A plasma membrane is permeable to specific molecules that a cell needs. Transport proteins in the cell membrane allow for selective passage of specific molecules from the external environment.

Like the exterior walls of a house, the plasma membrane is semi ... of carbon dioxide (Figure 3). The particular energy pathway that a cell employs depends in large part on whether that cell ...

This takes place when molecules diffuse across the cell membrane by travelling through specific transport proteins. It occurs down a concentration gradient - molecules move from an area of high to ...