins in the membrane are always being ___________ and ________.

Answer 28

Proteins in the membrane are always being inserted and removed.

Question 29

If you increase the number of transporters in the membrane, you increase the _________ of that ion

Answer 29

permeability

Question 30

The fraction of time that channels stay open for and the number of channels is modified by what three things?

Answer 30

1. specific molecules 2. the membrane potential 3. mechanical deformation

Question 31

How do specific molecules modify the fraction of time that channels stay open for and the number of channels in the membrane? Give examples?

Answer 31

ACh, cAMP, [Ca2+]i they can cause the opening of more channels eg. calcium induced calcium release causes the activation of calcium release channels in the SR ACh can bind to gated channels which causes the open probability to increase

Question 32

How does the membrane potentials modify the fraction of time that channels stay open for and the number of channels in the membrane? Give examples?

Answer 32

Voltage gated Na+ opens and the membrane potential changes which is how you get propagation of action potentials. A change in membrane potential activates things that will further alter the membrane potential

Question 33

How does mechanical deformation modify the fraction of time that channels stay open for and the number of channels in the membrane? Give examples?

Answer 33

Touch receptors respond to pressure and mechanical changes which cause the opening of calcium channels

Question 34

What is secondary active transport and what are the two types? What do these mean?

Answer 34

Uses the downhill gradient of one solute to transport another solute "uphill" - cotransporter: both solutes are moving in the same direction - counter-transporter: both solutes moving in the opposite direction

Question 35

What do carriers normally transport?

Answer 35

ions and something else (eg, glucose or amino acids), or two different ions

Question 36

What do carriers contain?

Answer 36

a binding site that determines the selectivity

Question 37

What is the type of system that carriers are?

Answer 37

they are a turn-style type system

Question 38

Describe a turn-style type system

Answer 38

Binding site(s) is only accessible from one side of the membrane at a time. It binds 1-2 molecules and then it changes conformation transporting solute across the membrane

Question 39

Explain carriers transporting a molecule

Answer 39

The carrier is open to the outside and the molecule enters from the outside and binds at a binding site. The outer gate closes and the molecule becomes occluded, still attached to its binding site. The inner gate opens with the molecule still bound. The molecules exits and enters the inside of the cell. The outer gate closes, occluding an empty binding site. This cycle can also flow in reverse order.

Question 40

Carriers are prone to _______ and ______-

Answer 40

competition | inhibition

Question 41

Carriers have _______ kinetics

Answer 41

saturation (there is only a limited number of molecules that can bind per second)

Question 42

The types of carriers are defined in terms of their what?

Answer 42

substrate whether it is the solute transported against electrochemical gradient or the binding of substrates

Question 43

Explain the na+/glucose cotransporter

Answer 43

2 Na+ molecules and a glucose and then it will open

Question 44

What is the purpose of the Na+/H+?

Answer 44

to maintain charge difference and to control acid secretion

Question 45

Where can a summary of gated channels and carriers are found?

Answer 45

slide 26

Question 46

What is the purpose of NKCC1?

Answer 46

Cl- transporting into the cell coupled to Na+ so when CFTR opens, Cl- can exit the membrane Na+ is pumped out through Na+/K+ ATPase, and a K+ channel so K+ can leave the cell