Membrane Transport 2 Flashcards
Describe the general characteristics of ion channels.
- integral, polytopic membrane proteins
- mediate facilitated passive transport
- transport 1000x faster than pumps
- highly selective and specific for ions
- responds to physical and chemical changes in their environment and undergo precisely timed conformational changes
- opening and closing determines membrane potential
What are the three types of ion channels and what do they respond to?
- Voltage gated: respond to change in membrane potential
- Chemically gated: respond to ligands such as neurotransmitters
- Mechanically gated: responds to mechanical stimuli like vibrations, pressure, stretch, touch, sounds, tastes, smells, heat, volume, and vision
What domains make up the pore in channels?
S5 to S6
What domains make up the voltage sensor? What domain speicifically senses?
S1-S4
S4 is specific
Describe how the selectivity filter works in the potassium channels.
As you go into the cavity, the channel becomes narrower
Potassium with water is too big to fit through the cavity so the potassium gets dehydrated
The dehydrated form of potassium is able to fit through and form covalent bonds with the channel (carbony bonds)
TVGYG signature sequence
Describe the bio energetic basis for ion selectivity in the potassium channel and why sodium will not go through channel.
The potassium going through the channel takes a lot of energy but the resolvation energy is greater than the desolvation energy and is therefore still energetically favorable
For sodium the resolvation energy is a lot less than the desolvation energy and is therefore not energy favorable for the cell
How does the potassium channel have rapid transport?
The potassium since it has like charges repel each other once they get into the selectivity filter and therefore propel each other down the channel
Describe the ball and chain model.
When channel is closed then the S4 domain closes the pore with a little lever (inactive domain) and the ball is not concluding the channel
When the channel is open, the S4 domain opens the pore and the liver is parallel with the membrane allowing ions to flow through, the ball is attached to the chain but not occluding the hole
In the inactivated occluded state, the S4 domain may be open but the ball is plugging the hole and therefore the channel is blocked
Describe an acetylcholine receptor.
- ligand is acetylcholine
- receptor is present on post-synaptic side
- binding of Ach opens the channel
- channel is equally permeable to Na+ and K+
- Pentamer of 4 kinds of spanning subunits that form a ring with a pore through the membrane (alpha2, beta, gamma, and epsilon)
What enzyme is responsible for generating Neurotransmission?
Na+/K+ - ATPase
Describes the steps of an action potential.
- Resting membrane potential (-60 mV)
- Depolarizing stimulus
- Na+ channels open
- Rapid Na+ entry- further depolarization
- Na+ channels close, K+ open
- K+ move out
- Membrane hyper polarization
- K+ channels close
- Cells return to resting potential
Describe what happens when an action potential reaches the synaptic terminal.
- Action potential reaches presynaptic terminal
- Depolarization opens up the VGCC
- Calcium enters cell
- Causes fusion of synaptic vesicles
- Exocytosis-release of NT in synaptic cleft
- NT bind to its receptor on post-synaptic side
- Opens up a ligand-gated channel
What is the importance of gap junctions.
Provide cell to cell communication and passages between contiguous cells
Describe some properties of gap junctions.
- Small hydrophilic molecules and ions can pass through
- sugars, amino acids,and nucleotides can pass through
-Proteins, polysaccharides and nucleic acids cannot pass through
What is the structure of a gap junction?
Each channel is made of 12 molecules of Connexin
Six Connexin form a half channel (Hexagonally shaped) called a connexon
Two connexon join end to end in the intercellular space to form a functional channel.
