Reverse Reactions: Photosynthesis and Cellular Respiration
Purpose
Use Lego bricks to build molecules of carbon dioxide, water, oxygen and glucose in order to
model the chemical reactions of photosynthesis and cellular respiration, to understand how
energy is stored and released in plants, and how plants use these compounds to produce
biomass and carry on all life functions.
Key Questions
What do plants do with the “food” they produce via photosynthesis? Where does a plant get
its “energy” and what does it do with this energy? Where does a tree get its mass?
Student
Outcomes
1. Students will be able to write or diagram the chemical reactions for photosynthesis and
cellular respiration. (See equations below.)
2. Students will demonstrate understanding of how plants release the energy stored in the
chemical bonds of the glucose the plant produced as a result of photosynthesis.
3. Students will demonstrate understanding of what plants do with the food they
manufacture, including how biomass is produced.
4. Students will demonstrate understanding of how plants provide energy for consumers in
its role as a producer.
State Standards
1. Organisms in ecosystems exchange energy and nutrients among themselves and with the
environment. As a basis for understanding this concept:
a. Students know energy entering ecosystems as sunlight is transferred by producers
into chemical energy through photosynthesis and then from organism to organism
through food webs.
Key Concepts
1. The capture and use of energy in living systems is dominated by two processes:
photosynthesis and respiration.
2. Plants take in carbon dioxide and water, and using the energy from the sun, these two
gases combine in a chemical reaction to produce glucose and oxygen. The sun’s
energy is stored as chemical energy in the bonds of the glucose. This process of
producing glucose is called photosynthesis. It is represented by the equation:
6 CO2 + 6 H2O + energy (from sunlight)
C6H12O6 + 6 O2
3. Plants need energy to carry on all of life’s processes: growth, reproduction, gas
exchange, make food, respond to stimuli, movement, and excrete waste. Plants use
oxygen in the air (or water) to turn their food (glucose) into energy. This process of
using oxygen to release energy from food is called cellular respiration.
4. Cellular respiration occurs in the mitochondria of the plant cell. The glucose
molecule, using oxygen, is broken apart and turned back into carbon dioxide and
water, the same types of molecules that originally combined to make the glucose.
The solar energy that was used to make the glucose molecule is released to the cell
as chemical energy. This process of breaking down glucose for energy is called
cellular respiration. It is represented by the equation:
C6H12O6 + 6 O2
6 CO2 + 6 H2O + energy
5. Sugars created in photosynthesis can be later converted by the plant to starch for
storage, or it can be combined with other sugar molecules to form specialized
carbohydrates such as cellulose, or it can be combined with other nutrients such as
nitrogen, phosphorus, and sulfur to build complex molecules such as proteins and
nucleic acids. Cellulose is probably the single most abundant organic molecule in the
biosphere.
Materials
Procedure
1 Task Card
18 Lego 2x2 bricks in one color (white)
12 Lego 2x2 bricks in a second color (red)
6 Lego 2x2 bricks in black
1. Have the Lego bricks separated into individual bricks grouped by color.
2. Write the chemical formula for water on the board. (H2O) Ask students what they
think H and O stand for. (H is for hydrogen and O is for oxygen). A water
molecule is made out of oxygen atoms and hydrogen atoms. Have students look at
the chemical formula for water. Ask them how many hydrogen atoms and how
many oxygen atoms combine (bond) together to make a molecule of water (2
hydrogen, 1 oxygen)
3. Tell students that each Lego represents an atom, an individual building block of
matter. Each color represents atoms of a different element. Post the following
color/element combinations on the board:
White = oxygen
Red = hydrogen
Black = carbon
4. Ask the students to assemble the blocks to represent a water molecule. Explain
that in order to be considered a molecule, the atoms “blocks” must be “bonded,”
or snapped together.
5. Write the chemical formula for carbon dioxide (CO2) on the board. Ask students
to assemble the blocks to represent a molecule of carbon dioxide.
6. Ask students to make 6 water molecules and 6 carbon dioxide molecules. The
students now have the molecules a plant uses (H2O and CO2) to make glucose.
7. Write the chemical formula for glucose (C6H12O6). Using only the 6 water
molecules and 5 carbon dioxide molecules the students have made, challenge them
to make a glucose molecule. How many glucose molecules can they make with
the blocks they have? (Answer: 1)
8. What types of atoms are left after the glucose molecule has been made? (Answer:
12 oxygen atoms). Oxygen in the air does not like to float around by itself.
Oxygen forms a diatomic molecule (two atoms of the same element bonded
together). Ask the students to make as many diatomic oxygen molecules as they
can. They should be able to make six O2 molecules.
9. Students now have glucose and oxygen, the products of photosynthesis. Ask the
students to think about what the plant does with these products. Where does the
glucose molecule go and where does the oxygen molecules go? (The plant needs
glucose and will store it in its roots and stems in the form of starch. Later, the
plant will burn the glucose molecule for the energy the plant requires to carry on
all of the life processes, including growth. The oxygen molecule is a waste product
and is given off as a gas through the stomata in the leaves.)
10. Ask the students to reflect in their science notebooks about what they have done
and learned to this point. They should be able to write the photosynthesis reaction.
They may draw a picture of the reaction or what is taking place in the plant. They
may ask questions about what they still want to know or write something new that
they have learned.
11. Ask students to look at the glucose molecule. Explain to them that the glucose
molecule holds a lot of chemical energy in the bonds that were created to hold the
individual atoms together to form the molecule.
12. Ask the students what will happen when the plant or animal needs to use the
glucose molecule for energy? (The plant or animal will send the glucose to the
mitochrondia in the cell. The oxidation of the glucose by organisms is called
respiration. This process occurs in both plants and animals. Respiration in plants
releases the energy required for all life processes. It requires 6 O2 molecules to
break the bonds of the glucose molecule, thereby releasing the energy it held in its
chemical bonds. 6 CO2 and 6 H2O molecules are waste products. YES, plants DO
GIVE OFF CO2 when they respire. Respiration, which produces carbon dioxide
and water, is constantly being balanced against photosynthesis in nature and in
agriculture. If the carbon dioxide fixed via photosynthesis were exactly the equal
to the carbon dioxide produced by respiration in the plant, it would contribute
nothing to the buildup of biomass in the plant. Photosynthesis must exceed
respiration by a considerable amount to accumulate biomass. In agriculture,
farmers attempt to achieve a photosynthesis/respiration ratio as high as 40:1.
www.biologie.uni-hamburg.de)
13. Ask students to think about what we need for energy. (Food and oxygen). How do
we take in the items we need (ingest via the mouth; breathe via the lungs). Where
does the reaction that gives us our energy take place? Why do we breathe out
CO2?
14. Students can simulate the increase in biomass by snapping several glucose
molecules together, forming specialized carbohydrates called cellulose, which
gives plants their structure. Energy from cellular respiration is needed for the plant
to combine glucose molecules into cellulose, which is a form of growth.
15. By combining other nutrients such as nitrogen, phosphorus with glucose, the plant
can build complex molecules such as proteins and nucleic acids. This can be easily
modeled by adding a 4th and 5th color Lego brick to the glucose molecule.