Laboratory Review ❮ 251
Results
You should find that, no, energy transfer isn’t perfect. All of that plant’s energy was not, in
fact, transferred to and used by the caterpillars.
Key Concepts
- Understand how this lab relates to the idea of how living organisms use free energy.
- Biomass is the mass of living tissue minus any water weight.
Investigation 11: Transpiration
This experiment takes the concepts found in Chapter 6 of the text and applies them to the
material in Chapter 14, Plants. You might want to review the material on plant anatomy
and vascular tissue before you begin.
Here is 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 contributes to this
transpiration-induced driving force of water through the xylem of the plants. Water mol-
ecules like to stick together, and when one of their kind is pulled in a certain direction,
the rest seem to follow.
This experiment examines various environmental factors that affect the rate of transpi-
ration: 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 bulb shin-
ing on your neck, and then a 100-watt bulb shining on your neck. The higher-wattage bulb
will probably cause you to sweat more. The same thing with plants: the higher the inten-
sity 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.
Basic Setup
One easy and straightforward way to measure water loss is by measuring the entire plant’s
mass every day for about a week. This “whole plant” method requires you to tightly seal a
plastic sandwich bag around the root ball so the only water loss is through the leaves.
Furthermore, if the poor plant is even thinkingabout flowering, you need to ensure that it
does not do so (pull off any flowers or buds). Once you understand this basic setup, the real
fun begins: inquiry! Think of some variables that may affect transpiration from your poor
plants. Sun? Wind? It’s up to you. One plant will be your control, and every other plant
KEY IDEA
BIG IDEA 4
Interactions
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