Amaple leaf falls from a tree branch.
from whose reference point is the leaf moving away from the observer?
a. an insect clinging to the leaf
b. a squirrel sitting beneath the tree
c. a bird perched on the top of the tree
d. a bird standing beneath the tree

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.