Answer:
diffusion
Explanation:
The process of random movement of ions and molecules from region of their higher concentration gradients to the region of lower concentration gradient is known as diffusion. The kinetic energy the molecules process make them to move one direction to another in order to gain free movement
it takes place only in liquid and gaseous medium, and It is affected by the diffusion distance, but proportional to the concentration gradient,and the surface area exposed for the diffusion to take place.( based on Fick's Law
In cell membrane because of the composition of the layer, polar molecules can diffuse freely down across the membrane because of their polarities and small sizes. Water, C02 and oxygen are examples..However some polar molecules are too big to move freely, and therefore need special passages or channels of protein to pass through. e,g glucose, Amino acid.