Sodium Ions Cannot Diffuse Passively Through The Plasma Membrane

1. Due to their charged nature, sodium ions cannot diffuse passively through the plasma membrane, which is a lipid bilayer. The sodium ion and its cloud of polarized water are capable of interacting with the polar hydrophilic heads of the phospholipids but not with the hydrophobic tails, so they could only cross the plasma membrane though ions channels, integral membrane proteins that form ion-conducting pores in the lipid bilayer, or pumps. The size of the pore and they way it interacts with ions, gives the channel its ion selectivity, so they allow only one ion to pass through. The sodium ion channels are voltage gated channels, meaning they response to voltage changes across the membrane. The ion channel detects an electric stimulus, it opens to the extracellular space allowing sodium ions to pass though the membrane by simple diffusion. Ion channels allow ions to move passively, no ATP is needed for their transport; however ATP is used to open and close the channels.

2.

The sodium ion channel is a single polypeptide chain made up of more than 1800 amino acids. It’s comprised of 4 domains and each domain has 6 transmembrane a helices segments, (S1-S6) contiguous with each other, folded into a cluster with the channel pore being in the center. The S4 segment is known to act as a voltage sensor. Both the N-terminus and C-terminus are inside the cytoplasm. The N-terminus protrudes into the cytosol and forms an inactivating particle that is on the inside of the cell. The inactivation particle sticks on the channel acting like a plug that prevents the ions from constantly moving though, so the inactivating particle only gets removed when there’s a change in

potential.
3. Sodium channels since they are voltage gated, they open and close in response to some electric stimulus, a phenomenon known as channel gating. The S4 one of the transmembrane segments acts as a voltage sensor, meaning it detects changes in membrane potential allowing the channel to open. Once opened, sodium ions are allowed to pass inside the cell, increasing the membrane’s voltage, what is known as depolarization. At some point, when Na+ concentration is high enough, the channel inactivates itself by the binging of the inactivating particle. Once the inactivating particle binds to the channel, no more ions can pass though,(channel inactivation) so as a result only a certain amount of ions are able to pass though when the channel is open. This is also important because as action potential travels down the axon, this prevents the channel from reopening up and sending the signal to opposite direction. It is essential to keep the channels closed for certain period of time so that sodium ions are not continuously allowed to cross the membrane.
4. The difference in anion and cation concentration between the cytosol and the extracellular fluid when the neuron is at rest, results in a differential charge, called resting membrane potential. At rest, there is a negative charge inside the cytosol because of the large negatively charged molecules such as RNA and proteins, which are trapped on the cell, and a positive charge outside the cell. The resting membrane potential is a combined result of simple diffusion of ions through their concentration gradients, the permeability of the membrane itself and the electrical attraction between anions and cations. Plasma membrane is more permeable to potassium ions than any other ions, that’s why the potassium concentration is grater on inside on the cell and less on the outside, while the opposite is true for sodium ions.
5. Action potential refers to the change of membrane potential when an impulse, an electrical signal is being transported form the cell body down the axon of the neuron. When a neuron receives an electric stimulus, that signal must be enough to change the membrane potential to a critical value, the threshold potential, which it is the minimum needed for voltage-gated channels to open and the signal to be transmitted across the axon. Once threshold value is reached, the neuron starts firing now an action potential, by opening many voltage gated sodium ion channels more quickly and letting sodium ions to diffuse down their concentration gradient, while opening potassium ion channels more slowly. This results in depolarization of the membrane as the inside now has a positive charge, but as the rising potential reaches 0 mV, sodium ion channels are inactivated and begin to close. By the time they all closed, membrane has reached its peak potential and the K+ channels are fully open, so K+ repelled by the negative cytosol, exit the cell, repolarizing the