Photosynthesis & Cellular Respiration Chemistry
Biology relies on chemical processes that happen constantly in and around living things. Two of those
processes are photosynthesis and cellular respiration. These chemical reactions supply cells with the
energy they need. Photosynthesis converts solar energy into chemical energy. Cellular respiration
converts the chemical energy stored in sugars into ATP, which is a useable form of energy in cells.
Photosynthesis Chemical Reaction
6CO2
+ 6H2O + light 
carbon dioxide
water
C6H12O6
glucose
+
6O2
oxygen
In a chemical reaction equation there are two types of numbers: coefficients and subscripts. The
coefficients are the numbers in front of the chemicals (like the 6 in front of CO2) and they say how
many of that substance is in the reaction. The subscripts are the little numbers in the chemical formula
(like the 2 after the oxygen in CO2) and they say how many atoms of each element are in the
compound. To find out the total number of atoms of each element in the reaction you multiply the
coefficient by the subscript!
Questions
1. What is the subscript on carbon in glucose?
2. How many hydrogen atoms are in glucose?
3. How many oxygen atoms are in a molecule of oxygen gas?
4. If you have 6 carbon dioxide molecules, how many oxygen atoms do you have?
5. How many carbon atoms are in 1000 glucose molecules?
The reaction equation is like a recipe for the substances in the reaction and what they make.
The reaction equation tells us the ratio for how many we need of each reactant and how many we can
make of each product. For example, if you have a recipe that calls for 1 cup of chocolate chips for
each dozen cookies, then you can make 2 dozen cookies with 2 cups of chocolate chips. One way to
solve these problems is using ‘dimensional analysis’, as shown below. The units on the given, starting
value cancel out, so you end up with different units on the answer. So, for example, you can find out
how many dozens of cookies can be made from 4 cups of chocolate chips.
4 cup chocolate chip
|
|
1 dozen cookies
1 cup chocolate chips
=
4 dozen cookies
Questions (cont….)
6. What is the ratio of carbon dioxide to glucose in the photosynthesis equation?
7. If you start with 18 carbon dioxide molecules, how many glucose molecules can you make?
Show your work like the example above.
8. How many water molecules are needed for a plant cell to make 60 glucose molecules? Show
your work like the example above.
Cellular Respiration Chemical Reaction
C6H12O6 + 6O2

6CO2
+ 6H2O
+ 36 ATP
9. How many oxygen gas molecules are needed to make 72 ATP molecules (based on the above
equation)?
10. If a cell uses 1000 glucose molecules and has unlimited oxygen gas, how many ATP molecules
will be produced?