Passive transport of substances through the membrane: a description, features

What is passive transport? Transmembrane movement of various high-molecular compounds, cellular components, supramolecular particles that are unable to penetrate channels in the membrane, is carried out through special mechanisms, for example, by phagocytosis, pinocytosis, exocytosis, and transfer through the intercellular space. That is, the movement of substances through the membrane can occur through various mechanisms, which are divided according to the characteristics of participation in them of specific vectors, as well as energy costs. Scientists subdivide the transport of substances into active and passive.

Basic modes 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 a gradient. At the same time, energy is consumed. Approximately 30-40% of the energy that is generated as a result of the 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 consumed oxygen is spent on active transport.

Passive transport of substances

it implies the transfer of various substances through biological membranes along a variety of gradients. Such gradients can be:

• gradient of electrochemical potential;
• concentration gradient of the substance;
• gradient of the electric field;
• gradient of osmotic pressure and others.

The process of implementing passive transport does not require any energy costs. It can occur with the help of light and simple diffusion. As we know, diffusion is a random movement of molecules of matter in various media, which is due to the energy of thermal vibrations of matter.

If the particle of the substance is electrically neutral, the direction in which diffusion will occur is determined by the difference in the concentration of substances contained in media that are separated by the membrane. For example, between the compartments of the cell, inside the cell and outside it. If the particles of matter, its ions have an electric charge, the diffusion will depend not only on the difference in concentrations, but also on the charge of the 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 does membrane transport provide?

Passive membrane transport is possible, due to the presence of concentration gradients of the substance, the osmotic pressure arising between 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 erythrocytes is about 12 times greater. Such a gradient, expressed in the concentration difference, is able to create a driving force that ensures the transfer of sodium molecules to the red blood cells from the 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 ions of this substance. Much more permeability, this membrane has against potassium ions. The energy of cellular metabolism is not used to perform the process of simple diffusion.

Diffusion rate

Active and passive transport of substances through the membrane is characterized by diffusion rate. 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 at which diffusion takes place, and ΔC expresses the difference in the concentration of substances from different sides of the biological membrane, while x characterizes the distance that exists between the diffusion points.

Obviously, through the membrane, those substances that diffuse simultaneously along concentration gradients and electric fields will most easily move. An important condition for the diffusion of matter through the membrane is the physical properties of the membrane itself, its permeability for each particular substance.

Due to the fact that the bilayer of the membrane is formed by hydrocarbon radicals of phospholipids with hydrophobic properties, substances of hydrophobic nature easily diffuse through it. In particular, this refers to substances that readily dissolve in lipids, for example, thyroid and steroid hormones, as well as certain substances of a narcotic nature.

Mineral ions and low molecular substances having a hydrophilic nature diffuse through the passive ion channels of the membrane, which are formed from the channel-forming protein molecules, and sometimes through the defects in the packing of the membrane 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, significant quantities of the substance can penetrate the bilayer membrane only if they are nonpolar and have a small size. Otherwise, the transfer occurs through carrier proteins. Such processes involving the carrier protein are called not diffusion, but transport of matter through the membrane.

Light diffusion

Light diffusion, like simple diffusion, occurs along the concentration gradient of a substance. The main difference is that a special protein molecule, called the carrier, participates in the process of substance transfer.

Light diffusion is a kind of passive transport of molecules of matter through biomembranes, carried out on a gradient of concentration by means of a carrier.

Protein carrier states

The carrier protein can be in two conformational states. For example, in state A, a given protein may have an affinity for the substance that it carries, its binding sites with matter are inwardly expanded, thereby forming a pore open to one side of the membrane.

After the protein binds to the transferred substance, its conformation changes and its transition to the B state occurs. With this transformation, the affinity for the carrier is lost with the substance. Because of 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. A similar transport of matter through the membrane is called uniport.

Speed ​​with light diffusion

Low molecular weight substances like glucose can be transported through the membrane by facilitated diffusion. Such transport can occur from blood to the brain, to cells from interstitial spaces. The rate of transport of matter with this type of diffusion is able to reach up to 108 particles through the channel in one second.

As we already know, the rate 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 concentration from different sides of the membrane continues to increase. The attainment of such a maximum velocity point in the process of facilitated diffusion can be explained by the fact that the maximum speed involves the involvement of all available carrier proteins in the transport process.

Which concept still includes active and passive transport through membranes?

Exchange diffusion

A similar kind of transport of molecules of matter through the cell membrane is characterized by the fact that molecules of the same substance that are on different sides of the biological membrane participate in the exchange. It should be noted that with this transport of substances, the concentration of molecules on both sides of the membrane does not change at all.

The type of exchange diffusion

One of the varieties of exchange diffusion is an exchange in which a molecule of one substance changes into two or more molecules of a different substance. For example, one of the ways in which the removal of positive calcium ions from the smooth muscle cells of the bronchi and vessels from the contractile myocytes of the heart occurs is to exchange 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 kind of passive transport through the cell membrane is called an antiport. It is in this way that the cell is able to release calcium ions, which are in abundance. This process is necessary in order for smooth myocytes and cardiomyocytes to relax.

In this article, active and passive transport of substances through the membrane was considered.