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Transport of substances and membranes in the cell
Medical expert of the article
Last reviewed: 04.07.2025
Substances circulate in the cell, being packed in membranes ("movement of the contents of the cell in containers"). Sorting of substances and their movement are associated with the presence of special receptor proteins in the membranes of the Golgi complex. 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 expenditure, active transport is energy-dependent.
Passive transport of uncharged molecules occurs along the concentration gradient by diffusion. Transport of charged substances depends on the potential difference on the surface of the cytolemma. As a rule, the inner cytoplasmic surface of the membrane carries a negative charge, which facilitates the penetration of positively charged ions into the cell.
The movement of ions or molecules from a zone where these substances are in higher concentration to a zone with lower concentration is called diffusion. Specific transport proteins built into the membrane transport small molecules across it. Each transport protein transports 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. "Facilitated" diffusion occurs with the participation of specific carrier proteins that bind the substance and transport it across the membrane. "Facilitated" diffusion is faster than neutral diffusion.
Active transport is carried out by carrier proteins. This consumes energy arising from 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 with the help of the 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. Thanks to ATPase, Na + ions are transferred through the membrane and excreted into the extracellular environment, and K + ions are transferred into the cell. ATPase also carries out active transport of amino acids and sugars.