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, by 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 participation of specific carriers in them, as well as according to energy consumption. Scientists divide the transport of substances into active and passive.
The main types of transport
Passive transport is the transfer of matter through a biological membrane along a gradient (osmotic, concentration, hydrodynamic and others) that does not require energy consumption.
Active transport is the transfer of a substance through a biological membrane against the gradient. This consumes energy. About 30 - 40% of the energy that is generated as a result of a metabolic reaction in the human body is spent on the active transport of substances. If we consider the functioning of human kidneys, then about 70 - 80% of the consumed oxygen is spent on active transport in them.
Passive transport of substances
it implies the transfer of various substances through biological membranes along various gradients. These gradients can be:
- gradient of electrochemical potential;
- 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 using lightweight 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 the 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 media that are separated by a membrane. For example, between cell compartments, inside and outside the cell. If particles of a substance, its ions have an electric charge, then diffusion will depend not only on the difference in concentrations, but also on the magnitude of the charge of this substance, the presence and signs of charge on both sides of the membrane. The magnitude of the electrochemical gradient is determined by the algebraic sum of the electric and concentration gradients on the membrane.
What provides transport through the membrane?
Passive membrane transport is possible due to the presence of gradients of the concentration of the substance, osmotic pressure arising between the different sides of the cell membrane or electric charge. For example, the average level of Na + ions contained in the blood plasma is about 140 mM / L, and its content in red blood cells is approximately 12 times higher. A similar gradient, expressed in a difference in concentrations, 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 low permeability of ions of this substance for the cell membrane. This membrane has much greater permeability with respect to potassium ions. The energy of cellular metabolism is not used to accomplish 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 = -kSΞC / 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 is equal to the area over which diffusion occurs, and ΞC expresses the difference in the concentration of substances from different sides of the biological membrane, while x characterizes the distance that exists between the points of diffusion.
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 to each specific substance.
Due to the fact that the bilayer 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 are readily soluble in lipids, for example, thyroid and steroid hormones, as well as certain substances of a narcotic nature.
Mineral ions and low molecular weight substances of a hydrophilic nature diffuse through passive ion channels of the membrane, 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 fluctuations.
Passive transport through the membrane is a very interesting process. If the conditions are normal, then significant amounts of the substance can penetrate through the bilayer membrane only if they are non-polar and have a small size. Otherwise, the transfer occurs through carrier proteins. Similar processes involving the carrier protein are called not diffusion, but the transport of matter through the membrane.
Light 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 transfer of matter.
Facilitated diffusion is a type of passive transfer of molecules of a substance through biomembranes, carried out along a concentration gradient using a carrier.
Carrier Protein Conditions
The carrier protein can be in two conformational states. For example, in state A, this protein may have an affinity with the substance that it transfers, its sites for binding to the substance are turned inward, due to which a pore is formed 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's affinity for the substance is lost. From the connection with the carrier, it is released and moves into the pore already on the other side of the membrane. After the substance is transferred, the carrier protein again changes its conformation, returning to state A. Such a transport of a substance through a membrane is called a uniport.
Light diffusion speed
Low molecular weight substances like glucose can be transported through the membrane through facilitated diffusion. Such transport can occur from blood to the brain, to cells from interstitial spaces. The rate of transfer of a substance with this type of diffusion can reach up to 10 8 particles through the channel in one second.
As we already know, the speed of active and passive transport of substances with simple diffusion is proportional to the difference in the concentration of the substance on both sides of the membrane. In the case of facilitated diffusion, this velocity increases in proportion to the increasing difference in the concentration of the substance to a certain maximum value. Above this value, the speed does not increase, even though the difference in concentrations from different sides of the membrane continues to increase. The achievement of such a maximum speed point in the process of facilitating diffusion can be explained by the fact that the maximum speed involves the involvement of all available carrier proteins in the transfer process.
What concepts still include active and passive transport across membranes?
Exchange diffusion
This type of transport of molecules of a substance through a cell membrane is characterized by the fact that molecules of the same substance, which are located on different sides of the biological membrane, participate in the exchange. It should be noted that with such a 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 types of exchange diffusion is exchange, in which the molecule of one substance changes into two or more molecules of another substance. For example, one of the ways in which positive calcium ions are removed from smooth muscle cells of the bronchi and blood vessels from contractile myocytes of the heart is by exchanging them for sodium ions located outside the cell. One sodium ion in this case is exchanged for three calcium ions. Thus, sodium and calcium move through the membrane, which is interdependent in nature. This type of passive transport through the cell membrane is called antiport. In this way, the cell is able to free itself from calcium ions, which are in excess. This process is necessary for smooth myocytes and cardiomyocytes to relax.
In this article, active and passive transport of substances through the membrane was considered.