What Happens to ATP & NADPH? Introduction to the Dark Reaction (AP Biology)
- Introduction: What is the Biosynthetic Phase?
- The Power Source: Role of ATP and NADPH in Food Synthesis.
- The Carbon Fixation Secret: How CO2 enters the cycle.
- C3 vs C4 Plants: The first stable product of CO2 fixation (3-PGA vs OAA).
- Why call it the "Dark Reaction"? (Clearing the common myth).
- Melvin Calvin’s Discovery: The use of Radioactive Carbon-14.
- Check Your Understanding: Unit 2 Practice Questions
- Advanced Thinking: Critical Questions
- Data Analysis: Interpreting Graphs
- The biosynthetic phase of photosynthesis is the second major phase, also called the dark reaction, It is a light-independent reaction, also called as carbon fixation phase.
- It occurs in Stroma of chloroplast. It does not directly require light, but depends on products of light reaction. So it usually occurs in light because ATP and NADPH are available only in light.
- It is a biosynthetic phase because it synthesizes organic compounds, i.e., glucose/sugars, from inorganic CO₂. It’s the phase where actual food is made.
💡 Related study To understand the
Key Experiments of Photosynthesis: From Priestley to Van Niel (AP Biology Unit 3)
- It is a dark reaction name given by Blackman. It does not t occur only in dark. It just does not need light directly. Stops in dark because light reaction stops supplying ATP + NADPH. Overall equation of biosynthetic phase:
- In the previous lessons, we saw how sunlight was converted into chemical energy. Now, let's see how this energy is actually "spent" to build food.
ATP: The Energy Currency
- Think of ATP (Adenosine Triphosphate) as the cash needed to run the machinery of the Biosynthetic phase.
- ATP provides the energy required for the chemical reactions that fix Carbon Dioxide (CO2).
- It is used in two critical steps of the Calvin Cycle: Reduction and Regeneration. Without ATP, the cycle simply stops, and the plant cannot "reset" its machinery to take in more CO2.
NADPH: The Power of Electrons
- While ATP provides the "push," NADPH (Nicotinamide Adenine Dinucleotide Phosphate) provides the "stuff" (electrons and hydrogen).
- NADPH acts as a powerful Reducing Agent.
- It donates high-energy electrons to convert 3-PGA (a 3-carbon acid) into G3P (a 3-carbon sugar). This is the exact moment where inorganic carbon officially becomes organic energy-rich food.
| Energy Molecule | Provided By | Function in Biosynthetic Phase |
|---|---|---|
| ATP | Photophosphorylation | Provides energy for CO2 fixation & regeneration of RuBP. |
| NADPH | Non-Cyclic Electron Transport | Acts as a reducing agent to convert 3-PGA into Glucose. |
The "Light-Independent" Myth
- A common confusion for students is calling this the "Dark Reaction.
- While these reactions do not use light directly, they are 100% dependent on the products of light (ATP and NADPH). If the light goes out, the "Power Source" dries up, and food synthesis stops shortly after.
- Most students think plants just "absorb" CO2, but the real magic happens at the molecular level inside the stroma of the chloroplast.
The Entry Point: Stomata
- Before the cycle begins, CO2 gas enters the leaf through tiny pores called Stomata.
- From there, it diffuses into the mesophyll cells and finally into the stroma.
The Secret "Acceptor" Molecule
- CO2 doesn't just float around; it needs a "partner" to bind with.
- Many scientists originally thought the first molecule was a 2-carbon compound.
The "Fixation" Step
- When 1 molecule of CO2 (1 Carbon) combines with RuBP (5 Carbons), it briefly forms an unstable 6-carbon intermediate.
- This intermediate immediately splits into two molecules of 3-Phosphoglyceric Acid (3-PGA).
- Because the first stable product has 3 Carbon atoms, this pathway is known as the C3 Pathway.
The Master Enzyme, RuBisCO
- This entire "secret" operation is controlled by an enzyme called RuBisCO (Ribulose Bisphosphate Carboxylase-Oxygenase).
- Plants have evolved different strategies to fix carbon depending on their environment.
- The classification depends on the number of carbon atoms in the first stable compound formed.
- First Product of calvin cycle is 3-phosphoglyceric acid (3-PGA) after fixation of carbon dioxide. It is 3 Carbon atom containing compound . C3 cycle is found in the majority of plants like Wheat and Rice.
- First Product of calvin cycle is Oxaloacetic acid (OAA) after fixation of carbon dioxide. It is 4 Carbon atom containing compound . C4 cycle is found in the majority of plants like tropical plants like Maize and Sugarcane to minimize photorespiration.
Comparison of Fixation Pathways
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First Stable Product | 3-PGA (3 Carbon) | OAA (4 Carbon) |
| Primary Enzyme | RuBisCO | PEP Case & RuBisCO |
| Leaf Anatomy | Normal | Kranz Anatomy |
- The term "Dark Reaction" is perhaps the most misleading name in plant physiology.
- Let’s set the record straight: the Biosynthetic Phase is not a reaction that happens only at night.
The Origin of the Name
- Historically, scientists divided photosynthesis into "Light" and "Dark" reactions because the second phase does not directly require photons (light energy) to proceed.
- This led to the popular myth that plants perform light reactions during the day and dark reactions at night.
The Reality: It's Light-Dependent!
- While the enzymes of the Calvin Cycle don't need light to function, they are 100% dependent on the products of the light reaction (ATP and NADPH).
- As soon as the sun goes down, the supply of ATP and NADPH stops. Within a very short time, the "Dark Reaction" also grinds to a halt because it runs out of fuel.
- Interestingly, several key enzymes in the Biosynthetic phase (like RuBisCO) are actually light-activated. They work most efficiently when the sun is shining!
- Melvin Calvin worked by using radioactive 14C in the process of photosynthesis in algae. Malvin Calvin demonstrated that when carbon dioxide is fixed in a plant then the product of 3-carbon organic acid is formed.
- Hence it was called the Calvin cycle. after him. This 3 carbon organic acid was 3-phosphoglyceric acid.
- Now scientists started to work that in every plant the first product was 3-Phosphoglyceric acid to be formed after the fixation of carbon dioxide.
- With the reference of various experiments that were conducted by the scientist, Another group of plants was discovered .
- In this group of plants, the first product was four carbons that were obtained after the fixation of CO2. This product was oxaloacetic acid (OAA).
📝 Test Paper : 1 What Happens to ATP & NADPH? Introduction to the Dark Reaction (AP Biology)
Total Marks: 20 | Time: 1.5 Hours
Section A: Multiple Choice Questions (5 Marks)
1. The biosynthetic phase of photosynthesis is called "dark reaction" because:
b) It does not require light directly but depends on products of light reaction
c) It occurs in darkness inside soil
d) It releases energy instead of storing it
2. The bio synthetic phase of photosynthesis occurs in which part of the chloroplast?
a) Thylakoid lumen
b) Stroma
c) Inner membrane
d) Grana
3. During the Calvin cycle, ATP and NADPH produced in light reaction are used for:
a) Splitting of water and release of O₂
b) Reduction of CO₂ to form carbohydrates
c) Excitation of chlorophyll in PS II
d) Synthesis of chlorophyll
4. In C₃ plants, the first stable product of CO₂ fixation is:
a) Oxaloacetic acid (OAA), a 4-carbon compound
b) Phosphoglyceric acid (3-PGA), a 3-carbon compound
c) RuBP, a 5-carbon compound
d) Glucose, a 6-carbon compound
5. Melvin Calvin used which radioactive isotope to trace the path of carbon in photosynthesis?
a) C-12
b) C-13
c) C-14
d) N-15
Section B: Short Answer Questions (9 Marks)
1.What is the biosynthetic phase of photosynthesis? Why is the term "dark reaction" misleading for this phase?[3]
2. State the role of ATP and NADPH in the Calvin cycle. From where are these molecules obtained?[3]
3. How does CO₂ enter the Calvin cycle in C₃ plants? Name the enzyme and the first stable product formed.[3]
Section C: Long Answer Questions (6 Marks)1. Melvin Calvin used radioactive Carbon-14 to trace the path of carbon in photosynthesis. Name the technique he used and the plant material used in his experiment.[6]
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📝 Test Paper : 2 What Happens to ATP & NADPH? Introduction to the Dark Reaction (AP Biology)
Total Marks: 20 | Time: 1.5 Hours
Section A: Multiple Choice Questions (5 Marks)
1.In the Calvin cycle, the 5-carbon CO₂ acceptor molecule that combines with CO₂ is:
a) PGA
b) RuBP
c) PEP
d) OAA
2. The enzyme that catalyzes the first step of CO₂ fixation in C₃ plants is:
a) PEP carboxylase
b) RuBisCO
c) NADP reductase
d) ATP synthase
3. The main difference between C₃ and C₄ plants is:
a) C₄ plants lack RuBisCO
b) C₄ plants perform Calvin cycle in bundle sheath cells
c) C₃ plants first fix CO₂ into a 4-carbon compound OAA
d) C₄ plants do not require ATP for CO₂ fixation
4. The first stable product of CO₂ fixation in C₄ plants like maize and sugarcane is:
a) 3-Phosphoglyceric acid (3-PGA), a 3-carbon compound
b) Oxaloacetic acid (OAA), a 4-carbon compound
c) RuBP, a 5-carbon compound
d) Phosphoenolpyruvate (PEP), a 3-carbon compound
5.The term "dark reaction" for the biosynthetic phase is considered a misnomer because:
a) The reaction occurs only in complete darkness
b) The reaction does not use light directly but stops in dark due to lack of ATP and NADPH
c) The reaction produces CO₂ in dark
d) The reaction destroys chlorophyll in dark
Section B: Short Answer Questions (9 Marks)
1. What is meant by "biosynthetic phase" of photosynthesis? In which part of the chloroplast does it occur?[3]
2. Why is ATP and NADPH called the "power source" for the biosynthetic phase? What would happen to the Calvin cycle if light reaction stops?[3]
3. Melvin Calvin used Carbon-14 to study photosynthesis. What did his experiment prove about the path of CO₂? Name the first stable compound he detected.[3]
Section C: Long Answer Questions (6 Marks)
1. The biosynthetic phase is called dark reaction but it is not correct." Justify this statement. Explain how CO₂ enters the Calvin cycle in C₃ plants, mention the role of ATP and NADPH in converting CO₂ to sugar, and state Melvin Calvin’s contribution in understanding this phase.[6]
📝 Advanced Thinking: Critical Application Questions:
Question 1 : . If a plant is kept in continuous light but CO₂ supply is cut off, why will the biosynthetic phase stop even though ATP and NADPH are still being made?
Answer: Calvin cycle needs CO₂ as the raw material for fixation with RuBP. No CO₂ → RuBisCO has no substrate → cycle halts. ATP and NADPH will accumulate but won’t be used, and RuBP regeneration stops. So sugar synthesis stops even in light.
Question: 2. Why do C₄ plants have an advantage over C₃ plants in hot, dry climates based on the first product of CO₂ fixation?
Answer: C₄ plants first fix CO₂ into OAA using PEP carboxylase, which has very high affinity for CO₂ and no oxygenase activity. So they avoid photorespiration, a wasteful process in C₃ plants where RuBisCO binds O₂ at high temp. This makes C₄ photosynthesis more efficient in hot, dry conditions.
Question : 3. Melvin Calvin supplied C-14 labelled CO₂ to algae for only 5 seconds. Why did he use such a short time and what did he find?
Answer: He used short exposure to catch the first compound formed after CO₂ enters. After 5 sec, C-14 appeared in 3-PGA. This proved 3-PGA is the first stable product of CO₂ fixation in C₃ plants. Longer time would show later products like glucose, hiding the first step.
Question : 4.If Di Nitro Phenol, a proton uncoupler, is added to isolated chloroplasts in light, will the Calvin cycle run? Explain using the role of ATP.
Answer: No, Calvin cycle will stop. DNP collapses the proton gradient across thylakoid membrane, so no ATP is made in light reaction. Calvin cycle needs ATP for 1. reduction of 3-PGA and 2. regeneration of RuBP. Without ATP, even if NADPH and CO₂ are present, sugar synthesis stops.
📝 Data Analysis: Interpreting Graphs
| Time after 14CO2 supply | Compounds showing radioactivity |
|---|---|
| 5 seconds | Only 3-PGA |
| 10 seconds | 3-PGA + Glucose phosphate + Fructose phosphate |
| 60 seconds | 3-PGA + Sugar phosphates + Sucrose + Amino acids |
Answer: A After 5 seconds, radioactivity appears only in *3-PGA*. This proves that 3-PGA is the first stable product formed when ¹⁴CO₂ is fixed by RuBP using RuBisCO in C₃ plants. Since no other compound is labelled yet, 3-PGA must be the primary acceptor product.
Answer: B Sucrose and amino acids are end products formed much later in the pathway. ¹⁴CO₂ first forms 3-PGA → then sugar phosphates like G3P, glucose phosphate → then these are used to make sucrose. Some 3-PGA also enters amino acid synthesis via transamination. This multi-step conversion takes time, so radioactivity reaches sucrose and amino acids only after 60 seconds, not immediately.
Answer : C If it were linear, ¹⁴C would keep moving forward and never return to the starting compound. But in this data, RuBP , the CO₂ acceptor also gets labelled later, showing it is being regenerated from 3-PGA using ATP. The cycle continues because the starting material RuBP is remade at the end. This regeneration of RuBP proves it’s a cyclic pathway, not a one-way linear path.[1][2]
Question : 2 The graph shows specific activity of PGA in
Chlorella after ¹⁴CO₂ was supplied in light.
a) Why does specific activity of PGA increase rapidly from 0 to 18 minutes?
b) What likely happened at the 20-minute mark to cause the sharp decline in PGA activity?
c) What does this graph prove about the requirement of light for the biosynthetic phase?[1][2]
Answer a : ¹⁴CO₂ is continuously fixed by RuBisCO → 3-PGA in light using ATP/NADPH. In light, ¹⁴CO₂ is continuously fixed with RuBP by RuBisCO to form 3-PGA. ATP and NADPH from light reaction are used to keep the cycle running, so 3-PGA accumulates → specific activity rises.
Answer b : At 20 min, light was likely switched off. So light reaction stopped → no ATP/NADPH formed → Calvin cycle halts. No new 3-PGA is made, while existing 3-PGA is converted to sugars → PGA radioactivity drops sharply.
Answer C : The graph proves that the biosynthetic phase/C3 cycle, though called light-independent, depends indirectly on light. Without light, ATP and NADPH are not made → 3-PGA synthesis stops. So the biosynthetic phase requires products of light reaction to continue.
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