Activity: The Chemistry of RESPIRATION and PHOTOSYNTHESIS
Objective: I will be able to write balanced equations for respiration and photosynthesis and explain how
the two equations are related.
Class
Copy!
Directions: Carefully read through this handout and answer all questions in complete sentences.
Introduction:
The chemistry of RESPIRATION and PHOTOSYNTHESIS are probably two of the most important concepts in biology. In
order to understand many of the processes that take place in living organisms, each student will need to have a complete
understanding of the details of these processes.
At the end of this laboratory activity you will be expected to write the equations for combustion, respiration and
photosynthesis. You should also be able to list differences and similarities between the equations.
The processes of ANIMAL RESPIRATION and PLANT PHOTOSNYTHESIS are key processes to the understanding of most
concepts and processes to follow in this course. It is therefore essential that you take extra time now to thoroughly
comprehend these processes.
PROCESS 1: “COMBUSTION:”
Combustion is the process of burning something. You may be familiar with this term because it is the process that is used all
around us to provide us with energy. Combustion of fossil fuels powers our cars, heats our homes, and powers factories.
The equation for combustion is:
Oxygen + Fuel Water + Carbon dioxide + Carbon + Energy (heat and light)
PROCESS 2: “CELLULAR RESPIRATION:”
Let’s compare the above process with that of animal respiration (cellular respiration). You have learned that animals require
oxygen and food. Food is a type of “fuel” for animals. You also learned that animals produce carbon dioxide and water vapor.
This is apparent when you blow your breath on a cold car window.
Is it also apparent that you, as well as other animals, produce energy. You do not produce light energy, but you do produce
energy that is the form of heat and muscle energy. Humans do not produce carbon (C) in the respiration process.
1. Complete the following chart for cellular respiration using the information above.
Animals need the following to produce energy.
Reactants (goes in)
Animals produce these products during the process.
Products (comes out/is produced)
2. Using the information in question 1, write an equation for the process of animal respiration. Do not worry about chemical
formulas yet. You can simply write everything out in words.
3. Compare the equation for respiration (answer to #2) to the combustion equation. How are they similar? How are they
different?
PROCESS 3: “PHOTOSYNTHESIS:”
Now, let’s think about photosynthesis, the process that plants go through to make food (sugars) using energy from the sun.
4. Think about what plants need in order to grow. What would you need to provide a plant in order for it to grow? (NOTE:
Soil minerals are plant requirement, but they do not react in this particular reaction. So they are not included in the
photosynthesis equation. You will learn more about this later.)
5. Think about what plants produce and combine that with the answer to question 4 to complete the following chart.
Plants need the following to produce food.
Reactants (goes in)
Plants produce these products during the process.
Products (comes out/is produced)
6. Using the information from the answer to question 5 to write an equation for the process of photosynthesis. Once again,
do not worry about chemical formulas yet.
PUTTING IT ALL TOGETHER: “The relationship between CELLULAR RESPIRATION and PHOTOSYNTHESIS:”
7. What do you notice when you compare the reactants of “respiration” with the products of “photosynthesis”?
8. What do you notice when you compare the products of “respiration” with the reactants of “photosynthesis”?
9. When you compare the RESPIRATION equation to PHOTOSYNTHESIS equation, one thing noticeable is that one is just
the _______________of the other.
The interdependence between these animals and plant processes can be simply diagrammed as follows:
Photosynthesis
produces “food”
and oxygen
“Food”
& O2
CO2 & H2O
Cellular respiration
produces carbon
dioxide and water.
Plants do both Photosynthesis
and Cellular respiration
10. What is the simplest food molecule (carbohydrate) that we have learned about?
Raise your hand to get
these questions
stamped off
MODELING IT!!!
To further understand how animals can obtain and use the “energy” in the food they eat, and how plants can trap “light
energy” and impart the light energy into food, it will be necessary to study a little basic chemistry. The specific food
substances from which animals get energy is a sugar called GLUCOSE. Glucose has the chemical formula C 6H 12O6 and is
probably the most important molecule you will learn about in this course. You will learn a lot about the compound, and about
how it chemically reacts with oxygen (O2) in your body to produce carbon dioxide (CO2), water (H 2O) and energy.
On your lab table are colored balls that represent atoms. The gray connectors represent the bonds which will hold the atoms
together. Locate the colored balls and figure out which ball represents which atom. Write the following information on your
paper:
Red = _______________ White= __________________ Black= _____________________
Remember that the empirical (scientific) formula for glucose is C 6H 12O6. The
structural formula for glucose is
An important thing to notice is that the molecule has a “ring” structure composed
of 5 C atoms and 1 O atom. The 6th atom is seen sticking up from the ring.
11. What is the molecular formula for glucose? ________________________
In actual atoms, there are no tubes to hold them together. In the kinds of atoms
that you made today, the atoms are held together by electrons from each atom that is shared by the two atoms. Each single
bond represents the sharing of 2 electrons. This kind of bond is called a COVALENT BOND.
Review:
12. How many bonds does carbon make?
13. How many bonds does oxygen make?
14. How many bonds does hydrogen make?
You will now make one of the most important molecules found in living systems. Make a glucose molecule by following the
next few steps. First study the structural formula for glucose for a moment.
C
O
C
C
First, make the basic “ring” of 5 C atoms and 1 O atom as shown to the left.
Second, add the remaining atoms. No prongs should be left open. Also count the
number of C, H, and O atoms to be sure that you have not missed any.
When the molecule is complete you might admire it for a moment since they are
not easy to build. Remember, this is what scientists accept the molecules of
glucose to look like, even though no one has ever seen a glucose molecule. If you
were to connect this glucose molecule to hundreds of others, in the chain, the result would be starch. Save this glucose
molecule.
C
C
Now, construct six molecules of O2. Remember that O2 contains double bonds. Save
the O2 molecules for the next step.
Raise your hand to get
your models stamped off
15. Write the equation for cellular respiration using the molecular formula for each molecule involved. Ex: substitute the word
“food” with C 6H 12O6 .
16. The molecules of O2 and C 6H 12O6 that you have just constructed are the reactants for which process?
With these molecular requirements, you can demonstrate to yourself what happens when O2 and C 6H 12O6 react. This will
visually show you where the products CO2, H 2O and energy come from. It should help you understand the respiration
reaction much more clearly.
Before you do this, it is helpful to know that this process is occurring in your cells at the moment. The glucose in you is carried
to each cell by your blood stream. The O2 entering your lungs is carried to each cell by the blood stream. There the glucose
molecules are pulled apart by enzymes and the parts of the glucose molecule rearranged into CO2 and H 2O.
Proceed to break apart both C 6H 12O6 and the O2 to form CO2 and H 2O molecules. This demonstrates the breaking of sugar
to produce energy. Break apart only those bonds needed to create the CO2 and H 2O. Use all the O2 and the glucose to form
as many CO2 and H 2O molecules as you can. Make sure to follow the rules on how many bond each type of atom makes
17. When you have completed the reaction, count the number of CO2 molecules
and H2O Molecules that you have. There are________CO2 molecules and
________H2O molecules.
Raise your hand to get
your models stamped off
18. You have just demonstrated which process? ___________________ When you pulled apart the glucose molecule and O2
molecules with your hands, that represented what the cell accomplishes using enzymes.
TO SUMMARIZE CELLULAR RESPIRATION:
How many molecules of glucose did you start with? ___________
How many molecules of O2 did you start with? _______________
How many molecules of CO2 did you end up with? ____________
How many molecules of H2O did you end up with? ____________
Do not take apart these molecules.
19. The equation for cellular respiration can be rewritten with the number of molecules needed, substituted for the blanks
below
_____O2 + C6H12O6  _____CO2 + _____H2O + energy
The above is what is called a BALANCED EQUATION, because it accounts for every molecule required and shows exactly
how many molecules of CO2 and H2O are produced. In any chemical equation, the number appearing before the molecule’s
formula tells how many molecules are involved. When one sees a formula as follows: 6 CO2 . The number 6, which precedes
the molecule, tells the reader the number of CO2 molecules. The subscript 2, tells the number of atoms of O in 1 CO 2
molecule.
20. In nature, what becomes of these products of respiration given off by you and by other animals?
In plant leaf cells, where photosynthesis occurs, the CO2 and H2O react. The light energy is absorbed by chlorophyll and is
used for breaking apart the molecules of CO2 and H2O. Enzymes aid in this break up, and other enzymes put the pieces of
CO2 and H2O together into 1 large glucose molecule. The remaining oxygen atoms combine as O2. The sun provides the
energy used to bond the atoms together into glucose molecule. This energy becomes stored in the BONDS of glucose
molecule.
Try breaking apart and combining your CO2 and H2O molecules to make 1 large glucose molecule. Remember to break just
enough bonds to be able to construct the glucose molecule.
21. How many oxygen atoms remain on the table? ____________________
How many O2 molecules are produced? __________________________.
Raise your hand to get
your models stamped off
It’s interesting to contemplate that what you just performed during this class period is taking place in a plant leaf. It takes only
seconds in the cells of the plant.
22. Write the number of molecules of each kind involved in the photosynthesis reaction below:
____CO2+____H2O + Energy (sun)  ____C6H12O6 + ____O2
This is the balanced equation for photosynthesis!
Notice that the energy from the sun is now locked back into the glucose molecule. When an animal eats the leaf, it takes in
glucose. Through the respiration process, the animal breaks the bonds of glucose again and obtains energy to move. And so
the process of respiration repeats itself again, and the SAME ATOMS of C, H and O are shuttled back and forth between
plants and animals for as long as this planet exists. A brief summary follows.
6 O2 + C6H12O6
6 CO2 + 6 H2O +E
This “reversible” reaction continues; respiration in animals cells and photosynthesis in
plant cells.
Clean up by dismantling your molecules and placing the atoms and tubes in their
appropriate containers.
Raise your hand to get
your clean up stamped
off.
REVIEW QUESTIONS:
1. Write the balanced equation for respiration. _______________________________________________________
2. What kind of organisms undergo respiration? _____________________________________________________
3.
Write the equation for combustion. _____________________________________________________________
4. The combustion reaction, which is so similar to respiration, occurs in the same chemical manner as does respiration. In
fact, fuel such as wax, wood and gasoline contain C, H and O.
List each substance found in the equation for cellular respiration under the following headings:
COMPOUNDS
MOLECULES
ATOMS
5. a) Write the balanced equation for photosynthesis. ________________________________________________
b) What type or form of energy is required by plant cells to put the glucose molecule together?
____________________________________________________________________________________________
c) A substance that helps put the atoms together and arrange them as glucose is called___________________.
d) The carbon atoms in the photosynthesis product, C6H12O6. come from what reactant molecule? ___________
e) All hydrogen atoms found in glucose come from what reactants? ____________________
6. In photosynthesis, how many oxygen molecules are produced for every 1 glucose molecule produced? _________
7. What “type” of formula is CO2? _________________ What “type” of formula is O=C=O? ____________________
8. What kinds of bonds are seen in the CO2 molecule? (O=C=O) ___________________________________________
9. Chlorophyll, which is the molecule responsible for absorbing sunlight that is needed for photosynthesis, is found in what
plant cell structure? ___________________________
10. The definition for compound is: A substance that is composed of ________________________________________. Can
it be broken down into a simpler substance? _________________________
11. a) TRUE or FALSE? An element can be separated into simpler substances. ____________________
b) A molecule is composed of_______________________________
c) The “products” of photosynthesis are the _______________for respiration. The energy that an animal obtains from
respiration comes from___________________.
** If you do not finish the review questions during class, please complete them at home for homework. The class
copy is attached under files on Connect.**