The "plasma membrane," which controls what comes in and goes out of a cell, is integral to maintaining homeostasis. Cell transport of small molecules across the cell membrane happens in several different ways. Some small, nonpolar molecules cross the plasma membrane along the concentration gradient directly through the "phospholipid bilayer," a barrier around the cell composed of two sheets of lipid molecules. Other smaller charged particles, such as water molecules and charged ions, cross the membrane via channel proteins through the process of "facilitated diffusion." Some substrates may need to be pumped across the membrane against the concentration gradient, which requires an energy input or the help of carrier proteins to cross the membrane via "active transport." In this unit, high school biology students designed a functional, three-dimensional model of a plasma membrane for cell transport. The model plasma membrane needed to allow different substrates to cross it via a variety of transport proteins. The model imitated the function of the phospholipid bilayer and incorporated channel proteins and carrier proteins to transmit four materials that represented different types of substrates that would need to enter or exit a cell. As students used the engineering design process to design and build an artificial cell membrane, they learned about the structures and functions of each part of the phospholipid bilayer and the processes of "osmosis," "diffusion," "facilitated diffusion," and "active transport."