Download Biology 2: Homeostasis in Plants Investigating the Effects of Gravity

Biology 2: Homeostasis in Plants
Investigating the Effects of Gravity on Root and Shoot Formation
Background: Gravitropism (also known as geotropism) is a movement by a plant- typically the root or shoot
tip- in response to gravity. Root growth occurs by division of stem cells in the root meristem located in the tip of
the root, and the subsequent expansion of cells in a region just proximal to the tip known as the elongation
zone. Differential growth during tropisms mainly involves changes in cell expansion verses changes in cell
division. Gravity is sensed in the root tip and this information must then be relayed to the elongation zone so as
to maintain growth direction and mount effective growth responses to changes in orientation and continue to
grow its roots in the same direction as gravity.
Abundant evidence demonstrates that roots bend in response to gravity
due to a regulated movement of the plant hormone auxin. Auxin exists
in nearly every organ and tissue of a plant. In roots, an increase in
auxin concentration generally inhibits cell expansion. Therefore the
redistribution of auxin toward the lower flank of a root, that has been
reoriented in the gravity field, can initiate differential growth resulting in
root curvature.
A similar mechanism is known to occur in plant stems, except that the shoot cells have a different response
with respect to auxin. In shoots, increasing the local concentration of auxin promotes cell expansion; this is the
opposite of root cells. The differential sensitivity to auxin helps explain observations that stems and roots
respond in the opposite way to gravity. In both roots and stems auxin accumulates towards gravity on the lower
side. In roots, this results in the inhibition of cell expansion on the lower side and the curvature of the roots
towards gravity (positive gravitropism). In stems, the auxin also accumulates on the lower side, however in this
tissue it increases cell expansion and results in the shoot curving up (negative gravitropism).
This laboratory activity will investigate the concept of tropisms, gravitropism in particular. Briefly describe how
researchers have found that gravity affects the formation of roots and shoots in actively growing plants? What
is happening at the cellular level?
Objectives: In this lab you will compare the direction of growth shown by young corn roots and stems when
positioned in different orientations to the force of gravity.
Hypothesis: If (what does gravity do to plant cells?), then (what will happen to roots and shoots when the
orientation of the cells is altered?)
Procedure:
1. Place four pre-soaked corn seeds in 4 different orientations on the bottom of the petri dish. These will be like
numbers on a clock: 12, 3, 6, and 9. See the figure to the right.
2. Using a petri dish as a guide; cut a piece of filter paper to fit inside the dish.
3. Carefully place the filter paper over the seeds
4. Place several cotton balls on top of filter paper to hold seeds in place.
5. Pour 25ml of water over cotton balls. Add more or less water if necessary.
Be sure all contents of petri dish are thoroughly damp.
6. Place cover on petri dish and tape closed. Be sure seeds have not shifted in
orientation
7. Label petri dish with group name and hour.
8. Tape petri dish in dark cabinet as designated by teacher
9. Make two observations of seeds (after 48 and 120 hours).
Examine the seeds for the direction of root and stem growth. Draw pictures of each day that you observe
changes. This is your data.
Name: _____________________________ Hour: 1
2
Lab Report - Investigating the Effects of Gravity on Root and Shoot Formation
Background: Gravitropism (also known as geotropism) is movement by a plant- typically the root or shoot tipin response to gravity. This laboratory activity will investigate the concept of gravitropism.
Question: What happens to newly developing roots and shoots when _______________________________
_______________________________________________________________________________________?
Hypothesis: If _______________________________________________________________________, then
________________________________________________________________________________________
________________________________________________________________________________________
Independent variable: _____________________________________
Dependent variable: _______________________________________
Controlled variables (List at least three): ______________________________________
____________________________________, _____________________________________________
Data Table:___________________________________________________________ (Title)
Seed
After 48 Hours
After 72 Hours
Notes/Observations
Orientation
“6:00”
“9:00”
“12:00”
“3:00”
Claim: Based on this lab activity, describe in one sentence how roots and shoots grow in relation to gravity.
Analysis Questions:
1.
In your own words, define gravitropism.
2. Plants also exhibit phototropism. The prefix “photo” means light. What does it mean for a plant to
exhibit phototropism?
3. Which of the following drawings is accurate? Explain your answer using scientific terms.
4. Explain why the petri dishes were kept in a dark cupboard.
5. Contrast positive gravitropism with negative gravitropism.
6. In a paragraph, explain why it is necessary for roots to exhibit positive gravitropism and for stems to
exhibit negative gravitropism. Include how this helps the plant maintain homeostasis. (4 sentence
minimum)