As you can see from the equation, photosynthesis is essentially the opposite of cellular respiration. Photosynthesis uses six molecules of carbon dioxide, six molecules of water, and light energy to produce one molecule of glucose and six molecules of oxygen. Photosynthesis occurs with or without light in the light-dependent and light-independent reactions respectively. The light-independent reactions are called the Calvin Cycle. Photosynthesis happens in the chloroplasts of plant cells. Light-dependent reactions start with, well, light. Light photons travel to the chlorophyll and excite electrons, this is called "photoexcitation." This chlorophyll is part the first of four protein complexes used by the light-dependent reactions, called Photosystem II (PSII). These four protein complexes all function on the membrane of the thylakoid. The electron that is excited during photoexcitation is then carried along the Electron Transport Chain by mobile electron carriers. When the electron leaves the chlorophyll in PSII, the chlorophyll splits a water molecule to replace the electron it lost. This also releases hydrogen ions into the cell and oxygen into the air. The next stop for the excited electron is the Cytochrome Complex. This protein complex uses some of the energy from the electron to pump more hydrogen ions into the cell. This, along with the water splitting, helps to build a concentration gradient that will be able to power ATP Synthase and produce ATP. From the Cytochrome Complex, the electron, now a little lower on energy, is carried to Photosystem I (PSI). PSI facilitates the "re-energization" of electrons by more photons of light. When these electrons are re-energized they are carried to and used by NADP+ Reductase. NADP+ Reductase combines NADP+ with a hydrogen ion and two electrons to make NADPH, a  valuable energy carrier. 

Now, let's talk about the Calvin Cycle and Light-Independent Reactions. The Calvin Cycle begins in the Stroma of the of the chlorophyll. The cycle begins with carbon fixation. Carbon fixation is the process of binding a CO2 molecule to a five-carbon molecule of RuBP. This process is helped along by an enzyme we like to call "RuBisCo." When these two molecules are combined, they are extremely unstable and break apart into two molecules of 3-Phosphoglycerate. ATP adds a phosphate group to each of these and NADPH adds some electrons, forming G3P. G3P can be used in the cell for forming almost any type of carbohydrate. However, five G3Ps are needed to replace the three RuBPs used to start with. Nine molecules of ATP and six molecules of NADPH are also needed. So although six G3Ps are created, only one can leave the cycle because the other five are needed for regeneration of the three RuBPs that were used at the start. 

Calvin Cycle picture from Khan Academy LINK

Calvin Cycle picture from Khan Academy LINK

Practice Multiple Choice Questions:

1. Which of these is a reactant in the photosynthesis equation?

  • A. Oxygen
  • B. Carbon Dioxide
  • C. Glucose
  • D. RuBisCo

2. Put these parts of the light-dependent reactions in order.

I      Photosystem I

II     Photoexcitation

III     Photosystem II

IV     Cytochrome Complex

  • A. I, II, III, IV
  • B. II, I, IV, III
  • C. II, III, I, IV
  • D. IV, III, II, I

3. True or False: The Calvin Cycle is part of the light-dependent reactions.

  • A. True
  • B. False

4. What is the name of the enzyme used in carbon fixation?

  • A. G3P
  • B. RuBisCo
  • C. 3-PGA
  • D. ATP synthase

5. How many molecules of G3P are made and how many leave the cell?

  • A. 1 is made, 1 leaves
  • B. 4 are made, 2 leave
  • C. 6 are made, 6 leave
  • D. 6 are made, 1 leaves

Practice Free Response Questions:

6. How many protein complexes are used in the light-dependent reactions and what are they called?


7. Photons are utilized by two complexes in the light-dependent reactions, which are they and what is the photon used for?


8. Why are hydrogen ions important in the light-dependent reactions? Make sure to explain where they come from and what they do in your response.


9. Briefly name and explain the three stages of the Calvin Cycle.


10. How many of each molecule used in the Calvin Cycle needs to be regenerated in the regeneration stage?



1. B is correct because carbon dioxide is the only of the options listed that is a reactant.

2. C is the correct answer because the light-dependent reactions begin with photoexcitation, then continue on to PSII, Cytochrome Complex, PSI, and eventually NADP+ Reductase.

3. The answer is "B. False" because the Calvin Cycle does not require photons of light to function, therefore it is classified as part of the light-independent reactions.

4. B is correct because RuBisCo is the enzyme used in carbon fixation.

5. D is correct because five of the six G3Ps created are used to regenerate the 3 used molecules of RuBP and one G3P leaves the cycle to be used by the plant.

6. There are four protein complexes: PSII, PSI, Cytochrome Complex, and NADP+ Reductase.

7. Photons are used in PSII for photoexcitation and start the process of light-dependent reactions. PSI utilizes photons to re-energize electrons so that they can be used to power NADP+ Reductase.

8. Hydrogen ions are released into the cell when PSII splits a water molecule after photoexcitation. Hydrogen ions are also pumped into the cell by the Cytochrome Complex using the energy from the excited electron. The hydrogen ions are used to create a concentration gradient in the cell which powers ATP Synthase and creates ATP to be used to transport energy throughout the cell.

9. Carbon fixation is the process of RuBisCo binding a molecule of RuBP with a molecule of CO2 to create an unstable six-carbon molecule. Reduction is the process by which ATP and NADPH attach a phosphate group and electrons to 3-PGA, creating G3P. Regeneration occurs after six G3Ps have been formed. Five of the G3Ps are use to regenerate to the three used RuBPs.

10. Three molecules of RuBP need to be regenerated, as well as nine molecules of ATP and six molecules of NADPH.

Posted on June 1, 2016 .