Microbiology, 2021

3.3 • Unique Characteristics of Prokaryotic Cells 95
(Figure 3.24). A major difference between passive and active transport is that active transport requires
adenosine triphosphate (ATP) or other forms of energy to move molecules “uphill.” Therefore, active transport
structures are often called “pumps.”
Figure 3.22
Mariana Ruiz Villareal)
Simple diffusion down a concentration gradient directly across the phospholipid bilayer. (credit: modification of work by
Figure 3.23
Ruiz Villareal)
Facilitated diffusion down a concentration gradient through a membrane protein. (credit: modification of work by Mariana
Figure 3.24
Mariana Ruiz Villareal)
Active transport against a concentration gradient via a membrane pump that requires energy. (credit: modification of work by