Welcome to AN!
As you probably already know, the Light-Independent Reactions (from here "LIR") are the second phase of the basic components of photosynthesis. From the name, we can see that this stage does not utilise light energy as a direct means of driving reactions.
The LIR are also known as the 'Calvin-Benson Cycle' (or more commonly as the 'Calvin Cycle'), and they are characterised by the process of fixating atmospheric CO2 and converting it, through a number of intermediary products, to glucose (C6H12O6). The general purpose of the CC can effectively be stated as "the assimilation of CO2 (and inorganic N and S) into organic matter (i.e. glucose).
Because these reactions are anabolic (we are building up larger molecules from smaller components), we're going to need an input of energy. Of course, the easiest means of energy usage by the plant is going to be the energy-relevant outputs of the Light-Dependent Reactions, namely ATP and NADPH (a reduced photosynthetic-specific proton carrier). This energy initiates the reactions of the Calvin Cycle.
Thank you so much!! That really helps! So basically, during Calvin Cycle, CO2 and H+ from the light dependent stage is used to make glucose? I always find the Calvin Cycle part confusing, everything else is pretty straight forward.So photosynthesis occurs in two stages, Light dependent and Light independent reactions.
Light Independent:
- Light Dependent:
- Occurs on the grana, and light energy is absorbed by different pigments (Chlorophyll, Xanthophylls, etc.)
- The energy that is absorbed by chlorophyll (light energy) causes electrons within the molecule to become excited. This energy is used to split Water molecules into H+ ions and O2 gas.
- The electrons flow through the electron transport chain to provide energy for ATP synthesis. Unloaded electron acceptor molecule NADP+, accepts H+ ions to form NADPH.
- Inputs: H2O, NADP+, ADP + Phosphate
Outputs: NADPH, O2, ATP
- Occurs in the stroma of the chloroplast.
- The Calvin Cycle occurs at this stage, during which H+ ions from the previous stage and CO2 are used to synthesise sugars. The energy for this reaction is provided by the ATP produced in the previous reaction.
- Inputs: ATP (from previous reaction), Hydrogen Ions 24H+ (from NADPH), CO2
Outputs: C6H12O6, H2O
Alondouek explains the entire thing quite well, but really ForeverYeti's post is all you actually need to know (for 3/4 Bio). You might want to do some further reading so you can better understand what actually happens, but that's up to you.So photosynthesis occurs in two stages, Light dependent and Light independent reactions.
Light Independent:
- Light Dependent:
- Occurs on the grana, and light energy is absorbed by different pigments (Chlorophyll, Xanthophylls, etc.)
- The energy that is absorbed by chlorophyll (light energy) causes electrons within the molecule to become excited. This energy is used to split Water molecules into H+ ions and O2 gas.
- The electrons flow through the electron transport chain to provide energy for ATP synthesis. Unloaded electron acceptor molecule NADP+, accepts H+ ions to form NADPH.
- Inputs: H2O, NADP+, ADP + Phosphate
Outputs: NADPH, O2, ATP
- Occurs in the stroma of the chloroplast.
- The Calvin Cycle occurs at this stage, during which H+ ions from the previous stage and CO2 are used to synthesise sugars. The energy for this reaction is provided by the ATP produced in the previous reaction.
- Inputs: ATP (from previous reaction), Hydrogen Ions 24H+ (from NADPH), CO2
Outputs: C6H12O6, H2O
This diagram might help:
(http://www.southtexascollege.edu/modeh/1408webpage_files/CH%207_files/image011.jpg)
Thank you so much!! That really helps! So basically, during Calvin Cycle, CO2 and H+ from the light dependent stage is used to make glucose? I always find the Calvin Cycle part confusing, everything else is pretty straight forward.Basically yes. The carbon dioxide is combined with a 5-carbon compound (the Ribulose Bisphosphate) which eventually splits into your two G3Ps (the hydrogens are used at some point in the cycle as you can see from alondoek's diagram). The Ribulose Bisphosphate is generated as well for use in future cycles, hence it being called the Calvin "cycle".
I also find an Input/Output table pretty useful for these complicated processes
So, for the Light Independent stage:
INPUTS: CO2, Loaded carrier (NADH+), ATP
OUTPUTS: Carrier (NADP), ADP+Pi, Glucose
If you're asked to provide an output of the Calvin cycle in the exam, write glucose, not G3P. You'll probably be marked wrong if you write G3P. For VCE Bio you won't have to worry about G3P :)Doesn't it depend on the question? I remember a past exam had a picture of the Calvin Cycle itself with the different intermediates labelled in terms of the number of carbons. It asked you what the output was and the correct answer was the 2 PGALs (though they had also accepted glucose and water if I remember correctly...)
Doesn't it depend on the question? I remember a past exam had a picture of the Calvin Cycle itself with the different intermediates labelled in terms of the number of carbons. It asked you what the output was and the correct answer was the 2 PGALs (though they had also accepted glucose and water if I remember correctly...)