Photosynthesis: The Calvin Cycle

Summary

The Calvin cycle, a critical part of photosynthesis, operates independently of light and relies on ATP and NADPH from light reactions. It involves three phases: carbon fixation, where CO2 is added to RuBP; reduction, where ATP and NADPH convert 3-PGA to G3P; and regeneration, where G3P is used to regenerate RuBP. This cycle enables the production of glucose and other organic compounds essential for plant and algal growth, supporting all life on Earth.

Takeaways

• 🌿 The Calvin cycle is part of photosynthesis that occurs after the light-dependent reactions.
• 🌑 It is sometimes called the light-independent or dark reactions, as it doesn't require light directly.
• ⚙️ ATP and NADPH, produced during the light reactions, are essential for the Calvin cycle to function.
• 📍 The Calvin cycle takes place in the stroma, the fluid surrounding the thylakoids.
• 🔄 Phase one is carbon fixation, where CO2 is fixed by an enzyme called Rubisco onto a five-carbon compound, RuBP.
• 🔄 Phase two is reduction, where ATP provides energy and NADPH provides electrons and hydrogen to produce G3P.
• 🔄 Phase three is regeneration, where five G3P molecules are converted back to three RuBP molecules, requiring ATP.
• 🔢 For every three CO2 molecules, the Calvin cycle produces one G3P molecule, with a net gain of three carbons.
• 🌱 Two G3P molecules are needed to make one glucose molecule, which is a key product of the Calvin cycle.
• 🌳 The glucose and other organic compounds produced through photosynthesis support the growth of plants and algae, and indirectly, all life.

Q & A

What is the Calvin cycle and where does it occur in the process of photosynthesis?

The Calvin cycle is the second set of reactions in photosynthesis that occur after the light-dependent reactions. It is sometimes called the light-independent or dark reactions because it does not require light directly. It takes place in the stroma, the fluid surrounding the thylakoids.

Why are ATP and NADPH necessary for the Calvin cycle?

ATP provides the energy, and NADPH provides the electrons and hydrogens needed to reduce carbon dioxide and build sugars during the Calvin cycle.

What is the role of the enzyme Rubisco in the Calvin cycle?

Rubisco is an enzyme that takes CO2 from the air and adds it to a five-carbon compound called RuBP, a process known as carbon fixation.

What is the significance of the six-carbon compound that forms after carbon fixation?

The six-carbon compound that forms after carbon fixation immediately splits into two molecules of three-phosphoglycerate, which is a key step in the Calvin cycle.

How does the reduction phase of the Calvin cycle contribute to the formation of G3P?

In the reduction phase, ATP provides energy to create an intermediate compound, which then gains electrons and hydrogen from NADPH to be reduced and form glyceraldehyde 3-phosphate (G3P).

What is the net gain of G3P for every three molecules of CO2 fixed in the Calvin cycle?

For every three molecules of CO2 fixed in the Calvin cycle, there is a net gain of one G3P.

What is the purpose of the regeneration phase in the Calvin cycle?

The regeneration phase is where the five remaining G3P molecules are converted back to RuBP, the starting compound, to continue the cycle.

How many carbon atoms are there in five G3P molecules, and how does this relate to the formation of RuBP?

There are 15 carbon atoms in five G3P molecules (5 molecules * 3 carbons each). This allows for the formation of three RuBP molecules, which is necessary to continue the Calvin cycle.

Where does the energy come from to convert G3P back to RuBP?

The energy required to convert G3P back to RuBP comes from ATP.

How many G3P molecules are needed to make one glucose molecule?

Two G3P molecules are needed to make one glucose molecule.

What is the overall significance of the Calvin cycle in the context of photosynthesis and life on Earth?

The Calvin cycle is crucial for producing glucose and other organic compounds in photosynthesis, which allows plants and algae to grow and support almost all life on Earth, including humans.