Membrane transport 1

Question 1

Cell membranes act as barriers how

Answer 1

Allow a few solutes to pass by diffusion (O2 and CO2)

The vast majority cannot and rely on membrane transport proteins

Question 2

What are the two major classes of transport proteins

Answer 2

Carrier proteins

Channel proteins

Question 3

Channel Proteins

Answer 3

Bind to a solute molecule on one side of the membrane and deliver it to the other by a conformational change

Question 4

Channel proteins

Answer 4

Form small hydrophilic pores and solutes pass through by diffusion.
These are also called ion channels

Question 5

How do membranes transport

Answer 5

The membrane proteins transport by lining the cell membrane enabling protection for hydrophilic molecules

Question 6

How do channel proteins discriminate

Answer 6

On the basis of Size and Charge

Carrier proteins specifically bind solute molecules –> selection is by specific binding

Question 7

Passive transport

Answer 7

Selective
Also known as facilitated diffusion. This arises when solute moves spontaneously across the membrane with a concentration gradient

Question 8

Active transport

Answer 8

This involves solute movement across a concentraton gradient and requires energy

Question 9

What is the most studied system across membranes

Answer 9

Glucose across the gut layer.

Question 10

The Glucose carrier

Answer 10

An example of a passive carrier protein
12 membrane pass alpha helix

This protein adopts 2 conformations

Glucose binds to the carrier when the concentration is high. This causes a structural protein change which delivers the molecule to the other side

The recognition of glucose is very specific

Only occurs with D glucose not L glucose

Question 11

Transporting uncharged molecules

Answer 11

Involves electrochemical gradient. The cells have a membrane potential and are usually negative inside
Sodium has a strong electrochemicial gradient and Potassium has a weak electrochemical gradient

Question 12

Electrochemical gradient

Answer 12

Combination of chemical concentration gradient and membrane potential –> Controls the direction the solute moves in

Question 13

Membrane potential

Answer 13

The difference in the electric potential between the inside and outsides of the cell

Question 14

Passive transport

Answer 14

This arises when the solute moves spontaneously across a membrane along the electrochemical gradient –> along a favourable direction

Question 15

Active transport

Answer 15

This involves the solute moving against the electrochemical gradient and requires energy being inputted.
This is travelling in the unfavourable direction

Question 16

How does active transport gain its energy

Answer 16

This obtains energy in three different ways

1. ATP
2. Light
3. Couples

Question 17

Passive transport of charged molecules

Answer 17

This involves and electrochemical gradient
The cells have a membrane potential.
If they have a strong electrochemical gradient they can enter cells more easily.
If they have a weak electrochemical gradient they enter the cell less easily,

Question 18

How does active transport move solutes against the electrochemical gradient

Answer 18

1. Coupled Pumps
   Couple uphill transport of one solute with the downhill transport of another. One example of this is a sodium-potassium pump.
2. ATP Driven Pumps
   Couples uphill transport to ATP hydrolysis
3. Light Driven Pumps
   Couples uphill transport to light input ( bacteria and plants )

Question 19

Types of Transport

Answer 19

Symport- Carriers move solutes in only one direction
Antiport- Carriers move solutes in opposite directions
Uniport - Carriers move only one type of solure e.g glucose

Question 20

Symport

Answer 20

Inward flow of sodium plays an important role in animal cells to drive other molecules in

e.g. glucose in the gut

Question 21

Active Uniport

Answer 21

The Ca2+ concentration inside cells is kept low compared to the outside. This is maintained by ATPases
An influx of Ca2+ (via the Ca2+ Channels) is often used to control signalling

Ca2+ has a hih affinity with many proteins

Question 22

Antiport

Answer 22

An example of this is a sodium/potasium pump

Na+ binds to cytoplasmic side and initiates ATP mediated protein phosphorylation
This causes a conformational change
Na+ is then released to the extracellular matrix side allowing the K+ to bind
k+binding causes protein dephospholoation and return to the original protein conformation

Question 23

How is pH maintained in cells

Answer 23

H+ ATPases are found in lysosomal membranes

This pumps H- out of the cytoplasm and into the lysosome

This keeps the cytoplasm neutral and the lysosome acidic.

Question 24

What are some of the differences between carrier proteins and ion channels

Answer 24

Much faster than carrier proteins
Ion channels are not continuously open, most are gated.

Cannot couple ion flow to an energy source for active transport but operate a membrane transiently permeable –> This means ions move rapidly through their channels via their electrochemical gradient.

The movement of ions alters voltage across membranes ( the membrane potential) this results in a specific function because membrane potential is the basis for all electrical activity in cells

Question 25

Resting membrane potential

Answer 25

This arises due to the composition and concentration of ions between cytosol and external environment

The ion movement is recilitated by electrochemical gradient and control membrane potential

Question 26

What is the resting membrane potential in steady state

Answer 26

-20 -200mV

Question 27

How do you calculate the Resting membrane potential

Answer 27

The Nerst Equation

v=62log10(C0/C1)

Question 28

What influences resting membrane potential

Answer 28

Reflected by k+ concentration gradient as membranes are highly permeable to k+

If an event causes channels to become permeable to Na+ ions They will enter and change the RMP.

Question 29

Measuring Ion channel activity

Answer 29

Electrical recordings can be made by PATCH CLAMP recording

Question 30

What is patch clamp recording

Answer 30

Fluid filles glass micro electrode used to contact cell membrane

Wire passes through the electrode

Current entering the membrane is detected by the wire and recorded.

This is a very sensitive technique –> Clamp refferes to the setting of the voltage across the membrane

It is possible to measure single ion channels–> These snap on and off when conducting and represent the conformational change of the protein.

Question 31

What are the types of gated channels

Answer 31

Voltage gated- This is controlled by membrane potential
Ligand gated - This is controlled by ligand binding
Stress activated - This is controlled by mechanical force

Question 32

How do neurons work

What type of channels

Answer 32

Ligand gated and voltage gated

Neuron signals always consist of a change in potential across a membrane

An action potential ‘boosts’ the transmission signal so that membrane potentials are propagated along axons

The action potentials are triggered by local membrane depolarisation to a less negative value–> If this depolarisation is large enough the voltage gates Na+ channels open temporarily
The membranes then become locally positive and then temporarily inactive before closing

The action potentials move without any loss in voltage

Question 33

How do synapses work

Answer 33

These relay electrical signals to chemical signals at the nerve terminal

When an action potential reaches a terminal, synaptic vesicles that store chemical neurotransmitters are released by exocytosis.

This causes the voltage gates Ca2+ channels to open and it enters the cell down the concentration gradient

This causes the vesicles to fuse with the presynaptic membrane
Neurotransmitters diffuse acrosss the cleft and binds to the postsynaptic neurotransmitter receptors –> This induces an action potential

The neurotrasmitter is rapidly removed by enzymatic breakdown or reuptake.

They then go through transmitter gated ion channels

Question 34

Transmitter gates ion channels

Answer 34

These are rapid neurotransmitter receptors which convert chemical signals to electrical signals –> This channel opening is transient and allows ions to move in and establishing an action potential

Question 35

Neurotranmitter responses can be

Answer 35

Excitatory or inhibitory

Question 36

Excitatory neurotransmitter response

Answer 36

Acetylcholine and glutamine allow the passage of NA+ and Ca2+ ions

These are blocked by Curare

Question 37

Inhibitory neurotransmitter response

Answer 37

y-aminobutarate (GABA) and glycine allow the passage of Cl-

This is blocked by strychnine

Question 38

What does an Na+ K+ pump maintain in cells

Answer 38

Osmotic pressure balance

If enough water moved in then cells can burst