Cells grown in culture are often used to investigate the role of organelles. As culture conditions can be easily manipulated, cells can be subjected to different chemicals and their responses studied and analyzed. In one such experiment, mouse cells were incubated in the presence of high concentrations of sucrose. The cells were examined under the microscope. Crystals of sucrose were found in the lysosomes of all the cells. When sucrose was depleted from the culture medium, the sucrose crystals in the lysosomes persisted for many days, but did not affect the growth of the cells. Eventually, the sucrose crystals disappeared. The rate of disappearance is shown in the graph below (note: all graphs are the same scale).
Researchers performed two additional experiments:
Immediately after sucrose was removed from the culture medium, the enzyme invertase was added. Invertase catalyzes the cleavage of sucrose into monosaccharides. The cells were then examined for the presence of sucrose crystals. The results are shown below.
Mouse cells grown in the presence of invertase (cell type A) were mixed with cells grown in the presence of sucrose (cell type B). A reagent that fuses cells was also added to the culture. After fusion, the cells (cell type AB) were grown in the absence of sucrose. The cells were then examined for the presence of sucrose crystals. The results are shown below.
Another sugar, mannose, was added to mouse cells, and mannose crystals formed inside lysosomes. Similar to sucrose crystals, the mannose crystals disappeared in a few days after the sugar was removed from the medium. If invertase were added to the culture immediately after mannose was removed, what would you predict would happen?
|A||The mannose crystals would disappear in less than two days.|
|B||The mannose crystals would disappear within a few days.|
|C||Sucrose crystals would form.|
|D||Mannose crystals would appear in the cytoplasm.|
Recall that enzymes are specific for their substrates. Invertase would probably not catalyze the break down of mannose. Therefore, even in the presence of invertase, the mannose crystals would take several days to be eliminated from the cells.