What happens when blood cells are placed in pure water?

The water molecules rapidly move into blood cells.

When blood cells are placed in pure water, the water molecules will rapidly move into the blood cells due to a process called osmosis.

The correct option is:

(2) The water molecules rapidly move into blood cells.

Osmosis is the movement of water molecules from a region of lower solute concentration (pure water, in this case) to a region of higher solute concentration (inside the blood cells). Blood cells contain various solutes, including salts, proteins, and other molecules, which create a higher solute concentration inside the cells compared to pure water.

As a result, water molecules from the surrounding pure water will move across the semipermeable cell membrane and into the blood cells, causing them to swell or even burst under certain conditions. This process helps equalize the concentration of solutes inside and outside the cells. However, if the solution outside the cells were a hypotonic solution (lower solute concentration than the cells), the cells could lose water and shrink.