Photosynthesis

How do light-dependent and light-independent reactions provide food for a plant?

Why?
Plants are the original solar panels. Through photosynthesis a plant is able to convert electromagnetic
(light) energy into chemical energy. This energy is used not only to keep the plant alive, but also to sustain
all creatures that rely on the plant for food and shelter. Plants and photosynthetic algae are also the source
of all oxygen on Earth, allowing the inhabitants of Earth to benefit from our most plentiful renewable
energy resource.

Model 1 – Chloroplast
6CO2 + 12H2O + sunlight energy ⎯→ C6H12O6 + 6O2 + 6H2O

Outer membrane

Inner membrane

Stroma
12H2O Site of the Calvin cycle, or light-
independent reactions.
6CO2
Thylakoid
Site of the light-dependent reactions.
Contains chlorophyll—a colored
6O2 compound that absorbs light.

Glucose (C6H12O6)

  1. Consider the organelle illustrated in Model 1.

a. What is the name of this organelle?
This is the chloroplast.

b. Is this organelle more likely to be found in animal cells or plant cells?

It's more likely to be found in plant cells

  2. The structures inside the organelle in Model 1 are called thylakoids. What compound necessary
for photosynthesis is contained in the thylakoids?

Chlorophyll is necessary and is in the

Photosynthesis 1
   3. Consider the chemical reaction in Model 1. This represents photosynthesis.
a. What substances are the reactants in photosynthesis? Include the name and chemical formula
of each substance in your answer.

Carbon dioxide(CO2) and water(H2O)

b. Where in the organelle are these molecules stored before they are used in photosynthesis?

In the Stroma

c. Is photosynthesis an endergonic or exergonic reaction? Support your answer with evidence

from Model 1.

Endergonic, since the process is absorbtion rather than expulsion of energy.

d. What is the energy source for photosynthesis?

Sunlight

  4. Photosynthesis occurs in two parts—the light-dependent reactions and the light-independent

reactions.
a. What is another name for the light-independent reactions?
The Calvin cycle

b. In what part of the chloroplast do the light-dependent reactions occur?

The thylakoid

c. In what part of the chloroplast do the light-independent reactions occur?

Inside of the stroma

  5. Considering your answers to Question 4, what compound is best able to absorb the light energy
from the Sun and convert it into chemical energy?

Chlorophyll

  6. What substances are produced during photosynthesis? Include the name and chemical formula of
each substance in your answer.
Oxygen(O2), water(H2O), and glucose(C6H12O2)

  7. Why is it necessary to have six CO2 entering the chloroplast?

To balance the reaction of carbons and hydrogens

2 POGIL™ Activities for AP* Biology

Model 2 – The Light-Dependent Reactions

ht

Lig
Lig

h
2 NADPH (to Calvin Cycle)

t
Photosystem II H+ Chlorophyll
2 NADP+ + 2H+

ETC

High
Potential H+ Photosystem I
Energy H+
2H2O H+ H+
Electron O2 H+ H+ H+ Inner
H+ Stroma
H+ H+ H+ H+
Thylakoid
H+

ATP Synthase
3 ATP (to Calvin Cycle)

3 ADP + 3 Pi
H+

  8. In Model 2, what shape or symbol represents a single electron?

The small black dots

  9. In the light-dependent reactions electrons are released from molecules in two ways.
a. Find two places in Model 2 where electrons are released from chlorophyll by a photon of light
coming from the Sun.

Through the stroma on both photosystem I &II

b. Find one place in Model 2 where electrons are released from water molecules.

Through the inner thylakoid in photosystem II

c. When the electrons are released from water molecules, what other products are formed?

Oxygen(O2) and Hydrogen(H2)

Photosynthesis 3
Read This!
The light-dependent reactions of photosynthesis include three major processes:
A. Excited electrons leave chlorophyll and reduce NADP+ into NADPH.
B. Excited electrons moving through the electron transport chain provide the free energy needed
to pump hydrogen ions into the inner thylakoid.
C. Hydrogen ions flowing out of the thylakoid via a protein channel provide the free energy
needed to convert ADP to ATP.

10. In Model 2, label the diagram with “A,” “B,” and “C” to indicate where the three steps in the
Read This! box are occurring.
11. The light-dependent reactions include an electron transport chain system that works in a very
similar fashion to the electron transport chain in respiration. Briefly describe how this system
works and what job it performs in the light-dependent reactions. (Your answer should include a
discussion about concentration gradient.)

The ETC uses energy from excess electrons to move excess H+ ions against their consentration gradient,
creating a high concentration of H+ within the thylakoid membrane

12. Refer to Model 2.

a. Name the embedded protein complex found in the thylakoid membrane that uses excited
electrons to reduce NADP+ into NADPH.
Photosystem I

b. Name the embedded protein complex found in the thylakoid membrane that provides excited
electrons to the electron transport chain.

Photosystem II

c. Name the embedded protein complex found in the thylakoid membrane that converts ADP
to ATP using free energy from a flow of hydrogen ions.

ATP synthase

4 POGIL™ Activities for AP* Biology

13. Once a chlorophyll molecule has released electrons it is no longer useful until those electrons are
replaced.
a. According to Model 2, what is the source of replacement electrons for those released from
photosystem I?
Electrons from the ETC path

b. According to Model 2, what is the source of replacement electrons for those released from
photosystem II?
Electrons from the splitting water molecules

14. Is carbon dioxide involved in the light-dependent reaction?

No

15. Refer to Model 2.

a. Write a chemical reaction that summarizes all of the chemical reactions in the light-dependent
reactions of photosynthesis starting with two water molecules.
2H2O + 2NADP+ + 3ADP + 3P = O2 + 2NADPH + 3ATP

b. In the photosynthesis reaction in Model 1, twelve water molecules are shown as reactants, but
six water molecules are shown as products. Are any of the twelve water molecules products of
the light-dependent reactions?
No

c. Calculate the total number of oxygen, NADPH, and ATP molecules that are produced when
twelve water molecules complete the light-dependent reactions.
6 Oxygen, 12 NADPH, 18 ATP

16. Where do the ATP and NADPH produced during the light-dependent reactions go when the
process is complete?

To the Calvin cycle

Photosynthesis 5
Model 3 – The Light-Independent Reactions (Calvin Cycle)
O C O
Carbon dioxide
(CO2)

3 CO2

O– OH O– O O–
O– P O CH 2 C CH CH CH 2 O P O– O– C CH CH 2 O P O–
O O OH O OH O
3 RuBP 6 PGA Phosphoglycerate
Ribulose biphosphate
(RuBP) Carbon (PGA)
Fixation
3 ADP + 3 Pi
Regeneration Reduction 6 ATP
3 ATP

6 ADP

5 PGAL 6 NADPH

6 PGAL 6 NADP+ + 6H2O + 6 Pi

1 PGAL
O O–
(a three-carbon sugar H C CH CH 2 O P O–
used by the cell to
produce glucose and OH O
other biomolecules) Phosphoglyceraldehyde
(PGAL)

17. According to Model 3, what are the three phases of the Calvin cycle?

Carbon fixation, regeneration, and reduction

18. Find the compound ribulose biphosphate (RuBP) in Model 3.

a. How many RuBP molecules are used in one turn of the Calvin cycle?

3 RuBP

b. How many carbon atoms are in each RuBP molecule?

5

c. Calculate the total number of carbon atoms represented in all of the RuBP molecules used in
one turn of the Calvin cycle.
15 carbon molecules

6 POGIL™ Activities for AP* Biology

 19. Ribulose biphosphate (RuBP) combines with carbon dioxide (CO2) to form phosphoglycerate
(PGA) during the carbon fixation phase of the Calvin cycle.
a. How many CO2 molecules are used in one turn of the Calvin cycle?
3
b. How many PGA molecules are made in one turn of the Calvin cycle?
6

c. How many carbon atoms are in each PGA molecule?

3
d. Calculate the total number of carbon atoms represented in all of the PGA molecules used in
one turn of the Calvin cycle.
18 carbon molecules

20. Explain what happened to the carbon atoms from the carbon dioxide molecules that entered the
Calvin cycle.
CO2 was split out to create the PGA molecules

21. Consider the term “carbon fixation.” Think individually for a moment what this term might
mean, then share ideas among the group. Record the group’s consensus definition for carbon
fixation here.
Carbon fixation is the process in which carbon atoms get removed from the atmosphere and are reincorperated into
larger molecules within a system

Read This!
Model 3 is a simplified version of the Calvin cycle. Each of the three phases in the cycle consist of
multiple reactions that are catalyzed by enzymes specific to that reaction. These enzymes have names like
RuBisCo, phosphoglycerate kinase, and PGAL hydrogenase.
22. Refer to the reduction phase of the Calvin cycle in Model 3.
a. What molecule does the PGA molecule turn into during this phase of the Calvin cycle?
PGAL

b. Describe specifically how the structures of the two molecules in part a are different.
When PGA molecules lose one oxygen atom, they turn into PGAL

c. Identify the types and numbers of molecules that provide the free energy necessary for the
reduction of the PGA molecules.
6 NADPH and 6 ATP provide the free energy needed

d. Is the total number of carbon atoms present in the Calvin cycle changed during the reduction
phase? Support your answer with evidence from Model 3.
No, the amount of oxygen molecules between PGA and PGAL tay the same, keeping the carbon the same between the
molecules
Photosynthesis 7
 23. Water is a product of the reduction phase of the Calvin cycle.
a. How many water molecules are produced?

6

b. Explain where the hydrogen and oxygen atoms in these water molecules originated.
When PGA is formed into PGAL, it losses an oxygen and allowed for it to be
repurposed into water, then the hydrogen comes from the conversion of NADP+ to
NADPH

24. Refer to the regeneration phase of the Calvin cycle in Model 3.

a. How many PGAL molecules continue on to the regeneration phase of the Calvin cycle?

5

b. Identify the types and numbers of molecules that provide the free energy necessary for the
regeneration of these molecules.

3 ATP molecules

c. How many total carbon atoms remain in the Calvin cycle at this point?

15 carbon molecules

d. What molecule(s) are “regenerated” in this phase of the cycle?

3 RuBP molecules

e. How many total carbon atoms leave the Calvin cycle before the regeneration phase?

3 carbon molecules

f. What happens to the PGAL molecule that does not continue on in the Calvin cycle?

It is used as energy for the cell

Read This!
As you have learned from your careful study of the Calvin cycle illustrated in Model 3, three atoms of car-
bon enter the cycle as carbon dioxide and three carbon atoms leave the cycle as PGAL. It is easy to assume
that the three atoms that leave are one and the same with the three that entered, but that is incorrect. It
may be that none of the carbon atoms from the carbon dioxide become incorporated into a molecule of
PGAL that leaves the cycle. Alternatively, it is also possible that one of the carbon atoms from the carbon
dioxide will become part of a PGAL molecule that leaves the cycle. Eventually all of the carbon atoms that
enter the cycle will leave as part of a PGAL molecule, but they must wait their turn.

8 POGIL™ Activities for AP* Biology

25. The reaction in Model 1 shows glucose (C6H12O6) as a product of photosynthesis.
a. How many PGAL molecules will it take to make one molecule of glucose? Justify your answer
with a discussion of numbers of carbon atoms.
2 PGAL molecules because they each contain 3 carbon molecules, but glucose requires six, therefore
2 are needed.

b. How many turns of the Calvin cycle will it take to make one molecule of glucose?
2 turns

c. Calculate the total number of ATP and NADPH molecules used in the production of one
molecule of glucose.
18 ATP and 12 NADPH molecules are required for one molecule of glucose

26. Where do the ADP and NADP+ go after they are used in the Calvin cycle?

They are recycled back into the Calvin cycle

27. Explain in detail, using complete sentences, how the two reactions (light-dependent and light-
independent) depend on each other.

The light cycle is what creates the free energy(18 ATP and 12 NADPH) for the light independent cycle, which are
then returned and recycled back into the light dependent cycle

28.Under each molecule in the equation below, indicate whether it is involved (either used or
produced) in the light-dependent reactions or the Calvin cycle.
6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O
Calvin cycle Light dependent Light dependent Calvin cycle
Calvin cycle
cycle cycle

29. Throughout photosynthesis, energy is transferred from light to several molecules with increas-
ingly higher potential energy. Use the words below to summarize the order in which the energy
flow occurs.
electrons ATP glucose sunlight
4 1
2 3

30. Although photosynthesis does produce some ATP, these molecules are not used to do the work
of the plant cells. What other process occurs in the cells that provides the ATP necessary to do
cellular work such as make proteins, divide cells, and move substances across membranes?

Cellular respiration

Photosynthesis 9
Extension Questions
Model 4 – The Study of Photosynthesis

Carbon dioxide
Oxygen
Concentration RuBP
of molecules

Time

31. When algae are undergoing photosynthesis, the concentrations of various molecules change with-
in the cells. These concentrations can be monitored and graphed. In complete sentences, explain
the shape of each line on the graph in Model 4.
carbon dioxide— Since carbon dioxide is a reactant in this reaction, it will start off at a higher concentration but as
the reaction occurs, will start to steadily decrease

oxygen—Since oxygen is a product in this reaction, it will start with a low concentration but as the reaction occurs,
the concentration will steadily increase

Since RuBP is constantly being recycled back into the cycle, when there is no activity the concentration will
RuBP—
start low, but gradually increase as activity is started. If the cycle is in constant turn, then the concentration of
RuBP will be at a constant

32. Photosynthesis is typically represented by a simple equation.

6CO2 + 6H2O + light ⎯→ C6H12O6 + 6O2
a. Compare and contrast this simplified equation with the one presented in Model 1.

In this equation, water is a reactant where as in model 1 it is a product.

b. Using the information from this activity explain why the equation above is a vastly oversim-
plified representation of the actual process.

Photosynthesis itself is a very complicated reaction, involving many steps and other reactions in order to occur. These
reactions can be separated out and take time to complete fully, and takes more steps than shown in the actual equation
such as splitting the carbon dioxide and water, then recombining to create new molecules

10 POGIL™ Activities for AP* Biology