18. An organ is composed of several tissues that work together for a specific function. Which of the following is an example of an organ?
A. A leaf producing food
B. A tendon connecting two bones
C. A rootlet absorbing water
D. A heart muscle fiber contracting

19. Cells from a zebra and a horse both have
A. impermeable cell membranes.
B. large central vacuoles.
C. exactly the same DNA.
D. membrane bound organelles.

20. In Figure 28.1, what does the starburst represent?

A. Glucose
B. Chemical energy
C. The sun
D. A phosphate group

21. If your body cells do not have enough oxygen because you are exercising hard, which process takes over for cellular respiration?
A. Calvin cycle
B. Chemosynthesis
C. Fermentation
D. Glycolysis

22. Almost every cell in an organism has a full set of the same genes, yet every cell does not function in the same way. Why?
A. In the process of cell division, the genes undergo many different mutations.
B. The few cells that do not have a full set of DNA are the most influential.
C. Each cell uses only the specific genes it needs to carry out its function.
D. The same series of nucleotide pairs code for different proteins each time they occur.

23. Cells containing chlorophyll working together to absorb sunlight form an example of a(n)
A. chromosome.
B. tissue.
C. organelle.
D. organ.

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