A cell is a structural unit of all living things on our planet and an open system. This means that for its vital activity a constant exchange of substances and energy with the environment is necessary. This exchange is carried out through the membrane - the main border of the cell, which is designed to maintain its integrity. It is through the membrane that cell metabolism is carried out and it either follows the concentration gradient of a substance or against it. Active transport through the cytoplasmic membrane is a complex and energy-intensive process.
Membrane - barrier and gateway
The cytoplasmic membrane is part of many cellular organelles, plastids and inclusions. Modern science is based on a liquid-mosaic model of the membrane structure. Active transport of substances through the membrane is possible due to its specific structure. The base of the membranes is formed by the lipid bilayer - these are mainly phospholipids, arranged in accordance with their hydrophilic-hydrophobic properties. The main properties of the lipid bilayer are fluidity (the ability to incorporate and lose sites), self-assembly and asymmetry. The second component of the membrane is proteins. Their functions are diverse: active transport, reception, fermentation, recognition.
Proteins are located both on the surface of the membranes and inside, and some penetrate it several times. The property of proteins in the membrane is the ability to transfer from one side of the membrane to the other (“flip-flop” jump). And the last component is the saccharide and polysaccharide chains of carbohydrates on the surface of the membranes. Their functions are debatable today.
Types of active transport of substances through the membrane
Such transfer of substances across the cell membrane, which is controlled, takes place with energy costs and goes against the concentration gradient (substances are transferred from a low concentration region to a high concentration region) will be active. Depending on which energy source is used, the following types of transport are distinguished:
- Primarily active (the source of energy is the hydrolysis of adenosine triphosphoric acid ATP to adenosine diphosphoric ADP).
- Secondarily active (provided by secondary energy created as a result of the mechanisms of primary active transport of substances).
Helper squirrels
In both the first and second cases, transport is not possible without carrier proteins. These transport proteins are very specific and are designed to transfer certain molecules, and sometimes even a specific variety of molecules. This was proved experimentally on mutated bacterial genes, which led to the impossibility of active transport through the membrane of a certain carbohydrate. Transmembrane carrier proteins can be carriers themselves (they interact with molecules and directly carry it through the membrane) or channel-forming (form pores in membranes that are open to specific substances).
Sodium and Potassium Pump
The most studied example of the primary active transport of substances through the membrane is Na + -, K + -pump. This mechanism provides a difference in the concentrations of Na + and K + ions on both sides of the membrane, which is necessary to maintain the osmotic pressure in the cell and other metabolic processes. The transmembrane carrier protein, sodium potassium ATPase, consists of three parts:
- On the outer side of the membrane of the protein are two receptors for potassium ions.
- On the inside of the membrane are three receptors for sodium ions.
- ATP activity is characteristic of the inner part of the protein.
When two potassium ions and three sodium ions bind to protein receptors on both sides of the membrane, ATP activity is activated. The ATP molecule is hydrolyzed to ADP with the release of energy, which is expended on the transfer of potassium ions inward, and sodium ions outward of the cytoplasmic membrane. It is estimated that the efficiency of such a pump is more than 90%, which in itself is quite surprising.
For reference: the efficiency of an internal combustion engine is about 40%, electric - up to 80%. Interestingly, the pump can work in the opposite direction and serve as a phosphate donor for the synthesis of ATP. For some cells (for example, neurons), up to 70% of all energy is spent on removing sodium from the cell and pumping potassium ions inside. According to the same principle of active transport, pumps for calcium, chlorine, hydrogen and some other cations (ions with a positive charge) work. For anions (negatively charged ions) such pumps were not found.
Cotransport of carbohydrates and amino acids
An example of secondary active transport is the transfer of glucose, amino acids, iodine, iron and uric acid into cells. As a result of the potassium-sodium pump, a gradient of sodium concentrations is created: the concentration is high on the outside and low on the inside (sometimes 10-20 times). Sodium seeks to diffuse into the cell and the energy of this diffusion can be used to transport substances out. This mechanism is called cotransport or associated active transport. In this case, the carrier protein has two receptor centers from the outside: one for sodium, and the other for the transported element. Only after activation of both receptors does the protein undergo conformational changes, and the diffusion energy of sodium introduces the transported substance into the cell against the concentration gradient.
The value of active transport for the cell
If the usual diffusion of substances through the membrane proceeded indefinitely, their concentrations outside and inside the cell would equalize. And this is cell death. After all, all biochemical processes must occur in an environment of electric potential difference. Without active, against the concentration gradient, transport of substances neurons would not be able to transmit a nerve impulse. And muscle cells would lose the ability to contract. A cell would not be able to maintain osmotic pressure and would flatten. And metabolic products would not be brought out. And hormones would never get into the bloodstream. After all, even an amoeba spends energy and creates a potential difference on its membrane using all the same ion pumps.