What is concentration? In a broad sense, this is the ratio of the volume of a substance and the number of particles dissolved in it. This definition is found in a wide variety of branches of science, from physics and mathematics to philosophy. In this case, we are talking about the use of the concept of "concentration" in biology and chemistry.
Gradient
Translated from Latin, this word means “growing” or “walking”, that is, it is a kind of “pointing finger”, which shows the direction in which any value increases. As an example, you can use, say, the height above sea level at different points on the Earth. Its (height) gradient at each individual point on the map will show a vector of increasing value until reaching the steepest ascent.
In mathematics, this term appeared only at the end of the nineteenth century. It was introduced by Maxwell and proposed his own designations for this quantity. Physicists use this concept to describe the intensity of an electric or gravitational field, a change in potential energy.
Not only physics, but also other sciences use the term "gradient". This concept can reflect both qualitative anda quantitative characteristic of a substance, such as concentration or temperature.
Concentration gradient
What is the gradient is now known, but what is the concentration? This is a relative value that shows the proportion of the substance contained in the solution. It can be calculated as a percentage of the mass, the number of moles or atoms in a gas (solution), a fraction of the whole. Such a wide choice makes it possible to express almost any ratio. And not only in physics or biology, but also in the metaphysical sciences.
And in general, the concentration gradient is a vector quantity, which simultaneously characterizes the amount and direction of change of a substance in the environment.
Definition
Can you calculate the concentration gradient? Its formula is a particular between an elementary change in the concentration of a substance and a long path that a substance will have to overcome in order to achieve equilibrium between two solutions. Mathematically, this is expressed by the formula С=dC/dl.
The presence of a concentration gradient between two substances causes them to mix. If particles move from an area with a higher concentration to a lower one, then this is called diffusion, and if there is a semi-permeable obstacle between them, it is called osmosis.
Active transport
Active and passive transport reflects the movement of substances through the membranes or layers of cells of living beings: protozoa, plants,animals and humans. This process takes place with the use of thermal energy, since the transition of substances is carried out against a concentration gradient: from smaller to larger. Most often, adenosine triphosphate or ATP is used to carry out such an interaction - a molecule that is a universal source of energy in 38 Joules.
There are different forms of ATP that are located on cell membranes. The energy contained in them is released when molecules of substances are transferred through the so-called pumps. These are pores in the cell wall that selectively absorb and pump out electrolyte ions. In addition, there is such a transport model as a symport. In this case, two substances are simultaneously transported: one leaves the cell, and the other enters it. This saves energy.
Vesicular transport
Active and passive transport involve the transport of substances in the form of bubbles or vesicles, hence the process is called, respectively, vesicular transport. There are two types of it:
- Endocytosis. In this case, bubbles are formed from the cell membrane in the process of absorption of solid or liquid substances by it. Vesicles may be smooth or bordered. Eggs, white blood cells, and the epithelium of the kidneys have this way of eating.
- Exocytosis. As the name implies, this process is the opposite of the previous one. There are organelles inside the cell (for example, the Golgi apparatus), which “pack” substances into vesicles, and they subsequently exit throughmembrane.
Passive transport: diffusion
The movement along the concentration gradient (from high to low) occurs without the use of energy. There are two types of passive transport: osmosis and diffusion. The latter is simple and lightweight.
The main difference between osmosis is that the process of movement of molecules occurs through a semi-permeable membrane. And diffusion along the concentration gradient occurs in cells that have a membrane with two layers of lipid molecules. The direction of transport depends only on the amount of substance on both sides of the membrane. In this way, hydrophobic substances, polar molecules, urea penetrate the cells, and proteins, sugars, ions and DNA cannot penetrate.
During diffusion, molecules tend to fill the entire available volume, as well as equalize the concentration on both sides of the membrane. It happens that the membrane is impermeable or poorly permeable to the substance. In this case, osmotic forces act on it, which can both make the barrier denser and stretch it, increasing the size of the pumping channels.
Facilitated diffusion
When a concentration gradient is not a sufficient basis for the transport of a substance, specific proteins come to the rescue. They are located on the cell membrane in the same way as ATP molecules. Thanks to them, both active and passive transport can be carried out.
In this way, large molecules (proteins, DNA) pass through the membrane,polar substances, which include amino acids and sugars, ions. Due to the participation of proteins, the transport rate increases several times compared to conventional diffusion. But this acceleration depends on some reasons:
- gradient of matter inside and outside the cell;
- number of carrier molecules;
- substance-carrier binding rates;
- rate of change in the inner surface of the cell membrane.
Despite this, transport is carried out thanks to the work of carrier proteins, and ATP energy is not used in this case.
The main features that characterize facilitated diffusion are:
- Fast transfer of substances.
- Transport selectivity.
- Saturation (when all proteins are busy).
- Competition between substances (due to protein affinity).
- Sensitivity to specific chemical agents - inhibitors.
Osmosis
As mentioned above, osmosis is the movement of substances along a concentration gradient across a semipermeable membrane. The osmosis process is most fully described by the Leshatelier-Brown principle. It says that if a system in equilibrium is influenced from the outside, then it will tend to return to its previous state. The first time the phenomenon of osmosis was encountered in the middle of the 18th century, but then it was not given much importance. Research on the phenomenon began only a hundred years later.
The most important element in the phenomenon of osmosis is a semi-permeable membrane that allows only certain molecules to pass through it.diameter or properties. For example, in two solutions with different concentrations, only the solvent will pass through the barrier. This will continue until the concentration on both sides of the membrane is the same.
Osmosis plays a significant role in the life of cells. This phenomenon allows only those substances that are necessary to maintain life to penetrate into them. The red blood cell has a membrane that allows only water, oxygen and nutrients to pass through, but the proteins that are formed inside the red blood cell cannot get out.
The phenomenon of osmosis has also found practical application in everyday life. Without even suspecting it, people in the process of s alting food used precisely the principle of the movement of molecules along a concentration gradient. The saturated saline solution "pulled out" all the water from the products, thereby allowing them to be stored longer.
