Polar molecules are unable to go across unless they are crossed with the help of the protein channel.
Due to the semi-permeable nature of the plasma membrane polar molecules and molecules with high molecular weight are unable to cross the membrane.
Their transportation across the cell membrane is facilitated by integral transmembrane proteins or carrier proteins which form channels across the membrane.
For example, fructose in the small intestine is transported into the cell with the help of GLUT5 transporter.
Membranes of different organisms are rich in lipids, a bio molecule, which is hydrophobic in nature. Due to this reason, amino acids which are hydrophobic in nature will become part of proteins, residing on the external surface of the membrane, so that they can interact with the membrane through hydrophobic interactions. Hydrophobic amino acids, such as leucine and phenylalanine interact with hydrophobic phospholipid and help in holding the protein together.
And the amino acids, which are having charged side chains will become part of the proteins, present inside the cell.A trans membrane pump (transporter protein) is associated with membrane. It helps in the movement of nutrients from one side to another by some components of cells, such as vesicles. It carries out a facilitated diffusion process.
