What is passive transport? Transmembrane movement of various macromolecular compounds, cellular components, supramolecular particles that are not able to penetrate through the channels in the membrane, is carried out through special mechanisms, for example, using phagocytosis, pinocytosis, exocytosis, transfer through the intercellular space. That is, the movement of substances through the membrane can occur using various mechanisms, which are divided according to the signs of the participation of specific carriers in them, as well as energy consumption. Scientists divide the transport of substances into active and passive.
Main modes of transport
Passive transport is the transfer of a substance through a biological membrane along a gradient (osmotic, concentration, hydrodynamic and others), which does not require energy consumption.
Active transport is the transfer of a substance across a biological membrane against a gradient. Whereinenergy is consumed. Approximately 30 - 40% of the energy that is formed as a result of metabolic reactions in the human body is spent on the implementation of active transport of substances. If we consider the functioning of the human kidneys, then about 70 - 80% of the consumed oxygen is spent on active transport.
Passive transport of substances
it involves the transfer of various substances through biological membranes along a variety of gradients. These gradients can be:
- electrochemical potential gradient;
- substance concentration gradient;
- electric field gradient;
- osmotic pressure gradient and others.
The process of implementing passive transport does not require any energy consumption. It can occur by facilitated and simple diffusion. As we know, diffusion is a chaotic movement of molecules of a substance in a variety of media, which is due to the energy of thermal vibrations of a substance.
If a particle of a substance is electrically neutral, then the direction in which diffusion will occur is determined by the difference in the concentration of substances contained in the media that are separated by the membrane. For example, between compartments of the cell, inside the cell and outside it. If the particles of a substance, its ions have an electric charge, then diffusion will depend not only on the concentration difference, but also on the magnitude of the charge of the given substance, the presence and signs of charge on both sides of the membrane. The magnitude of the electrochemical gradientis determined by the algebraic sum of the electrical and concentration gradients across the membrane.
What provides transport across the membrane?
Passive membrane transport is possible due to the presence of substance concentration gradients, osmotic pressure arising between different sides of the cell membrane or electric charge. For example, the average level of Na+ ions contained in blood plasma is about 140 mM/l, and its content in erythrocytes is about 12 times higher. Such a gradient, expressed as a difference in concentration, is able to create a driving force that ensures the transfer of sodium molecules to red blood cells from blood plasma.
It should be noted that the rate of such a transition is very low due to the fact that the cell membrane is characterized by low permeability for the ions of this substance. This membrane has much greater permeability in relation to potassium ions. The energy of cellular metabolism is not used to complete the process of simple diffusion.
Diffusion rate
Active and passive transport of substances through the membrane is characterized by the rate of diffusion. It can be described using the Fick equation: dm/dt=-kSΔC/x.
In this case, dm/dt is the amount of the substance that diffuses in one unit of time, and k is the coefficient of the diffusion process, which characterizes the permeability of the biomembrane for the diffusing substance. S equals the area over which diffusion occurs, and ΔC expresses the differenceconcentrations of substances from different sides of the biological membrane, while x characterizes the distance between the diffusion points.
Obviously, those substances that diffuse simultaneously along the gradients of concentrations and electric fields will most easily move through the membrane. An important condition for the diffusion of a substance through a membrane is the physical properties of the membrane itself, its permeability for each specific substance.
Due to the fact that the bilayer of the membrane is formed by hydrocarbon radicals of phospholipids with hydrophobic properties, substances of a hydrophobic nature easily diffuse through it. In particular, this applies to substances that dissolve easily in lipids, such as thyroid and steroid hormones, as well as some narcotic substances.
Mineral ions and low molecular weight substances that are hydrophilic in nature diffuse through passive membrane ion channels, which are formed from channel-forming protein molecules, and sometimes through membrane packing defects of phospholipid molecules that arise in the cell membrane as a result of thermal fluctuation.
Passive transport across the membrane is a very interesting process. If the conditions are normal, then significant amounts of a substance can penetrate the bilayer membrane only if they are non-polar and have a small size. Otherwise, the transfer occurs through carrier proteins. Similar processes involvingcarrier protein is not called diffusion, but the transport of a substance through the membrane.
Facilitated diffusion
Facilitated diffusion, like simple diffusion, occurs along the concentration gradient of a substance. The main difference is that a special protein molecule, called a carrier, takes part in the process of substance transfer.
Facilitated diffusion is a type of passive transfer of molecules of a substance through biomembranes, carried out along a concentration gradient using a carrier.
Transfer protein states
The carrier protein can be in two conformational states. For example, in state A, this protein may have an affinity for the substance it carries, its binding sites for the substance are turned inward, thereby forming a pore that is open to one side of the membrane.
After the protein has bound to the transferred substance, its conformation changes and its transition to state B occurs. With this transformation, the carrier loses its affinity for the substance. From the connection with the carrier, it is released and moves to the pore already on the other side of the membrane. After the substance has been transferred, the carrier protein changes its conformation again, returning to state A. This transport of the substance across the membrane is called a uniport.
Facilitated Diffusion Velocity
Small molecular weight substances like glucose can be transported throughmembrane through facilitated diffusion. Such transport can occur from the blood to the brain, to cells from the interstitial spaces. The rate of transfer of matter with this type of diffusion can reach up to 108 particles through the channel in one second.
As we already know, the rate of active and passive transport of substances in simple diffusion is proportional to the difference in the concentrations of the substance on both sides of the membrane. In the case of facilitated diffusion, this rate increases in proportion to the increasing difference in the concentration of the substance up to a certain maximum value. Above this value, the rate does not increase, even though the difference in concentrations from different sides of the membrane continues to increase. The achievement of such a maximum rate point in the process of facilitated diffusion can be explained by the fact that the maximum rate implies the involvement of all available carrier proteins in the transfer process.
What other concept does active and passive transport across membranes include?
Exchange diffusion
This type of transport of substance molecules through the cell membrane is characterized by the fact that molecules of the same substance that are located on different sides of the biological membrane participate in the exchange. It should be noted that with such transport of substances, the concentration of molecules on both sides of the membrane does not change at all.
A type of exchange diffusion
One of the varieties of exchange diffusion is an exchange in whicha molecule of one substance is exchanged for two or more molecules of another substance. For example, one of the ways in which positive calcium ions are removed from the smooth muscle cells of the bronchi and vessels from the contractile myocytes of the heart is their exchange for sodium ions located outside the cell. One sodium ion in this case is exchanged for three calcium ions. Thus, there is a movement of sodium and calcium through the membrane, which is interdependent. This kind of passive transport across the cell membrane is called an antiport. It is in this way that the cell is able to get rid of calcium ions, which are present in excess. This process is necessary for smooth myocytes and cardiomyocytes to relax.
This article examined the active and passive transport of substances through the membrane.
