When a marine fish was taken from the ocean and put in a tank of fresh water, it died after a short period because?

Step 1: Analyze the question A marine (saltwater) fish is moved into fresh water and dies quickly. We need to find the reason, which is an osmosis question.

Step 2: The concept Osmosis is the movement of water from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a semi-permeable membrane.

Key terms:

• Hypotonic: a solution with a lower solute concentration than the cell (more dilute).
• Hypertonic: a solution with a higher solute concentration than the cell.
• Isotonic: equal concentration.

Step 3: Apply it

• A marine fish's body fluids are adapted to the salty ocean. The surrounding sea water is roughly balanced with, or more concentrated than, its body fluids.
• Fresh water has very few dissolved salts, so it is hypotonic to the fish's body fluids and cells.
• Water therefore moves by osmosis from the fresh water into the fish's cells, through the gills and body surface.
• Marine fish have no efficient way to pump out this excess water or to retain enough salts. The cells swell, become overloaded, and eventually burst (lyse). This disrupts normal body function and the fish dies.

Step 4: Check the options

• A. Tank size has nothing to do with it. Wrong.
• B. Cells swell and burst because the fresh water is hypotonic. Correct.
• C. This is the reverse of what happens. Shrinking (crenation) occurs when cells are placed in a hypertonic solution, and fresh water is not hypertonic to the fish. Wrong.
• D. Starvation takes far longer than a "short period." Wrong.

Exam tip: Remember the direction of water movement: water follows salt. Water moves toward the more concentrated solution.

• Cell in hypotonic (dilute) surroundings: water enters, the cell swells and may burst.
• Cell in hypertonic (concentrated) surroundings: water leaves, the cell shrinks.

A similar JAMB question uses a red blood cell or a plant cell in distilled water. The same logic applies, though plant cells resist bursting because of their cell walls.