What Transport Protein Establishes The Membrane Potential Of An Animal Cell?

Resting Membrane Potential -
Establishment of the Membrane Potential

Our goal is to first sympathise how the membrane potential comes about. Later on, nosotros will discuss the factors that govern the value of the membrane potential. Ii factors are important for the generation of the membrane potential. In fact, these two factors are the minimum essential features that must be present in a system before a membrane potential tin be established. The ii factors are:

1. Asymmetric distribution of ions across the plasma membrane (i.e., ion concentration gradients); and
2. Selective ion channels in the plasma membrane. K⁺, Na⁺, and Cl⁻ channels are the most important aqueduct types for near cells; all the same, there are many cells in which other channels are of import too.

Permit�s see how the membrane potential tin can be established in the first place. After, we will run into how we can calculate and fifty-fifty manipulate the value of the membrane potential. We volition start by because a hypothetical case of ii solution compartments separated by a membrane. This example can later exist extended to empathise how the membrane potential is established in real cells. Assume that we take 2 solution compartments that are separated by a membrane, whose lipid and poly peptide compositions can be altered depending on the unique cases considered (meet Figs. i–3). Because we want to empathize the situation in biological cells, nosotros will designate one compartment as the inside or intracellular compartment (i in Figs. 1–3) and another as the exterior or extracellular compartment (o in Figs. 1–3). These compartments represent the intracellular and extracellular fluid compartments, respectively. In the intracellular compartment (i), we place a solution of 150 mM KCl. In the extracellular compartment (o), nosotros take five mM KCl. Finally, we will set up a simple apparatus to measure the potential difference (membrane potential of V _m) across the membrane that separates the ii compartments.

Here, our focus will be on G⁺ ions, and we have chosen these concentrations to resemble the physiological concentrations of K⁺ in intracellular and extracellular solutions. Besides for the sake of this exercise, we will ignore osmotic forces that are caused by the differences in the concentration of solutes across the membrane. In real cells, the osmolarity of the intracellular and extracellular solutions must, of course, exist the same in order to avoid prison cell swelling or shrinkage. Also, delight call back that in real cells, the majority of intracellular anions are in fact carried on large proteins and, moreover, the extracellular Cl⁻ concentration is much college (~110 mM). However, the KCl solutions used hither have been chosen to simplify the numbers and, as we will see later in this lecture, the statement used here applies directly to real cells.

Effigy 1. Two solution compartments separated by a pure lipid bilayer.

If two solution compartments are separated by a pure lipid bilayer, there can exist no movement of ions across this bilayer from one compartment to another, even if a concentration slope exists between the compartments. Therefore, no separation of charge tin can take place beyond this pure lipid bilayer and, consequently, no voltage difference (i.e., membrane potential) can be established beyond this membrane. Thus, V _m = 0 mV in this scenario.

Allow'due south first consider the state of affairs where the membrane separating the two compartments is a pure lipid bilayer (i.eastward., devoid of membrane proteins) (Fig. 1). In this instance, the voltage difference betwixt the 2 compartments (i.e., the membrane potential) is zero. This is because in each compartment electroneutrality is perfectly obeyed. That is in each compartment, the sum of positive charges equals the sum of negative charges. Although, the KCl concentration is college in the intracellular compartments, no improvidence of ions tin can occur due to the fact that no ionic pathways are present in the membrane for One thousand⁺ and Cl⁻. Remember that ions cannot cross biological membranes without the aid of membrane transport proteins. Therefore, there can exist no accuse separation across the membrane and, therefore, in that location is no potential difference between the two compartments (Fig. ane). Although there is a concentration slope, because in that location are no ionic pathways to let the passage of ions, accuse separation across the membrane cannot be established. Recall that a potential deviation can merely be established if in that location is charge separation across the membrane.

We now innovate large holes in the lipid bilayer that separates the 2 compartments (Fig. two). Assume that the holes are big in bore and not-selective, which will let all solutes to pass through. In this example, both K⁺ and Cl⁻ diffuse from compartment i to o downwardly their respective concentration gradients. In order to maintain electroneutrality, the same number of Yard⁺ and Cl⁻ movement together from i to o. The motility of K⁺ and Cl⁻ from i to o will go along until equilibrium is established. At equilibrium, the KCl concentration volition be the same in both compartments, and will be the algebraic average of the initial values ([KCl]_i = [KCl]_o = 77.5 mM). Once more, electroneutrality is maintained in both compartments. No accuse separation has taken place beyond the lipid bilayer and, thus, no voltage difference exists across the membrane. Information technology is emphasized again that a potential divergence tin can just be established if at that place is accuse separation across the membrane.

Figure 2. 2 solution compartments separated by a membrane that contains non-selective pores.

If two solution compartments are separated past a membrane that contains not-selective pores, each solute diffuses down its own concentration gradient until the solute concentration is the same on both sides (i.due east., until the concentration gradient is dissipated). Because of the non-selective nature of the pores, movement of Thou⁺ from i to o will exist followed and balanced by Cl⁻. Therefore, no separation of charge can take place across this membrane and, consequently, no voltage divergence (i.e., membrane potential) can exist established across this membrane. Thus, V _m = 0 mV in this scenario.

We will at present movement to a scenario that is closer to what is seen in real cells (Fig. 3). If we insert Yard⁺-selective channels in the membrane, because of the concentration gradient, Thou⁺ ions will tend to motility from compartment i to o. Therefore, at that place is a chemical gradient that causes Thousand⁺ ions to move downward their concentration gradient from compartment i to o. Cl⁻ cannot follow because the channels are highly selective for Thousand⁺ and do not allow Cl⁻ to laissez passer. Therefore, movement of K⁺ from i to o leads to accuse separation beyond the membrane. Every K⁺ ion that moves from i to o adds a cyberspace positive accuse to the extracellular side of the membrane and, simultaneously, leaves backside a cyberspace negative charge on the intracellular side of the membrane (Fig. 3A). This accuse separation leads to the generation of an electrical gradient (i.e., electrical field), which forms the basis for the establishment of the membrane potential. In this instance, compartment i is negative with respect to compartment o. Note that the electric gradient did not be initially (prior to accuse separation across the membrane), and only came into existence every bit Yard⁺ diffusion from i to o led to charge separation across the membrane (Fig. 3B). Because of the magnitude of the chemical gradient, K⁺ diffusion from i to o continues leading to excess positive accuse on the extracellular side of the membrane and excess negative accuse on the intracellular side of the membrane. Every bit more 1000⁺ ions diffuse, the size of the electrical gradient increases. Ultimately, a bespeak will be reached when diffusion of K⁺ ions from i to o down its concentration gradient is opposed past the excess positive and negative charges on the membrane (i.east., repelled by backlog positive charge on opposite side of membrane and attracted by backlog negative charge on the same side of the membrane). Thus, at some point the size of the electrical gradient is big enough to exactly residue and stop the net movement of K⁺ from i to o downwards its concentration gradient (Fig. 3B).

Information technology is of import to note that prior to the opening of the Thousand⁺ channels, no potential departure existed between the two compartments (i.eastward., V ₁₀₀₀ = 0 mV). As soon every bit the first One thousand⁺ ion diffuses from i to o, a small membrane potential is established, which grows in size until the magnitude of the electrical slope equals the magnitude of the chemic gradient (Fig. 3, B and C). When the electric slope exactly balances the chemical gradient, K⁺ is said to exist at electrochemical equilibrium. Based on the intracelluar and extracellular concentrations of M⁺ used in this example, the last (i.e., equilibrium) value of the membrane potential is about −ninety mV (intracellular side negative with respect to the extracellular side). In the adjacent two sections of this lecture, we will learn how to calculate the value of the membrane potential.

To summarize, while G⁺ is diffusing from i to o down its concentration gradient, a negative membrane potential is existence established that favors the memory of K⁺ in compartment i. Therefore, ii forces are now interim on K⁺:

Figure 3. Ii solution compartments separated by a membrane that contains potassium (G⁺) channels.

( A ) If two solution compartments are separated past a membrane that contains selective ion channels for only one ion species (in this case, K⁺ channels), the ion (Yard⁺) can improvidence downward its concentration gradient from one compartment to the other (i to o). The counterion (Cl⁻) cannot follow because no selective channels be for this ion. Thus, the net movement of K⁺ from i to o leads to the addition of internet positive charge to compartment o and leaves behind cyberspace negative charge in compartment i. This leads to accuse separation and, hence, a voltage potential difference beyond the membrane. ( B ) As K⁺ diffuses from i to o, an electrical slope (voltage potential difference) is established across the plasma membrane that grows in size until information technology exactly balances the K⁺ chemical slope (i.e., concentration gradient). When the chemical and electrical gradients are equal in size, the ion is said to be in electrochemical equilibrium, and the membrane potential established is the equilibrium potential (V _eq.) for the ion. ( C ) In the organization described to a higher place, opening of K⁺ channels, leads to the establishment of a membrane potential, which grows in size until information technology reaches the Thousand⁺ equilibrium potential (Five _Chiliad). The magnitude of V _M depends on the K⁺ concentration gradient across the membrane. In this scenario, based on the intracellular (150 mM) and extracellular (5 mM) concentrations of One thousand⁺, 5 _m = Five _K = −90 mV.

1. Chemical gradient, which arises from the concentration difference between the two compartments; and
2. Electrical slope, which initially does not exist, simply is established as presently equally K⁺ begins to move from compartment i to o, and gets larger and larger every bit more and more K⁺ ions diffuse down their concentration gradient from compartment i to o. Therefore, it can exist seen that the larger the chemical slope (i.eastward., concentration gradient) across the membrane, the larger the resulting electrical gradient will exist in order to balance the diffusion of the ion from the compartment with high concentration to the compartment with low concentration.

At some betoken, a large enough electrical gradient will be established such that it volition completely balance the chemical gradient driving Chiliad⁺ from compartment i to o. At this indicate, One thousand⁺ is said to be in thermodynamic equilibrium. The membrane potential that is reached at the thermodynamic equilibrium is called the thermodynamic equilibrium potential (V _eq.) for Thou⁺. The equilibrium potential is the potential at which the chemic and the electrical forces acting on K⁺ ions exactly balance one another. At equilibrium, there is no internet movement of Thou⁺ between the two compartments. Because we are considering K⁺, this potential is referred to as the Yard⁺ equilibrium potential, and is designated Five _Thou.

The higher up example considered a situation where there were just G⁺-selective channels in the membrane. As a upshot, the established membrane potential (Five _m) is at the Thou⁺ equilibrium potential (V _M). Nosotros can follow a similar logic and ascertain the equilibrium potential for Na⁺ (V _Na), Cl⁻ (5 _Cl), Ca²⁺ (V _Ca), etc. The important point to recollect is that the membrane potential would only be at the equilibrium potential for any given ion if there are selective channels in the membrane only for that detail ion. Of course, the channels must be open to allow the passage of the ion. In real cells, the situation is more complicated because many different types of ions channels exist in the membrane. We will consider this case in a later department of this lecture (meet In Real Cells, Multiple Ions Contribute to the Membrane Potential).

While we have focused on a hypothetical system in which only K⁺ ions contribute to the membrane potential, we will run into shortly that Thousand⁺ also plays the dominant role in most cells (with smaller contributions past Cl⁻ and Na⁺ ions). Before we discuss the state of affairs in real cells, our next goal is to gain a quantitative understanding of the value of the membrane potential.

Posted: Saturday, Feb 15, 2014

Source: https://www.physiologyweb.com/lecture_notes/resting_membrane_potential/resting_membrane_potential_establishment_of_the_membrane_potential.html

Posted by: keaslertheraid.blogspot.com

Share this post