CHAPTER 8
PHOTOSYNTHESIS
PART 2
Below is a diagram illustrating an overview of the Light Reactions.
1. Where do the Light Reactions occur?
2. How many “photosystems” are involved in the Light Reactions?
3. What pigment is each photosystem made of (hint: look at the color)?
4. Are NADPH and ATP formed inside the thylakoid or outside the thylakoid (in stroma)?
The diagram below shows the electron transport chain of the Light Reactions.
Photosystem II:
5. What is occurring in Step #1?
6. Due to Step #1, what happens in Step #2?
7. Since Step #2 removes 2 electrons from chlorophyll, how are they replaced in Step #3?
8. What does Step #3 produce from the chemical equation of photosynthesis?
Electron Transport Chain:
9. What is “moving” in Step #4?
10. From the “overview diagram”, what moves through the integral protein in Step #4?
11. What is formed indirectly from the movement of these particles?
Photosystem I:
12. Where does the energy come from that initiates Photosystem I?
13. Due to this energy, what happens in Step#5?
14. There is another short electron transport chain following Step #5. Where do the electrons end
their journey? Forming what molecule?
Here is another diagram representing the Light Reactions.
ATP Phosphorylation:
15. During the Light Reactions, hydrogen ions are moved into the thylakoid. This builds up a
hydrogen ion gradient inside the thylakoid. What ends up happening to these hydrogen ions,
along with the ones formed from splitting water?
16. What is the name of the enzyme involved in this process?
17. What is the molecule formed from this process?
18. Where do NADPH and ATP go following the Light Reactions?
Below is a diagram representing the Calvin Cycle. Understanding the Calvin Cycle is a “Carbon Atom
Counting Game”. Since it is a “cycle” the amount of carbons that goes in must come out. Read the
following description of the 3 phases of the Calvin Cycle and ensure you are keeping track of the
number of carbons in each step. You may have to read this description several times and even
compare it to the textbooks description of the Calvin Cycle.
Phase 1: Carbon Fixation
Two carbon based molecules will “combine” in this step: A 5-carbon molecule (RuBP) and a 1-carbon
molecules (CO2). This “carbon fixation” is aided by an enzyme known as Rubisco. You will notice that
there are 3 CO2 molecules (a total of 3 carbons) that are fixed to 3 RuBP molecules (a total of 15
carbons) which would initially form 3 6-carbon molecules (a total of 18 carbons). These 3 6-carbon
molecules would be quickly split into 6 3-carbon molecules (still a total of 18 carbons) known as PGA
molecules.
Phase 2: Reduction
The energy from ATP and NADPH help “rearrange” the 6 PGA molecules into 6 G3P molecules (these
molecules contain more energy than the PGA molecules). One 3 carbon molecules (G3P) will exit the
cycle to become a building block of Glucose (it takes two spins of the Calvin Cycle to make the 6 carbon
molecule of glucose). This will leave 5 3-carbon molecules (a total of 15 carbons due to 3 carbons
exiting) to advance to phase 3.
Phase 3: Regeneration
In phase 3, there are 5 3-carbon G3P molecules remaining (total of 15 carbons). With the aid of the
energy from ATP, these 5 3-carbon molecules will be rearranged into 3 5-carbon molecules known as
RuBP (still a total of 15 carbons). These 3 5-carbon molecules (total of 15 carbons) will be ready to
accept the carbon from carbon dioxide (total of 18 carbons) with the aid of the Rubicso enzyme.
To form the 6-carbon molecule of glucose (C6H12O6), the Calvin Cycle has to “spin” two times due to
forming 1 3- carbon sugar (G3P) per spin. Following the numbers, that would require 6 CO2 as given in
the chemical equation.
19. What 3 factors (“limiting factors”) can affect the rate of photosynthesis?
20. How does each of these factors affect the rete?