Is named cell transport to the exchange of substances between the interior of the cell and the external environment in which it is found. For instance: gas diffusion, sweating, phagocytosis, exocytosis. This occurs through the plasma membrane, which is a semi-permeable barrier that delimits the cell.
Cell transport is vital for the intake of nutrients and substances dissolved in the medium, and the expulsion of waste or metabolized substances inside the cell, such as hormones or enzymes. According to its direction of movement of matter and its energy cost, we will talk about:
- Passive transport. By going in favor of the concentration gradient, that is, from a more concentrated medium to a less concentrated one, it occurs by diffusion through the membrane and has no energy cost, since it takes advantage of the random movements of the molecules (their kinetic energy ). There are four types of passive transport:
- Simple diffusion. The material moves from the most concentrated area to the least concentrated until the levels are equal.
- Facilitated dissemination. Transport is carried out by special transport proteins found inside the cell membrane.
- Filtration. The plasma membrane has pores through which material of a specific size can leak into its interior by hydrostatic pressure.
- Osmosis. Similar to simple diffusion, it depends on the passage of water molecules through the membrane, due to the pressure of the medium and its selectivity.
- Active transport. Unlike the passive, it works against the concentration gradient (from a less concentrated area to a more concentrated one), so it has a cost of cellular energy. This allows cells to accumulate the material they need for their synthesis processes.
Examples of passive transport
- Dissolution in the phospholipid layer. Thus, many elements enter the cell, such as water, oxygen, carbon dioxide, fat-soluble vitamins, steroids, glycerines, and low-molecular-weight alcohols.
- Entrance through whole protein channels. Some ionic substances (with an electrical charge), such as sodium, potassium, calcium or bicarbonate, cross the membrane guided by channels and special proteins for this purpose, of very small size.
- The renal glomeruli. They filter the blood in the kidneys, stripping it of urea, creatinine and salts, through an ultrafiltration process carried out by the capillaries, preventing the passage of larger elements and excreting the smaller ones thanks to the pressure of the medium itself.
- Glucose absorption. The cells are always kept with a low concentration of glucose, causing it to always flow by diffusion into their interior. To do this, transporter proteins carry it in and then turn it into glucose-6-phosphate.
- The action of insulin. This hormone secreted by the pancreas enhances the diffusion of glucose in the blood into the cells, reducing the presence of sugar in the blood, fulfilling a hemoregulatory role.
- Gas diffusion. Simple diffusion allows the entry of gases produced by respiration, from the outside to the inside of the cells from their concentration in the blood. In this way the CO is expelled2 and oxygen is used.
- Sweating. The excretion of sweat through the skin is carried out by osmosis: the liquid flows out and carries toxins and other substances with it.
- Plant roots. They have selective membranes that allow water and other minerals to enter the interior of the plant, and then send it to the leaves to photosynthesize.
- Intestinal absorption. The epithelial cells of the intestine absorb water and other nutrients from the stool, without allowing them to enter the bloodstream. Said selectivity also occurs passively, through the electrolyte gradient.
- The release of enzymes and hormones into the bloodstream. It is often produced by the mechanics of high intracellular concentration, at no cost of ATP.
Examples of active transport
- Sodium-potassium pump. It is a cell membrane mechanism that allows, through a carrier protein, sodium to be expelled from the interior of the cell and replaced with potassium, maintaining ion gradients (low sodium and abundant potassium) and the appropriate electrical polarity.
- Calcium pump. Another transport protein present in the cell membrane, allows calcium to be carried against its electrochemical gradient, from the cytoplasm to the outside.
- Phagocytosis. The white blood cells that defend the organism incorporate, through sacs of their plasma membrane, the foreign particles that we will later expel.
- Pinocytosis. Another process of phagocytization proceeds through invaginations in the membrane that allow the entry of environmental fluid. It is something that the ovum does during its maturation.
- Exocytosis. Contrary to phagocytization, it expels elements of the cellular content through membranous sacs that move outwards, until they fuse with the membrane and open outwards. This is how neurons communicate: transmitting ionic contents.
- HIV infection. The AIDS virus enters cells by taking advantage of their membrane, binding to glycoproteins present in their outer layer (CD4 receptors) and actively penetrating their interior.
- Transcytosis. A mixture of endocytosis and exocytosis, it allows the transport of substances from one medium to another, for example, from the blood capillaries to the surrounding tissues.
- Sugar phototransferase. A typical process of certain bacteria like coli, which consists of chemically modifying the substrates inside to attract others by covalent bonding and thus save a lot of energy.
- Iron uptake. Many bacteria take up iron by secreting siderophores such as enterobactin, which binds to iron, forming chelates and is then absorbed by affinity into the bacteria, where the metal is released.
- LDL uptake. This lipoprotein with cholesterol esters is captured by the cell thanks to the action of an apoprotein (B-100) that allows it to enter the membrane and subsequently break down into amino acids.