Photosynthesis

(a 3-5 Ecosystems lesson plan)

From the unit: How do things live in my neighborhood?

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Abstract
Students examine the effect of light on plant growth.
Standards and Benchmarks
AAAS Benchmarks
  • Almost all kinds of animals' food can be traced back to plants.
Objectives
  • Students will understand the role of light in the process of photosynthesis.
  • Students will observe the effects of light on plants.
  • Students will understand that plants use the sun's energy to produce food through photosynthesis.
Class Time Needed
One week
Teacher Preparation
  1. The teacher needs to plant pea seeds ahead of time (two plants per pair of students). First, the seeds need to germinate. To do this:
    1. Place pea seeds onto damp paper towels on a tray or shallow dish. Make sure they are covered lightly with water.
    2. A "hook" should appear in two to three days. After the "hook" appears, the seeds are ready to be planted in the pots.
  2. Transfer the seed to the pots. To do this:
    1. Fill each pot with soil up to 2.5 inches from the top.
    2. Place the seeds on top of the soil. Cover with 1/4 to 1/2 inches of soil mix.
    3. Sprinkle water over the top until well saturated.
Materials
Per group of two:
  • 2 pots
  • Soil
  • Several pea seeds
  • Watering cans
  • Paper towel and water (to germinate seeds)
Science Background

Photosynthesis
In order for seeds to germinate (start to grow) they need moisture, air, and warmth. Under the conditions which meet these requirements, seeds will move from a dormant state (which began after the seed developed) into an active state.

Plants do not take in food from the outside, but instead manufacture food themselves. In order for plants to make the food necessary for growth, they need water, light energy, and air. Light energy from the sun absorbed by the chlorophyll in a plant's leaves converts water (transported to the leaves by the roots and stem) and carbon dioxide (from the air) into the necessary food (called glucose) for the plant through the process of photosynthesis. Oxygen is given off as a byproduct.

Students' Alternative Ideas

Energy

Alternative idea: Elementary school students often believe that organisms and things in the environment are made up of completely different substances that are not transformable into each other. They also often believe that energy only has to do with humans, is gone when it is used up, and is not measurable.

Scientific idea: All living organisms are made up of atoms and have energy. Atoms and energy cannot be created or destroyed. When a chemical change occurs (such as when something gets eaten, when photosynthesis occurs, etc.), atoms get rearranged and energy is transformed.

Dealing with the alternative idea: The Energy Transformations Lesson Plan will help students understand that energy is not created or destroyed - it simply is transformed.

The Atom Lesson Plan assists students in understanding that all living organisms are made up of the same thing - atoms.


Food

Alternative idea: Elementary school students often use the term "food" as humans do and do not use the biological meaning. They often think food includes substances such as water, air and minerals.

Scientific idea: In any given food chain, energy (in the form of food) is passed from organism to organism. Photosynthesis is the process by which plants make glucose, which serves as their energy source. Many students will not view this as food energy because it is not something they typically eat.

Dealing with the alternative idea: The Photosynthesis Lesson Plan will help students understand that energy comes in many different forms, not just pizza or hamburgers!


Photosynthesis

Alternative idea: Many elementary school students believe that plants obtain food from their environment and do not manufacture it internally. They also often confuse energy with concepts such as force and temperature. Therefore, some students may not understand the energy conversion that takes place during photosynthesis.

Scientific idea: Plants do not take in food from the outside, but instead manufacture food themselves. In order for plants to make the food necessary for growth, they need water, light energy, and air. Light energy from the sun absorbed by the chlorophyll in a plant's leaves converts water (transported to the leaves by the roots and stem) and carbon dioxide (from the air) into the necessary food (called glucose) for the plant through the process of photosynthesis. Oxygen is given off as a byproduct.

Dealing with the alternative idea: The Photosynthesis Lesson Plan

Description
  1. Place students into groups of two.
  2. Pose the following question to students: What do plants need in order to grow? Record their responses on the board. It is important to allow as many students as possible the opportunity to share their thoughts with the class. Try not to correct any misunderstandings at this point. Students will probably mention soil, water and light.
  3. [?] Why should my students ask and answer questions in science?
  4. [?] How can I help my students ask and answer questions in science?
  5. Ask students to think about why light is important to plants. Have them share their responses with the class. Having students share their responses will give you a better indication of students' prior knowledge of photosynthesis. Many students do not know that plants make their own food - they often think it comes from the soil. Don't correct students' misunderstandings at this point.
  6. Tell students that each pair of students is going to get two pots. One pot will go in a lit area and the other will go in a darkened area, like a closet.
  7. Ask students to make predictions about what they think will happen to the plant in each pot.
    • Will they grow?
    • What will the leaves, stems, and roots look like for each plant?
  8. Have students share their predictions with the class. Ask them to give a reason for their predictions.
  9. At the end of one week or ten days, remove the pot from the designated dark area and compare it with the pots grown in the light.
  10. Students should make observations about each of the plants. How do the leaves, stems, and root system differ for each plant? You may want to have students draw pictures to illustrate their observations
  11. [?] Why should students collect evidence to answer questions?
  12. [?] How can I help my students collect evidence?
  13. Have a whole class discussion around these findings. Ask students:
    • What happened to the plants in each pot? Why do you think this happened?
    • How do these results compare with your predictions?
    • Why do you think the plants that received light grew better than the plants that didn't receive light?
  14. By the end of the discussion most students should understand that plants need sunlight to make their own food (this is called "photosynthesis"). Because the plants in the dark did not receive much (if any) sunlight, they could not make their own food. So, they did not grow as much as the plants that did receive sunlight. During this discussion it is important to listen for the following alternative conceptions:
    • Plants "eat" their food
    • Plants get their food from the soil or air and do not make it internally
    • Energy is not transformable
  15. [?] Why should students communicate and justify their findings?
  16. [?] How can I help my students communicate and justify their findings?
Assessment
Have students write a response to the following question: What would happen in an ecosystem if photosynthesis did not take place? Students' responses should include:
  • plants (producers) could not make food
  • plants would not have any energy to carry out daily functions
  • plants would eventually die
  • animals (primary consumers) would eventually die also because they feed off of the plants
  • the animals that feed on other animals (secondary consumers) would eventually die also because there would be no animals to feed off of
  • the entire ecosystem would break down
-- note that this is an impossibility; photosynthesis will not cease to occur --
Images of Inquiry
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This lesson focuses on Questioning & Predicting, Explanations & Evidence, and Communicating & Justifying. (more)

How Kirsten taught this lesson
In preparing to teach this lesson, Kirsten was concerned that her students might have a difficult time making connections between the photosynthesis activity and what actually happens in the natural world. Since most of her students probably hadn't observed plants in closets, she decided to put the 'dark area' pea plants under a counter and then put some big plants she already had in her classroom in front of them.

Even though it wouldn't be as dark as it would have been in the closet, she felt like this might better represent natural growing conditions that plants might face. In the class discussion following the activity, she probed her students' ideas about how this set-up might represent real situations in nature. Many students understood that bigger plants may block light from smaller plants and that non-living parts of ecosystems, like rocks, can do the same thing.

Author(s): CASES Team

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