This type of system does not interact with the environment around it. It recycles all of the mass on its own. A. No system
B. Open system
C. Isolated system
D. Closed system

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom

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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.