Pinocytosis is a type of endocytosis. During pinocytosis, the cell takes up extracellular fluid through plasma membrane invagination and vesicle formation. The process is energetically costly and requires many molecules of ATP. Pinocytosis is a non-specific process, meaning that materials are not selectively taken up. In other words, extracellular fluid is engulfed along with any or all particles within it. The fluid taken up by pinocytosis is already digested and degraded; therefore, the process is not accompanied by the action of lysosomes.

Active transport is the movement of molecules against their concentration gradient (from an area of low concentration to an area of high concentration, requiring energy, usually in the form of ATP. Passive transport is the movement of molecules with their concentration gradient (from an area of high concentration to an area of low concentration), and does not require energy input. Facilitate diffusion is the movement of molecules with their concentration gradient across the cell membrane using transmembrane proteins (carrier proteins or channels), and does not require energy. Osmosis is the movement of a solvent (usually water), from an area with a lower concentration of solute to an area of higher concentration of solute; this process does not require energy.

This is the definition of passive transport. Active transport requires energy for molecules to move. Also, the Na+/K+ pump requires energy, and thus is a form of active transport. Vesicle transport, including endocytosis and exocytosis, also requires energy.

 Only small, nonpolar molecules and small, uncharged, polar molecules can freely diffuse across the lipid bilayer of the cell. Glucose is a large molecule that cannot freely diffuse across the lipid bilayer, even if this would be favored by the concentration gradient. Facilitated diffusion is the movement of molecules across the lipid bilayer using carrier proteins or channels, which does not require energy input.

A hypotonic solution is a solution in which there is a lower concentration of solutes in the solution than in the cell. Thus, water (the solvent) will enter the cell, causing the cell to swell.

 As the size of a cell increases, the surface area to volume ratio decreases, as surface area is a squared function, while volume is a cubic function. Due to the decreasing surface area to volume ratio, there is less area for the diffusing molecules to actually enter the cell, thus decreasing the rate at which diffusion can occur.

The cell membrane is a phospholipid bilayer, where the hydrophobic heads prevent hydrophilic molecules (such as charged ions) from crossing. Small, uncharged molecules are able to passively diffuse across the cell membrane when favored by the concentration gradient.