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Transport of substances and membranes in a cell
Medical expert of the article
Last reviewed: 19.10.2021
Substances circulate in the cell, being packed into membranes ("movement of the cell's contents in containers"). Sorting of substances and their movement are associated with the presence in the membranes of the Golgi complex of special receptor proteins. Transport through membranes, including through the plasma membrane (cytolemma), is one of the most important functions of living cells. There are two types of transport: passive and active. Passive transport does not require energy costs, active transport is volatile.
Passive transport of uncharged molecules is carried out by a concentration gradient through diffusion. Transport of charged substances depends on the potential difference on the surface of the cytolemma. As a rule, the internal cytoplasmic membrane surface carries a negative charge, which facilitates penetration of positively charged ions into the cell.
The transition of ions or molecules from the zone where these substances are in a higher concentration to the zone with a lower concentration is called diffusion. Specific transport proteins, built into the membrane, carry small molecules through it. Each transport protein carries the transport of one class of molecules or only one compound. Transmembrane proteins are either carriers, or form "channels". Diffusion can be neutral when uncharged substances pass between lipid molecules or through cytolemma proteins that form channels. "Light" diffusion occurs with the participation of specific carrier proteins that bind the substance and carry it through the membrane. "Light" diffusion proceeds faster than neutral diffusion.
Active transport is carried out by carrier proteins. In this case, energy is expended due to the hydrolysis of ATP, as well as different potentials (charges) on different surfaces of the membrane. Active transport occurs against the concentration gradient. The membrane potential is maintained on the cytolemma using a sodium-potassium pump. This pump, which pumps K + ions into the cell against concentration gradients, and Na + ions into the extracellular space, is an ATPase enzyme. Due to ATPase Na + ions are transported through the membrane and are released into the extracellular environment, and the K + ions are transported inside the cell. ATPase also carries active transport of amino acids and sugars.