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Laboratory Experiment 9: Transpiration for AP Biology

By — McGraw-Hill Professional
Updated on Oct 24, 2011

This experiment takes the principles of water transport covered in cellular respiration, and applies them to the material in plants. You might want to review the material on plant anatomy and vascular tissue.

For a quick review on cellular repiration and plants, refer to the following concepts:

Just a quick reminder of how water moves from the soil to the leaves and branches of a plant. Three minor players in the transport of water are capillary action, osmosis, and root pressure. Water is drawn into the xylem (the water superhighway for the plant) by osmosis. The osmotic driving force is created by the absorption of minerals from the soil, increasing the solute concentration within the xylem. Once in the xylem, root pressure aids in pushing the water a small way up the superhighway. The main driving force for the movement of water in a plant from root to shoot is transpiration. When water evaporates from the plant, it causes an upward tug on the remaining water in the xylem, pulling it toward the shoots. The cohesive nature of water molecules contribu es to this transpiration-induced driving force of water through the xylem of the plants. Water molecules like to stick together, and when one of their kind is pulled in a certain direction, the rest seem to follow.

The first part of the experiment examines various environmental factors that affect the rate of transpiration: air movement, humidity, light intensity, and temperature. The rate of transpiration increases with increased air movement, decreased humidity, increased light intensity, and increased temperature. It is not hard to remember that increased temperature leads to increased transpiration—think about how much more you sweat when it is hot. It also makes sense that decreased humidity would lead to an increase in the rate of transpiration. When it is less humid, there is less moisture in the air, and thus there is more of a driving force for water to leave the plant. Imagine that you are standing with a 40-watt (W) bulb shining on your neck, and then a 100-W bulb shining on your neck. The higherwattage bulb will probably cause you to sweat more. The same thing with plants—the higher the intensity of the light, the more transpiration that occurs. Air movement is less obvious. If there is good airflow, then evaporated water on leaves is removed more quickly, increasing the driving force for more water to transpire from the plant.

To design an experiment to test the effects of various environmental factors on the rate of transpiration, measure the amount of water that evaporates from the surface of plants over a certain amount of time under normal conditions. You can do this using a piece of equipment known as a potometer, a device that aids in the measurement of water loss by plants. Then compare the normal rate with the rates obtained when the temperature, humidity, airflow, or light intensity are altered. (Remember: Change only one variable at a time!) If you run a lab of this nature, it is important to measure the surface area of the leaves involved because larger surface areas can transpire more water more quickly.

The rest of this experiment examines the structure of various cells found in plants.

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