4. Many fresh-water one-celled organisms have structures called contractile vacuoles. These
structures collect and pump out excess water that accumulates in the cell. Name the process that
causes water to flow into these organisms.
Explain why contractile
vacuoles would be of little value to one-celled organisms living in the ocean (salt water).

The one-celled organisms have some structures called contractile vacuoles which help them to remove excess of water from their body.
The water enters in the body through osmomis.
Osmosis is the movement or transfer of water or other solvent through a plasma membrane from a region of low solute concentration to a region of high solute concentration, by which equal concentration is achived on the both sides. Osmosis is passive transport which means it does not require any energy.
There is high solute(cell organels in this case) concentration inside the cell and low solute concentration in surrounding water and hence water enters inside the cell.
It is necessary to expel excess of water from the cell as if the water is not expled from the body then water will continuously enter inside the cell as a result of which the cell will burst(same as raisins burst in water when they are kept for too long).
The concentration of salt water is similar to the concentration of the cell hence contractile vacuoles has a little value in the ocean (salt water).

Result: the process of movement of water or other solvent through a plasma membrane from a region of low solute concentration to a region of high solute concentration is known as osmosis.