The cell membrane's main trait is its selective permeability, which means that it allows some substances to cross it easily, but not others. Small molecules that are nonpolar (have no charge) can cross the membrane easily through diffusion, but ions (charged molecules) and larger molecules typically cannot.
Charged Ions
An ion is a molecule that is charged because it has lost or gained an electron. The cell membrane is made of a bilayer of phospholipids, with an inner and outer layer of charged,hydrophilic "heads" and a middle layer of fatty acid chains, which are hydrophobic, or uncharged. Charged ions cannot permeate the cell membrane for the same reason that oil and water don't mix: uncharged molecules repel charged molecules. Even the smallest of ions -- hydrogen ions -- are unable to permeate through the fatty acids that make up the membrane. If ions "want" to enter the cell due to a high concentration of that type of ion on one side of the cell, they can do so by entering through the protein channels that are embedded between the lipids.
Large Polar Molecules
Large uncharged molecules, such as glucose, also cannot easily permeate the cell membrane. Although they do sometimes manage to slip across the membrane through diffusion, the process is extremely slow due to the size of the molecules. In order for these molecules to cross the membrane at a normal rate of speed, they need to resort to ion channels and specific transporters, which require energy output from the cell.
Explanation:
The identification is based primarily on the analysis of one or two regions of the ribosomal DNA: sequencing of the D1 / D2 domains of the 28S rRNA gene and / or of the ITS region (ITS1 and ITS2 intergenic spaces and that includes the 5.8S rRNA gene ). Its comparison with the sequences available in public databases allows the strains to be assigned to a specific species when the homology percentage of its sequences is greater than or equal to 99%.
Contraction is the answer
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