1. Peter was preparing some potatoes for cooking. He put some water in a pot and added some salt. He cut up the potatoes and put them in the salt solution. Then the telephone rang and he spent 30 minutes talking to George. When he returned to put the potatoes on the stove they were flaccid (soft). Explain what happened to the potatoes. The concentration of free-moving water molecules is higher in potato cells, the cytoplasm of the cell is hypertonic. The water molecule will flow from higher concentration to lower concentration, so that they get out of potato cells through osmosis. The plasma membrane of plant cell will collapse as the water inside the cell become less and less, that cause plasmolysis and the cell become flaccid.
2. A salt water fish may die if placed in fresh water. Why?
In order to keep an isotonic condition, the salt water fish’s cells have the same concentration with salt water. When they are placed in fresh water, concentration of water molecule in fresh water is higher than that in fish’s cell, fresh water is hypertonic. Due to osmosis, the water molecule will move from fresh water to the cell. The saltwater fish has no mechanism to get the extra water out. The cell will explode, or salt water fish can adapt that much water inside their body, and they may die.
3. Explain why a salt solution is a good antiseptic (kills bacteria)? The concentration of water molecules is higher inside the bacteria, and the cytoplasm of bacteria is hypertonic. Due to osmoses, the water molecule will move from bacteria to salt solution. The bacterial cells will shrivel and die, as water leaving the cell.
4. A small amount of fertilizer will make the grass of a lawn grow. Too much fertilizer will “burn” kill the grass. Why? Using too much fertilizer will create a hypotonic surrounding for grass cell. The water molecule will move out of the grass, and cause plasmolysis of cell. The grass will then become yellow, flaccid, and die.
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