Membrane Transport of Ions: Videos & Practice Problems

Membrane transport involves the movement of ions down their electrochemical gradients, influenced by both chemical and electrical gradients. The transmembrane potential, or ΔΨ, represents the charge difference across a cell membrane, typically negative inside. Five types of ion channels facilitate this transport: leakage channels (always open), ligand-gated channels (open with extracellular ligands), signal-gated channels (open with intracellular signals), voltage-gated channels (open with transmembrane potential changes), and mechanically gated channels (open with physical stimuli). Understanding these mechanisms is crucial for grasping cellular function and signaling.

Membrane Transport of Ions

Hey, guys. We're going to quickly revisit our map of the lesson on membrane transport just to get a better idea of what we've covered and where we're headed. And so, of course, we know that we're exploring this map by following the leftmost branches. So we've talked about molecular transport of small molecules, specifically, passive transport, distinguishing simple from facilitated passive transport. And then we talked about carriers and transporters and specific types of carriers and transporters, including the erythrocyte glucose uniporter GLUT 1 and the erythrocyte chloride bicarbonate antiporter. And so now we're going to explore a new branch here talking more about these points and channels, and we're going to build up the knowledge that we need to understand the 5 types of ion channels that we have here on our map. And so, now that we have a better idea of where we're headed, I'll see you guys in our next video.

Membrane Transport of Ions

In this video, we're going to begin talking about membrane transport of ions. And really the main takeaway of this video is that charged ions flow down their electrochemical gradients. And we'll talk about exactly what that means very shortly. Now the direction that charged ions diffuse across membranes actually depends on two different factors: Number 1, it depends on the actual charge of the ion, whether or not it is positively or negatively charged; and Number 2, it depends on the electrochemical gradient as we've already mentioned. But what exactly is this electrochemical gradient?

Well, the electrochemical gradient is actually based on two different roots, and so you can see that it has this electro prefix referring to electrical, and then it also has this chemical root as well. And so the electrochemical gradient is really just a combination and a balance of the following two gradients: The first gradient is the chemical gradient, and the second gradient is the electrical gradient. And so what you can see down below in our image is that if you take the chemical gradient and add the electrical gradient right here, what you'll get is the electrochemical gradient.

And so let's talk a little bit more about each of these gradients to distinguish them. The chemical gradient, as you guys are already familiar with, is the standard chemical concentration gradient that we usually refer to; it is referring to a difference in chemical concentration between two different regions, where one region would either have a higher or a lower chemical concentration than the other. Recall that chemicals have this natural tendency to flow down their chemical concentration gradients from areas of high chemical concentration down to areas of low chemical concentration, and they will continue to do that until they reach chemical equilibrium, which would mean that the chemical concentrations in both areas are equal. If we take a look at this part of our image down below, it's dedicated to the chemical gradient, and what you'll notice is we have a membrane separating these two different regions, the left region from the right region. The left region has a high chemical concentration of this blue substance, and the natural tendency is for this substance to diffuse down its chemical concentration gradient from an area of high concentration to an area of low concentration, as indicated by this blue arrow.

But how about this electrical gradient? How does this work? The electrical gradient is different from the chemical gradient. The electrical gradient is not based on a difference in chemical concentration between two regions; it's based on a difference in the sum of electrical charges between two regions where one region would have a different charge than another region, a different net charge. And so charged ions are going to respond to the electrical gradient whereas uncharged ions won't respond to the electrical gradient. And so charged ions, they will specifically flow towards the oppositely charged regions, and they will continue to do that until they reach electrical equilibrium instead of chemical equilibrium. Electrical equilibrium is established when the net charge of that region is equal to 0. Essentially, the charges are balanced out between these regions. If we take a look at our image down below right here, notice this part of our image is dedicated to the electrical gradient. The first thing to notice here is that we have our membrane, and we have two different regions. We have this left side which is a positively charged region with a net positive charge, and then we have this right side of the membrane which has a net negative charge. And so again, only charged ions are going to respond to the electrical gradient. Here we have a positively charged ion and it's going to respond to this electrical gradient by flowing towards the oppositely charged region. And of course, positively charged molecules cannot cross the membrane through simple diffusion; they have to cross the membrane through facilitated diffusion, which is why we have this membrane protein right here.

And again, because the electrochemical gradient is a combination of these two gradients, it's really just a balance of these two different forces. So, you can see that we have this balance right here; we have the chemical gradient on one end and the electrical gradient on the other. Charged ions are going to respond to both of these gradients, whereas uncharged ions only respond to the chemical gradient. And so in some cases, the chemical gradient will overpower the electrical gradient, and in other cases, the electrical gradient will overpower the chemical gradient. And really, the electrochemical gradient is a balance of the two.

This here concludes our introduction to how charged ions flow down their electrochemical gradients, and we'll be able to talk more and more about this as we move along in our course. But for now, this concludes this video, and I'll see you guys in our next one.

Membrane Transport of Ions

In this video, we're going to introduce the transmembrane potential. The transmembrane potential is also commonly referred to as the transmembrane voltage. It can be abbreviated either as ΔΨ or as V_M, depending on your textbook. Moving forward in our clutch prep biochemistry course, we're going to use ΔΨ to represent the transmembrane potential. The transmembrane potential or transmembrane voltage is defined as the difference in electrical charge between the inside and the outside of a cell's plasma membrane.

If we take a look at our image on the left-hand side, notice that we're showing you two different ways to represent the transmembrane potential ΔΨ. The delta symbol, the Greek symbol Δ, indicates the final value minus the initial value. Delta Ψ will differ depending on what we're calling the final side of the membrane and what we're calling the initial side of the cell's plasma membrane. If we consider the inside of the cell as the final side (ψ_in) and the outside of the cell as the initial side (ψ_out), then the transmembrane potential ΔΨ will have a negative value. Conversely, if the outside of the cell is considered the final side (ψ_out) and the inside as the initial side (ψ_in), then ΔΨ will have a positive sign.

In textbooks and by professors, the transmembrane potential or voltage is usually presented from the relative position of the inside of a cell's membrane, calling the final side of the membrane the inside of the cell. Thus, the transmembrane potential is generally expressed in units of volts or millivolts (V or mV). Typically, the inside of cells is more negative relative to the outside of the cell, which is more positive. This means that the transmembrane potential is usually presented as a negative value.

Examples of resting transmembrane potential, such as -70 millivolts, are often seen in textbooks when discussing the resting membrane potential for neurons. On the right image, notice we have a cell's plasma membrane. It's important to note that the inside of the cell is more negative with respect to the outside of the cell. This means that when we change what we're calling the final side and the initial side, it changes the overall sign of the transmembrane potential.

If the transmembrane potential is negative, we're looking at it from the perspective of the inside of a membrane (ψ_in minus ψ_out). If it's a positive value, then that means we're looking at it from the perspective of the outside of the membrane subtracting the inside (ψ_out minus ψ_in). When the transmembrane potential does not equal a value of 0, it establishes opposite electrical gradients for cations and anions. Anions are negatively charged and will diffuse towards the oppositely charged region on the outside of the cell, which is more positive. Cations have an opposite electrical gradient and will diffuse towards the negative charge inside the cell.

The main takeaway from this video is that the transmembrane potential can be expressed either as a negative value or as a positive value depending on what we're calling the final and initial sides of the membrane. Usually, it is associated with a negative value. When the transmembrane potential is not equal to 0, it establishes electrical gradients for anions and cations in opposite directions. This concludes our introduction to the transmembrane potential, and as we move forward in our course, we'll be able to apply the transmembrane potential in different scenarios. I'll see you guys in our next video.

Membrane Transport of Ions

So now that we know a little bit about the membrane transport of ions in terms of charged ions diffusing down their electrochemical gradients and in terms of charged ions diffusing across a membrane with respect to the transmembrane potential, we're now going to move on and talk about the 5 types of ion channels that allow ions to passively diffuse across a membrane. And those 5 ion channels include the ion channels that are listed here on our map. And so again, we'll talk about these 5 ion channels in our next lesson video. So I'll see you guys there.

Membrane Transport of Ions

In this video, we're going to talk about the different types of ion channels. And so, as their name implies, ion channels will selectively and passively transport very specific ions, or charged atoms such as sodium ions, potassium ions, and chloride ions across a membrane. And again, they will do this in a passive manner, which means that no energy is required. And so, there are 5 types of ion channels that you guys should know, which we're going to describe down below, and of course, the numbers for each of these ion channels correspond with the numbers that you see down below in our image. And so the very first type of ion channel that you guys should know is the leakage ion channel. And as its name implies, the leakage ion channel remains open, which means that it's always going to allow the leakage of ions down their electrochemical gradients. And so if we take a look at this part of our image over here on the left-hand side, notice that we're showing you an example of a leakage ion channel, which is this orange structure that you see here embedded in the membrane. And this ion channel is specific to potassium ions, K⁺ here. And because the leakage ion channels are always open, they're always going to allow the potassium ion here to diffuse across the membrane down its electrochemical gradient. And really the biggest takeaway here of the leakage ion channel is that they always remain open.

Now moving on to the other types of ion channels, 2, 3, 4, and 5, what's important to note is that these are all gated ion channels, and the gated part is pretty much exactly what it sounds like. They have pretty much a gate that can open and close to allow the ions to diffuse across and then also to block the ions from diffusing across. And so what's important to note about these gated ion channels, 2, 3, 4, and 5, is that the gated ion channels here will all open and close in response to various stimuli, and we'll talk about those stimuli here very shortly. And so the second type of ion channel that you guys should know is the ligand-gated ion channel. The ligand-gated ion channel is going to open and close due to regulation by an extracellular ligand molecule. And so what you'll notice is, if we take a look at the ligand-gated ion channel down below here, on the left-hand side, it is in its closed port version. And so this closed version does not allow the transport of ions across the membrane. However, the ligand-gated ion channel will open in response to an extracellular ligand. And so when this ligand molecule binds to the ligand-gated ion channel, as it is over here, it will open up the gate so that the ion is actually able to cross the membrane down its electrochemical gradient. And so again, ligand-gated ion channels respond to extracellular ligands to open up.

Now the 3rd type of ion channel that you guys should know is the signal-gated ion channel, which is pretty similar to the ligand-gated ion channel, except there's one difference. And that is that these will open and close due to regulation by an intracellular signaling molecule. And so what you'll notice with the signal-gated ion channel here is that again it has its closed version over here that does not allow the transport of ions across the membrane. But, of course, once a signal molecule, an intracellular signal molecule binds to it, it will open up to allow the transport of the ion across the membrane down its electrochemical gradient. And so really the biggest difference between the ligand-gated and the signal-gated is that the ligand-gated is responding to an extracellular ligand, whereas the signal-gated is responding to an intracellular signal.

Now moving on to the 4th type of ion channel that you guys should know, it is the voltage-gated ion channel. And so the voltage-gated ion channel is going to open and close due to changes in the transmembrane potential, or the transmembrane voltage, Δψ. And so if we take a look at number 4 down below, the voltage-gated ion channels, notice that it has its closed version over here on the left-hand side that will not allow the transport of ions across the membrane. And that is going to be closed at a very specific transmembrane potential. Here, for example, a transmembrane potential of -50 millivolts. But, of course, when that transmembrane potential changes to a different value, such as, for example, -70 millivolts, then the voltage-gated ion channel can open up so that the ion is able to be transported across the membrane down its electrochemical gradient.

The 5th type of ion channel that you guys should know is the mechanically gated ion channel, and the mechanically gated ion channel is going to open and close due to a mechanical stimulation, such as, for example, touch, sound, or pressure. And so if we take a look at ion channel number 5 down below, the mechanically gated ion channel. Again, notice on the left-hand side, it has its closed version that will not allow the transport of ions across the membrane. But, of course, the mechanically gated ion channel will respond to a mechanical stimulation such as, for example, sound. And so sound could cause this mechanically gated ion channel to open up so that the transport of ions is possible across the membrane. And so really this here concludes our lesson on the 5 different types of ion channels and as we move forward in our course, we'll be able to get some practice applying these concepts. So I'll see you guys in our next video.

Facilitated diffusion of charged ions across a biological membrane is __________________:

• A. Generally irreversible.
• B. Endergonic.
• C. Driven directly by ATP.
• D. Not specific with respect to the type of ion.
• E. Driven by a difference in the electrochemical gradient.

Answer: (E)

Which of the following statements is false about a signal-gated ion channel receptor?

• A. They are present in the cell membrane.
• B. They respond to the presence of intracellular signaling molecules.
• C. Differences in membrane potential can affect whether the channel receptors are open or close.
• D. They are a type of gated-ion-channel that can open and close under different conditions.

Answer: (C)

The voltage-gated potassium channels associated with an action potential provide an example of what type of membrane transport?

• A. Simple diffusion.
• B. Facilitated diffusion.
• C. Coupled transport.
• D. Primary active transport.
• E. Secondary active transport.

Answer: (B)

Here’s what students ask on this topic:

What is the electrochemical gradient and how does it influence ion transport across membranes?

The electrochemical gradient is a combination of two gradients: the chemical gradient and the electrical gradient. The chemical gradient refers to the difference in the concentration of ions across a membrane, while the electrical gradient refers to the difference in charge across the membrane. Together, these gradients influence the movement of ions. Ions will move down their electrochemical gradient, meaning they will move from areas of high concentration to low concentration (chemical gradient) and from areas of like charge to opposite charge (electrical gradient). This movement is crucial for various cellular processes, including nerve impulse transmission and muscle contraction.

What is the transmembrane potential and how is it measured?

The transmembrane potential, also known as transmembrane voltage (ΔΨ or V_M), is the difference in electrical charge between the inside and outside of a cell's plasma membrane. It is typically measured in volts (V) or millivolts (mV). The transmembrane potential is calculated as the difference between the electrical potential inside the cell (Ψ_in) and the electrical potential outside the cell (Ψ_out). In most cells, the inside is more negative compared to the outside, resulting in a negative transmembrane potential. This potential is crucial for the function of excitable cells like neurons and muscle cells.

What are the different types of ion channels and how do they function?

There are five main types of ion channels: leakage channels, ligand-gated channels, signal-gated channels, voltage-gated channels, and mechanically gated channels. Leakage channels are always open, allowing ions to passively diffuse down their electrochemical gradients. Ligand-gated channels open in response to extracellular ligands binding to them. Signal-gated channels open in response to intracellular signaling molecules. Voltage-gated channels open and close in response to changes in the transmembrane potential. Mechanically gated channels open in response to physical stimuli such as touch, sound, or pressure. Each type of channel plays a specific role in cellular signaling and function.

How do chemical and electrical gradients contribute to the electrochemical gradient?

The electrochemical gradient is the sum of the chemical and electrical gradients. The chemical gradient is created by differences in ion concentration across a membrane, causing ions to move from high to low concentration areas. The electrical gradient is created by differences in charge across the membrane, causing ions to move towards regions of opposite charge. Together, these gradients drive the movement of ions across membranes, influencing processes such as nutrient uptake, waste removal, and signal transmission in neurons. The balance between these gradients determines the direction and rate of ion flow.

What role do ion channels play in maintaining the transmembrane potential?

Ion channels are crucial for maintaining the transmembrane potential by regulating the flow of ions across the cell membrane. Leakage channels allow ions to passively diffuse, helping to maintain a steady state. Voltage-gated channels respond to changes in the transmembrane potential, opening or closing to adjust ion flow and stabilize the membrane potential. Ligand-gated and signal-gated channels respond to specific molecules, allowing for precise control of ion movement. Mechanically gated channels respond to physical stimuli, contributing to the dynamic regulation of the membrane potential. Together, these channels ensure the proper function of excitable cells like neurons and muscle cells.