Membrane transport 1 Flashcards
Cell membranes act as barriers how
Allow a few solutes to pass by diffusion (O2 and CO2)
The vast majority cannot and rely on membrane transport proteins
What are the two major classes of transport proteins
Carrier proteins
Channel proteins
Channel Proteins
Bind to a solute molecule on one side of the membrane and deliver it to the other by a conformational change
Channel proteins
Form small hydrophilic pores and solutes pass through by diffusion.
These are also called ion channels
How do membranes transport
The membrane proteins transport by lining the cell membrane enabling protection for hydrophilic molecules
How do channel proteins discriminate
On the basis of Size and Charge
Carrier proteins specifically bind solute molecules –> selection is by specific binding
Passive transport
Selective
Also known as facilitated diffusion. This arises when solute moves spontaneously across the membrane with a concentration gradient
Active transport
This involves solute movement across a concentraton gradient and requires energy
What is the most studied system across membranes
Glucose across the gut layer.
The Glucose carrier
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
Transporting uncharged molecules
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
Electrochemical gradient
Combination of chemical concentration gradient and membrane potential –> Controls the direction the solute moves in
Membrane potential
The difference in the electric potential between the inside and outsides of the cell
Passive transport
This arises when the solute moves spontaneously across a membrane along the electrochemical gradient –> along a favourable direction
Active transport
This involves the solute moving against the electrochemical gradient and requires energy being inputted.
This is travelling in the unfavourable direction
How does active transport gain its energy
This obtains energy in three different ways
- ATP
- Light
- Couples
Passive transport of charged molecules
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,
How does active transport move solutes against the electrochemical gradient
- Coupled Pumps
Couple uphill transport of one solute with the downhill transport of another. One example of this is a sodium-potassium pump. - ATP Driven Pumps
Couples uphill transport to ATP hydrolysis - Light Driven Pumps
Couples uphill transport to light input ( bacteria and plants )
Types of Transport
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
Symport
Inward flow of sodium plays an important role in animal cells to drive other molecules in
e.g. glucose in the gut
Active Uniport
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
Antiport
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
How is pH maintained in cells
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.
What are some of the differences between carrier proteins and ion channels
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
Resting membrane potential
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
What is the resting membrane potential in steady state
-20 -200mV
How do you calculate the Resting membrane potential
The Nerst Equation
v=62log10(C0/C1)
What influences resting membrane potential
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.
Measuring Ion channel activity
Electrical recordings can be made by PATCH CLAMP recording
What is patch clamp recording
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.
What are the types of gated channels
Voltage gated- This is controlled by membrane potential
Ligand gated - This is controlled by ligand binding
Stress activated - This is controlled by mechanical force
How do neurons work
What type of channels
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
How do synapses work
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
Transmitter gates ion channels
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
Neurotranmitter responses can be
Excitatory or inhibitory
Excitatory neurotransmitter response
Acetylcholine and glutamine allow the passage of NA+ and Ca2+ ions
These are blocked by Curare
Inhibitory neurotransmitter response
y-aminobutarate (GABA) and glycine allow the passage of Cl-
This is blocked by strychnine
What does an Na+ K+ pump maintain in cells
Osmotic pressure balance
If enough water moved in then cells can burst
