Thinking Scientifically

1. A liver cell has a volume of 5,000 μm³. Its total membrane area, including the inner membranes lining organelles as well as the cell membrane, fills an area of 110,000 μm² . A cell in the pancreas that manufactures digestive enzymes has a volume of 1,000 μm³ and a total membrane area of 13,000 μm² . Which cell is probably more efficient in carrying out activities that require extensive membrane surfaces, and why?

2. What advantages does compartmentalization confer on a large cell?

3. Why does a muscle cell contain many mitochondria and a white blood cell contain many lysosomes?

4. The amoeba Pelomyxa palustris is a single-celled eukaryote with no mitochondria, but it contains symbiotic bacteria that can live in the presence of oxygen. How does this observation support or argue against the endosymbiont theory of the origin of eukaryotic cells?

Additional Questions and Terms

1. In an inherited condition called glycogen cardiomyopathy, teenagers develop muscle weakness, including heart muscle. Affected muscle cells contain huge lysosomes, swollen with glycogen. What is the biochemical basis of the glycogen buildup?

2. Cells of the green alga Chlamydomonas reinhardtii are grown for a few hours in a medium that contains amino acids "labeled" with radioactive hydrogen, called tritium. The Chlamydomonas cells are then returned to nonradioactive medium. At various times, the cells are applied to photographic film (which radiation from the tritium exposes, producing dark silver grains). The film is then examined under an electron microscope. After 3 minutes, the film shows radioactivity in the rough ER; after 20 minutes, in the smooth ER; after 45 minutes, in the Golgi apparatus; after 90 minutes, in vesicles near one end of each of the cells. After 2 hours, no radioactive label is evident. What cellular process has this experiment traced? How might a similar technique be used to follow a lysosome's activity?

3. Discuss why the cell theory and the endosymbiont theory are considered to be theories, and how they are still evolving.