Why does the cell sometimes have to expend energy to move individual molecules across the cell membrane?

Normally, molecules move naturally down a concentration gradient [From an area of high concentration to an area of low concentration] by a process called diffusion. However, sometimes the concentration of the needed substance is higher in the cell than out of it, so the cell uses a different process called Active Transport. Unlike Diffusion, Active transport requires energy, as it is going against a concentration gradient.

Cells expend energy to move cells across the cell membrane as they use Active transport rather than diffusion.

answer and explanation;

volcanic rocks are igneous rocks which form from lava, molten rock which has been thrust out of a volcano onto the surface of a rocky body such as an asteroid, planet, or dwarf planet.  

plutonic rocks are igneous rocks which form from subterranean chambers of molten rock or magma. plutonic rocks make up the base of continental crust as well as oceanic crust. their slow rate of formation causes them to be coarse grained since there is plenty of time for large crystals to form before the magma solidifies into rock.

the textural difference is that plutonic rocks are more coarse grained than volcanic rocks. plutonic rocks are coarse grained while volcanic are fine grained ; glassey or fragmental. since fine and coarse are adjectives the decision as to whether a rock is fine or coarse grained is generally made on the bases of whether or not the individual crystals can be seen in most if the rock.