What is concentration? Speaking in a broad sense, this is the ratio of the volume of a substance to the amount 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 indicates the direction in which any value increases. As an example, we can use, for example, the height above sea level at different points on the Earth. Its (height) gradient at each individual point on the map will show the vector of increasing the value until it reaches the steepest climb.
In mathematics, this term appeared only at the end of the nineteenth century. He was introduced by Maxwell and proposed his notation for this quantity. Physicists use this concept to describe the strength 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 and quantitative characteristics of a substance, for example, concentration or temperature.
Concentration gradient
What is the gradient now known, and 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 metaphysical sciences.
But in general, the concentration gradient is a vector quantity, which simultaneously characterizes the quantity and direction of change of a substance in the medium.
Definition
Is it possible to calculate the concentration gradient? Its formula is the particularity between an elementary change in the concentration of a substance and the long path that a substance will have to overcome in order to achieve equilibrium between two solutions. Mathematically, this is expressed by the formula C = dC / dl.
The presence of a concentration gradient between the two substances is the reason for their mixing. If particles move from a region with a higher concentration to a lower one, then this is called diffusion, and if there is a semipermeable obstacle between them, it is called osmosis.
Active transport
Active and passive transport reflects the movement of substances through membranes or layers of cells of living creatures: protozoa, plants, animals and humans. This process takes place using thermal energy, since the transition of substances is carried out against the concentration gradient: from smaller to larger. Most often, adenosine triphosphate or ATP, a molecule that is a universal source of energy of 38 Joules, is used to carry out such an interaction.
There are various forms of ATP that are located on cell membranes. The energy contained in them is released during the transfer of molecules of substances 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 transported simultaneously: one leaves the cell, and the other enters it. This saves energy.
Vesicular transport
Active and passive transport involves the transport of substances in the form of vesicles or vesicles, so 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 can be smooth or have a border. Eggs, white blood cells, and kidney epithelium have this type of nutrition.
- Exocytosis. Based on the name, this is the opposite process to the previous one. Inside the cell there are organelles (for example, the Golgi apparatus) that “pack” substances into the bubbles, and they subsequently exit through the membrane.
Passive transport: diffusion
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 moving molecules occurs through a semipermeable membrane. And diffusion along the concentration gradient occurs in cells having 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 into the cells, and proteins, sugars, ions and DNA cannot penetrate.
In the process of diffusion, the molecules tend to fill the entire available volume, as well as even out 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 pump channels.
Light diffusion
When the 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), polar substances, which include amino acids and sugars, ions, pass through the membrane. Due to the participation of proteins, the transport rate increases several times, compared with ordinary diffusion. But this acceleration depends on some reasons:
- the gradient of the substance inside and outside the cell;
- the number of carrier molecules;
- the binding rate of the substance and the carrier;
- rate of change of 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 substance transfer.
- Selectivity of transport.
- Saturation (when all proteins are occupied).
- 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 through a semipermeable membrane. The osmosis process most fully describes the Leshelier-Brown principle. It says that if a system in equilibrium is influenced from the outside, it will tend to return to its previous state. The first time they encountered the phenomenon of osmosis in the middle of the 18th century, but then they did not attach much importance to it. Studies of the phenomenon began only a hundred years later.
The most important element in the phenomenon of osmosis is a semi-permeable membrane that passes only molecules of a certain diameter or properties through itself. 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 cell life. This phenomenon allows them to penetrate only those substances that are necessary to maintain life. The red blood cell has a membrane that allows only water, oxygen and nutrients to pass through, but the proteins that form inside the red blood cell cannot get out.
The phenomenon of osmosis has also found practical application in everyday life. Without even knowing it, people in the process of salting food used exactly the principle of movement of molecules along the concentration gradient. A saturated saline solution “pulled” all the water from the products onto itself, thereby allowing them to be stored longer.