in chemical composition among archaea, bacteria, and eukaryotes, allowing scientists to use them to characterize
unique species.
Plasma membranes from different cells types also contain unique phospholipids, which contain fatty acids. As
described inUsing Biochemistry to Identify Microorganisms (http://cnx.org/content/m58817/latest/),
phospholipid fatty acid analysis (PLFA) profiles can be used to identify unique types of cells based on differences in
fatty acids. Archaea, bacteria, and eukaryotes each have a unique PFLA profile.
Membrane Transport Mechanisms
One of the most important functions of the plasma membrane is to control the transport of molecules into and
out of the cell. Internal conditions must be maintained within a certain range despite any changes in the external
environment. The transport of substances across the plasma membrane allows cells to do so.
Cells use various modes of transport across the plasma membrane. For example, molecules moving from a higher
concentrationtoalowerconcentrationwiththeconcentrationgradientaretransportedbysimplediffusion,alsoknown
as passive transport (Figure 3.22). Some small molecules, like carbon dioxide, may cross the membrane bilayer
directly by simple diffusion. However, charged molecules, as well as large molecules, need the help of carriers or
channels in the membrane. These structures ferry molecules across the membrane, a process known as facilitated
diffusion (Figure 3.23).
Figure 3.22 Simple diffusion down a concentration gradient directly across the phospholipid bilayer.
Chapter 3 | The Cell 103