Final answer:
A blood cell in fresh water swells due to a hypotonic environment, potentially causing it to burst; in concentrated salt water, it shrinks due to a hypertonic environment. Both effects are due to osmosis, which is the movement of water across a permeable membrane to balance solute concentrations.
Step-by-step explanation:
When a red blood cell is placed into fresh water, it will swell and potentially burst due to the influx of water in an attempt to balance the solute concentration inside and outside the cell; this is known as hemolysis. Conversely, if placed in concentrated salt water, the cell will lose water and shrink, which is called crenation. This occurs because of osmosis, the movement of water across a semi-permeable membrane.
In the case of fresh water, the environment is hypotonic to the blood cell, meaning it has a lower concentration of solutes compared to the inside of the cell, leading to water entering the cell. However, in concentrated salt water, the environment is hypertonic, having a higher concentration of solutes than the cell's interior, causing water to leave the cell. Without proper tonicity, the imbalance of water movement can disrupt cell function and structure, leading to the aforementioned hemolysis or crenation.