How do phospholipids move to allow water and other molecules to pass through into or out of the cell?

Phospholipids have a hydrophobic tail, and a hydrophilic head.


Hydrophilic means water loving, hydrophobic means water hating.





The cell membrane is made up of a phospholipid bilayer (two layers).





The hydrophilic tails aggregate together leaving the heads (which are water loving) to stick out.


Or more precisely, this structure maximizes contact of hydrophilic regions of proteins and phospholipids with water in the cytosol and extracellular fluid, while providing their hydrophobic parts with a nonaqueous environment.





In the fluid mosaic model of a cells membrane, proteins are bobbing through a fluid bilayer of phospholipids.





Nonpolar molecules such as hydrocarbons, carbon dioxide, and oxygen, are hydrophobic and can therefore dissolve in the lipid bilayer membrane (the hydrophobic middle) and can pass it easily.





However, the hydrophobic core impedes the direct passage of ions and polar molecules which are hydrophilic, through the cell membrane.





Polar molecules such as glucose and and other sugars pass only slowly through a lipid bilayer and even water,an extremely small polar molecule, does not cross very rapidly.





Thus, the lipid bilayer is selective in this manner and this is where we get the characteristic of ';selectively permeable';.





The cell membrane is permeable to specific ions and some polar substances, but for the most part they rely on transport proteins to get inside the cell. These span the membrane and rely on both passive and active transport to get the cell its necessary nutrients.


One example is the aquaporin. Aquaporins are protein channels that allow water to more easily enter the cell.





Other than proteins, cells have something called phagocytosis, pinocytosis, and cell-mediated endocytosis to exchange materials with the environment where the cells membrane actually surrounds many sbstances and breaks to form a vesicle with the substances to move around inside the cell. And other vesicles may fuse with the cell membrane to release the substances they have to the environment.