Transport: How Electrifying Flashcards
Lecture 18
What are the 3 ways the free energy of gradients can be utilized?
- Co-transport - Up gradient transport of other molecules (symporters, antiporters)
- Production of electrical signals
- Chemiosmotic coupling
What are the 3 ways potential energy is stored?
- High energy bonds - ATP, GTP (phosphoanhydride bonds cleaved, release energy)
- Concentration gradients (high to low concentration of ions can drive the movement of other molecules)
- Charge gradients (electric potential
Which is easier, for Na+ to leak inside the cell or for K+ to leak outside the cell?
There is an open leak channel for K+ ions to exit the cell.
How many K+ ions are pumped into the cell per ATP by the sodium-potassium pump?
2 K+
How many Na+ ions are pumped out of the cell per ATP by the sodium-potassium pump?
3 Na+
Why is the plasma membrane permeable to potassium, but not sodium?
There is a potassium leak channel in the PM that is always open and moves K+ down its concentration and electrochemical gradient.
What is membrane potential? What establishes it?
Membrane potential is an electric gradient or gradient of charge.
MP is established by the conductance of charge via ion transport across a membrane (separation of negative charges on one side and positive charges on other side of a membrane).
Is the concentration gradient and the membrane potential of potassium across the plasma membrane allied or opposing?
Opposing. K+ is flowing down the concentration gradient via the potassium leak channel, but the membrane potential moves in the opposite direction because only the positive K+ charges are moving past the membrane. The positive charges want to rejoin the remaining negatively charged macromolecules, in opposition to their concentration gradient.
Why do only positive ions leak across the membrane?
Because K+ ions are smaller than the large macromolecules with negative charges (proteins and nucleic acids).
Membranes act as capacitors. What are capacitors?
How thick are biological membranes? What electrochemical implication does this have?
4-6 nm. Membrane thinness means negative and positive charges can sense excess charges on other side of membranes, causing them to line up.
What is the resting (equilibrium) membrane potential? Why?
-60 to -200 mV. Cells at rest usually have an excess of negative ions with an excess of positive ions outside the plasma membrane.
Why is the resting membrane potential negative (-60 to -200 mV)?
Due to K+ leaking (diffusing) out of the cell, leaving negative charges inside the cell. This resting potential is maintained by resting K+ channels.
What is the charge of the bulk solution of cytoplasm around the cell?
Neutral, no charge due to an even mix between positive and negative charges
How to measure the separation of charge across a membrane?
Put an electrode inside the cell and another outside of the cell. Measure the voltage difference across the membrane with a potentiometer.
Which two factors influence the membrane potential?
- electrochemical gradient
- rate at which ions are conducted across the membrane
Where is the calcium concentration greatest, inside or outside the cell?
3-4 orders of magnitude greater outside of the cell
Where is the sodium concentration greatest, inside or outside the cell?
An order of magnitude greater outside the cell
Where is the potassium concentration greatest, inside or outside the cell?
inside the cell
Where is the synapse?
the gap between the pre-synaptic and post-synaptic cell
What kind of signal moves through a synpase?
chemical signal
What kind of signals are processed by the neurons?
electrical signals
Describe the process of cell signaling between neurons.
- Depolarization event in first neuron. An influx of positive charges in the cell causes the internal voltage of the first neuron to become positive.
- Step 1 activates a voltage-gated calcium channel, causing calcium to enter the cell.
- Calcium influx triggers regulated exocytosis; The calcium binds to proteins associated with the 20 neurosecretory vesicles that are already filled with neurotransmitters and docked at the plasma membrane via SNARE proteins. After binding, the vesicles move to the end of the neuron and release the acetylcholine neurotransmitters.
- Acetylcholine binds to a ligand-gated sodium channel protein on the postsynaptic neuron, causing it to open.
- After Step 4, sodium rushes into the cell, but does not cause a massive depolarization in the cell. The resting membrane potential begins to increase, having a big local effect (but a small broader effect).
- After the small influx of Na+ ions, local depolarization causes voltage-gated sodium channels to open, resulting in a massive influx of Na_ ions and a change in membrane potential from -60 mV to +40 mV.
- Repolarization
- Hyperpolarization
- Return to normalcy
What is a depolarization event?
when the membrane potential of a neuron becomes less negative
Which voltage-gated channels are open during the resting state?
Neither.
Describe the state of the voltage-gated channels during depolarization.
Na+ channels are open and K+ channels are closed.
Describe the state of the voltage-gated channels during repolarization.
Na+ channels are inactivated and K+ channels are open.
Describe the state of the voltage-gated channels during hyperpolarization.
Na+ channels are closed and K+ channels are open; ends when the K+ channel closes and ions move passively
What is a hyperpolarization event?
when the membrane potential of a neuron becomes more negative
What causes the voltage-gated Na+ channel to open?
initial localized change in membrane potential across the post-synaptic membrane
What causes the ligand-gated channel to open?
when a particular molecule binds to the channel
What causes the initial localized change across the post-synaptic membrane that ultimately causes the voltage-gated Na+ channel to open?
The acetylcholine neurotransmitter is released from the secretory vesicles and binds to the receptor protein (ligand-gated channel), causing it to open, allowing a small influx of Na+ ions.
Describe how voltage-gated Na+ channels work.
- Initial depolarization due to influx of Na+ ions causes a conformational change in voltage-sensing alpha helices, resulting in the opening of the channel in the span of less than 0.1 ms.
- After the channel opens, the voltage-sensing alpha helices return to a resting position and the channel is inactivated with a hydrophobic plug over the span of 0.5-1.0 ms.
- The inactive Na+ channel experiences a refractory period.
- The membrane is repolarized, the channel-inactivating segment is displaced, and the gate closes over the span of several ms.
How does the membrane potential change after the depolarization event at the voltage-gated Na+ channel?
-60 mV -> +40 mV
What keeps the voltage-gated sodium channels from remaining open indefinitely? What is necessary to open the channels?
Occluded by hydrophobic amino acids within the structure of the channel; to open, must move the amino acids out of the way by changing the charge of the membrane (from negative to positive), resulting in the alpha helices changing conformation
What happens to a voltage-gated sodium channel when a membrane changes charge from negative to positive?
The alpha helices change conformation, resulting in the removal of the hydrophobic plug. This allows sodium ions to flood in from the outside environment.
How many hydrophobic plugs are there in the voltage-gated sodium channel?
two; after the first is removed, a smaller plug remains
What happens if the membrane potential near a voltage-gated sodium channel rapidly changes for a second time?
Due to the additional hydrophobic plug, the channel will not immediately reopen. It has a refractory period and must reset and experience another voltage change before it can open again.
How much time does the initial depolarization, movement of voltage-sensing alpha helices, and the opening of a voltage-gated sodium channel take?
less than 0.1 ms
How long does it take for the voltage-sensing alpha helices to return to a resting position while the channel is inactive?
0.5-1.0 ms
When the voltage-gated sodium channels open, is there a sodium influx or efflux in the cell?
Influx within 3-4 ms.
Why is there a loss of charge from the cell when Na+ moves in during a membrane depolarization event?
Because K+ ions move out of the cell.
After a membrane depolarization event due to the voltage-gated Na+ channels allowing an influx of Na+ ions, what causes the membrane potential to return to a resting state (repolarization)?
Voltage-gated K+ channels allow K+ ions to leave the cell.
Why does the change in potential only move in one direction along the length of a cell?
Because the voltage-gated channels have refractory periods. After they’ve already been fired, they close, so sodium doesn’t continue coming inside temporarily.
What are the two types of glia that wrap axons with insulating myelin?
Oligodendrocytes (CNS) and Schwann cells (PNS)
Standard electrical measurement of neurons measures changes in overall cellular voltage, which reflects the opening and closing of many channels at any given time. What technique allows the measurement of ion flow through a single ion channel. How does it work?
Patch clamp electrophysiology AKA single-channel recording.
Lets you measure flow of ions and manipulate different ion concentrations.
Take a tiny tube or syringe (micropipette) and place it onto one channel. Suction causes a tight seal to form between the pipette and the plasma membrane, allowing a patch of membrane (with one or a few ion channels) to be sealed off from the surrounding medium. Allows manipulation of the current to those individual channels (in vivo or in vitro).
During the experiment, an amplifier maintains voltage across the membrane with an electronic feedback circuit, keeping the cell at a fixed membrane potential by injecting current as needed. The voltage clamp measures tiny changes in current flow from the patch pipette.
What does Botox do to receptors of neurons?
Once internalized, the reducing environment inside the endosome cleaves the disulfide bond between the two chains, allowing the light chain to enter the cytoplasm. The light chain, now in an activated form, can interact with proteins found in the presynaptic nerve terminal. The catalytic light chain of each of the various toxins cleaves the v- and t-SNARE proteins. The cleavage of one or more of these proteins means that the machinery necessary for vesicle fusion is no longer present. This results in inhibition of acetylcholine release into the synaptic cleft. If the neuron innervates a muscle, prevention of acetylcholine release in turn blocks muscle contraction.
What are dendrites?
extensions to neurons that receive signals and combine them with signals received from other neurons
What are axons?
extensions to neurons that conduct signals, sometimes over long distances
How are voltage-gated potassium and sodium channels structurally different? How are they similar?
Voltage-gated potassium channels are multimeric - have four separate protein subunits that form a central pore in the membrane
Voltage-gated sodium channels are monomeric proteins (single large polypeptide) with four separate domains
Each domain in the sodium channel is similar to one of the subunits of the potassium channel.
In both kinds of channels, each subunit or domain contains 6 transmembrane alpha helices.
What are the points of contact between neurons?
Synaptic boutons of the axons of a transmitting neuron and the dendrites of a receiving neuron
How do action potential move due to myelination of axons?
They jump from node to node instead of via a steady ripple along the membrane.
Describe an electrical synapse.
The presynaptic neuron is connected to the postsynaptic neuron via gap junctions. As ions move back and forth between the two cells, the depolarization in one cell spreads passively to the connected cell.
Describe a chemical synapse.
The presynaptic and postsynaptic neurons are connected by cell adhesion proteins (NOT gap junctions). The pre and post-synaptic membranes are divided by the synaptic cleft (20-50 nm).
How do neurotransmitters work?
An action potential causes the neurotransmitter to be secreted into and diffuse across the synaptic cleft. The neurotransmitter molecules bind to receptors (ligand-gated ion channel) in the postsynaptic neuron and are converted back into electrical signals. This either stimulates or inhibits the production of an action potential in the postsynaptic neuron.
