Electrochemical gradients and the membrane potential. Primary and secondary active transport. Na+/K+ pump.
Passive transport is a great strategy for moving molecules into or out of a cell. It's cheap, it's easy, and all the cell has to do is sit there and let the molecules diffuse in. But...it also doesn't work in every situation. For instance, suppose the sugar glucose is more concentrated inside of a cell than outside. If the cell needs more sugar in to meet its metabolic needs, how can it get that sugar in?
Here, the cell can't import glucose for free using diffusion, because the natural tendency of the glucose will be to diffuse out rather than flowing in. Instead, the cell must bring in more glucose molecules via active transport. In active transport, unlike passive transport, the cell expends energy (for example, in the form of ATP) to move a substance against its concentration gradient.
Here, we’ll look in more detail at gradients of molecules that exist across cell membranes, how they can help or hinder transport, and how active transport mechanisms allow molecules to move against their gradients.
We have already discussed simple concentration gradients, in which a substance is found in different concentrations over a region of space or on opposite sides of a membrane. However, because atoms and molecules can form ions and carry positive or negative electrical charges, there may also be an electrical gradient, or difference in charge, across a plasma membrane. In fact, living cells typically have what’s called a membrane potential, an electrical potential difference (voltage) across their cell membrane.
Image depicting the charge and ion distribution across the membrane of a typical cell. Overall, there are more positive charges on the outside of the membrane than on the inside. The concentration of sodium ions is lower inside the cell than in the extracellular fluid, while the reverse is true for potassium ions.
An electrical potential difference exists whenever there is a net separation of charges in space. In the case of a cell, positive and negative charges are separated by the barrier of the cell membrane, with the inside of the cell having extra negative charges relative to the outside. The membrane potential of a typical cell is -40 to -80 millivolts, with the minus sign meaning that inside of the cell is more negative than the outside. The cell actively maintains this membrane potential, and we’ll see how it forms in the section on the sodium-potassium pump (below).
As an example of how the membrane potential can affect ion movement, let’s look at sodium and potassium ions. In general, the inside of a cell has a higher concentration of potassium (K) and a lower concentration of sodium (Na) than the extracellular fluid around it.
- If sodium ions are outside of a cell, they will tend to move into the cell based on both their concentration gradient (the lower concentration of Na in the cell) and the voltage across the membrane (the more negative charge on the inside of the membrane).
- Because K is positive, the voltage across the membrane will encourage its movement into the cell, but its concentration gradient will tend to drive it out of the cell (towards the region of lower concentration). The final concentrations of potassium on the two sides of the membrane will be a balance between these opposing forces.
The combination of concentration gradient and voltage that affects an ion’s movement is called the electrochemical gradient.
Active transport: moving against a gradient
To move substances against a concentration or electrochemical gradient, a cell must use energy. Active transport mechanisms do just this, expending energy (often in the form of ATP) to maintain the right concentrations of ions and molecules in living cells. In fact, cells spend much of the energy they harvest in metabolism to keep their active transport processes running. For instance, most of a red blood cell’s energy is used to maintain internal sodium and potassium levels that differ from those of the surrounding environment.
Active transport mechanisms can be divided into two categories. Primary active transport directly uses a source of chemical energy (e.g., ATP) to move molecules across a membrane against their gradient. Secondary active transport (cotransport), on the other hand, uses an electrochemical gradient – generated by active transport – as an energy source to move molecules against their gradient, and thus does not directly require a chemical source of energy such as ATP. We’ll look at each type of active transport in greater detail below.
Primary active transport
One of the most important pumps in animal cells is the sodium-potassium pump, which moves Na out of cells, and K into them. Because the transport process uses ATP as an energy source, it is considered an example of primary active transport.
Not only does the sodium-potassium pump maintain correct concentrations of Na and K in living cells, but it also plays a major role in generating the voltage across the cell membrane in animal cells. Pumps like this, which are involved in the establishment and maintenance of membrane voltages, are known as electrogenic pumps. The primary electrogenic pump in plants is one that pumps hydrogen ions (H) rather than sodium and potassium.
The sodium-potassium pump cycle
Figure showing the transport cycle of the sodium-potassium pump.
The sodium-potassium pump transports sodium out of and potassium into the cell in a repeating cycle of conformational (shape) changes. In each cycle, three sodium ions exit the cell, while two potassium ions enter. This process takes place in the following steps:
- To begin, the pump is open to the inside of the cell. In this form, the pump really likes to bind (has a high affinity for) sodium ions, and will take up three of them.
- When the sodium ions bind, they trigger the pump to hydrolyze (break down) ATP. One phosphate group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product.
- Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer likes to bind to sodium ions (has a low affinity for them), so the three sodium ions are released outside the cell.
- In its outward-facing form, the pump switches allegiances and now really likes to bind to (has a high affinity for) potassium ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2.
- With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell.
- In its inward-facing shape, the pump loses its interest in (has a low affinity for) potassium ions, so the two potassium ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.
This may seem like a complicated cycle, but it just involves the protein going back and forth between two forms: an inward-facing form with high affinity for sodium (and low affinity for potassium) and an outward-facing form with high affinity for potassium (and low affinity for sodium). The protein can be toggled back and forth between these forms by the addition or removal of a phosphate group, which is in turn controlled by the binding of the ions to be transported.
How the sodium-potassium pump generates a membrane potential
How, exactly, does the sodium-potassium pump establish a voltage across the membrane? It’s tempting to simply make an argument based on stoichiometry: for every three ions of sodium that move out, only two ions of potassium move in, resulting in a more negative cell interior. While this charge ratio does make the cell’s interior slightly more negative, it actually accounts for only a tiny fraction of the sodium-potassium pump’s effect on membrane potential.
Instead, the sodium-potassium pump acts primarily by building up a high concentration of potassium ions inside the cell, which makes potassium’s concentration gradient very steep. The gradient is steep enough that potassium ions will move out of the cell (via channels), despite a growing negative charge on the interior. This process continues until the voltage across the membrane is large enough to counterbalance potassium’s concentration gradient. At this balance point, the inside of the membrane is negative relative to the outside. This voltage will be maintained as long as K concentration in the cell stays high, but will disappear if K stops being imported.
For more explanation of how the voltage across the membrane is established, take a look at the membrane potential article in the neurobiology section.
Secondary active transport
The electrochemical gradients set up by primary active transport store energy, which can be released as the ions move back down their gradients. Secondary active transport uses the energy stored in these gradients to move other substances against their own gradients.
As an example, let's suppose we have a high concentration of sodium ions in the extracellular space (thanks to the hard work of the sodium-potassium pump). If a route such as a channel or carrier protein is open, sodium ions will move down their concentration gradient and return to the interior of the cell.
In secondary active transport, the movement of the sodium ions down their gradient is coupled to the uphill transport of other substances by a shared carrier protein (a cotransporter). For instance, in the figure below, a carrier protein lets sodium ions move down their gradient, but simultaneously brings a glucose molecule up its gradient and into the cell. The carrier protein uses the energy of the sodium gradient to drive the transport of glucose molecules.
Diagram of a sodium-glucose cotransporter, which uses the energy stored in a sodium ion gradient to transport glucose "uphill" against its gradient. The cotransporter accomplishes this by physically coupling the transport of glucose to the movement of sodium ions down their concentration gradient.
In secondary active transport, the two molecules being transported may move either in the same direction (i.e., both into the cell), or in opposite directions (i.e., one into and one out of the cell). When they move in the same direction, the protein that transports them is called a symporter, while if they move in opposite directions, the protein is called an antiporter.
Simple diagram of a symporter (carrying two molecules in the same direction) and an antiporter (carrying two molecules in opposite directions).
Want to join the conversation?
1. How is the energy from the sodium ions transferred to the sodium/glucose symporter?
2. Does the energy from the sodium ions increase the affinity of the symporter to actively collect glucose? or does the symporter simply wait for a glucose to fall into place in its own time?
3. Can the symporter be activated by two sodiums without a glucose present? if not, does the symporter store energy from sodium ions until a glucose arrives? or do the sodium ions and glucose need to interact with the symporter simultaneously?(21 votes)
- 1) The "sodium/glucose symporter", known as SGLT (sodium glucose linked transporter), receives the energy needed to perform its transport from the electrochemical gradient established by the sodium/potassium pump. Because of the difference in sodium concentration (between the inside and outside of the cell) and due to the electrical potential difference (between the inside and outside of the cell), the SGLT is able to perform its function.
2) The affinity of the symporter to collect glucose is not dependent on the sodium concentration, but rather the concentration of glucose (you can imagine there would be a higher probability of glucose being able to cross the system if more glucose was near the system). The purpose of the sodium ions is to establish the electrochemical gradient which provides the energy for the SGLT to perform its function.
3) The exact mechanism for the transporter is not fully understood - the diagram, which indicates 2 sodium ions and 1 glucose ion, is meant for gaining a conceptual understanding of the system. If you're curious about the topic, I have linked an article and a video below. SGLT is typically seen in action in the intestines and kidneys.
- how does the antiporter provide energy for molecules to go in the "opposite" direction? in a symporter it makes sense that the flow of one ion should cause other molecules to go along with it, however in an antiporter, what causes the other molecule to move up its conc. gradient?(15 votes)
- In an antiporter it uses the energy of the ions flowing down their concentration gradient. Imagine this just like a hydroelectric power station uses the energy of water flowing down the waterfall to create electricity. This electricity can do work.
In case of the cell the energy is used directly to move the other molecule out of the cell against it´s concentration gradient.(9 votes)
- if there are more Potassium ions in a cell, how is the cell negatively charged? shouldn't it be positively charged? this is in relation to the electrochemical gradient section. thank you(8 votes)
- There are certainly more numbers of potassium ions in the cells, but remember that there are greater numbers of sodium ions outside the cell (since for every three sodium ions moved out of the cells, two potassium ions enter the cell), therefore, resulting in a net positive charge outside the cell and net negative charge inside the cell.(7 votes)
- what is the main difference between active transport using a carrier protein, and passive transport using facilitated diffusion?(3 votes)
- Main difference: In facilitated diffusion, the solute moves down the concentration gradient, from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. This process does not require energy. Conversely, in active transport, the solute moves against the concentration gradient, from regions of lower to higher concentration. This process requires some form of chemical energy.(12 votes)
- Hi, I'm just a little curious, why can't 3 potassium ions be pumped out and 2 sodium ions pumped in(the other way around)? Can't that serve the same role?(4 votes)
- No, because the potassium is highly concentrated in the cell, and so that's where the molecules must go(based on active transport going low concentration to high concentration). And sodium is highly concentrated outside of the cell, so that's where it goes during active transport considering it is low to high concentration.
Hope this helped:)(9 votes)
- why wouldnt cells use atp for primary active transport of glucose directly, why go through a second step?(4 votes)
- Because it is faster to utilize already available ATP through the electrochemical gradient. Also, it is spatially more available than in the case of ATP.(4 votes)
- i dont quite understand the secondary active transport. how does the carrier protein "harness" energy from another molecules concentration gradient? does the glucose molecule just randomly attach itself to the protein when it's about to transport a sodium ion to the other side?(4 votes)
- If for example there are alot of sodium ions outside, their positive charges will repel and like atoms don't like being near eachother so they flow to the other side if there's a way to. Active transport works by using a phosphate group from ATP to change the shape of the protein and thus do work.(2 votes)
- Are these pumps negatively charge since they have a high affinity for the positive ions? when it releases the ions the pump no longer has a affinity for the molecules, is this because the energy used to change the conformation of the pump knocks off electrons or does the phosphates make the pump more negative due to the negative charges on the phosphates itself?(4 votes)
- Great question — since proteins are very large and complex they almost always have regions with different characteristics — including patches that have negative or positive charges.
Since the residue being phosphorylated is an aspartate (a negative charged amino acid), it is unlikely to be as simple as that directly effecting the binding of the cations.
The exact region where the cations bind in the two forms has been identified and it seems likely that the shape changes associated with phosphorylation disrupt/recreate regions that almost perfectly fit the two different ions.
That being said — as far as I can tell the exact molecular mechanism for the sodium-potassium pump is still not completely clear, but very sophisticated molecular simulations are being used to figure this out:
- Why does the diffusion of the solute down its concentration gradient release energy?(2 votes)
- Because a solute moving down its concentration gradient is spontaneous. Spontaneous activity releases energy, non-spontaneous activity (like moving something against its concentration gradient) requires energy.(4 votes)
- what's the difference between the primary and secondary active transport?(2 votes)
- Primary active transport uses energy that comes directly from ATP hydrolysis. Secondary active transport uses the energy from another chemical gradient that was itself created through active transport and ATP hydrolysis.(3 votes)