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Student Experience 4: What do plants need to grow?

Posted By Courtney Nelson On January 2, 2018 @ 11:35 am In | Comments Disabled

^[1]

Purposes

To demonstrate for students that plants use resources around them to grow: light, water, air (particularly carbon dioxide), and nutrients ^[2].
To help students relate what producers do in an aquatic system (e.g., a pond or jars from Student Experience 3) to what more familiar land-based producers ^[3] do.

Description

Recall that students have monitored the growth of plants exposed to different sunlight and water conditions. [Click here to view the instructions for the initial set up of this experiment. ^[4]] A short video showing the growth of green bean plants exposed to the same conditions as used in this student experience is available here ^[5]. Students watch this video as a quick refresher of what they observed over the three weeks. The students then review and discuss their recorded observations of this experiment. In particular, they make comparisons across the different samples for different time points to draw conclusions about the effects of the abiotic factors (i.e., light and water) they tested.

Using a simulation is another option for exploring how changing light, water, and nutrient conditions affect plant growth. One possibility is the “Growing Plants” Gizmo: (https://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=615 ^[6]). This simulation allows the user to vary light and water conditions and see the effects on plant height and mass.

Once the students recall the evidence showing plants need light and water, tell them that plants also need air (particularly carbon dioxide) to make food and grow. Then introduce the term “producer”; that is, plants produce their own food using these resources. Highlight that producers also make oxygen in the process of making their food. Students then view a video that reiterates the inputs producers use to make food and the products of the process (sugar and oxygen). Examples include:

“Photosynthesis” by Science Up Close (http://www.harcourtschool.com/activity/science_up_close/311/deploy/interface.swf ^[7]).
“Photosynthesis” by NOVA (https://unctv.pbslearningmedia.org/resource/tdc02.sci.life.stru.photosynth/photosynthesis/#.WV-uiFGQypo ^[8])

Questions to Ask Students After They Collect Plant Growth Data

What do the plants you tested need from the environment to grow well? How do you know? What is your evidence?
How do your conclusions here relate to, or help explain results from, the pond water experiment? What was needed to cause the greenish cloudy growth? What kind of organism do you think accounts for the cloudy growth? (A key outcome here is that students recognize that both the plants in this student experience and the algae from Student Experience 3 are producers and thus have the same basic needs.)

Science Practices

Constructing Explanations ^[9] Students use evidence gathered from their own investigations (algae growth and plant growth) and possibly a simulation to explain what plants need to grow. Students should cite specific evidence from these experiences to support their claims.

Crosscutting Concepts

Patterns ^[10] Similar to the previous pond water/algae growth investigation, it is important to have students attend to patterns in their observation data. It may be helpful to suggest that students consider one characteristic at a time (e.g., height, number of leaves) and look both at patterns across varied conditions and over time.

Student Thinking

Some students believe that plants take in their food from the soil or another part of their environment. Students may think of light, air, or water as being generally helpful to plants but not essential for growth. Many students have difficulty understanding the term “producer” as it relates to ecosystems. Because the word has other uses, students tend to adopt broader meanings and apply them to the ecosystem context. For example, students may view humans as producers because we make our food from basic ingredients (i.e., prepare a recipe) or because we raise food crops. Alternatively, students may use “producer” for the appropriate organisms (e.g., plants) but think this describes them because they produce food for other organisms. The use of the term “produce” to label fresh vegetables at supermarkets probably encourages students to think of the term “producer” inappropriately.

Implementation Tips

Ideally, students would also observe the effects of growing plants with no air/carbon dioxide. We have omitted this condition here because it is not practical in classrooms. However, it may be helpful to discuss this difficulty as a way to address the role of carbon dioxide in plant growth.
With a subscription to explorelearning.com ^[11], the Growing Plants Gizmo is free. Even without a subscription, the simulation can be used for free for 5 minutes each day.
Emphasize that plants need each of the resources to grow. Students may find it easier to accept that plants need to make their own food, rather than taking it from their environment (e.g., soil), if they grapple with the evidence showing no single resource can provide it. Seeing that the seeds germinate without sunlight (buried in the soil), students may raise questions about the differences between seed germination and later plant growth. Although germination is not a focus of the pathway, informing students that seeds have sufficient food in them to support germination may be helpful.
Because some students believe that plants take in their food from soil, emphasize the observation that being in soil was not sufficient to support healthy growth for the plants lacking water or sunlight.
If students observed bubbles in samples with algae growth in Student Experience 3, be sure to remind them of the observation as evidence when they view the videos that address plants producing oxygen.