Question

explain how molecule and ions get into and out of cells

Answer 1

The internal composition of the cell is maintained because the plasma membrane is a selectively permeable structure. The plasma membrane forms a barrier that blocks the free exchange of molecules between the cytoplasm and the external environment of the cell.

The plasma membrane allows the passage of various polar molecules such as ions, sugars, amino acids and others. Special classes of transport proteins (carrier and channel) are responsible for transferring such solutes across cell membranes. Thus the transport across plasma membrane can occur either through the lipid bilayer or through the transport proteins.

Transport across PM is of two types: 1. passive diffusion and 2. Active transport

Passive diffusion occurs along the concentration gradient and without the use of metabolic energy. It may be simple diffusion or facilitated diffusion.

In simple diffusion, molecules simply dissolve in the bilayer, diffuses across it. No membrane proteins are involved. The direction of transport is determined by the relative concentrations of the molecule inside and outside the cell. Gases such as oxygen and carbon dioxide, hydrophobic molecules such as benzene, small polar but uncharged molecules such as water and ethanol are able to diffuse across the plasma membrane.

Facilitated diffusion involves the movement of molecules in the direction determined by their relative concentrations inside and outside the cell. However, the passage is mediated by transport protein called permease and is selective in nature. Facilitated diffusion allows polar and charged molecules , such as carbohydrates, amino acids, nucleosides, and ions to cross the membrane.

Active transport occurs against concentration gradient and is mediated by carrier proteins. Metabolic energy is used to move ions or molecules against a concentration gradient. Active transport results in the accumulation of solute molecules on one side of membrane.