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From Seed to Seed:
Plant Science for K-8 Educators

Why Is Transpiration So Important?

Transpiration serves several important functions in the plant.

First of all, transpiration drives the "circulatory" system of the plant. Imagine a vein at the tip of a leaf. Through the intricate network of vascular tissues, this leaf is connected to the roots-similar to the way capillaries in your fingertips are connected to your heart. In humans, the force driving circulation is the heartbeat; in plants, it's transpiration. Water vapor escaping the leaf surface creates a tension that draws more water up to replace it. Through the work of veins, this process occurs in every leaf throughout the entire plant.

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Secondly, transpiration cools the plant. Many animals use water to regulate internal body temperature. When we get too hot, we perspire, and as the water on the surface of our skin evaporates, it cools us. The same is true of plants. Transpiration cools the plant, which may prevent the plant from overheating, especially in direct sunlight.

Finally, transpiration is crucial to maintaining water pressure within the cells, keeping them rigid so that they can support the plant. The water pressure inside the cells is called turgor pressure, and it is maintained by a process called osmosis.

the movement of water across a differentially permeable membrane from a place where water concentration is higher to one where the concentration is lower

Fluids like to reach a state of equilibrium. If you pour milk into your coffee, it doesn't all stay in one place but diffuses throughout the cup. If your students put a drop of food coloring in a basin of water, it will diffuse until all of the water is tinted.

Plant cells maintain a delicate balance of water and dissolved salts and sugars. If the fluid inside the plant cell is "saltier" than the surrounding fluid, water molecules move into the cell to try to reach an equilibrium between the inside and the outside of the cell. If there were no cell membrane, then, at the same time, the salty water would diffuse out, until the salt concentrations inside and outside the cell were equal.

But, the cell membrane is "differentially permeable," meaning that the water molecules can enter, but the salt molecules are too large to escape. The result is that water pressure builds inside the cell, causing the cell membrane to exert pressure on the cell wall. You can demonstrate this for your students by inflating a balloon inside a box. Eventually the balloon will exert pressure on all sides of the box, much as the cell membrane exerts pressure on the cell wall.

These rigid, stacked "boxes" keep the plant upright. What would happen if the balloon deflated? The box would collapse. Plants must maintain their internal water pressure, or turgor pressure, to keep stems rigid and leaves expanded to the sunlight. This means that water must be available to the plant whenever it needs it. If water isn't available, the cells collapse and the plant wilts.

Rates of transpiration and water loss vary depending on the temperature of the air, humidity, wind, and the amount of leaf surface area. On a hot, dry, sunny day with a warm breeze, plants with large leaves lose a tremendous amount of water. On a cool, cloudy, humid day, plants transpire far less. Explain to your students that by noting how they feel, they can get a sense of a plant's transpiration rate. If they need to drink lots of fluids on hot, sunny days, so do the plants!

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