Active transport is the movement of molecules from a lower concentration to a higher concentration. This process is vital for living organisms and is important for the following reasons:
(1) Absorption of most nutrients from the intestine, (2)Rapid and selective absorption of nutrients by cells, (3)Maintaining a membrane potential, and (4)Maintaining water and ionic balance between cells and extracellular fluids.
Basic Types of Active Transport
Primary Active Transport
Primary active transport uses a direct source of chemical energy – for example, ATP – in order to move the molecules across their gradient and across a membrane. Because the energy source of the transport process comes from ATP, it is considered primary active transport.
The sodium-potassium pump is a very important pump in animal cells, and it moves NA+ out of the cells and K+ into the cells. This sodium-potassium pump maintains the right concentrations of NA+ and K+ in living cells; however, it plays a very important role in generating the right amount of voltage across the cell membrane in animal cells as well.
These types of pumps are involved in maintaining and establishing membrane voltages and they are also known as electrogenic pumps. A plant’s primary electrogenic pump pumps hydrogen ions (H+) rather than sodium and potassium.
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The Cycle of the Sodium-Potassium Pump
A pump that transports sodium out of the cell and potassium into the cell, it consists of a cycle that repeats itself and involves conformational, or shape, changes. With each of the cycles, three sodium ions leave the cell and two potassium ions enter the cell. This is a multi-step process that can be explained in the following six steps.
- For the first step, the pump is open all the way to the inside of the cell because in this form, the pump likes to bind, i.e., has an affinity for, sodium ions and in fact, will take up three of them.
- The sodium ions will bond and trigger the pump to break down, or hydrolyze, the ATP. One of the phosphate groups from ATP then attaches itself to the pump and ADP is released as a by-product.
- The pump changes shape because of the phosphorylation process and it then re-orients itself in order to open up towards the extracellular space. During the conformation, the pump doesn’t like to bind anymore to sodium ions, having a low affinity for them, which results in the three sodium ions being released outside of the cell.
- The pump in its outward-facing form switches its allegiances and it now likes binding to or has a high affinity for, the potassium ions. In fact, it binds two of them, which starts the process of removing the phosphate group that is attached to the pump in step two above.
- The phosphate group is now gone and therefore the pump changes back to its original form. It now opens towards the cell’s interior.
- The pump is now facing inwards and has lost its interest in, or has a low affinity for, the potassium irons. Because of this, the two potassium ions get released into the cytoplasm. The pump is then back to where it was in step one above, which means that the cycle now repeats itself.
This cycle may seem complex but it merely consists of protein going back and forth between two different forms: first, an inward-facing form that has a low affinity for potassium and a high affinity for sodium; and second, an outward-facing form with a low affinity for sodium and a high affinity for potassium.
The protein can go back and forth from the different forms simply by adding or removing a phosphate group. This, in turn, is controlled by the ions that need to be transported being bound.
Generation of a Membrane Potential from the Sodium-Potassium Pump
By now, most people wonder how this pump establishes the voltage across the membrane. You can make an argument based on stoichiometry; in other words, every time that three ions of sodium move out, only two potassium ions move in, which results in a cell interior that is more on the negative side.
This type of charge ratio makes the interior of the cell a little more negative but in reality, it only accounts for a tiny part of the pump’s effect on membrane potential.
In fact, a sodium-potassium pump acts mainly to build up a high concentration of potassium ions on the inside of the cell. This makes the concentration gradient of the potassium very steep. The gradient is so steep that the potassium ions move out of the cell via channels even with the interior’s growing negative charge.
The process continues until the voltage found across the membrane is big enough to counterbalance the concentration gradient of the potassium. When this balance occurs, the membrane’s inside is negative as related to the outside.
As long as the K+ concentration in the cell remains high, the voltage is maintained. However, if K+ stops being imported, it will remain high.
Secondary Active Transport
Also called cotransport, secondary active transport uses an electrochemical gradient as the energy source in order for the molecules to move against their gradient. Generated by active transport, this type of transport does not directly require an energy source such as ATP.
It stores energy through its electrochemical gradients that are set up by the primary active transport process itself and this can be released as the ions start to back down their gradients. This type of active transport also uses the gradients’ stored energy in order to move other materials against their own gradients.
Here is another way to look at it. Let’s say that we have sodium ions at a high concentration located in the extracellular space because of the sodium-potassium pump’s hard work. If a certain route, for instance, a carrier protein or a channel, open up, these sodium ions move down their concentration gradient and then return to the cell’s interior.
With secondary active transport, the sodium ions move down their gradient and are coupled with the other substances’ uphill transport, facilitated by a shared carrier protein. This carrier protein at this point is called a cotransporter.
In the figure below, the carrier protein allows the sodium ions to move down their gradient; however, they also simultaneously bring a glucose molecule up their gradient and into the cell. The carrier protein utilizes the sodium gradient’s energy to drive the transport of glucose molecules.
In secondary active transport, two molecules are transported and move either in the same direction. For example, they both move into the cell or in totally opposite directions, such as if one of them goes into the cell and one of them goes out of the cell.
If they move in the same direction, the symporter is the protein that transports them. The protein is called the antiporter when they move in opposite directions.
Three Main Types of Active Transport
Sodium Potassium Pump
This pump is actually a structure called a cell membrane pump and it uses energy to transport potassium and sodium ions in and out of a cell. The cell membrane pump has numerous variations but the sodium-potassium pump plays the biggest part in maintaining the homeostasis of a cell.
The pump uses the molecule ATP, or advancing triphosphate, for its power and this ATP allows the pump’s shape to change. Its contents are then emptied either into or out of the cell using the following five steps:
- Inside the cell, three sodium ions form and bind to the pump.
- Phosphate inside a molecule of ATP binds to the pump.
- The shape of the pump changes and outside of the cell, the sodium ions are released.
- Two potassium ions then bind to the pump.
- The pump releases the phosphate group and then changes shape again, finally releasing the ions into the cell’s interior.
In endocytosis, the cells absorb large, solid particles and then deposit them into a cell. Membrane-bound sacs form that pinch off from the cell membrane, which is how this happens. This process is often used to bring large particles, including glucose, into the cell.
The process is also used by white blood cells to ingest bacteria or viruses and then digest them in their lysosomes. Endocytosis comes in two sub-categories: pinocytosis, which brings several types of liquid into the cell; and phagocytosis, which transports solids such as large particles into the cell.
Exocytosis is sort of the opposite of endocytosis because it deposits materials to the outside of the cell from the inside instead of the other way around. Vesicles are then formed and are filled with the materials that are going to be sent outside of the cell. They then fuse with the cell membrane and release the contents outside of the cell.
Examples of Active Transport
This happens when plants’ root hair cells take in mineral ions and when humans take in glucose through the intestines. Substances moving from areas with low concentrations to areas with high concentrations is a good example of active transport. Generally, the substance is one that the cells need for sustenance, for example, ions, amino acids, or glucose.
Main Differences Between Primary and Secondary Active Transport
The difference between the two types of active transports depends on whether the transporter uses energy either directly or indirectly. In primary active transport, the ATP is used directly, which means that the energy comes from a high-energy phosphate bond being broken.
In secondary active transport, the ATP is not used directly and the energy comes from a gradient that was made by a primary active transport system that just happened to use ATP.
Active transport always refers to the moving of molecules across the cell membrane but against the concentration gradient. It is assisted by enzymes and uses cellular energy for the process to work. There are two main types of active transport and the difference lies in where the energy comes from when the molecules are transported.
In primary active transport, the breakdown of ATP is what causes the molecules to transport while in secondary active transport, the energy comes from one molecule’s concentration gradient. There are other differences, of course, but these are the major differences and the main ways to identify each of the transport types.
Check out our article “10 Different Types of Protists”
Glossary Terms Related to Active Transport
When materials move into and out of a cell that requires energy, is usually in the form of APT.
ATP (advancing triphosphate)
This is the compound that stores the energy that is released during the respiration of cells.
This refers to the random collision and motion of molecules in a solution; it was observed by the Scottish scientist Robert Brown in the year 1827, hence its name.
This is a protein that lines the cytoplasmic side of coated bits and is very fibrous.
These pits reside in plasma membranes and contain receptor proteins. They originate from vesicles that contain large essential substances during receptor-mediated endocytosis.
If you look at the difference between concentration that is high and a concentration that is low, this is an example of a concentration gradient.
This refers to materials’ movement from an area that has a high concentration to an area that has a low concentration.
This term describes the movement and collision of particles that are continuous and have no concentration changes.
This refers to materials’ movement into a cell through vesicles that are formed from the membrane’s plasma.
This refers to materials’ movement out of a cell through vesicles that are formed from the membrane’s plasma.
In this process, certain molecules diffuse across a membrane’s plasma as opposed to across a membrane’s plasma via transport proteins.
These are organic molecules made up of a carbon chain and one or more carboxyl (COOH) groups.
Fluid – Mosaic Model
This is a model made to explain the components and properties of a plasma membrane. These membranes always include a phospholipid bolster that has several types of protein embedded into it.
These are merely lipid molecules to attach to simple sugars.
Glycoproteins are protein molecules that simple sugars are attached to.
Homeostasis is the maintenance of a relatively constant environment within a cell even though there are fluctuations in the cell’s environmental surroundings.
This describes a water-loving compound that tends to form bonds made out of hydrogen and that is therefore easy to dissolve when placed in water.
Hypertonic solution is a solution whereby the concentration of dissolved substances is much greater than that of another solution.
Hypotonic solution is a solution whereby the concentration of dissolved substances is much less than that of another solution.
These are plasma membranes that can pump ions into and out of cells against a concentration gradient.
Isotonic solution is a solution whereby the concentration of dissolved substances is equal to that of another solution.
This is the movement of water across a semi-permeable barrier from an area with higher water potential to an area that has a lower water potential.
Osmotic pressure is defined as the change in pressure resulting from the water flow in osmosis. This pressure is equal to water’s tendency to enter a solution because of a concentration gradient.
This refers to the movement of materials when there is no energy used.
Phagocytosis is when bacteria or other materials are engulfed by cells.
A phosphate group is a molecule consisting of four oxygen atoms bound to a central phosphorus atom.
This is defined as a lipid molecule that consists of a phosphate group containing a hydrophilic head linked to two hydrophobic fatty acid tails. It is the fundamental component of a plasma membrane.
Pinocytosis results when liquid is engulfed by wrapping a membrane around a droplet so that it is taken into the cell.
The plasma membrane is the bilayer of protein molecules and phospholipid that surrounds the cytoplasm of cells.
Plasmolysis is the shrinking of cytoplasm in a cell resulting from the loss of water through osmosis to a hypertonic solution in which the cell is placed.
This is the process of endocytosis that enables the cell to swallow large molecules such as hormones and large proteins. The process involves the binding of large molecules to receptors in coated pits.
This term refers to the property of a plasma membrane that allows some molecules to pass freely through the membrane when other molecules can’t.
Transport proteins are proteins in the plasma membrane that allow materials to pass to and from a cell either by active transport or facilitated diffusion.
Turgor pressure refers to the internal pressure of a plant cell.