The Calvin Cycle: Detailed Notes chpt 8 assignment video 9/29/5
Photosynthesis: A Two-Step Process
Photosynthesis is divided into two main sets of reactions.
Light Reactions
These are the first reactions, responsible for capturing solar energy.
They convert solar energy from sunlight and energized electrons (taken from water) into the energy-carrying molecules ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
Oxygen (O_2) is released as a byproduct during this stage.
Calvin Cycle (Light-Independent Reactions)
This is the second set of reactions.
It utilizes carbon dioxide gas (CO_2) from the atmosphere and the chemical energy stored in the ATP and NADPH produced during the light reactions.
The primary goal of the Calvin cycle is to produce carbohydrates, which are organic molecules.
Initiation of the Calvin Cycle: Carbon Fixation
The Calvin cycle begins with a crucial process called carbon fixation.
Definition of Carbon Fixation: This is the process where carbon from inorganic carbon dioxide gas (CO_2) is incorporated into an existing organic molecule.
First Step Detailed:
CO_2 from the atmosphere is initially attached to a five-carbon organic molecule known as RuBP (ribulose-1,5-bisphosphate).
This attachment forms an unstable six-carbon molecule.
Almost immediately, this six-carbon molecule splits.
It forms a pair of three-carbon molecules, each called 3PG (3-phosphoglycerate).
Second Stage: Carbon Dioxide Reduction
This stage focuses on reducing the 3PG molecules formed in the previous step.
Energy Input: This reduction requires a significant input of energy.
The necessary energy is supplied by ATP, which was generated during the light reactions.
Electron Input (Reduction): Reduction, in a chemical context, involves the addition of electrons.
In the Calvin cycle, the electrons required for reduction are supplied by NADPH, also a product of the light reactions.
The addition of electrons from NADPH reduces the 3PG molecules.
Product Formation: The reduction of 3PG results in the formation of G3P (glyceraldehyde-3-phosphate).
Recycling of Energy Carriers:
The products of this stage, ADP (adenosine diphosphate) and NADP^+ (oxidized form of NADPH), are regenerated.
These molecules then cycle back to the light reactions to be re-energized and re-reduced, respectively.
Significance of G3P:
G3P is a critically important intermediate molecule in the Calvin cycle.
It serves as the direct precursor for the synthesis of a wide variety of organic molecules essential for the plant.
Primary Organic Molecules: Most prominently, G3P is used to form carbohydrates, such as:
Sucrose (a disaccharide, primary transport sugar)
Starch (a complex polysaccharide, energy storage)
Cellulose (a complex polysaccharide, structural component of cell walls)
Other Organic Molecules: Beyond carbohydrates, G3P can also be converted into:
Fatty acids (components of lipids)
Amino acids (building blocks of proteins)
Achieved Goal: At this point, the plant has successfully captured solar energy and utilized it to reduce inorganic carbon dioxide gas (CO_2) into versatile organic molecules.
Efficiency Consideration: This entire process would be highly inefficient if the intermediate molecules, particularly RuBP, could not be recycled.
Last Step: Regeneration of RuBP
This final stage is the crucial recycling phase of the Calvin cycle.
Purpose: To regenerate RuBP, the initial five-carbon CO_2 acceptor, allowing the cycle to continue.
Process: Some of the G3P produced in the reduction stage is not immediately converted into other organic molecules.
Instead, a portion of the G3P is modified.
This modification process also requires an input of energy, supplied by ATP (originating from the light reactions).
Outcome: The modification of G3P regenerates RuBP.
Cycle Continuation: The newly regenerated RuBP can then be used to fix more CO_2, thus ensuring the continuous operation of the Calvin cycle.