Which of the following is a characteristic of vertebrates? a. stinging cells b. an inner skeleton made of bone or cartilage c. a hard exoskeleton made of chitin d. a hard outer shell

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.