Question

The presence of a contractile vacuole indicates that a single-celled protist _______ a. is marine c. is photosynthetic b. lives in fresh water d. secretes a toxin

Short answer

A contractile vacuole indicates that the protist b. lives in fresh water.

Step-by-step solution

Understand the Function of a Contractile Vacuole

A contractile vacuole is an organelle found in some single-celled organisms, which functions to expel excess water that enters the cell. This organelle helps to maintain the cell’s osmotic balance and prevents it from bursting due to internal pressure.

Consider Environmental Conditions

Single-celled protists that live in hypotonic environments, such as fresh water, are constantly taking in water through osmosis. This is because the concentration of solutes inside the cell is higher than in the surrounding water, causing water to flow into the cell.

Connect the Dots

Since a contractile vacuole is used to expel excess water and maintain osmotic balance, organisms with this organelle likely live in environments where water influx is common, such as fresh water environments.

Select the Correct Answer

Based on the understanding of the contractile vacuole's role and environmental conditions where one might be necessary, the correct answer is that a single-celled protist with a contractile vacuole most likely b. lives in fresh water.

Key concepts

Single-celled protists

Single-celled protists are fascinating organisms because they are composed of only one cell, yet they manage to carry out all essential life functions that multicellular organisms do. These protists are highly adapted to their environments and can be found in various ecosystems, from oceans to freshwater ponds and even in moist soil.
They often have specialized structures that help them survive and thrive in their respective habitats:

• Many protists possess cilia or flagella, which are tiny hair-like structures. These help them move through their aquatic environments.
• Certain protists have light-sensitive eyespots that help them detect light and guide their movement.
• Some have protective external coverings or shells to shield themselves from predators or environmental changes.

These adaptations showcase the incredible diversity and complexity of life at the microscopic level.

Osmotic balance

Osmotic balance is crucial for the survival of single-celled organisms, especially those living in environments with varying concentrations of water and solutes. This balance refers to the ability of a cell to regulate the flow of water into and out of itself, ensuring it maintains proper internal conditions.

Here’s why it matters:

• Cells in hypotonic environments, like fresh water, face the challenge of excess water entering the cell. This can cause the cell to swell and might even lead to bursting.
• To prevent this, many single-celled protists have developed contractile vacuoles. These organelles act like tiny pumps, squeezing out excess water to prevent cell damage.
• Osmoregulation is not just about removing excess water. It also involves retaining or expelling solutes as needed to maintain equilibrium within the cell.

Understanding osmotic balance helps explain why many single-celled protists can live in freshwater environments without being destroyed by water influx.

Freshwater environments

Freshwater environments present unique challenges and opportunities for single-celled organisms. Unlike marine environments, freshwater bodies like ponds, lakes, and rivers generally have lower concentrations of salts and solutes.
In such settings:

• Water naturally tends to move into cells through osmosis because the solute concentration outside the cell is lower than inside.
• This influx of water can upset the internal balance of the cell, but structures like the contractile vacuole help to manage this process.
• Freshwater habitats are often dynamic, with varying temperatures, pH levels, and nutrient availability.

These factors make it necessary for single-celled protists to be highly adaptable and equipped to maintain homeostasis despite environmental changes. This adaptability allows them to flourish in diverse freshwater ecosystems.