2. What do geologists mean when they refer to the “grains" that make
up a rock?

The scale and arrangement of the minerals or grains that make up a rock are referred to as its texture. Textures of Igneous Rock Compositions and Igneous Rock.

Explanation:

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.