Summary

In this lesson, students will explore a simple, but key, biochemical reaction: photosynthesis. Many students (and adults) have the misconception that the disciplines of chemistry and life science are completely separate, and many students who are attracted to life sciences are less attracted to chemistry and vice versa. This divide can be bridged by showing students how the two overlap. Students will explore the concept of photosynthesis and build molecules of water (H₂O), carbon dioxide (CO₂), and oxygen (O₂). They then explore plant cells and create models of plant cells and leaves. Finally, students will model the process of photosynthesis using their molecule models and leaf models.

Standards

NGSS and Cross-Disciplinary Extensions addressed in this lesson.

Objectives

By the end of this lesson, students should be able to

• Describe the importance of plants on Earth.
• Identify the reactants (carbon dioxide and water) and products (oxygen and sugar) of photosynthesis.
• Explain the importance of carbon dioxide, water, and sugar to plants.
• Describe where photosynthesis occurs.

Chemistry Topics

This lesson supports students’ understanding of the following topics in chemistry:

• Matter
• Chemical changes
• Reactions
• Conservation of matter
• Photosynthesis

Time

Engage: 10 minutes
Explore A: 15 minutes
Explore B: 15 minutes
Explore C: 30 minutes
Explore D: 20 minutes
Explain: 20–60 minutes

Materials

Engage

• Photos of plant cells, chloroplasts

Molecule Models

• Photosynthesis Molecules handout
• Diagrams of CO₂, H₂O, and O₂ molecules
• Scissors
• Clear tape
• Glue
• Possible bonding supplies: pipe cleaners cut into five pieces or black construction paper cut into strips, paper clips with small bar magnets or small round magnets
• Alternative: instead of paper models; students can make 3-D models using small magnetic balls in three colors

Plant Cell & Leaf Models
For each group:

• 2 paper lunch bags
• Construction paper (any color)
• Small balloon
• 2 pieces of green construction paper, 2 ¼”× 7 ½”
• Small envelope or sheet of paper to make an envelope

Photosynthesis Model

• Flashlight
• Plant cell and leaf models
• Molecule models
• Science journal

Safety

• If using the purchased magnetic balls, inform the students that these are not candy and they are not to be placed in their mouths.

Vocabulary Terms

Keywords

photosynthesis, glucose, chlorophyll, stoma, light reaction, pigment

Teacher Notes

Science Background

• Photosynthesis is a chemical reaction in which carbon dioxide and water are converted into glucose (sugar) and oxygen. The simplified balanced reaction for photosynthesis is

6CO₂ + 6H₂O + light energy ⇾ C₆H₁₂O₆ + 6O₂

Note, however, that the reaction is actually quite complex and involves a number of steps.

• Photosynthesis occurs within the chloroplasts of plant cells.

Source: OpenStax Biology

• Molecules can be modeled on paper or in 3-D. The following models can be used for reference during this lesson.

Water (H2O)	carbon dioxide (CO2)
oxygen gas (O2)	glucose (C6H12O6)

• Additional information can be found at:
  • http://www.biology4kids.com/files/plants_photosynthesis.html
  • http://scienceline.ucsb.edu/getkey.php?key=153
  • http://www.youtube.com/watch?v=3pD68uxRLkM
  • http://www.nature.com/scitable/topicpage/photosynthetic-cells-14025371

Tips

• If you have enough bags, the students may make a plant cell with a stoma and without.
• Have a large flat area available on the floor for placing many plant cells if you have 30 or more students.

Lesson

Engage

Assess what students already know about photosynthesis and introduce them to the idea that photosynthesis involves a chemical reaction.

1. Assess what students already know about photosynthesis:
   • What do plants need to live and grow?
   • Why do you need plants?
     
     Guide students to understand that plants need water, air, and sunlight to grow. More specifically, they need carbon dioxide in the air. Humans and other animals need plants because plants are also a source of food, and are also the source of oxygen that humans and animals need to breathe to stay alive.
   • How are the things that plants need (water, carbon dioxide, and light) related to the things that humans need plants for (oxygen and food)? Guide students to understand that plants use water, light, and carbon dioxide to produce oxygen gas and sugar. This is a chemical reaction known as photosynthesis.
   • What would happen to humans if none of the plants on Earth had enough water, carbon dioxide, or sunlight? Make sure that the students realize that plants and humans survive because of each other.
2. Assess what students already know about plant cells and where photosynthesis takes place.
   • Where does photosynthesis happen? What part of the plant? [the leaf]
   • What are leaves made of? [cells]
   • What are cells made of? What are some different parts of a plant cell? Show the students pictures of a plant cell and point out various parts, including the cell wall, cell membrane, nucleus, chloroplasts.
   • Why are most healthy leaves green? Guide students to understand that photosynthesis takes place in special parts of a leaf cell known as chloroplasts. These are green because they contain a green pigment, or coloring, called chlorophyll.

Explore

A. Making Molecules:

H₂O, CO₂, and O₂

Have students use the Photosynthesis Molecules handout, scissors, tape, and other supplies to make models of H₂O, CO₂, and O_2.

1. Review or introduce the idea that water is made of tiny pieces of water called molecules and that each molecule is made of atoms: two atoms of hydrogen and one of oxygen.
2. Make sure students understand that “H₂O” is a “formula,” which is an abbreviation for water, and that H stands for hydrogen while O stands for oxygen.
3. Have students cut out the elements and then put them together to make the models of each molecule. Before telling them what the models should look like, challenge them to infer, based on the formula and previous knowledge.
4. Correct any misunderstandings and have them compare their models to images of molecular models of H₂O, CO₂, and O₂ (see additional models in the Teacher Notes).

Download the Teacher Guide to view the rest of this lesson.