<span>President Abraham Lincoln issued the Emancipation Proclamation on January 1, 1863, whereas Dr. King gave his famous "I Have A Dream" speech on August 28, 1963. Therefore, over 100 years had passed between the two events, and while slavery had been formally abolished for that duration, the Civil Rights movement showed that there was still tremendous work to be done in ensuring equality and fair treatment for African Americans.</span>
You have to give 2 breaths to each child.
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.
For the desalination of salt water a draw back is that it's more costly than fresh water
