19. Mark found an organism in a pond, and he thinks it’s a freshwater sponge. His friend Ralph thinks it looks more like an aquatic fungus. How can they decide whether it is an animal or a fungus? (A) See if it can swim. (B) See if it is a eukaryote or a prokaryote. (C) Look for cell walls under a microscope. (D) Determine whether it is unicellular or multicellular. (E) Figure out whether it is autotrophic or heterotrophic.

21. A major difference in the mechanism of action between steroid and peptide hormones is that (A) steroid hormones mainly affect the synthesis of proteins, whereas peptide hormones mainly affect the activity of proteins already in the cell. (B) target cells react more rapidly to steroid hormones than they do to peptide hormones. (C) steroid hormones enter the nucleus, whereas peptide hormones stay in the cytoplasm. (D) steroid hormones bind to a receptor protein, whereas peptide hormones bind to G protein. (E) steroid hormones affect metabolism, whereas peptide hormones affect membrane permeability.

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