Cell is a structural unit of all life on our planet and an open system. This means that its life requires a constant exchange of matter and energy with the environment. This exchange is carried out through the membrane - the main border of the cell, which is designed to preserve its integrity. It is through the membrane that cellular metabolism is carried out and it goes either along the concentration gradient of a substance, or against it. Active transport across the cytoplasmic membrane is a complex and energy-intensive process.
Membrane - barrier and gateway
The cytoplasmic membrane is part of many cell organelles, plastids and inclusions. Modern science is based on the fluid mosaic model of membrane structure. Active transport of substances across the membrane is possible due to itsspecific building. The basis of membranes is formed by a lipid bilayer - mainly phospholipids arranged in accordance with their hydrophilic-hydrophobic properties. The main properties of the lipid bilayer are fluidity (the ability to embed and lose sites), self-assembly, and asymmetry. The second component of membranes 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 of them penetrate it several times. The property of proteins in a membrane is the ability to move from one side of the membrane to the other (“flip-flop” jump). And the last component is saccharide and polysaccharide chains of carbohydrates on the membrane surface. Their functions are still controversial today.
Types of active transport of substances across the membrane
Active will be such a transfer of substances through the cell membrane, which is controlled, occurs with energy costs and goes against the concentration gradient (substances are transferred from an area of low concentration to an area of high concentration). Depending on what source of energy is used, the following modes of transport are distinguished:
- Primary active (energy source - hydrolysis of adenosine triphosphoric acid ATP to adenosine diphosphoric acid ADP).
- Secondary active (provided with secondary energy created as a result of the mechanisms of primary active transport of substances).
Proteins-assistants
In both the first and second cases, transport is impossible without carrier proteins. These transport proteins are very specific and are designed to carry certain molecules, and sometimes even certain types of molecules. This was proved experimentally on mutated bacterial genes, which led to the impossibility of active transport across the membrane of a certain carbohydrate. Transmembrane transporter proteins can be self-transporters (they interact with molecules and directly carry them across 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 across the membrane is the Na+ -, K+ -pump. This mechanism ensures the 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 protein membrane there are two receptors for potassium ions.
- There are three sodium ion receptors on the inside of the membrane.
- The inner part of the protein has ATP activity.
When two potassium ions and three sodium ions bind to protein receptors on either side of the membrane, ATP activity is turned on. The ATP molecule is hydrolyzed to ADP with the release of energy, which is spent on the transport of potassium ionsinside, and sodium ions outside the cytoplasmic membrane. It is estimated that the efficiency of such a pump is more than 90%, which in itself is quite amazing.
For reference: The efficiency of an internal combustion engine is about 40%, electric - up to 80%. Interestingly, the pump can also work in the opposite direction and serve as a phosphate donor for ATP synthesis. For some cells (for example, neurons), up to 70% of all energy is spent on removing sodium from the cell and pumping potassium ions into it. Pumps for calcium, chlorine, hydrogen and some other cations (ions with a positive charge) work on the same principle of active transport. No such pumps have been found for anions (negatively charged ions).
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 operation of the potassium-sodium pump, a gradient of sodium concentrations is created: the concentration is high outside, and low inside (sometimes 10-20 times). Sodium tends to diffuse into the cell and the energy of this diffusion can be used to transport substances out. This mechanism is called cotransport or coupled active transport. In this case, the carrier protein has two receptor centers on the outside: one for sodium and the other for the element being transported. Only after the activation of both receptors, the protein undergoes conformational changes, and the diffusion energysodium 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 for an arbitrarily long time, their concentrations outside and inside the cell would equalize. And this is death for the cells. After all, all biochemical processes must proceed in an environment of electrical 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. The cell would not be able to maintain osmotic pressure and would collapse. And the products of metabolism 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 the same ion pumps.
