PLANT SYSTEMS QUESTION 1991: L. PETERSON/AP BIOLOGY
A group of students designed an experiment to measure transpiration rates in a
particular species of herbaceous plant. Plants were divided into groups and were
exposed to the following conditions.
Group I - Room conditions (light, low humidity, 200 C, and little air
movement)
Group II - Room conditions with increased humidity
Group III - Room conditions with increased air movement (fan)
Group IV - Room conditions with additional light
The cumulative water loss due to transpiration of water from each plant was
measured at 10-minute intervals for 30 minutes. Water loss was expressed as
milliliters of water per square centimeter of leaf surface area. The data for all
plants in Group I (room conditions) were averaged. The average cumulative
water loss by the plants in Group I is presented in the table below.
Average Cumulative Water Loss by the Plants in Group I
Time (minutes) Average Cumulative Water Loss
(milliliters H2/centimeter2)
10 3.5 x 10-4
20 7.7 x 10-4
30 10.6 x 10-4
a. Construct and label a graph using the data for Group I. Using the same set of
axes, draw and label three additional lines representing the results that you
would predict for Groups II, III, and IV.
b. Explain how biological and physical processes are responsible for the
differences between each of your predictions and the data for Group I.
c. Explain how the concept of water potential is used to account for the movement
of water from the plant stem to the atmosphere during transpiration.
STANDARDS:
A. GRAPH: (Max of 3 points)
Average Cumulative Water Loss by Plants
15
ml water loss 10
per cm2 x 10-4
5
0
0 10 20 30
Time (minutes)
__ Correct Orientation of X (independent) and Y (dependent) axes
__ Scale and proper labels
__Group I - properly graphed (3.5/7.7/10.6)
B. BIOLOGICAL/PHYSICAL PROCESSES FOR DIFFERENCES:
(Max of 5 points)
__Control (I) - function of
__Transpiration / Photosynthesis (description of)
__Humidity (II) - thus increase water potential surrounding leaf - thus curve will be
BELOW control (inside constant/outside increased)
__Fan (III) - water potential decreased surrounding leaf - therefore curve will be
ABOVE control (inside constant/outside decreased)
__Fan (III) - wind shear (fan close) stomates closed - therefore curve BELOW control
__Fan (III) - cools because of evaporation (decreases kinetics) - therefore curve
BELOW control
(Max of 2 points for Fan)
__Light (IV) - stomates open (or) increase water usage via photosynthesis - curve
ABOVE control
__Light (IV) - heat - physical (increased kinetic energy) - curve ABOVE control
__Light (IV) - heat - stomates closed - therefore curve BELOW control
(Max of 2 points for Light)
__Curves (graphs) - all correspond to description (hypothesis testing)
__Mechanism of stomates close/open (CO2 / K+ flux / etc.)
C. CONCEPT OF WATER POTENTIAL: FROM STEM - LEAF - ATMOSPHERE:
(Max of 4 points)
__Concept of water potential (high to low)-(diffusion/osmosis)-relationship to solute
__Water movement (Cohesion-Tension Theory)
Properties of water (adhesion, cohesion, hydrogen bonding, capillary action)
Tension "transpirational pull" - gradient xylem to outside
Negative pressure
__Pressure potential/Osmotic potential
__Air space of leaf has higher water potential than air outside leaf
Water moves out through stomata
__Mesophyll cells higher water potential than surrounding air spaces of leaf
Water moves out into leaf air spaces
__Solute in mesophyll cells becomes more concentrated when less water present -
therefore, less water potential (Mesophyll hypertonic to xylem)
__Xylem has higher water potential than mesophyll cells - water moves out of xylem
into mesophyll cells