The Calvin Cycle

Summary

This tutorial explains the Calvin cycle, a crucial part of photosynthesis that converts atmospheric carbon dioxide into carbohydrates. Occurring in the stroma of chloroplasts, the Calvin cycle is divided into three phases: carbon dioxide fixation, reduction, and regeneration of RuBP. The enzyme Rubisco captures CO2, which is transformed into G3P, a carbohydrate. ATP and NADPH, produced by light reactions, provide the energy for this process. The cycle must turn six times to form one glucose molecule, demonstrating its dependence on the light reactions for energy and electrons.

Takeaways

• 🌿 The plant's leaves have cells that carry out photosynthesis.
• 🔬 Chloroplasts are the organelles where the reactions of photosynthesis occur.
• 💡 Photosynthesis consists of two primary steps: the light reactions and the Calvin cycle reactions.
• 🌞 The Calvin cycle's main purpose is to convert atmospheric carbon dioxide into carbohydrates (sugars) for the plant's cellular activities.
• 🌀 The Calvin cycle takes place in the stroma of the chloroplast and uses ATP and NADPH from the light reactions.
• 🌿 The Calvin cycle has three phases: carbon dioxide fixation, carbon dioxide reduction, and regeneration of RuBP.
• 🌱 In the carbon dioxide fixation phase, CO2 is attached to RuBP, producing an unstable 6-carbon molecule that quickly splits into two 3PG molecules.
• ⚡ During carbon dioxide reduction, electrons from NADPH and energy from ATP are used to convert 3PG into G3P, a carbohydrate.
• 🍬 One G3P molecule is set aside as a building block for glucose, while the rest move forward into the regeneration phase.
• 🔄 To form one glucose molecule, the Calvin cycle must turn six times, adding one carbon atom per cycle, relying on the light reactions for ATP and NADPH.

Q & A

What is the primary purpose of the Calvin cycle?

The primary purpose of the Calvin cycle is to convert carbon dioxide from the atmosphere into carbohydrates (or sugars) that the plant needs to power its cellular activities and build new plant structures.

Where do the Calvin cycle reactions occur within the chloroplast?

The Calvin cycle reactions occur within the stroma of the chloroplast.

What molecules produced by the light reactions are used in the Calvin cycle?

The Calvin cycle uses ATP and NADPH produced by the light reactions to convert carbon dioxide into carbohydrates.

What are the three phases of the Calvin cycle?

The three phases of the Calvin cycle are carbon dioxide fixation, carbon dioxide reduction, and regeneration of RuBP.

What enzyme is involved in the carbon dioxide fixation phase of the Calvin cycle?

The enzyme involved in the carbon dioxide fixation phase is called Rubisco.

What is the initial product formed after carbon dioxide fixation?

The initial product is an unstable 6-carbon molecule that quickly splits into two 3-carbon molecules called 3-phosphoglycerate (3PG).

What happens during the carbon dioxide reduction phase?

During the carbon dioxide reduction phase, NADPH and ATP from the light reactions supply electrons and energy to reduce 3PG into G3P, a carbohydrate.

What is G3P, and what role does it play in the Calvin cycle?

G3P is a 3-carbon carbohydrate formed during the carbon dioxide reduction phase. One G3P molecule is set aside to build glucose, while the rest are used in the regeneration of RuBP.

How many turns of the Calvin cycle are required to form one glucose molecule?

Six turns of the Calvin cycle are required to form one glucose molecule because each turn adds only one carbon atom from the incoming carbon dioxide.

Why is the Calvin cycle dependent on the light reactions?

The Calvin cycle is dependent on the light reactions because they provide NADPH and ATP, which supply the electrons and energy needed to reduce carbon dioxide and form carbohydrates.

Outlines

00:00

🌿 Photosynthesis Overview: Light Reactions and Calvin Cycle

This paragraph introduces photosynthesis, explaining that it occurs in the chloroplasts of plant cells. It highlights the two main stages: the light reactions and the Calvin cycle, focusing on the latter in this tutorial. The purpose of the Calvin cycle is to convert atmospheric carbon dioxide into carbohydrates, which the plant uses for energy and growth. The reactions take place in the stroma of the chloroplast, using ATP and NADPH from the light reactions to convert CO2 into sugars.

🔄 Calvin Cycle Phases: CO2 Fixation, Reduction, and RuBP Regeneration

This section zooms into the Calvin cycle, outlining its three main phases: carbon dioxide fixation, carbon dioxide reduction, and the regeneration of RuBP. In the first phase, CO2 is captured and attached to RuBP, forming an unstable 6-carbon molecule that quickly splits into two 3-carbon molecules called 3PG. This initiates the process of transforming CO2 into a usable form.

⚡ Carbon Dioxide Reduction: NADPH and ATP in Action

The paragraph explains the carbon dioxide reduction phase, where energy and electrons are added to CO2. NADPH and ATP, produced by the light reactions, provide the necessary electrons and energy. Through a sequence of reactions, 3PG is reduced to G3P, a carbohydrate. The leftover ADP and NAD+ are recycled back into the light reactions. One G3P molecule is set aside as a glucose precursor, while the remaining molecules continue to the next phase.

🔁 Regeneration of RuBP and the Completion of the Calvin Cycle

Here, the focus is on the final phase of the Calvin cycle, where ATP is used to regenerate RuBP from G3P molecules. This allows the cycle to continue, enabling additional CO2 molecules to be captured. To produce one glucose molecule, the Calvin cycle must turn six times, with each turn incorporating one carbon atom from the captured CO2. The Calvin cycle is dependent on the light reactions for the necessary NADPH and ATP.

Keywords

💡Photosynthesis

Photosynthesis is the process by which plants convert light energy into chemical energy in the form of sugars. It consists of two main stages: the light reactions and the Calvin cycle. In this script, photosynthesis is introduced as the key biological process that enables plants to produce carbohydrates for growth and energy.

💡Chloroplast

Chloroplasts are specialized organelles found in plant cells where photosynthesis takes place. The script mentions chloroplasts as the location where the reactions of photosynthesis occur, specifically within the stroma during the Calvin cycle reactions.

💡Calvin Cycle

The Calvin cycle is the set of chemical reactions that take place in the stroma of chloroplasts during photosynthesis. It uses ATP and NADPH from the light reactions to convert CO2 into carbohydrates. The video focuses on explaining this cycle's three phases: carbon fixation, reduction, and regeneration of RuBP.

💡Stroma

The stroma is the fluid-filled space inside the chloroplast where the Calvin cycle reactions take place. It is in this environment that carbon dioxide is converted into carbohydrates using energy from ATP and NADPH, both produced by the light reactions.

💡Carbon Dioxide (CO2)

Carbon dioxide is a gas that plants capture from the atmosphere, and it serves as a critical input for photosynthesis. In the Calvin cycle, CO2 is fixed by attaching to RuBP and is eventually transformed into sugars, which are vital for plant energy and growth.

💡RuBP (Ribulose bisphosphate)

RuBP is a five-carbon molecule that plays a central role in the first phase of the Calvin cycle. CO2 is attached to RuBP in the carbon fixation phase, forming an unstable six-carbon molecule that quickly splits into two molecules of 3-phosphoglycerate (3PG).

💡Rubisco

Rubisco is an enzyme that catalyzes the first step of the Calvin cycle—carbon fixation. It attaches carbon dioxide to RuBP, initiating the process that eventually leads to the creation of carbohydrates. Rubisco is one of the most abundant enzymes in plants due to its critical role in photosynthesis.

💡3-phosphoglycerate (3PG)

3-phosphoglycerate (3PG) is the first stable product formed after carbon fixation in the Calvin cycle. The six-carbon molecule created by the addition of CO2 to RuBP quickly splits into two molecules of 3PG, which are further processed in the cycle to form carbohydrates.

💡NADPH

NADPH is a molecule produced during the light reactions of photosynthesis that provides the electrons and energy required for carbon dioxide reduction in the Calvin cycle. It donates electrons to 3PG to reduce it to G3P, which is a precursor for glucose production.

💡ATP

ATP, produced during the light reactions, provides energy for various stages of the Calvin cycle. It is used to convert 3PG into G3P and later in the regeneration phase to recycle RuBP, enabling the cycle to continue.

Highlights

Plant leaves contain cells that carry out photosynthesis.

Zooming into a photosynthetic plant cell shows chloroplasts, where photosynthesis occurs.

Photosynthesis consists of two primary steps: light reactions and the Calvin cycle reactions.

The Calvin cycle's main purpose is to convert atmospheric carbon dioxide into carbohydrates (sugars) that the plant uses for energy and structure.

Calvin cycle reactions take place in the stroma of the chloroplast.

ATP and NADPH, produced by light reactions, are used to convert CO2 into carbohydrates.

The Calvin cycle consists of three phases: carbon dioxide fixation, reduction, and RuBP regeneration.

In carbon dioxide fixation, CO2 is attached to RuBP, a five-carbon molecule.

Rubisco is the enzyme responsible for fixing CO2 to RuBP, forming an unstable six-carbon molecule that splits into two 3PG molecules.

In the reduction phase, NADPH and ATP from the light reactions are used to convert 3PG into G3P, a carbohydrate.

ADP and NAD+ return to the thylakoids to be converted back into ATP and NADPH during light reactions.

One G3P molecule is reserved as a building block for glucose.

Most G3P molecules are used in the third phase to regenerate RuBP molecules.

The Calvin cycle must turn six times to form one glucose molecule, as each turn fixes only one carbon atom.

The Calvin cycle is dependent on light reactions to provide the NADPH and ATP needed for carbohydrate production.

Transcripts

The leaves of this plant have cells that carry out photosynthesis.

If we zoom in on this photosynthetic plant cell, we can see the chloroplasts

where the reactions of photosynthesis occur. Photosynthesis

consists of two primary steps: the light reactions and the

Calvin cycle reactions. In this tutorial, we'll focus exclusively

on the Calvin cycle reactions. The overall purpose of the Calvin cycle

is to convert carbon dioxide from the atmosphere into

carbohydrates (or sugars) which the plant needs to power its cellular activities

and build new plant structures. The Calvin cycle reactions occur

within the stroma of the chloroplast. The reactions use the ATP

and NADPH produced by the light reactions to convert the CO2

to carbohydrate. Let's zoom into the Calvin cycle to take a closer

look. The Calvin cycle can be divided into 3 phases:

carbon dioxide fixation, carbon dioxide

reduction, and regeneration of RuBP.

The first phase, carbon dioxide fixation, basically

captures the CO2 from the atmosphere, so it can be used in the reactions.

To do this, CO2 is attached to RuBP,

a five carbon molecule. The enzyme used in this reaction

is called Rubisco, and the result of the reaction is an unstable

6-carbon molecule that quickly splits into two 3-carbon molecules

called 3-phosphoglycerate, or 3PG.

Next, the process of converting the CO2 to carbohydrate

begins. This phase is called the carbon dioxide reduction phase,

because we're adding electrons and energy to the CO2 molecule.

During this phase, a sequence of reactions uses NADPH

and some of the ATP from the light reactions.

These molecules supply the needed electrons and energy for CO2

reduction. Electrons are added from NADPH

and through a series of reactions, 3PG is reduced to

form G3P, a carbohydrate. ADP and

NAD+ return to the thylakoids, to be converted back

ATP and NADPH by the light reactions.

One of the G3P molecules is set aside as a building block

for glucose. But the majority of the G3P molecules move

forward into the third phase of the Calvin cycle. In this phase,

ATP is used to combine the rest of the G3P molecules to

form RuBP molecules.

This RuBP can then combine with additional carbon dioxide molecules

continuing the carbon reactions.

To form a glucose molecule, the cycle actually has to turn 6 times,

because each turn of the cycle adds only one carbon atom from the incoming

carbon dioxide. It's important to remember that the

Calvin cycle is dependent on the light reactions, which provide

NADPH and ATP, which in turn provide the electrons

and energy needed to form carbohydrates.