20. most of today’s organisms could not have lived on earth 3.6 billion years ago because there was no in the air then.

answer; /// i believe the correct answer is (thrombocytopenia); (a)

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.