Name: ____________________
AP Biology
Light Reactions Table Activity
Introduction
Photosynthesis is the process, by which autotrophs (plants) make organic compounds, such as
sugars. Photosynthesis occurs in two phases: Light-Dependent Reactions & The Calvin Cycle (also
called the Light-Independent Reactions). The Light-Dependent Reactions occur within the
thylakoid membranes of the chloroplast and the Calvin Cycle occurs in the stroma, the fluid space
that surrounds the grana (stacks of thylakoids).
In the figure below label the following: thylakoid, stroma & grana.
double membrane
Purpose
The purpose of this activity is to investigate the first phase of photosynthesis: The Light-Dependent
Reactions. The light-dependent reactions capture light energy and store it temporarily in two
energy storage molecules—NADPH and ATP—that will be used in the second phase of
photosynthesis.
Main Reactants of the Light-Dependent Reactions: Light Energy
&
H2O
eMain Products of the Light-Dependent Reactions: ATP
&
NADPH
&
O2
Overall Photosynthesis Equation:
Question: In the equation above, circle the main reactants and main products of the LightDependent Reactions. Why are ATP and NADPH missing from the overall equation?
Procedure
Initial Set Up:
-Place two electrons (e-) in each Photosystem (in the designated circle)
-Place 4 H+ out in the stroma and 8 H+ in the thylakoid space
-Place the H2O molecule to the left of the arrow under Photosystem II (PS II)
-Place NADP+ and an H+ to the left of the arrow above NADP+ Reductase
-Place ADP and P to the left of the arrow below the ATP Synthase
(In each of the steps below, the actions you are supposed to perform are italicized and bolded.)
Step 1: Light energy is absorbed by special pigments called chlorophyll in Photosystem II (PS II).
The light energy excites a pair of electrons. The excited electrons are released from the PS II to
the first electron carrier of the electron transport chain (ETC). The electron carrier’s name is
Plastoquinone (Pq). Move the excited electrons to the first e- carrier (Pq). The electron carrier
is mobile (meaning it can move) and its job is to transport electrons through the ETC.
Question: What is the function of light in this step?
Step 2: PS II lost electrons to the ETC and splits a molecule of water (H2O) into hydrogen ions,
oxygen atoms, and electrons. The electrons released from the split water replace the electrons lost
to the ETC. Split the H2O molecule into 2H+, ½O2, and 2e-. Move the 2e- to PS II. The ½O2
combines with another ½O2 to become an O2 molecule. Move the O2 out of the thylakoid.
Question: Why do plants require water?
Question: What vital gas do plants give off and what is the source of that gas?
Step 3: Back to the first electron carrier. The electrons are then passed to the H+ (hydrogen ion)
pump, called b6f cytochrome complex. (Pay attention: This is REALLY important!!!) Move the
electrons to the H+ Pump. When the electrons are passed to the H+ Pump they give off energy.
The energy is used by the H+ Pump to pump H+ into the thylakoid space. This is not an easy job. It
requires lots of energy because the H+ are in higher concentration in the thylakoid space (they are
being pumped up the concentration gradient). Remember this … it will be important in a second.
Pump H+ into the thylakoid space.
Question: How does the H+ Pump get the energy it requires to pump H+ into the thylakoid space?
Step 4: After the H+ Pump e- are passed to the next electron carrier. This electron carrier’s name is
Plastocyanin (Pc). Move the electrons to the next e- carrier (Pc). The electrons will soon be
passed to Photosystem I (PS I), but not until it loses its electrons.
Step 5: At PS I, light energy excites electrons (similar to PS II) and electrons are passed to the
electron carrier. This electron carrier’s name is Ferredoxin (Fd). Move the electrons from PS I to
the e- carrier (Fd).
Question: Explain the job of the electron carriers.
Question: What does “ETC” stand for? How does the name explain the function?
Step 6: Electrons (e-) are then passed to the NADP+ Reductase. This is a special enzyme that
combines NADP+, H+, and e- to form NADPH. Move the electrons to NADP+ Reductase, combine
NADP+, H+, and 2e- to form NADPH. NADPH is an energy storage molecule that temporarily stores
the high energy electrons until they will be used in the Calvin Cycle.
Step 7: PS I lost electrons, but they are quickly replaced by the electrons at the second electron
carrier. Move the electrons to PS I, excite the electrons with light energy, and then pass the
electrons to the e- carrier (Fd) and then to NADP+ reductase. Form a second NADPH.
Question: Where do the electrons that are stored in the NADPH molecule originally come from?
Step 8: Okay, almost done, but we still have to make some ATP! Remember how the H + Pump used
energy from electrons to pump H+ into the thylakoid space? The pumping in of H+ generates a
concentration gradient, meaning there is a higher concentration of H+ inside the thylakoid space
than there is outside in the stroma.
Question: If H+ are in a higher concentration inside the thylakoid, then what direction must they
flow to reach equilibrium?
Step 9: H+ cannot pass through the thylakoid membrane. The only place that the H+ ions can flow
down their concentration gradient is through a special enzyme called ATP Synthase. H + ions flow
through a channel in the ATP Synthase. This flow of H+ powers the ATP Synthase (much like a water
wheel) and the ATP Synthase (now activated) attaches a free phosphate (P) to ADP (adenosine
diphosphate) to make ATP (adenosine triphosphate). Move the H+ down their concentration
gradient through the ATP Synthase and add a P to ADP to make ATP.
Question: In your own words explain how the H+ gradient is generated and how it functions in
generating ATP.
Step 10: Begin again, move all materials back to their position as instructed in the initial set up.
Go through the process of making NADPH and ATP until you can do it without looking at the
instructions. As you move through the process again use your finger to trace the movement of
electrons through the ETC.
Question: The purpose of the light-dependent reactions was to make ATP and NADPH for the Calvin
Cycle. What are the ATP and NADPH used for in the Calvin Cycle?
Step 11: What you have just been simulating is called “noncyclic” electron flow. Now, let’s model
“cyclic” electron flow, an alternative path of electron flow. Cyclic electron flow is a short circuit
that uses Photosystem I but not Photosystem II. The electrons excited from PS I cycle back from the
e- carrier Ferredoxin (Fd) to the b6f cytochrome complex (H+ Pump) and from there back to PS I.
Start with the electrons in PS I. Excite the electrons with light energy and then pass the
electrons to the e- carrier (Fd). From here, pass the electrons to the H+ Pump. As you pass the
electrons to the next e- carrier (Pc), move H+ into the thylakoid space. Finally, pass the
electrons back to PS I. Now, move the H+ down their concentration gradient through the ATP
Synthase to make ATP.
There is no production of NADPH or oxygen in cyclic electron flow. However, cyclic electron flow
does generate ATP. This extra production of ATP is necessary because noncyclic electron flow
produces ATP and NADPH in roughly equal quantities, but the Calvin cycle consumes more ATP than
NADPH. Since cyclic electron flow produces ATP but no NADPH, it makes up the difference.
Question: If a chloroplast runs low on ATP for the Calvin cycle, the Calvin cycle begins to slow
down and NADPH begins to accumulate. This build-up of NADPH is thought to stimulate a shift from
noncyclic to cyclic electron flow. How does this shift allow the Calvin cycle to continue?
Step 12: When you are finished put all materials back into the plastic bag.