Photosynthesis
Equation:
The Light Reactions
Read the following description of the light reactions. Each time you come to a letter in parentheses STOP find that letter on the
corresponding picture and color in the letter appropriately.
Take a look at the diagram you have in front of you. This is the thylakoid membrane. The thylakoid is a hollow oval shaped
structure inside of chloroplasts. It is where photosynthesis begins. The thylakoid membrane, like all membranes, is composed of a
phospholipid bilayer (B).
The first and most important event in photosynthesis is the capturing of light energy (E) by pigments. This occurs in two places
along the membrane:
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Photosystem II (P2) along with Pigment 680
Photosystem I (P1) along with Pigment 700
When light energy is absorbed in photosystem II electrons (e) become excited and jump to higher energy levels. These lightenergized electrons leaving photosystem II are pass along to the electron acceptors (EA). These acceptors pass the excited electrons to a
proton pump (PP) embedded in the membrane. This proton pump moves protons (H+) atoms into the thylakoid interior. The
movement of these protons takes some energy, which it gets from the energized electrons, because they are being moved from a low
concentration to a high concentration.
To prevent the chlorophyll in photosystem II from running out of electrons there is a system to replace lost electrons. Two water
molecules bind to an enzyme that splits water into hydrogen ions (H+) and an oxygen atom (O2). This process is called PHOTOLYSIS
and is illustrated by the arrows labeled “L.”
2H2O
2H+ + 2e- + O2
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The 2 electrons (e) are donated to the chlorophyll in photosystem II.
The 2 protons (H+) remain in the thylakoid interior.
The oxygen (O2) is released as a gas.
The electrons from photosystem II eventually make there way to photosystem I by way of more electron acceptors (EA). By the
time they get there they have lost some of the energy to the proton pump. However, light absorbed by photosystem I re-energizes the
electrons and passes them to the enzyme NADP Reductase (R). NADP reductase transfers the electrons to a molecule of NADP
(NADP), which takes the electrons and some hydrogen to make NADPH (NADPH). The electrons are no longer in the electron
transport chain – they are being carried by NADPH to the dark reactions.
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Note: All of the electrons that pass down the membrane come from water! The path the electrons took is marked by an X – color in
the Xs the same color you used for e.
As electrons are moving down the membrane, H+ are building up in the thylakoid interior. Remember: Photosystem II pumped H+
into the thylakoid membrane & H+ were released into the interior when the water was split. The thylakoid membrane is NOT permeable to
protons, so as the H+ build up inside of the thylakiod interior a concentration gradient forms. The H+ want out!
The protons only have one exit via a special channel provided by ATP Synthase (S). The protons move through the ATP
synthase from the interior, which has a high concentration, to the stroma, which has a low concentration. This movement fuels ATP
synthesis. As protons pass through the ATP synthase, ADP has a P added to make ATP. This ATP is now on its way to the dark reactions
where it will be used to generate glucose.
 Note: The arrows labeled "Y" mark the path of protons through the membrane. Color in the Ys the same color you used for H +.
Questions:
1. What is needed for the light reactions to occur?
2. What is produced by the light reactions?
3. Summarize the flow of electrons through the thylakoid membrane.
4. Summarize the flow of protons across the thylakoid membrane.
5. NADP Reductase is a protein in the cell membrane that makes
6. ATP Synthase is a protein in the cell membrane that makes
7. Explain how the concentration gradient affects the process of ATP synthesis.
The Light-Independent Reactions / The Dark Reactions / The Calvin Cycle
Read the following description of the Calvin Cycle. Each time you come to a BOLDED TERM STOP find that term on
the corresponding picture and color in the term appropriately.
Take a look at the diagram you have in front of you. Unlike the diagram you colored for the light reactions, this diagram is not of a
physical thing, but instead of a process. The process is called the Calvin Cycle. It occurs in the stroma, the area of the chloroplast
outside of the thylakoid. The products of the light reactions (located in the stroma) along with CO2 are used as the reactants in the
Calvin Cycle.
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Step 1: The first event in the Calvin Cycle is when 3 molecules of CO2 combine with 3 molecules called RuBp. Each RuBP
molecule has 5 carbon atoms and each CO2 molecule has 1. When they come together they form 3 molecules with 6-carbon
atom each.
Step 2: The 3 molecules formed in step 1 immediately split in half. Since the molecules had 6 carbon atoms each when they
split in half they end up with 3 carbons each. The end result is 6 molecules called 3-PGA.
Step 3: The 6 molecules of 3-PGA go through a series of transformations that require six ATP and six NAPDH. After the
transformation you have six G3P molecules.
Step 4: Of the 6 G3P molecules 1 will leave the cycle to become sugar while the other 5 will continue on.
Step 5: The five remaining G3P molecules turn back into 3 RuBp molecules. This reaction requires three more ATP molecules.
To make 1 molecule of sugar the Calvin Cycle has to spin twice.
Questions:
8. List all of the things needed for the dark reactions to occur & where they come from …
What is needed
CO2
ATP
Where it came from
Cycles Through the Calvin Cycle
Light Reactions
9. List all of the things produced by the dark reactions & where they go…
What is produced
Where it goes
Back to the light reactions
To the cytoplasm to fuel the cell
RuBP