Back To CourseAP Biology: Exam Prep
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Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.
Humans sweat - a lot. Whether we jog up the stairs, are stuck in a crowded elevator, or just have too many layers of clothing on, we release water in an effort to keep our body temperature relatively constant. This relatively stable set of internal conditions is called homeostasis. Plants also release water as part of their need to maintain homeostasis. However, instead of perspiration, it's called transpiration, technically defined as the loss of a plant's water to its environment through evaporation.
Let's start by reviewing a few terms crucial to understanding properties of water. First, recall that water moves from an area of high concentration to an area of low concentration through osmosis. Water molecules also exhibit the properties of cohesion, in which water molecules stick to one another, and adhesion, in which water molecules stick to surfaces. One way to remember the difference between these two terms is to think that in adhesion you 'add' a surface, while cohesion works when water molecules 'cooperate' with one another. Together, cohesion and adhesion cause capillary action, the movement of a liquid due to forces between molecules of that liquid. Capillary action and surface tension, the pressure on the surface of a liquid due to intermolecular forces, allow water to move up a narrow tube, like xylem in plants, against the force of gravity.
But how does this process really work? First - think 'TACT': transpiration, adhesion, cohesion, and tension. All of these things combine to draw water from the root up and through the shoot. How? Well, first, recall that gas exchange takes place primarily in openings in the leaves called stomata. Although the guard cells that surround these stomata have some control over how much water can escape through the stomata, a significant amount of water enters the atmosphere by evaporation. This is the technical definition of transpiration, and the first 'T' in 'TACT.'
As this happens, water that's already in the leaves rushes in to fill the space left behind by the evaporating water molecules, which stick to one another and to the inner surfaces of vessels in the leaves and stem. These are the 'A' and 'C' of 'TACT.' These moving water molecules, in turn, pull the water from the xylem. Tension, the final 'T,' between the water molecules pulls water up the xylem from the roots. It's not unlike pulling on a line of people holding hands.
What are some variables that you think might affect transpiration? Let's start at the top. If you poke one hole in a bucket of water, it will leave the bucket very slowly. If you poke 20 holes, on the other hand, it will empty the bucket pretty quickly. Likewise, it's reasonable to expect that the number of stomata per leaf will have an effect on the rate of transpiration. Creating microscope slides using dermal tissue squares of equal size from the upper and lower epidermis, counting the number of stomata that appear, and estimating the number of stomata per plant per unit of surface area will give you an estimate of that plant's ability to complete transpiration.
A better and quantitative way to measure the rate of transpiration is to create a potometer, a set-up designed to measure the rate of transpiration. Water loss is measured by using a pipette, and the rate of transpiration is determined by dividing the measured water loss by a specific period of time. You can then subject the plants of equal leaf surface area to more than one potometer with different environmental conditions to see what variables affect the rate of transpiration.
What are some of these variables? Let's consider four set-ups:
|1||room conditions (control)|
|4||increased wind (fan)|
Think about what you would predict for each of these conditions, considering what is going on in the leaves. How would each of these conditions compare to the control condition? These graphs show some data you might get from such an experiment:
What relationship do you see between the rate of transpiration and the ability of the water to leave the stomata? The greatest rate of transpiration occurred in condition three because light does two things: it makes water molecules move more quickly and, therefore, evaporate more rapidly due to an increase in temperature, and it also increases the rate of photosynthesis, which, as you recall, requires water. The increased wind (via the fan) moves water away from the leaves of the plant quickly, again, allowing more water to exit the stomata. Finally, the increased humidity decreases the rate of transpiration because an increase in the concentration of water around the leaves causes a drop in evaporation through the stomata.
Let's conclude by summarizing this lab exercise. Transpiration is defined as the loss of a plant's water to its environment through evaporation. Transpiration is just the first 'T' in 'TACT', in the mechanisms necessary to move water through plants. Adhesion, the property of water molecules that makes them stick to surfaces, like the inside of xylem, and cohesion, the 'sticking together' of water molecules by hydrogen bonding are the 'A' and the 'C.' Tension, the pressure on the surface of a liquid due to intermolecular forces, is the final 'T' that helps water move through the xylem from the roots to the shoots, where it exits by evaporation through the leaf stomata.
A simple experiment to determine the rate of respiration in a plant can be accomplished by using a potometer. If you know the approximate surface area of the leaves in square centimeters, and use the potometer to measure the amount of water lost to the air over a specific period of time, you can calculate the rate of transpiration in units of volume, such as milliliters per time (hours or minutes). Conditions that increase the rate of transpiration are those that favor water loss from the leaves, like higher temperatures, increased light, and increased wind. A condition that decreases the rate of transpiration is high humidity, because evaporation of water from the stomata is decreased.
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Back To CourseAP Biology: Exam Prep
29 chapters | 298 lessons