cell bio exam case study

When nerve cells are at rest, there is an unequal amount of positive and negative charges on either side of a nerve cell membrane. This charge difference creates an electrical potential. Describe how the resting membrane potential (resting potential) is generated.

Resting membrane potential: When a neuron is not sending any signals, it is said to be in rest. There is unequal distribution of ions across the cytoplasmic membrane due to the action of ion specific transporters. The inside of the neuron is negative relative to the outside. When the neuron is at rest, K+ ions can cross through the membrane, while Cl- and Na+ ions are not readily permeable. The Na+/K+ pump creates the ion concentration gradient by moving 3 Na+ out of the cell while moving 2K+ into the cell, against concentration gradient. Due to this, more sodium ions accumulate outside the cell leaving the cell inside more negative. This difference in the ion concentration creates a potential, called membrane potential. The resting membrane potential (of a neuron) is -70 mV.. which means that the inside of the neuron is 70 mV less than the outside.

Describe the function of the voltage-gated sodium ion channel. In your description explain what is meant by channel gating and channel inactivation.

1. Voltage gated Na channel means that these channels are sensitive to changes in the membrane potential. 2. Voltage gated Na channels lead to DEPOLARIZATION phase of an action potential. Thus it helps in conduction of action potential. 3. Voltage gated Na channels are present in the Na/ K pumps . These pumps are present in the cell membranes which enables a cell to maintain to maintain it's VOLUME. 4. Voltage gated Na channels help in the conduction of an impulse at neuromuscular junction. 5. Voltage Na channel has two gates OPENING and a CLOSING gate. These gates are quick to open and close. When both the gates close , it leads to INACTIVATION which means that the DEPOLARIZATION phase comes to an end no matter how strong a stimulus is.

What is the Kd? What does it tell you about the binding of TTX to the Na- Channel?

Binding affinity is a strength of binding interaction. Binding affinity is measured by the equillibruim dissociation constant (Kd). The smaller the Kd value greater the binding affinty. Binding affinity is influenced by noncovalent intermolecular interactions like hydrogen bonding, electrostatic interactions. TTX mimics the hydrated Na cation enters the mouth of the Na- channel peptide complex binds to a peptide glutamate side group and further tightens it. The TTX and Na channel binding site is extremely tight.(Kd= 10 -10nm).

Explain what is meant by an electrostatic interaction between two molecules. From what you know about these interactions, would you guess that the TTX effects on the channel are reversible or irreversible? Explain your answer

Electrostatic interaction is a interaction between or among anions and cations. These can be reversible or irreversible. The TTX irreversibly binds to the channel mimicing the hydrated sodium ions, blocks entry of Na+ ions.

Describe the structure of glutamate. How do you think guanidinium group of TTX and glutamate in the channel become involve in the binding of TTX to the channel?

Glutamate is a non-essential aminoacid naturally occuring in L- form. The molecualr formula of glutamate is C5H8NO4-. IUPAC name is (2R)- 2-amino-5-hydroxy-5-oxopentonoate. TTX is neurotoxin, blocks voltage gated Na+ channels on the surface of nerve membranes. TTX contain positively charged guanidium group that contains 3 nitrogen atoms and pyrimidine ring which contains hydroxyl groups. These hydroxyl groups help in tighting the bond between TTX and cahnnel. The binding of TTX to the Na channels involves bond between positively charged guanidino group of TTX and negatively charged carboxyl group of gluatamate present at the mouth of Na+ channel.

Describe the structure of a voltage-gated sodium ion channel.

It's a specific protein embedded in the plasma membrane which trigger by Na+ ion. This consists of four homologous domain. Voltage gated sodium ion channel open/close in response to change in membrane potential.

Explain why sodium ions need channels in order to move into and out of cells. describe the process by which this transport occurs.

Sodium need channels to move into cell because if cell will let every ion to move into it then it will become toxic. In order to prevent this nerve cells regulated the entry of ions via ion gated channels. Another reason is that sodium cannot cross the cell via simple diffusion,it needs to be facilitated via channels. There is a concentration gradient always maintained in a cell that is equal amount of pottassium and sodium in and out of the cell but when the amount of sodium in the extracellular space increases it causes the intracellular space to become less concentrated thus forming a concentration gradient which provides the potential for opening of the channels and there movement in the cell.

What would happen to a neuron if it was exposed to tetrodotoxin? Be specific regarding its effect on the ability of a neuron to communicate.

Tetrodotoxin specifically blocks the Na + ion channel and the flow of Na+ ions. Tetrodotoxin is very large in size than Na+ that acts as a plug to the channel via binding through +ve charged guanidino group on the tetrodotoxin to the -ve charged COOH- groups on side chains in the mouth of the channel. As we know that, if Na+ is blocked there is chance membrane depolarization for stimulus.

Explain how a conformation change in the channel complex might lead to tighter interactions between the TTX and the channel.

The confirmational changes in the channel complex might lesd to tighter interactions between the TTX and the channel. Confirmations changes leads to smaller the dissociation constant so binding affinity increases. So the bond becomes tightens.

What is happening to the electrical potential of a neuron when it generates an action potential? What is the function of the action potential in neurons?

The transfer of information in the neuron is through electrical signal. A booster system like action potential is required to conduct electrical signal in neuron. The electrical signal occurs by electrochemical caused by electrically charged ions. Neurons surrounded by semipermeable membrane contains charged protein molecules acts as a voltage gated ion transport between inside and outside the neuron cell. When the neuron is not sending information it is at resting membrane potential is measured as -70mV. At rest K+ ions pass through the ion channel in the membrane easily whereas Cl- and Na+ are difficult to pass through. Action potential causes different ions change across the membrane and travel down the axon. When the three molecules of sodium moves inside the neuron and two molecules of potassium comes out of the neuron causes depolarization and vice versa in repolarization. At stimulus, the membrane potential rises above the threshold potential -55 mV and when the stimulus applied, the action potential rises to peak + 40mV and drops to - 90mV and then finally reaches the resting membrane potential -70mV. Thus the action potential transmits the electrical signal in the neuron.

7. Describe how a sodium ion enters a voltage-gated sodium ion channel. How does this channel act selectively for this ion?

When the cell membrane is at its resting membrane potential,the activation gates of the voltage gated sodium ion channnels are closed and inactivation gates are open.Voltage gated potassium ion channels are closed. Depolarization is caused when a stimulus makes the membrane potential to become more positive, leading to the opening of voltage gated sodium ion channels.On reaching the threshold many sodium channels are opened. Sodium ions move across the membrane causing depolarizaion.Voltage gated potassium ion channels also begin to open, but very slowly. Therefore depolarization takes place because more sodium ions diffuse into the cell than potassium ions diffuse out of it.As the membrane potential approaches maximum depolarization, the inactivation gates of the voltage gated sodium ion channels start closing and diffusion of sodium ions starts closing. Potassium ion channels remain open and potassium ions continue to diffuse out of the cell. The extra efflux of potassium ions causes the membrane potential to become slightly more negative than the resting value. After the potassium ions close the sodium and potassium ions establish resting membrane potential. The voltage gated sodium ion channels consists of 4 subunits each having 6 transmembrane helices. Once activated these helices open the pore. Two of the six helices are separated by a loop that lines the pore and is responsible for the selective movement of sodium ions across the membrane.

Describe the role of sodium ions and sodium channels in the action potential?

When the membrane is polarized it has membrane potential of -70 to -90 mV. It is due to potassium ion. When a stimulus is given to axonal membrane, it's permeability for ion changes. And it becomes permeable for sodium ions as the sodium gated channels get open. Sodium ions influx starts from extra cellular fluid into axoplasm. Which cause increase in the membrane potential from -70 to + 35 mV causing membrane to depolarize. At this time membrane is completely impermeable for anions.