At which Loint is air pressure lowest? The image shows a representation of mountains of
various heights, numbered 1, 2, and 3. The plain is
labeled number 4.
O 1
2
3
04

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.