Because cell membranes are selectively permeable they are capable of interacting with the outside environment. this interaction enables the cell to

A selectively permeable membrane is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion" 

The higher you go up the less oxygen there is 

It depends on the surface of the levels. the rougher the surface, the greater is the frictional force. for rougher surfaces, you would need to exert greater force to move the object. the concept is that, you must overcome the frictional force in order to bring the object into motion. this is the reason why we add lubricants to the joints so they could easily move. this answer was on