Protists with contractile vacuoles
A. are algae.
B.use them to expel excess water from the cell.
C. typically live in salty seawater.
D. use them for motility. E. All of the choices are correct.
The Correct Answer and Explanation is :
The correct answer is B. use them to expel excess water from the cell.
Explanation:
Protists are a diverse group of eukaryotic microorganisms that can be unicellular or multicellular. Among the many types of protists, some possess contractile vacuoles, specialized organelles that help in osmoregulation. These vacuoles are crucial for maintaining the cell’s internal environment, especially in freshwater organisms.
Contractile Vacuoles:
A contractile vacuole is an organelle that actively expels excess water from the cell to maintain osmotic balance. Protists, particularly those living in freshwater environments, often experience an influx of water due to osmosis. Since the concentration of solutes inside the cell is higher than the surrounding freshwater, water constantly enters the cell. Without the contractile vacuole, the cell would burst from the pressure. The contractile vacuole collects the excess water from the cytoplasm and expels it through the cell membrane.
Why Other Choices Are Incorrect:
- A. Algae: While some algae are protists, not all protists with contractile vacuoles are algae. Algae are typically photosynthetic protists, and they don’t all require contractile vacuoles.
- C. Salty Seawater: Protists that live in salty seawater (marine environments) generally do not need contractile vacuoles because the osmotic balance between the cell and the surrounding seawater is more stable. In salty environments, cells may require different adaptations to manage water balance, such as salt-excreting mechanisms.
- D. Motility: Contractile vacuoles are not involved in motility (movement). Instead, they serve in the expulsion of excess water, which helps in maintaining the cell’s structural integrity.
Thus, B is the best answer because contractile vacuoles help protists regulate water, particularly in environments where they are at risk of taking in too much water.