sodium-potassium pump

 The sodium-potassium pump, which is also called Na+/K+ ATPase, transports sodium out of a cell while moving potassium into the cell.

ATPases are a group of enzymes that catalyze the hydrolysis of a phosphate bond in adenosine triphosphate (ATP) to form adenosine diphosphate (ADP). 

They harness the energy released from the breakdown of the phosphate bond and utilize it to perform other cellular reactions.

Harness:control and make use of (natural resources), especially to produce energy.

1. ATPases are a group of enzymes that 
2. catalyze the hydrolysis of phosphate bond in adenosine triphosphate 
3. to form ADP adenosine di phosphate 

sodium ions

 As an example, even though sodium ions (Na+) are highly concentrated outside of cells, these electrolytes are charged and cannot pass through the nonpolar lipid bilayer of the membrane.

extracellular environments are isotonic,

When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function).

Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell.

A solution that has a higher concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure 3.8).

Water also can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself. 

Osmosis is the diffusion of water through a semipermeable membrane (Figure 3.7).