Where Are the ATP and NADPH Used?
The light reaction of photosynthesis delivers three products: ATP, NADPH and O2. Only oxygen leaves the chloroplast — ATP and NADPH stay inside the stroma to pay for the synthesis of sugars. That spending phase is the biosynthetic phase, often loosely called the “dark reaction”.
In this quiz you will test your understanding of:
- Why the biosynthetic phase does not directly depend on light, yet depends absolutely on ATP and NADPH.
- The simple experiment: synthesis continues briefly after light is removed, then stops — and restarts when light returns.
- Melvin Calvin‘s use of radioactive ##\ce{^{14}C}## in algal photosynthesis studies.
- The first CO2 fixation product, PGA (3-phosphoglyceric acid), and the C3 and C4 pathways.
- The unexpected primary acceptor of CO2 — the 5-carbon ketose sugar RuBP.
Work carefully: several options are deliberately close to the truth. Read each question twice before answering.
Q1. Which of the following are the products of the light reaction of photosynthesis?
The light reaction yields ATP, NADPH and ##\ce{O2}##. PGA and ##\ce{RuBP}## belong to the biosynthetic (Calvin) phase, not to the light reaction itself.
Q2. Among the products of the light reaction, which one diffuses out of the chloroplast?
Of the three products, only oxygen diffuses out of the chloroplast. ATP and NADPH are retained and used inside the stroma to drive sugar synthesis.
Q3. ATP and NADPH produced in the light reaction are used to drive the processes leading to the synthesis of:
The lesson states that ATP and NADPH drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase of photosynthesis.
Q4. The biosynthetic phase of photosynthesis takes place in the:
The enzymes of carbon fixation reside in the stroma, the fluid space surrounding the thylakoid membranes. ATP and NADPH made in the membranes are spent just outside them, in the stroma.
Q5. The biosynthetic phase of photosynthesis does not directly depend on the presence of light, but it is dependent on:
The process does not directly depend on light but depends on the products of the light reaction, i.e. ATP and NADPH, besides ##\ce{CO2}## and ##\ce{H2O}##.
Q6. Immediately after light becomes unavailable, what happens to the biosynthetic process?
This is the simple verification given in the lesson: immediately after light becomes unavailable, the biosynthetic process continues for some time and then stops, because the accumulated ATP and NADPH are gradually exhausted.
Q7. If light is made available again after the biosynthetic process has stopped, what happens?
The lesson states that if light is then made available, the synthesis starts again, because fresh ATP and NADPH are produced by the light reaction.
Q8. Why is calling the biosynthetic phase the "dark reaction" regarded as a misnomer?
The phase does not directly depend on light, yet it depends absolutely on ATP and NADPH made in the light. Hence the label "dark reaction" is misleading, since the process is indirectly light-dependent.
Q9. Melvin Calvin's exemplary work, carried out just after World War II, used which radioactive isotope in algal photosynthesis studies?
Calvin used radioactive ##\ce{^{14}C}## in algal photosynthesis studies. This led to the discovery that the first ##\ce{CO2}## fixation product was a 3-carbon organic acid.
Q10. The first ##\ce{CO2}## fixation product identified by Calvin was:
The first product identified was 3-phosphoglyceric acid, or PGA in short. OAA is the first product in the C4 pathway, and ##\ce{RuBP}## is the acceptor, not the product.
Q11. How many carbon atoms does 3-phosphoglyceric acid (PGA) contain?
PGA is a 3-carbon organic acid, which is why plants using it as the first ##\ce{CO2}## fixation product are said to follow the C3 pathway.
Q12. In another group of plants, the first stable product of ##\ce{CO2}## fixation was found to be a 4-carbon organic acid. This acid is:
Experiments over a wide range of plants revealed a group in which the first stable product was a 4-carbon organic acid, identified as oxaloacetic acid (OAA). Such plants follow the C4 pathway.
Q13. Plants in which the first product of ##\ce{CO2}## fixation is PGA are said to follow the:
Plants whose first ##\ce{CO2}## fixation product is the C3 acid PGA follow the C3 pathway, while those whose first product is the C4 acid OAA follow the C4 pathway.
Q14. Studies showed, very unexpectedly, that the primary acceptor of ##\ce{CO2}## is:
The acceptor molecule turned out to be a 5-carbon ketose sugar, ribulose bisphosphate ##\ce{(RuBP)}##. Scientists had long believed it would be a 2-carbon compound and spent many years searching for one.
Q15. Before discovering ##\ce{RuBP}##, scientists spent many years trying to identify a compound with how many carbon atoms as the primary acceptor of ##\ce{CO2}##?
Because the first product was a C3 acid, scientists believed the primary acceptor would be a 2-carbon compound, and they spent many years trying to identify one before the 5-carbon ##\ce{RuBP}## was discovered.