Summer Research Program for Science Teachers
Eric Paul
1999
Growing Barley for Use in Biosynthesis Experiments
Focus
This is a biochemical action plan, applicable to both biology and chemistry. [Teaching Standard A- Work across disciplines] Students will first try to finding the optimum conditions for growing barley under certain restrictions imposed by the nature of the research. Then they will use their knowledge in assessing the effects of various substances, such as ammonium chloride and urea, on the plants. [Content Standard Unifying Concepts- Change, constancy, and measurement]
Background
In studies of biosynthesis, rare isotopes are often used as tags to trace pathways. [9-12 Content Standard E- Understandings about science and technology] Incorporated into compounds, the isotopes are presented to the organism being studied. For example, if a biochemist wants to discover whether any of the hydrogen in caffeine is derived from the soil, he or she might place in the soil of a coffee plant a compound rich in deuterium, a rare isotope of hydrogen. If deuterium turns up frequently in the caffeine of the plant, this will show that the soil is a source of hydrogen for the caffeine.
On the college level, the rare isotopes used are often radioactive, but this is forbidden on the high school level, so any rare isotope used in this project will be a stable one such as nitrogen-15, carbon-13, or deuterium (hydrogen-2). The rare isotopes undergo the same chemical reactions as their common counterparts, although deuterium has the unusual property of being slower in its reactions than ordinary hydrogen. [9-12 Content Standard B- Properties of matter] An organism restricted to 100% deuterium oxide (in which the hydrogen of ordinary water is replaced with deuterium) will die. (Deuterium and its compounds should be handled with care: as usual, nothing should be ingested in the lab.)
Before using the rare isotopes, studies must be made of ordinary plant growth patterns. This task will be given to the students. Also, before expensive tagged compounds are introduced, the effects of untagged versions of the compounds should be studied. For example, ammonium chloride tagged with nitrogen-15 can be added to the soil to see whether a plant uses nitrogen from the soil to assemble a particular amino acid, but a study should be made on the effect of ordinary ammonium chloride on the plant before using the tagged version.
Materials and Equipment
goggles
aprons
deuterium oxide, 100 g
balance
metric ruler
distilled water
refrigerator
aquarium tanks
paper towels
mortar and pestle
small containers (such as film cannisters)
30 petri dishes (diameter 10 cm)
7 beakers, 400-mL
scissors
barley seeds (obtainable from suppliers such as Raja Foods)
lab notebook, preferably with graph paper
model of atom
[Teaching Standard D- Make accessible science tools]
Purpose
The purpose of this project will be to study the growth patterns of barley under a number of restrictions. The activities of the project will be woven around other lesson plans, since the project itself will require several weeks, and often only a few minutes each day. [Teaching Standard B- Structure time for extended investigations] At the outset, conditions will be fairly predetermined, but as the project progresses students will be given a freer hand in designing portions of their own experiments. For the control setup, the restrictions are as follows:
1. The barley will be soaked overnight in beakers of distilled water. For uniformity, 10 g of seeds (the mass to be measured before soaking) will be soaked in a 400-mL beaker, with 200-mL of water.
2. After being soaked overnight, the barley seeds will be placed in 10-cm petri dishes lined with three layers of paper towels cut to fit the circular bottom of each dish.
3. Distilled water will be provided to each petri dish. It will be sufficient to touch all the seeds but not so much as to submerge any of them. Water will be added daily, except on weekends and holidays.
4. The light provided will consist of the artificial light of the classroom, plus any light that comes in from the windows. At night the artificial light will be turned off.
5. An aquarium disinfected by wiping all surfaces with rubbing alcohol will be placed upside-down over the petri dishes to protect them from attack of a fungus. A standard wooden pencil will be placed under the aquarium on one side to prop it up and allow for movement of air.
6. The plants will be harvested on the 15th day of their growth, with Day 1 being the day they were planted.
Implementation
Students will be placed in teams of four. The four members of the team will consist of:
(1) a safety monitor (who will also serve as the team leader)
(2) a recorder, who will record in his or her lab notebook, the names and responsibilities of the team members, everything the team does to the plants and observations of what how the plants respond
(3) a researcher who will look up information in books and on the internet for the team to use
(4) a measurements person who will perform measurements such as the amount of seeds placed to soak, the volume of the water added, and so forth. In addition, each team member will be required to keep a lab notebook describing his or her contributions to the group. Some classes may require a fifth person in some teams. This person can be the planter of the seeds. The teacher may want to appoint the safety monitors, or else he or she may decide to allow the team members to elect their own safety monitor. Each team will be responsible for at least one petri dish, and most teams will be assigned two. [Teaching Standard E- Nurture collaboration among students]
It will be a good idea to review with the students certain aspects of atomic structure, so that they can understand the concept of an isotope. [9-12 Content Standard B- Structure of atoms] Specifically, students should be shown a model of an atom, to show that protons and neutrons are found in the nucleus of an atom, while electrons are found in the electron cloud. Students should be aware that since electrons are the part of atomic structure that takes part in chemical reactions, the number of neutrons in the nucleus does not affect chemical properties. Show students a clear bottle of deuterium oxide, to show them what it looks like. Explain to them that the differences between isotopes will be used by scientists to trace the paths taken by atoms utilized by plants.
Although the procedure is outlined here, it is not a "cookbook" set of instructions. The students are to devise their own steps of procedure, and these steps must be logged into their laboratory notebooks. For example, one of the tasks students must perform is to mass the seeds before placing them in the beaker of water to soak. The students need to be aware of the fact that when 10 grams of seeds are specified, a balance needs to be used to mass those seeds before they begin soaking. It will therefore be important for the teacher to spend some time ensuring students have gotten used to the idea that they must select the appropriate measuring devise for a given measurement--the balance to measure mass, the metric ruler to measure length, the thermometer to measure temperature, and so forth. Also, the organization of the lab notebooks should be discussed. The teacher may want to plan some activities in advance to prepare students for such responsibilities. The teacher should also specify the format he or she wants for writing up the activities in the lab notebook.
There are a number of decisions the students will have to make from the outset. In the first phase of plant growing, a study will be made in terms of plant height. Ideally, fourteen dishes, numbered 2 to 15, would be set up just as the control except that only the control would be harvested on Day 15. All the other dishes would have their plants harvested on days leading up to Day 15. Since seeds are planted on Day 1, no dish need be designated Dish 1, but on Day 2 dish #2 would be harvested, on Day 3 dish #3 would be harvested, on Day 4 dish #4 would be harvested, and so forth, until on Day 15 the control, dish#15, would be harvested. At each of these harvests the plants would be measured in terms of height, number of seeds germinated, and so forth. The problem with the ideal plan is that on weekends there would be no one around to conduct the harvests. This would be a good thing to discuss with the class, to have each group try to decide how planting and harvesting the barley could be timed so that each of the days would be covered. Once the data is collected for all 15 days of growth, each group should come up with a way of presenting the data. Presentations may be made via tables, graphs, etc., with the members of the team making the best presentation receiving a few points extra credit. [9-12 Content Standard A- Use mathematics to improve scientific communication] Groups should be encouraged to show their presentations to other members of the class. Again, the students are being asked to make a decision about how to present the data, instead of having the decision made for them. [Teaching Standard B- Orchestrate scientific discourse]
In the second phase of plant growing, students would gain experience in using a controlled experiment. One group would be given custody of the control petri dish, whose parameters are predetermined, in addition to at least one experimental dish. The task of all the groups would be to vary conditions of the experimental dishes in such a way as to produce the most barley plants per petri dish. [9-12 Content Standard A- Design and conduct a scientific investigation] In this way the project would be open-ended. Only the amount of water delivered to the plants, the number of seeds planted per dish, and the amount of soaking given to the plants, would be allowed to vary, so each group would have to devise a hypothesis that could be tested by varying one of these parameters.
The hypothesis would have to meet certain standards: It would have to be an assertion, it would have to be stated in a complete sentence, it would have to be stated in such a way as to stand or fall based on the observation of plant growth. The experimental design would have to be specified in detail in the team's notebook, and it would have to be quite explicit which parameter was being used as the independent variable and which as the dependent variable. For example, a hypothesis might be as simple as, "If 15 grams of seeds are grown instead of the standard 10 grams, then more shoots exceeding 8 cm will be harvested on day 15 in our experimental dish than will be harvested on day 15 in the control dish." The criterion height chosen would have to be decided on in advance, probably by a vote of the class, so that all groups would be aiming to produce the maximum number of plants of a certain height. [9-12 Content Standard E- Abilities of technological design] The winning team in this contest would receive extra credit as determined by the teacher.
Technology can be very useful here, in the form of the internet. One of the persons in each group is a "researcher." With a resourceful researcher, a group can see what data has already been collected on the growth of barley, and armed with this data can make plans that will enhance their chances of winning. Other members of the group might come up with leads that the researcher might pursue in pursuing such information. It is important for the groups to be resourceful in this way.
In the third phase of plant growing, students would gain experience subjecting the plants to adverse conditions such as a certain percentage of deuterium oxide, a certain concentration of ammonium chloride, or a certain concentration of urea. Students should be reminded of standard safety precautions before beginning this phase, especially since many may be ignorant of the hazard of deuterium oxide, which could be harmful if ingested in large quantities. [Teaching Standard D- Ensure a safe working environment] Before beginning this phase, the class should vote on the conditions for the control setup, based on the results of the previous study. That is, if, say, 20 grams of seeds per dish produced the greatest number of plants of a desired height per dish, then the mass of seeds in the standard might be changed from 10 g to 20 g. The students would be given a sign-up sheet of various substances in various concentrations to deliver to the plants. The groups would again have a competition for extra credit to see who could grow the most barley plants of a certain height per dish, but this time the independent variable would be the concentration of a substance such as ammonium chloride delivered to the plants. This study would be of value for determining the extent to which such concentrations could be tolerated by the plants. By conducting this study, the students would be laying some of the groundwork for the research into biosynthesis, since the ability of the plants to tolerate ammonium chloride or other sources of nitrogen would be very useful for a scientist trying to decide which nitrogen compound to use to supply the barley with nitrogen-15. To avoid using up all the deuterium oxide too quickly, only one group would be allowed at first to sign up for it, and the percentage would be specified as 10%. The harvested plants from each group should be split into root parts and shoot parts, and these should be ground by use of mortar and pestle into separate pastes and placed in labeled containers and refrigerated for further analysis. (The paste from plants grown in 10% deuterium oxide should be passed on to a college laboratory equipped to analyze natural products.) [Teaching Standard A- Select content to meet students' abilities]
One of the frustrations of real research is that sometimes things do not go as planned. It is a good idea to prepare the students for this. If for example an attack of fungus occurs in one of the petri dishes, students should be encouraged to come up with ways of improving the setup to minimize the threat of fungus. Sometimes an obstacle is really an opportunity, if seen in this way. [9-12 Content Standard A- Identify questions and concepts that guide inquiry]