π The Calvin Cycle: Light-Independent Reactions Explained
The Calvin Cycle, also known as the light-independent reactions or the dark reactions, is a crucial part of photosynthesis. It's where plants use the energy captured during the light-dependent reactions to convert carbon dioxide ($CO_2$) into glucose ($C_6H_{12}O_6$), a type of sugar. This process occurs in the stroma of the chloroplasts.
π Objectives
- π― Understand the overall purpose of the Calvin Cycle.
- π± Identify the three main phases of the Calvin Cycle.
- π§ͺ Explain the inputs and outputs of each phase.
π Materials
- πΊοΈ Whiteboard or projector.
- ποΈ Markers or pens.
- π Handouts with diagrams of the Calvin Cycle (optional).
βοΈ Warm-up (5 mins)
Quick review of photosynthesis and the role of ATP and NADPH, asking students simple questions to gauge understanding. For example:
- π€ Where does photosynthesis take place? (Answer: Chloroplast)
- β‘ What are the two main energy-carrying molecules produced in the light-dependent reactions? (Answer: ATP and NADPH)
π¨βπ« Main Instruction
The Calvin Cycle consists of three main phases:
1. π³ Carbon Fixation
- βοΈ Initial Step: Carbon dioxide ($CO_2$) from the atmosphere enters the cycle.
- π€ Key Enzyme: An enzyme called RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the reaction between $CO_2$ and a five-carbon molecule called ribulose-1,5-bisphosphate (RuBP).
- π₯ Unstable Product: This reaction forms an unstable six-carbon compound that immediately breaks down into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA).
2. π Reduction
- β‘ Energy Input: Each molecule of 3-PGA receives a phosphate group from ATP (produced in the light-dependent reactions), becoming 1,3-bisphosphoglycerate.
- NADPH then reduces 1,3-bisphosphoglycerate, losing its phosphate group to become glyceraldehyde-3-phosphate (G3P). This step requires energy from NADPH (also from the light-dependent reactions).
- β¨ G3P Fate: G3P is a three-carbon sugar. For every six molecules of G3P produced, one exits the cycle and can be used to make glucose and other organic molecules. The other five G3P molecules are used to regenerate RuBP.
3. Regenerating RuBP
- π Complex Reactions: A series of complex enzymatic reactions uses ATP to convert the five molecules of G3P back into three molecules of RuBP.
- π Importance: This regeneration is crucial because RuBP is needed to continue the cycle and fix more carbon dioxide.
- β»οΈ Cycle Continuation: The cycle then repeats, fixing more $CO_2$ and producing more G3P.
π§ͺ Assessment
Let's check your understanding! Answer these questions to test what you've learned about the Calvin Cycle.
β Practice Quiz
- 𧬠What is the primary enzyme responsible for carbon fixation in the Calvin Cycle?
- β‘ What two energy-rich molecules from the light-dependent reactions power the Calvin Cycle?
- π Why is the regeneration of RuBP essential for the Calvin Cycle?
- π― In which part of the chloroplast does the Calvin Cycle take place?
- π³ How many molecules of $CO_2$ are needed to produce one molecule of G3P that exits the Calvin Cycle?
Answer Key: 1. RuBisCO, 2. ATP and NADPH, 3. To continuously fix carbon dioxide, 4. Stroma, 5. Three