When a Spirogyra cell is immersed in a salt solution more concentrated than its cell sap, it?

Step 1: Analyze the question A Spirogyra cell (a green alga) is placed in a salt solution that is more concentrated than its own cell sap. That means the surrounding solution is hypertonic to the cell.

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

• Hypotonic surroundings (more dilute than cell sap): water enters the cell and the cell becomes turgid.
• Isotonic surroundings (same concentration): no net movement of water.
• Hypertonic surroundings (more concentrated than cell sap): water leaves the cell.

Step 3: Apply it

• The salt solution outside is more concentrated than the cell sap, so the cell sap has the higher water concentration.
• Water moves out of the cell by osmosis, across the cell membrane, into the salt solution.
• The vacuole shrinks, and the cytoplasm and cell membrane pull away from the cell wall. This is called plasmolysis, and the cell becomes flaccid and shrivelled.

Step 4: Check the options

• A. Remains unchanged: wrong, because there is a concentration difference, so osmosis occurs.
• B. Takes up water and bursts: wrong. This would happen in a dilute solution, and plant cells don't burst anyway because of their cell walls.
• C. Absorbs a little water: wrong, because the water movement is outward.
• D. Loses water and shrivels: correct.

Tip: Water always moves toward the more concentrated solution, so remember "water follows salt". A concentrated outside means the cell loses water (plasmolysis). A dilute outside means the cell gains water (turgid). Plant cells don't burst because the cell wall protects them, while animal cells like red blood cells can.