A water sports science project from Science Buddies

Key Concept Biology Osmotic Cell Chemical Concentration Water Transport

Introduction Have you ever wondered how plants "drink" water from the soil? The absorption of water by plants is quite complicated. A process called osmosis helps water enter the roots of plants from the soil and then into the plant cells. In this activity, you will see for yourself how to make water flow through osmosis!

Most of the water in the background underground is not pure water. It usually contains dissolved mineral salts. Animals and plants need these salts (including calcium, magnesium, potassium, and the sodium salt you may be familiar with) to grow, develop, and maintain health. Different water sources carry different amounts of these salts. Nature wants to balance an unbalanced system. Therefore, if you mix two waters with different salt concentrations, the salt will not remain separated, but will be evenly distributed in the solution until the salt concentration is always the same.

If you use a semi-permeable membrane to separate the two salt solutions, you will find a similar reaction. The semi-permeable membrane is a barrier that only allows certain particles to pass through and prevents others. This type of membrane usually allows water to pass through, but does not allow the salts dissolved in the water to pass through. In this case, because only water can pass through the membrane, the water will start to move from the area with lower salt concentration (more water and less salt) to the area with higher salt concentration (less water and more salt). Only when both sides of the membrane When the concentration of salt and water are the same, this water movement will stop.

The process of passing water through a semi-permeable membrane is called osmosis. Plants use this process to absorb water. They create an environment of high salt concentration in the root cells in contact with the soil. The cell wall acts as a semi-permeable membrane that only allows water to pass through. Since the water outside the root cells has a lower salt concentration, the water starts to enter the root cells due to osmotic effect. The water that enters the plant fills the cells and can spread to the rest of the plant. However, the osmotic effect is two-way. If a plant is placed in water with a higher salt concentration than its intracellular concentration, the water will move out of the plant to balance the difference in concentration. As a result, the plant shrank and eventually died. In this activity using potatoes and different saline solutions, you will see this effect with your own eyes.

Observations and results Are your potato strips shrinking and swelling? In the beginning, all potato chips should have the same length and feel the same. However, when you put them into different solutions, the situation begins to change. The potato fries in the "0 gram" cup may become larger, while other potato fries may become shorter after they are placed in salted water for 30 minutes. (If you don't see any significant changes after 30 minutes, leave the potato strips in the brine for longer.)

The shrinkage and expansion of potato strips are due to osmosis. Potatoes are made of cells, and their cell walls act as semi-permeable membranes. A 0 gram solution contains less salt and more water than potato cells (more salt and less water). In order to balance these concentration differences, the water in the cup will enter the potato cells. The water entering the potato cell pushes the cell wall and makes the cell bigger. As a result, the entire potato chip became larger. The opposite is true in higher concentration salt solutions. If the salt concentration in the cup is higher than the salt concentration in the potato cells, water will flow out of the potato and enter the cup. This causes the potato cells to shrink, which explains why the length and diameter of the potato strips become smaller. Due to the contraction of the potato cells, the potato strips also become less firm. If you bend potato strips, you should notice that those potato strips in the solution with the highest salt content are more likely to bend than potato strips in water without salt.

If you made a chart, you might notice that there is a salt concentration at which the potato strips neither swell nor shrink. This should be where your data curve and your starting length line intersect. At this time, the salt concentration in the potato cells and in the cup is the same. Because the concentration is already balanced, no water moves.

Clean up the saline solution in the discarded sink. Throw the potato chips into the compost to clean up your work space. You can use other unused potato pieces for cooking.

More exploration penetration, from the Biology Dictionary Does fish drink water? Cucumber Chemistry from the Office of Science and Society of McGill University: Use a desiccant to capture moisture, from "Scientific American", soak it dry! How water passes through plants, from Science Buddies STEM activities for children, from Science Buddies

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