The World Around Us: Light Independent Reactions

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard: Understand how photosynthesis turns light energy into chemical energy.

Article includes ideas, images, and content from Troy Smigielski (2021-10)

Light Independent Reactions

(…and then… BAM! Magic!)

The photosynthesis process includes two distinct steps. The light dependent reactions and the light independent reactions. The net result of photosynthesis is the production of oxygen that is released and sugar that is stored to be used by the plant.

The light dependent reactions produce oxygen that gets released by the plant as a byproduct. It also produces ATP and the electron carrier NADPH which are used in the Light Independent Reactions.

Keep in mind that the oxygen produced in the light dependent reactions is NOT needed by the plant. It is a byproduct that is given off.

The goal of LDR is to create energy rich molecules for the Calvin cycle to use.

The light independent reactions are also called the Calvin cycle or dark reactions. These reactions take place in the stroma and do not require light.

The dark reactions use three reactants to produce the final product (sugar).

ATP (Adenosine triphosphate, C10H16N5O13P3)
NADPH (Nicotinamide adenine dinucleotide phosphate, C21H29N7O17P3)
CO2 (Carbon dioxide)

The Calvin Cycle uses the ATP and the NADPH made in the light dependent reactions for energy to fuel itself.

In the process, the ATP (plus water, H2O) is broken into ADP (Adenosine diphosphate, C10H15N5O10P2) and a phosphate ion (PO4^-3)

Source, 2021-10

Whereas the following represents the reaction going on in the diagram…

C10H16N5O13P3 + H2O --> C10H15N5O10P2 + PO4^-3

…a close look reveals that there are more hydrogen atoms on the reactant side. Remember that the ATP-ADP cycle is only PART of what is going on!

So, here's the big idea…

ATP + Water becomes ADP + phosphate + ENERGY! ENERGY! ENERGY! (and also some H atoms)

The Calvin Cycle also uses the CO2 that we exhale to make glucose.

Recall that the CO2 comes into the plant from the atmosphere. It enters the leaves through openings called stomata. Stomata are protected by guard cells that open during the day and close at night.

The Calvin Cycle Steps

So… there's a reason its called a cycle! It goes around and around as it produces the sugar. The process can be understood only by seeing it in motion. There's not exactly a starting place; all of the cycle parts are present when…

Okay, let's start with a CO2 coming in… what happens to it?

Step 1: One CO2 molecule enters the leaf. It combines with a 5-Carbon molecule called RuBP.

Step 2: The 6-Carbon molecule splits into two 3-Carbon molecules called 3-PGA (PGA).

Step 3: ATP and NADPH (high energy) from light dependent reactions are used to convert the PGA into G3P (aka PGAL). When ATP is used for energy, it becomes ADP.

When NADPH is used for energy, it becomes NADP+. These compounds go back (eventually) to the light reaction to be reenergized.

Step 4: One carbon from the G3P molecules goes on to start making a glucose. The other 5 carbons of the G3P are regenerated into RuBP to start the cycle over again.

Step 5: The cycle repeats 6 times to form one molecule of glucose.

The above diagram shows the reactants and products of the cycle happening 3 times (all in the same diagram). It has to happen 6 times to produce one glucose molecule. Each cycle adds one carbon to what will become the glucose chain (at the bottom).

Some diagrams you might see will show this cycle differently.

The above diagram shows the cycle after it has happened 6 times.

Light Independent Reactions: Bottom Line

Light independent reactions are also called the Calvin Cycle and dark reactions.
The Calvin Cycle happens in the stroma of a chloroplast.
CO2 from the atmosphere is used to start the Calvin Cycle.
Energy from the light dependent reactions is used in the light independent reactions.
The Calvin Cycle makes ADP and NADP+ to return to light dependent reactions.
After 6 rounds of the Calvin Cycle, one glucose molecule is made.