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11.7 - Where are the ATP and NADPH Used?

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Role of ATP and NADPH

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Teacher
Teacher

Today, we will discuss the roles of ATP and NADPH produced during the light reactions of photosynthesis. Can anyone tell me what these molecules are used for?

Student 1
Student 1

Are they used to make sugars?

Teacher
Teacher

Exactly! They drive the processes leading to sugar synthesis in a phase called the biosynthetic phase. What else do we know about this process?

Student 2
Student 2

Is it correct to call it the dark reactions?

Teacher
Teacher

That's a common misconception! While we refer to it as dark reactions, it doesn't mean light is not involved at all. In fact, ATP and NADPH are products of the light reactions, and they continue to work even in the absence of light for a short time.

Student 3
Student 3

So, if we turn off the light, photosynthesis stops immediately?

Teacher
Teacher

"Not immediately! The biosynthetic processes continue until the stored ATP and NADPH are used up. Let's remember this pattern:

The Calvin Cycle

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Teacher
Teacher

Who can remind us what the first stable product of CO2 fixation is in the Calvin Cycle?

Student 4
Student 4

Is it 3-phosphoglyceric acid, or PGA?

Teacher
Teacher

Spot on! The enzyme responsible for this reaction is RuBisCO. Can anyone explain what happens next after PGA is formed?

Student 1
Student 1

I think PGA is then converted into glucose using ATP and NADPH?

Teacher
Teacher

Precisely! During this phase, for every six CO2 molecules fixed, we need six turns of the cycle to produce one glucose molecule. This requires a lot of ATP and NADPH—specifically, 18 ATP and 12 NADPH!

Student 2
Student 2

So, ATP powers the reactions while NADPH reduces them?

Teacher
Teacher

Exactly! It's essential to understand that without these energy carriers, the synthesis of glucose would not be possible.

C3 and C4 Pathways

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Teacher
Teacher

Now, what are the two types of pathways we've discussed regarding carbon fixation?

Student 3
Student 3

The C3 and C4 pathways?

Teacher
Teacher

Correct! The C3 pathway begins with PGA, but C4 plants like maize initially produce oxaloacetic acid. Can anyone tell me why this difference is significant?

Student 4
Student 4

I believe C4 plants are more efficient under stress conditions or in high temperatures?

Teacher
Teacher

"Right again! They help avoid photorespiration, which is wasteful for C3 plants. Let's summarize this key point:

Introduction & Overview

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Quick Overview

ATP and NADPH produced during photosynthesis are utilized in the biosynthetic phase to synthesize sugars, primarily through the Calvin cycle.

Standard

The section highlights how ATP and NADPH, generated from the light reactions of photosynthesis, are crucial for the biosynthetic phase, where carbon dioxide is fixed into organic compounds. The Calvin Cycle is primarily responsible for this conversion, utilizing RuBP as the primary CO2 acceptor, resulting in the formation of glucose.

Detailed

In the biosynthetic phase following the light reactions of photosynthesis, ATP and NADPH play essential roles in converting carbon dioxide and water into glucose. This process occurs during the Calvin Cycle, also known as the dark reactions, where ATP is used for energy, and NADPH provides reducing power. The first stable product formed during CO2 fixation is 3-phosphoglyceric acid (PGA), facilitated by the enzyme RuBisCO, which also participates in photorespiration under certain conditions. The section contrasts C3 and C4 pathways, highlighting their differences in CO2 fixation and adaptation to environmental conditions. Ultimately, understanding the utilization of ATP and NADPH is vital for comprehending plant metabolism and growth.

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Audio Book

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Role of ATP and NADPH in Photosynthesis

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We learnt that the products of light reaction are ATP, NADPH and O2. Of these O2 diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars.

Detailed Explanation

In the light reactions of photosynthesis, energy from light is converted into chemical energy in the form of ATP and NADPH. These two molecules are crucial because they provide the energy and reducing power needed to convert carbon dioxide (CO2) and water (H2O) into glucose during the process of synthesizing food. While oxygen (O2) produced during light reactions is released into the atmosphere, ATP and NADPH are used in the darker biochemical reactions to form organic compounds.

Examples & Analogies

Think of ATP and NADPH as the fuel and electricity needed to run a factory. The factory in this analogy represents the chloroplast, where the raw materials (CO2 and H2O) are transformed into products (sugars). Just like a factory needs fuel and electricity to produce its goods, the chloroplast needs ATP and NADPH for photosynthesis.

Biosynthetic Phase of Photosynthesis

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This is the biosynthetic phase of photosynthesis. This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO2 and H2O.

Detailed Explanation

The biosynthetic phase, also known as the Calvin cycle, occurs in the stroma of the chloroplasts. Although it is commonly referred to as a 'dark reaction' because it does not require light directly, the process actually relies on the ATP and NADPH generated from the light-dependent reactions. This cycle involves fixing carbon dioxide into an organic form, enabling plants to synthesize sugars, which will serve as energy sources for various functions.

Examples & Analogies

Imagine making a homemade smoothie. You don’t need fruit (light) present at all times, but once you have the ingredients prepared (ATP and NADPH), you can blend and have a smoothie whenever you want. Similarly, the Calvin cycle can continue using the products (ATP and NADPH) from the light reactions, even when there is no light available.

Continued Activity in Absence of Light

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You may wonder how this could be verified; it is simple: immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops. If then, light is made available, the synthesis starts again.

Detailed Explanation

This statement emphasizes that the biosynthetic reactions can continue for a brief period even in the absence of light. This is possible because the ATP and NADPH produced during the last exposure to light can be used momentarily for CO2 fixation. However, as these reactants are consumed, the process eventually comes to a halt until sunlight is available again to replenish ATP and NADPH.

Examples & Analogies

Consider a battery-powered device. When you use it, it continues to work until the battery runs out. As long as you have power stored (like the ATP and NADPH), it functions even when not plugged in (absence of light). Once the power is depleted, you will have to charge it again with electricity (light) to use it.

The Calvin Cycle Overview

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Let us now see how the ATP and NADPH are used in the biosynthetic phase. We saw earlier that CO2 is combined with H2O to produce (CH2O) or sugars. It was of interest to scientists to find out how this reaction proceeded, or rather what was the first product formed when CO2 is taken into a reaction or fixed.

Detailed Explanation

During the Calvin cycle, carbon dioxide combines with a five-carbon sugar known as ribulose bisphosphate (RuBP) to form an unstable six-carbon compound, which quickly splits into two molecules of 3-phosphoglycerate (3-PGA). This signifies the initial step of incorporating CO2 into organic molecules, a process essential for sugar synthesis.

Examples & Analogies

Imagine a chef preparing a dish. The CO2 acts as the main ingredient, while RuBP is like a base that helps to bind all the flavors (energy) together. Once combined, they create a new flavor (sugar) essential for the final dish, similar to how the Calvin cycle produces the energy-rich glucose.

Discovery of the First Product of CO<sub>2</sub> Fixation

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The work of Melvin Calvin is exemplary. The use of radioactive 14C by him in algal photosynthesis studies led to the discovery that the first CO2 fixation product was a 3-carbon organic acid. He also contributed to working out the complete biosynthetic pathway; hence it was called Calvin cycle after him.

Detailed Explanation

Using radioactive isotopes, scientists like Melvin Calvin were able to trace the pathways of carbon assimilation in plants. His experiments demonstrated that the first stable product formed when carbon dioxide is fixed during photosynthesis is a three-carbon compound known as 3-phosphoglycerate (PGA). This significant finding allowed for a deeper understanding of the process of photosynthesis and how plants synthesize sugars.

Examples & Analogies

Think of Melvin Calvin's work like a detective using clues to solve a mystery. By using ‘labeled’ clues (the radioactive carbon), he pieced together the pathway plants take to convert carbon dioxide into sugar, just as a detective arranges bits of evidence to uncover the truth.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • ATP: The primary energy carrier used in various cellular processes.

  • NADPH: A key reducing agent in the Calvin Cycle.

  • Calvin Cycle: The process of converting CO2 into glucose.

  • RuBisCO: An important enzyme for fixing carbon in photosynthesis.

  • Photorespiration: An inefficient process that can decrease plant productivity.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • In plants like spinach (C3), the Calvin cycle produces a 3-carbon compound (PGA).

  • In maize (C4), the Calvin cycle starts with a 4-carbon compound (OAA), which is more efficient in hot climates.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • ATP and NADPH are a powerhouse pair,
    Fueling the sugars, in photosynthesis's care.

📖 Fascinating Stories

  • Imagine a factory where ATP is the energy source running the machines, and NADPH is the worker reducing materials to create sweet sugars. Together, they ensure the manufacturing of glucose from sunlight.

🧠 Other Memory Gems

  • Remember C3 for Carbon-3-PGA,
    C4 plants go OAA and stand tall at play.

🎯 Super Acronyms

C.A.R. - Carboxylation, ATP usage, Reduction for the key steps in the Calvin Cycle.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: ATP

    Definition:

    A nucleotide that serves as the primary energy carrier in cells.

  • Term: NADPH

    Definition:

    A coenzyme that acts as a reducing agent in the biosynthetic phase of photosynthesis.

  • Term: Calvin Cycle

    Definition:

    The series of biochemical reactions that convert CO2 into glucose using ATP and NADPH.

  • Term: RuBisCO

    Definition:

    An enzyme that catalyzes the first step of carbon fixation in the Calvin Cycle.

  • Term: Photorespiration

    Definition:

    A biochemical process where RuBisCO oxygenates RuBP, leading to a decrease in photosynthetic efficiency.