Download AP Biology: Evolution

(Copy the title, objective and summarized background neatly in your lab
notebook.)
AP Biology: Interactions
Investigation 11: Transpiration
______Big Idea 4
Lab Objective:
What factors, including environmental variables, affect the rate of transpiration in
plants? How do these environmental variables (chosen for this investigation)
affect the rate of transpiration in plants?
(Read the background below and summarize for your lab notebook)
Background:
Introduction: The amount of water needed daily by plants for the growth
and maintenance of tissues is small in comparison to the amount that is lost
through the process of transpiration and guttation. If this water is not
replaced, the plant will wilt and may die. The transport up from the roots in the
xylem is governed by differences in water potential (the potential energy of
water molecules). These differences account for water movement from cell to
cell and over long distances in the plant. Gravity, pressure, and solute
concentration all contribute to water potential and water always moves from an
area of high water potential to an area of low water potential. The movement
itself is facilitated by osmosis, root pressure, and adhesion and cohesion of water
molecules.
The overall process: Minerals actively transported into the root
accumulate in the xylem, increase solute concentration and decrease water
potential. Water moves in by osmosis. As water enters the xylem, it forces fluid
up the xylem due to hydrostatic root pressure. But this pressure can only move
fluid a short distance. The most significant force moving the water and dissolved
minerals in the xylem is upward pull as a result of transpiration, which creates
a negative tension. The "pull" on the water from transpiration is increased as a
result of cohesion and adhesion of water molecules.
The details: Transpiration begins with evaporation of water through the
stomata, small openings in the leaf surface which open into air spaces that
surround the mesophyll cells of the leaf. The moist air in these spaces has a
higher water potential than the outside air, and water tends to evaporate from
the leaf surface. The moisture in the air spaces is replaced by water from the
adjacent mesophyll cells, lowering their water potential. Water will then move
into the mesophyll cells by osmosis from surrounding cells with the higher water
potentials including the xylem. As each water molecule moves into a mesophyll
cell, it exerts a pull on the column of water molecules existing in the xylem all
the way from the leaves to the roots. This transpiration pull is caused by (1) the
cohesion of water molecules to one another due to hydrogen bond formation, (2)
by adhesion of water molecules to the walls of the xylem cells which aids in
offsetting the downward pull of gravity. The upward transpiration pull on the
fluid in the xylem causes a tension (negative pressure) to form in the xylem,
pulling the xylem walls inward. The tension also contributes to the lowering of
the water potential in the xylem. This decrease in water potential, transmitted all
the way from the leaf to the roots, causes water to move inward from the soil,
across the cortex of the root, and into the xylem. Evaporation through the open
stomata is a major route of water loss in the plant. However, the stomata must
open to allow the entry of CO2 used in photosynthesis. Therefore, a balance
must be maintained between the gain of CO2 and the loss of water by regulating
the opening and closing of stomata on the leaf surface. Many environmental
conditions influence the opening and closing of the stomata and also affect the
rate of transpiration. Temperature, light intensity, air currents, and humidity are
some of these factors. Different plants also vary in the rate of transpiration and
in the regulation of stomata openings.
(Copy the big idea, enduring understandings and learning objective codes in
your lab notebook)
Big Idea 4:
Biological systems interact, and these systems and their interactions possess
complex properties.
Enduring Understandings:
 1A2: Natural selection acts on phenotypic variations in populations.
• 2A3: Organisms must exchange matter with the environment to grow,
reproduce, and maintain organization.
• 2B1: Cell membranes are selectively permeable due to their structure.
• 2B2: Growth and dynamic homeostasis are maintained by the constant
movement of molecules across membranes.
• 2D1: All biological systems from cells and organisms to populations,
communities, and ecosystems are affected by complex biotic and abiotic
interactions involving the exchange of matter and free energy.
• 4A4: Organisms exhibit complex properties due to interactions between
their constituent parts.
• 4A6: Interactions among living systems and with their environment result
in the movement of matter and energy.
Learning Objectives:
• 1.5 The student is able to connect evolutionary changes in a population
over time to a change in the environment (1A2 & SP 7.1).
• 2.6 The student is able to use calculated surface area-to-volume ratios to
predict which cell(s) might eliminate wastes or procure nutrients faster by
diffusion (2A3 & SP 2.2).
• 2.8 The student is able to justify the selection of data regarding the type
of molecules that an animal, plant, or bacterium will take up as necessary
building blocks and excrete as waste products (2A3 & SP 4.1).
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2.9 The student is able to represent graphically or model quantitatively
the exchange of molecules between an organism and its environment, and
the subsequent use of these molecules to build new molecules that
facilitate dynamic homeostasis, growth, and reproduction (2A3 & SP 1.1,
SP 1.4).
4.9 The student is able to predict the effects of change in a component(s)
of a biological system on the functionality of an organism(s) (4A4 & SP
6.4).
4.14 The student is able to apply mathematical routines to quantities that
describe interactions among living systems and their environment that
result in the movement of matter and energy (4A6 & SP 2.2).
4.15 The student is able to use visual representation to analyze situations
or solve problems qualitatively to illustrate how interactions among living
systems and with their environment result in the movement of matter and
energy (4A6 & SP 1.4).
(Copy the following headings in your lab notebook and answer the four getting
started questions)
The Investigation:
Getting Started:
1. If a plant cell has a lower water potential than its surrounding
environment, make a prediction about the movement of water across the
cell membrane. In other words, will the cell gain water or lose water?
Explain your answer in the form of a diagram with annotations.
2. In the winter, salt is sometimes spread over icy roads. In the spring, after
the ice has melted, grass often dies near these roads. What causes this to
happen? Explain your answer in the form of a diagram with annotations.
3. Prepare a thin section of stem from your plant and examine it under the
microscope to identify the vascular tissues (xylem and phloem) and the
structural differences in their cells. Describe how the observed differences
in cellular structure reflect differences in function of the two types of
vascular tissue.
4. If you wanted to transplant a tree, would you choose to move the tree in
the winter, when it doesn’t possess any leaves but it’s cold outside, or
during the summer, when the tree has leaves and it’s warm and sunny?
Explain your answer.
(Copy the following headings, create a hypothesis, copy the materials and
procedures in your lab notebook)
The Whole Plant Transpiration Method:
Hypothesis:
Write a hypothesis about what you think what will happen based on your
knowledge of transpiration and plants.
Materials:
• Four purple waffle plants
• Non-zipper style gallon and sandwich size “food storage” bags
• String
• Rubber bands
Procedure:
1. Wrap the root ball of plant in a plastic bag
2. Tie bag around the base of the plant with string so that only the leaves
are exposed.
3. Over plant #2 place a bag that has been misted over the plant and tie it
in place.
4. Mass the plants and record the data in the chart marked Day 1. Also write
the initial mass of the plant and bag on the plastic bag with your team
initials.
5. Place plant #1 in the place designated for the control.
6. Place plant #2 in the place designated for the humid conditions.
7. Place plant #3 in the place designated for the light.
8. Place plant #4 in the place designated for the wind.
9. Mass and record the data in the data table provided for the next three
days.
(Copy the data heading in your lab notebook and attach the data table. Also
include an appropriate graph of your data.)
Data:
(Copy the analysis heading in your lab notebook and answer the following
questions in full sentence answers)
Analysis:
1. For this experiment, what were the independent variable and the
dependent variable? What was the control? What were some constants in
this lab?
2. Calculate the average rate of water loss per day for each of the
treatments. (Humidity, Light, Fan, Dark, Room or control).
3. Explain why each of the conditions causes an increase or decrease in
transpiration compared with the control. Cite data within your discussion.
4. How did each condition affect the gradient of water potential from the
stem to leaf in the experimental plant?
5. What is the advantage to a plant of closed stomata when water is in short
supply? What are the disadvantages?
6. Describe several adaptations that enable plants to reduce water loss from
their leaves. Include both structural and physiological adaptations.
7. Why did you need to calculate the % water loss each day instead of
graphing the total amount of water lost each day?
8. Draw a plant leaf cross section and label the following layers in this
diagram; cuticle, upper epidermis, palisade layer (mesophyll), spongy
layer (mesophyll), vascular bundle(s), lower epidermis, guard cells, and
stomata.
9. State the function of each of the structures listed in question # 8.
10. Make a prediction about the number of stomata in a leaf and the rate of
transpiration. What type(s) of experiments could you conduct to
determine the relationship between the number of stomata and the rate
of transpiration?
11. Create a diagram with annotation to explain how the TACT (transpiration,
adhesion, cohesion, tension) mechanism enables water and nutrients to
travel up a 100-ft. tree. Predict how a significant increase in ambient
(environmental) temperature might affect the rate of transpiration in this
tree. Explain your prediction in terms of TACT and the role of guard cells
in regulating the opening and closing of stomata.