Light independent stage of Photosynthesis

[T-Infinite]

Hey guys! I'm fairly new here. I've been lurking around this forum for about 2 days I think!
I'm in year 11 doing Year 12 Bio at the moment and I have my photosynthesis SAC +test in about 2 weeks. I'm having trouble fully understanding the light-independent stage of photosynthesis, could someone please explain is to me?~ Like, step by step as in what occurs at each stage including the inputs and outputs and so on T_T , the textbook is so confusing sometimes.

Thank you! 

[AbominableMowman]

So photosynthesis occurs in two stages, Light dependent and Light independent reactions.

• 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
Light Independent:

• 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:

[alondouek]

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.

Note: From this point, I'll be discussing the Calvin Cycle reactions in a fairly biochemically-heavy way. You're almost certainly NOT going to need most (or any) of this, but it's definitely useful to understand - if you can handle it - for biological context.

If you're not understanding anything from here - DON'T WORRY. It's almost definitely not VCE-level critical knowledge.

• The first reaction that occurs is the conversion of CO₂ to a molecule called 3PG (3-Phosphoglycerate). This reaction is facilitated by an enzyme called 'Rubisco' (Ribulose bisphosphate carboxylase oxygenase). The overall reaction for the first stage is as follows:
  
  
  
• Then, 3PG is reduced to a molecule known as G3P (glyceraldehyde-3-phosphate, which you might have seen as PGAL) in the following reaction:
  
  
  
  
  Side reactions that occur are
  
  • between 3GP and 1,3-bisphosphoglycerate
  • between 1,3-bisphosphoglycerate and G3P
• This G3P (or PGAL) is an output sugar. However, only one molecule is used as an output; the second is 'regenerated' to RuP ₂, hence completing the 'cycle'. Note that this reaction is also anabolic and so requires an input of ATP.
• Hence, we can say that the overall LIR is:
  
  
  

The G3P molecule is later converted into glucose, and used for numerous purposes in polymer form in the plant (i.e. starch/sucrose etc).

[T-Infinite]

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.

So photosynthesis occurs in two stages, Light dependent and Light independent reactions.

• 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
Light Independent:

• 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

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.

[psyxwar]

So photosynthesis occurs in two stages, Light dependent and Light independent reactions.

• 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
Light Independent:

• 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:
(Image removed from quote.)

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.

Just one thing though -- you should know that glucose isn't actually the output of the Calvin Cycle; it's your two G3Ps/ PGALs, or glyceraldehyde-3-phosphates. This level of knowledge is expected knowledge as far as I know, but VCAA tends to accept glucose as well so it's not that big of a deal.

EDIT:

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".

[ealam2]

Here's a link to a really simple video on Photosynthesis that may help you remember what are the inputs and outputs of each stage.

http://www.youtube.com/watch?v=OlzbVr7XEeU&feature=youtu.be

[jeffdng]

Another Great Video

http://www.youtube.com/watch?v=sQK3Yr4Sc_k

[ferrsal]

Hey! Just dug up my photosynthesis notes. This a concise summary (dot-points) of the light independent stage that me and my bio teacher constructed in case an exam question came up that required a brief (but informative) response

Light Independent Stage
- Occurs in the stroma (fluid matrix)
- 5C sugar acceptor molecule RuBP (Ribulose Biphosphate) binds to the CO2 dissolved in the stroma
- This process is catalyzed by the enzyme rubisco (RuBP carboxylase)
- This forms 2 molecules of 3C 3PG (3-phosphoglycerate) or phosphoglyceric acid (PGA)
- These 2 PGA molecules are converted to 2 PGAL molecules (glyceraldehyde 3 phosphate), which are 3 carbon sugars
- A PGAL formation is achieved by adding a phosphate group ATP, then breaking the phosphate bond and adding the H+ from the NADPH+

Therefore, 3 turns of the cycle = 3 molecules of CO2 used = production of 6 PGAL molecules
However, only 1 of 6 molecules exist as an output, because the other 5 enter a process that regenerates more rubisco to continue the cycle
Therefore, 6 turns of the cycle = 2 PGAL molecules = formation of one glucose molecule

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

[Snorlax]

It's probably been repeated but this is my understanding of it:
A 5-carbon molecule (RuBP) attaches itself with carbon dioxide from the atmosphere. This is called carbon fixation.
The process creates an unstable 6 carbon which immediately splits into two 3-carbons.
ATP produced from the Light dependant stage is the energy used to drive the cycle, as well as the NADPH donating a H to create a 3carbon sugar phosphate molecule which some leave the cycle as a carbohydrate or continue on with the cycle (as a 5 carbon RuBP)

Please correct me if anything incorrect, but I'm pretty sure thats the LID part of photosynthesis. + you may already know this, but this stage occurs in the stroma of chloroplasts.

[jeffdng]

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

Glucose is not actually an output of the Calvin Cycle, the it's your two G3Ps/ PGALs, or glyceraldehyde-3-phosphates as psyxwar stated before, glucose is actually synthesised in the cytosol  and  two G3Ps/ PGAL can also be joined to make fructose, modified into lipids, pretty much everything in plants can be synthesised from the two G3Ps/ PGALs.

Please correct me if I am wrong.

[Scooby]

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 

[psyxwar]

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...)

[Yacoubb]

(1) Carbon Dioxide that enters from the stroma combines with the 5-carbon compound RuBP (ribulose bi-phosphate) to form a 6-carbon compound.
(2) The reaction of carbon-fixation is catalysed by the rubisco enzyme ~ its actually called ribulose bi-phosphate carboxylase-oxygenase.
(3) The 6-carbon compound splits into 2 three-carbon compounds, called PGA or phosphoglyceric acid.
(4) Using ATP and NADPH from the light-dependent stage, the PGA is converted to PGAL (glyceraldehyde 3-phosphate).
(5) Some of the PGAL molecules leave the chloroplast and may be converted to simple sugars such as glucose and fructose.
NOTE: ALTERNATIVELY, THE SUGARS FORM DISACCHARIDES SUCH AS SUCROSE IN THE STROMA OF THE CHLOROPLAST ORGANELLE.
(6) The majority of PGAL (glyceraldehyde 3-phosphate) produced are converted to ribulose bi-phosphate (RuBP) with the use of ATP from the light reactions.
(7) The newly formed RuBP then continues the Calvin Benson Cycle, that reduces the carbon in Carbon-Dioxide to C₆H₁₂O₆.

[Scooby]

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...)

I just had a look over the question I think you're talking about. Only glucose and water were listed as accepted answers. Yeah, G3P/PGAL is more correct, but you're never gonna be marked wrong for writing glucose. There's a good chance you will be marked wrong for writing G3P/PGAL though