Which of the following sequences correctly represents the flow of electrons during photosynthesis?

A. NADPH -> O2 -> CO2

B. H2O -> NADPH -> Calvin cycle

C. NADPH -> chlorophyll -> Calvin cycle

D. H2O -> photosystem I -> photosystem II

E. NADPH -> electron transport chain -> O2

Diagram showing the process of photosynthesis
Diagram showing the process of photosynthesis

The correct answer is B. H2O -> NADPH -> Calvin cycle

The photosynthetic reactions only occur in autotrophic organisms that are able to synthesize sugar using sunlight. The overall equation for photosynthesis is 6 CO2 + 6 H2O → C6H12O6+ 6 O2.

Glucose molecules are synthesized in photosynthesis using the raw materials of carbon, hydrogen and oxygen atoms that are obtained from carbon dioxide gas and water molecules.

In eukaryotic cells, such as plants, photosynthesis occurs in organelles known as chloroplasts. These structures consist of a double membrane and an internal series of sacs called thylakoids.

The first part of photosynthesis involves the light reactions which depend on solar energy. These reactions take place in the thylakoids of the chloroplast. The dark reactions, or Calvin cycle, take place in the stroma (semi-fluid substance) of the chloroplast.

Light reactions occur in two photosystems which consists of a series of chlorophyll pigments and molecules that associate to enable the reactions to take place. It is important to note that water is split in the first set of reactions to provide electrons.

Reaction center chlorophylls absorb sunlight and excite an electron from the water. This electron that is excited is then accepted by an acceptor molecule.

The electrons then pass along a chain of molecules and provide the energy in the process, to enable the formation of ATP by photophosphorylation.

Electrons from photosystem II move to photosystem I where the process is repeated in the form of a second electron transport chain. More ATP is generated and NADPH is formed.

These two substances move into the Calvin cycle in which glucose is finally formed. Essentially electrons have flowed from water to NADP and finally into the dark reactions.

Photosynthesis basics

Photosynthesis is a process that only occurs in autotrophic organisms such as green algae and plants. It is the method by which light energy is captured and used to help make glucose through a series of reactions.

The general equation for photosynthesis is 6 CO2 + 6 H2O → C6H12O6+ 6 O2. Essentially photosynthetic organisms use carbon dioxide and water as raw materials for forming a glucose molecule, which is a six-carbon sugar.

Radiant energy from the sun is used to catalyze the start of the reactions, and in the end, oxygen is released from the splitting of water.

The photosynthetic reactions can be divided into two parts, the light reactions, and the dark reactions or Calvin cycle.

The first set of reactions do depend on light energy and thus only occur when there is solar energy available. The second set, the dark reactions can sometimes occur when there is no light available.

Electrons pass from water molecules to NADPH which is formed in the light reactions, and then into the Calvin cycle.

Photosynthesis occurs in the chloroplast in eukaryotic cells. This is a membrane-bound organelle which has a series of internal sacs called thylakoids, which group together to form grana. The semi-liquid part of the chloroplast is called the stroma.

The light reactions

These reactions occur in the grana of the chloroplast. The thylakoids of the grana contain groups of pigments called chlorophylls. Chlorophyll a and b are the main two types that are essential for the photosynthetic reactions of plants.

These pigments come together to form what is known as photosystems. In addition, some of the pigment molecules form what are called reaction centers, which also receive light energy absorbed by the other pigments that are known as antenna pigments.

Photosystem II and Photosystem I

Photosystem II actually occurs before photosystem I, which is a bit confusing. Nonetheless, the process involves these two electron transport chains, with photosystem II passing electrons on to photosystem I.

When light is absorbed by the reaction center, which is known as P680 in photosystem II, it excites an electron. This means that the light energy causes the electron to be elevated to a higher energy level.

This electron is obtained from a water molecule, which is split in the process that is known as photolysis. The excited electron then moves and becomes attached to an acceptor molecule.

These electrons are then passed from this acceptor molecule along a chain of molecules and in the process, the electrons provide the energy to pump protons across the thylakoid membrane from the stroma into the thylakoid.

These protons diffuse back through to the stroma via special proteins lined with ATP synthase enzyme. As these protons diffuse through, ATP is generated from ADP and an inorganic phosphate molecule in the process called photophosphorylation.

Electrons now pass to photosystem I where the reaction center is P700. Light energy absorbed here then elevates these electrons to an acceptor molecule and once again they pass along a transport chain. ATP is formed and some electrons pass to molecules of NADP to form NADPH.

Energy stored in the NADPH and the ATP is then carried into the stroma of the chloroplast for the next set of reactions, the Calvin cycle.

The dark reactions (Calvin cycle)

The Calvin cycle or dark reactions is the stage when sugar molecules are made. These reactions can occur provided there are adequate levels of ATP present.

The plant uses carbon dioxide which enters through stomata (pores) in the leaves. The carbon from the gas is used in a reaction to form the molecule ribulose 1,5-biphosphate (RuBP).

This molecule is further modified through a series of enzyme-based reactions that take place. The final product that is formed is the six carbon sugar C6H12O6 (glucose).

It is important to understand that the dark reactions are a cycle and the process begins once more when the starting molecule is regenerated. During the reactions of the Calvin cycle energy is provided by the oxidation of the NADPH and ATP molecules.


  1. H Lambers, JA Bassham (2018). Photosynthesis. Retrieved from Encyclopedia Britannica.
  2. RH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants, 4th edition. New York: USA, Worth Publishers.
  3. Editors of Encyclopedia Britannica (2018). Chloroplast. Retrieved from Encyclopedia Britannica.
  4. OFG Kilgour (1982). Mastering Biology.London: UK, MacMillan Press.
  5. C Rye, R Wise, V Jurukovski, J DeSaix, J Choi, Y Avissar (2017). Biology. Houston: USA, Rice University.


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