Passive diffusion and active transport are modes of transfer by which substances (ions, water and other molecules, etc.) enter and leave the cell across the cell membrane. Although both are involved in the movement of substances across the membrane, the mechanism by which the movement is accomplished differs between the two.
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Passive diffusion is a type of diffusion characterized by the movement of matter in the direction of the concentration gradient without the input of energy. Basically from an area of higher concentration to an area of lower concentration.
The following are some examples of passive diffusion:
Examples of passive diffusion
Osmosis
One of the best examples of passive diffusion is osmosis. Essentially, osmosis refers to the movement of a solvent (e.g. water) from an area of low solute concentration to an area of higher solute concentration through a membrane.
In a biological system, a semipermeable membrane separates the extracellular matrix from theZytoplasma. Here, the concentration of solutes within the cell, as well as in the extracellular matrix surrounding the cell, determines the direction in which the solvent moves.
illustration 1
*The arrows in Fig. 1 represent the direction of the solvent (water).
passive diffusion; How a higher solute concentration (represented by large red dots) on one side of the membrane affects solvent movement. Photo credit: MicroscopeMaster.com
Using the graph above as an example, you can see how higher solute concentration (represented by large red dots) on one side of the membrane affects solvent movement.
In general, the osmotic potential of pure water is zero. However, when solutes (e.g. sodium ions) are added, the osmotic potential of water is reduced and becomes slightly negative.
When solutes cannot pass through the membrane (diffusion), water is forced to move from the higher osmotic potential area to the lower osmotic potential area of water.
In Fig. 1, there is more solute on the right side of the membrane than on the left.
Because of this, the solvent on the right side of the membrane has a lower osmotic potential than the solvent on the left. Because of this osmotic gradient, water molecules are forced to move from the higher osmotic potential side to the lower osmotic potential side.
*In plants and animals, as wellmicroorganismsOsmosis is an important mechanism by which cells absorb water. The massive flow of water also supports the transport of small dissolved substances and dissolved nutrients into the cell (e.g. into the roots of plants).
In animals, the mechanism also plays an important role in water retention. For example, osmosis transports fluid from the renal tubules and gastrointestinal tract into the capillaries. This process prevents excess water from being excreted in the urine.
As mentioned earlier, water molecules easily pass through thecell membraneby osmosis. Being a form of passive diffusion, this mechanism does not require energy input nor is it heavily dependent on integral proteins involved in the transport of ions and larger polar molecules.
However, it's worth noting here that water molecules have a polarity that would affect their movement through the phospholipid bilayer, which has a hydrophobic region. Compared to other polar molecules, water molecules are small, which allows them to easily pass through the lipid layer.
Both in vitro and in vivo, the relative concentration of solutes (in a solution or in the extracellular matrix) has a direct impact on the direction and rate at which water enters and leaves the cell.
In a hypotonic solution, water enters the cell because the extracellular fluid or solution has lower osmolarity compared to the cytoplasm. Essentially, this means that the cytoplasm has a higher concentration of solutes compared to the extracellular matrix or solution. An isotonic solution, on the other hand, has the same osmolarity as the cell.
In this scenario, there is no osmotic gradient affecting a net movement of water in or out of the cell. Instead, water molecules enter and exit the cell at nearly the same rate.
Finally, in a hypertonic solution (or extracellular fluid under hypertonic conditions), high osmolarity conditions outside the cell force water molecules out of the cell. As a result, the cells begin to shrink.
For example, if the soil contains a very high salt concentration (compared to the salt concentration in a plant's root cells), water will be forced to move out of the root cells. As a result, the plant gradually withers and eventually dies.
*Although water can cross the lipid bilayer, it is also transported through channels known as aquaporin channels.
Factors affecting the movement of water in or out of the cell generally include:
- size of water molecules
- osmotischer Gradient
easy diffusion
Simple diffusion is also a type of passive diffusion. A good example of simple diffusion here is the movement of small lipophilic molecules across the cell membrane.
Essentially, lipophilic molecules include molecules that can dissolve in lipids, fats, steroid hormones, and various non-polar solvents. These molecules are not only lipophilic (lipid lovers) but also non-polar. As such, they can readily diffuse across the membrane on a concentration gradient.
This type of movement has also proven itself when transporting lipophilic drugs. Since some of the drugs tend to be weak organic acids or bases that are in unionized form, they can easily diffuse through the cell membrane to enter the cell.
In addition to being non-ionized and fat-soluble, the concentration gradient (high concentration of molecules outside the cell compared to inside the cell) forces these molecules to easily enter the cell.
Studies have shown that smaller drug molecules penetrate the membrane faster than larger ones. This is an example of passive diffusion, no energy is used to transport the molecules across the cell membrane.
In summary, some of the factors affecting the movement of these lipophilic molecules across the cell membrane are:
- molecular size
- Ionization State: Ionized molecules have low solubility.
- be lipophilic
- concentration gradient
*Facilitated diffusion is also considered a type of passive diffusion. Although it does not require energy (similar to other types of passive diffusion), facilitated diffusion relies on integral transmembrane proteins located in the cell membrane to transport substances in and out of the cell along their concentration gradient.
Active transportation
In contrast to passive diffusion (and even facilitated diffusion), where molecules move along a concentration gradient, active transport involves the movement of molecules against a concentration gradient.
So this means that molecules have to move from an area with low concentration of molecules to an area with high concentration. Thus, active transport can be said to prevent diffusion since it prevents molecules or ions from moving down their concentration gradient.
In neurons, for example, active transport prevents sodium and potassium ions from moving down their concentration gradient, thereby propagating the action potential (electrical signal).
There are two main types of active transport, including:
Primary active transport- In this type of active transport, the energy of ATP is used to move molecules against their concentration gradient.
secondary active transport- Unlike primary active transport, secondary active transport uses electrochemical energy to transport ions
Examples of active transport
Transport of calcium ions out of the cell
In general, the concentration of calcium ions inside cells is significantly low compared to the concentration of these ions outside the cell. The concentration of calcium ions is about 1000 times higher than that in cells. This is evidence that the cell is constantly pumping calcium ions out of the cell, thus moving calcium ions against its concentration gradient.
This is particularly important as it prevents the accumulation of calcium phosphate crystals that would otherwise kill the cell. Calcium phosphate crystals can form after a reaction between calcium ions and ATP molecules.
Increased entry of calcium ions into the cell (via calcium ion channels) can transmit a signal that causes changes within the cell.
For this reason, calcium ions must be pumped. This requires energy from ATP not only to close the cytosolic gate and open the extracellular gate, but also to cause a conformational change in the pump proteins that bind calcium ions during transport.
Below is a schematic representation of this means of transport:
Figure 2
The calcium ion in the cytoplasm quickly binds to the cytosolic gate of the calcium pump, causing it to open. Photo credit: MicroscopeMaster.com
In Figure 2, the calcium ion in the cytoplasm quickly binds to the cytosolic gate of the calcium pump, causing it to open. At this point, the pump proteins have a high affinity for calcium and therefore allow calcium ions to enter/bind.
In the next step, ATP hydrolysis at the calcium pump causes the cytosolic gate to close and the extracellular gate to open. In addition, it affects the conformational change of proteins that change their affinity for calcium ions.
This releases calcium ions into the extracellular matrix. Because proteins have no affinity for calcium ions, calcium ions in the extracellular matrix cannot bind to pump proteins to be transported into the cell.
*In contrast to calcium channels (also known as voltage-gated calcium channels), calcium pumps only transport calcium ions out of the cell against their concentration gradient.
*In ATP hydrolysis, an ATP phosphate binds to the pump protein and causes a conformational change.
*ATP is required for the active transport of sodium and potassium ions through the sodium-potassium pump. Here, ATP hydrolysis allows for the transport of three (3) sodium ions out of the cell via the sodium-potassium pump and the transport of two (2) potassium ions into the cell.
By binding to pump proteins, ATP phosphate increases its affinity for sodium ions, allowing them to be transported.
When phosphate is cleaved from proteins, they undergo a conformational change that causes them to lose their affinity for sodium ions and increase their affinity for potassium ions. This allows potassium ions to be transported into the cell.
vesicular transport
Vesicular transport is also an example of active transport. This transport mode is related to the transport of macromolecules through thePlasma Membrane.
For example, the transport of these molecules out of the cell (e.g., the transport of hormones out of cells by endocrine glands) is known as exocytosis, while the entry of macromolecules through the cell membrane is known asEndozytose(PhagozytoseÖpinocitosis).
In phagocytosis, studies have shown that the absence or reduction of ATP negatively affects the uptake of macromolecules. However, the availability of extracellular ATP has been shown to play an important role in transporting calcium ions and therefore invaginating the membrane to create a vesicle for macromolecules to enter the cell. Therefore, active transport in this case requires the use of energy.
Some of the other examples of active transport are:
- Transport of amino acids to the cells of the intestinal mucosa.
- pinocitosis
- Transport of some sugars against their concentration gradient
differences
Both passive diffusion and active transport are methods by which substances (molecules, ions, macromolecules, etc.) are transported into or out of a cell (or across a membrane).
They are important mechanisms that ensure that various materials required by the cell are successfully transported into the cell and that some substances (e.g. waste products or excreted substances in the cell) are removed from the intracellular environment.
Although both have the same features, the way this is achieved varies in many ways. The following are some of the main differences between the two transport mechanisms.
Passive diffusion depends on the concentration gradient
One of the main differences between passive diffusion and active transport is the fact that passive diffusion involves the movement of substances along their concentration gradient. This simply means that substances move from where they are more concentrated to places where they are less concentrated.
In active transport, substances are transported against their concentration gradient. This means that substances are actively transported from where they are less concentrated to the area where they are more concentrated. Because of this, energy is required.
This differs from passive diffusion in that the concentration gradient forces highly concentrated molecules to move towards the region where they are highly concentrated.
*Although the concentration gradient plays an important role in passive diffusion, the size and polarity of molecules also directly affect this mode of transport, particularly in biological systems.
For example, while very small molecules can easily diffuse across the cell membrane, other small substances, such as ionic substances, would not easily cross the membrane and therefore would have to be actively transported.
Active transport requires energy
As already mentioned, active transport plays an important role in moving substances against their concentration gradient. This means that it is involved in moving substances from an area of lower concentration to an area of high concentration. To do this, active transport consumes energy.
In addition to energy requirements, active transport also depends on integral transmembrane proteins that act as transporters. In contrast to passive diffusion, where a concentration gradient and the size and polarity of molecules affect the movement of substances across the membrane, the concentration gradient actually opposes active transport. Because of this, energy must be expended to move substances against the gradient.
Although ATP (adenosine triphosphate) is one of the most common forms of chemical energy, it is not directly involved in all forms of active transport. For this reason, active transport is divided into two main categories, which include primary and secondary active transport.
In primary active transport, the direct hydrolysis of an ATP molecule leads to the production of a phosphate, which causes the transport proteins to change their conformation, thereby promoting the transport of certain substances (e.g. ions). In this case, ATP is required for the protein transporters to function as required.
Secondary active transport (also called cotransport) does not directly depend on energy from ATP. Rather, it relies on the electrochemical gradient created during primary active transport, by which ions were pumped out of the cell against their concentration gradient.
The active pumping of ions (e.g. sodium ions) results in a higher concentration of these ions leaving the cell. As a result, an electrochemical gradient in the extracellular fluid around the cell increases.
Through this gradient, these ions are pushed into the cell via protein transporters along their concentration gradient. Due to their charge, however, they also bind to other substances (e.g. glucose molecules) and thus help to transport them into the cell.
So here the secondary active transport depends on the electrochemical gradient resulting from the primary active transport.
*Although facilitated transport requires protein transporters to move substances down their concentration gradient, no energy is consumed.
Active transport is involved in the transport of relatively larger substances.
As mentioned earlier, smaller non-polar molecules as well as lipophilic molecules (e.g. oxygen and carbon dioxide, etc.) can easily diffuse through the lipid bilayer to enter the cell. Due to their small size, they can easily pass.
Larger molecules, especially macromolecules, cannot easily diffuse. Therefore, many of these molecules rely on active transport to move across the membrane against their concentration gradient.
*Some of the larger ones are transported on their concentration gradient by integral proteins: facilitated diffusion and therefore no energy is consumed.
Active transport moves substances in one direction
Due to the way active transport works, substances are only transported in one direction. As mentioned above, this type of transport serves to move substances against their concentration gradient.
Energy is required for substances such as ions to move in a certain direction (e.g. out of the cell). Here, the opening and closing of the cytosolic gate and the extracellular gate of the ion pumps ensure that the ions cannot pass in the opposite direction after being transported through the membrane.
In passive diffusion, channels allow substances to move in any direction as long as a concentration gradient exists. In addition, substances can be slowly dispersed in and out of the cell by diffusion in the lipid bilayer at a concentration gradient.
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references
Alexey V. Melkikh and Vladimir D. Seleznev. (2012). Mechanisms and models of active ion transport and energy conversion in intracellular compartments.
Abumrad NA, Sfeir Z, Connelly MA, and Coburn C. (2000). Lipid Transporters: Membrane transport systems for cholesterol and fatty acids.
Ruchi Gaur, Lallan Mishra, and Susanta K. Sen Gupta. (2014). Diffusion and transport of molecules in living cells.
Yip Wah Chung. (2006). Introduction to materials science and engineering.
connections
https://www.sciencedirect.com/topics/chemistry/passive-diffusion
https://www.researchgate.net/publication/316786043_Difference_Between_Active_and_Passive_Transport
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