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Photosynthesis involves Light-dependent reactions, which are the noncyclic pathway, and the light-independent reactions, known as Calvin cycle. The sequence of events is described below.
NOTE: Since I could not find the complete question, I will provide a complete explanation of how photosynthesis occurs, so you will be able to arrange the events in your statement in the correct order.
What is the photosynthetic sequence of events?
Photosynthesis involves two stages: light-dependent and light-independent reactions.
LIGHT-DEPENDENT REACTION
It occurs in the thylakoids of chloroplasts. Involves the noncyclic pathway which is a flow of electrons along a chain located in the thylakoid membrane.
Photosynthetic pigments are in charge of light absorption. When sunlight reaches the pigments, electrons get excited and move along the electron transport chain.
During these reactions, oxygen is released, while ATP and NADPH are produced. Both of them are used during light-independent reactions.
LIGHT-INDEPENDENT REACTION
Light-independent reaction is known as the Calvin cycle, and it takes place in the stroma.
During the Calvin cycle, sugars or carbohydrates are synthesized.
When carbon dioxide, CO₂, enters the leaves through stomas, it diffuses to the chloroplast stroma.
Carbon atoms fixate, meaning that they incorporate into organic molecules. These molecules are used to produce 3-C sugars.
The whole process is impulsed by ATP and NADPH coming from light-dependent reactions.
SEQUENCE OF EVENTS
⇒ THYLAKOID MEMBRANE: The steps of light-dependent reactions are the followings,
1) Luminous energy is trapped by chlorophyll in Photosystem II.
2) When the pigment molecules absorb light, electrons provided by water molecules get to a higher energy level.
3) The excited electrons go through the electron transport chain from Photosystem II to a less energetic level in photosystem I.
4) When the excited electrons leave photosystem II, they are replaced by new electrons extracted from the water molecules.
5) The absorbed luminous energy moves the electrons from the photosystem I to another electron acceptor, from where they get transported again and used to produce NADPH molecules.
6) When electrons leave Photosystem I, they are replaced by new electrons coming from photosystem II.
7) When the water molecule breaks down, hydrogen ions remain in the thylakoid lumen, from where they are pumped to the stroma by the ATP synthase.
8) The released energy is used to produce ATP molecules.
9) Hydrogen ions go back from the stroma to the thylakoid compartment.
⇒ STROMA: The steps of light-independent reactions are the followings,
1) During the carbon fixation phase, a CO₂ molecule combinates with a ribulose-1,5-bisphosphate (5-C molecule) to form 6-C molecules, which will divide into two 3-phosphoglycerate molecules, 3-PGA (3-C molecules).
CO₂ + ribulose-1,5-bisphosphate ⟶ 6-C molecule ⟶ 2 3-PGA
2) During the reduction phase, ATP and NADPH are used to produce the sugar molecule glyceraldehyde-3-phosphate (G3P) from 3-PGA molecules.
NADPH donates its electrons to turn 3-PGA into glyceraldehyde 3-phosphate, G3P.
3-PGA + ATP + NADPH ⟶ G3P + ADP + NADP⁺
3) During the regeneration phase,
- some glyceraldehyde 3-phosphate molecules leave the cycle and go to the cytosol to form glucose.
- Other G3P get recycled to regenerate the RuBP acceptor. This last reaction needs ATP and a series of other reactions.
When three CO₂ enter the cycle, 6 G3P are produced. One of them leaves the cycle, and the remaining five are recycled and generate three RuBP.
⇒ CYTOSOL
1) Once in the cytosol, glyceraldehyde 3-phosphate molecules are used to form glucose and fructose.
2) These two molecules are the monosaccharides that form sucrose.
3) Once sucrose is formed, it is transported from the photosynthetic tissues to different parts of the plant by the phloem.
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