Diffusion, movement of particles across a membrane is driven by differences in
a. concentration.
b. shape.
c. force.
d. size.

A concentration.

In cells, there are two types of transportation of needed organic compounds. The active and passive transport.
The active transport requires energy or ATP while passive doesn't. Further, diffusion, a passive transport transfers compounds from higher to lower concentrations. 

Interacting group of various species in a common location. for example, a forest of trees and undergrowth plants, inhabited by animals and rooted in soil containing bacteria and fungi, constitutes a biological community.