Answers To Review Questions

1. E. All these statements about biological membranes are true.

2. A. Collagen is a major constituent of the extracellular matrix. Considering the other answers: B and C are wrong because phospholipid is found in membranes while RNA is intracellular. Answer D is wrong because the extracellular matrix is an open meshwork through which the aqueous extracellular medium and its dissolved solutes can easily pass.

3. B. Plants do not have gap junctions, indeed the presence of the cellulose cell wall means that even if the cells produced the protein subunits neighboring cells never approach each other closely enough to form gap junctions.

4. C. Proteins are synthesized using the information on RNA that must be exported from the nucleus to the cytoplasm through nuclear pores. None of the other choices is correct. In particular, the nuclear envelope with all its nuclear pores breaks down in cells undergoing mitosis.

5. D. The CO2 must diffuse across the inner mitochondrial membrane, the outer mitochondrial membrane, the plasma membrane of the first cell, the plasma membrane of the second cell, and the membrane of the Golgi apparatus.

6. D. The cell center is occupied by the centrioles (which are not organelles because they are not membrane-bound) and by the Golgi apparatus.

7. B. The vast majority of cells are able to release calcium ions from the endoplasmic reticulum when stimulated. The nuclear envelope, mitochondria, and lysosomes may be able to release calcium ions in certain more limited circumstances.

8. A. Most ribosomes that are not free in the cytoplasm are found on the rough endoplasmic reticulum, but some are found on the outer nuclear membrane, which is continuous with the membrane of the endoplasmic reticulum (see Fig. 3.5). Mitochondria and chloroplasts have their own ribosomes, but these are found inside these organelles, not on the outer surface.

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