Photosynthesis Study Guide


When you get hungry, what do you do? It’s a pretty easy question to answer–you probably eat some food. But where does that food come from? Do you make it yourself? Not as in cooking it, but making it. Humans rely on other organisms to sustain life, namely plants and animals (or just plants 🌱 –shoutout to all the vegans out there!). Plants are different from us–they actually produce their own food themselves through a process called photosynthesis. Let’s take a closer look at this process and learn how it all goes down.

Lesson Objectives

  • Understand the importance of photosynthesis.
  • Understand the light dependent and light independent reactions of photosynthesis.

What is Photosynthesis?

Photosynthesis is a process by which plants use sunlight, water, and carbon dioxide to synthesize food represented by the equation below:

  • 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2

Take special note of the oxygen and glucose produced by photosynthesis. They’re a huge deal for us humans (at least if you’re interested in being alive). Not only does photosynthesis produce the oxygen we need to breathe, but also the energy on which every organism on the planet survives!

Evolution of Photosynthesis

We don’t know exactly how the process of photosynthesis came to be, but we’re pretty sure it traces back to the bacterial domain. Somewhere around 3 billion years ago, the ancestors, of what we know today as cyanobacteria developed the ability to take carbon dioxide from the atmosphere and convert it into oxygen. That’s right–some bacteria can photosynthesize too!

This led to the Endosymbiosis Theory. Endosymbiosis, for which the theory is named, is a form of symbiosis in which one organism lives inside the other. In this case, we think that a larger eukaryotic cell engulfed a cyanobacteria. The cyanobacteria provided food (glucose) through photosynthesis for the eukaryotic cell, and in return, the eukaryotic cell provided protection and/or nutrients for the cyanobacteria. These photosynthetic cyanobacteria ancestors then evolved into what we know as chloroplasts today.


Chloroplasts are the cell organelles in which photosynthesis takes place. Chloroplasts contain Chlorophyll, a pigmented molecule that has the ability to absorb sunlight and convert it into energy–a key part of the photosynthetic process.

There are two types of chlorophyll involved in photosynthesis: Chlorophyll A and Chlorophyll B.

  • Chlorophyll A absorbs light from the orange-red and violet-blue wavelengths of the light spectrum and uses that energy to donate electrons to the electron transport chain. Basically, it converts sunlight into the energy necessary for photosynthesis to occur.

  • Chlorophyll B isn’t necessarily required for photosynthesis to occur, but it can help with the job. Chlorophyll B absorbs higher frequency blue light from the sun to contribute to photosynthesis. A chloroplast can get all of the energy it needs from chlorophyll A, so chlorophyll B’s role is mostly expanding the spectrum of light that can be converted into chemical energy.

What Happens During Photosynthesis?

Photosynthesis takes place in two stages: the light-dependent reactions and light-independent reactions. As you might’ve guessed, the light-dependent reactions rely on sunlight to occur, while the light independent reactions do not. Let’s go over each of them below.

The Light Dependent Reactions

During the light-dependent reactions, solar energy is absorbed by chlorophyll and converted into chemical energy in the form of NADPH and ATP. This process takes place in a multiprotein complex called a photosystem, which is found within the thylakoid membranes of chloroplasts.

  • Two types of photosystems are found in these membranes: Photosystem II (PSII) and Photosystem I (PSI). Each photosystem contains a light-harvesting complex and a reaction center.

  • In the light-harvesting complex of photosystem II, pigments pass energy from sunlight to a special pair of chlorophyll A molecules located in the reaction center. When this energy is received by the chlorophyll molecules, an electron is energized and passed onto the primary electron acceptor.

  • The electron is now passed through an electron transport chain, losing energy as it goes. This energy drives the synthesis of ATP in a process known as chemiosmosis.

  • From here the electron is transferred to photosystem I. It joins another pair of chlorophylls and is once again boosted to a high level of energy and transferred to an acceptor molecule.

  • It is replaced by the arrival of another electron from photosystem II. Once again, the electron travels down another electron transport chain, synthesizing NADPH in the process.

The Light Independent Reactions: The Calvin Cycle

Check out the simulation below to visualize the different steps of the Calvin cycle!

In the light-independent reactions, also known as the Calvin Cycle, carbon dioxide from the air is used to build sugars in the stroma of a chloroplast. Two other important chemicals found in the stroma help initiate the cycle: the enzyme RuBisCO and the molecule RuBP.

As the name suggests, this process is not driven by solar energy. Instead, it is fueled by the ATP and NADPH synthesized in the light-dependent reactions. The Calvin cycle takes place over three main stages: Carbon Fixation, Reduction, and Regeneration. Let’s go over them below.

  • During carbon fixation, RuBisCO catalyzes a reaction between a CO2 and RuBP molecule, forming a 6-carbon compound.

  • This compound is then split into two 3-carbon compounds, which we call 3-PGA.

  • In the reduction phase, the energy from the ATP and NADPH synthesized in the light-dependent reactions is then used to convert each of these E-PGA compounds into two different kinds of 3-carbon compounds called G3P molecules.

  • In the regeneration phase, one molecule from each G3P pair is used to form a carbohydrate molecule. Because a carbohydrate molecule is composed of six carbons, it takes six turns through the Calvin cycle to generate just one carbohydrate (generally in the form of glucose).

  • The remaining G3P molecule is used to regenerate RuBP, which will be used the next time carbon fixation occurs.

💡 Conclusion:

  • Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to synthesize food.

  • We use the Endosymbiosis Theory to explain the origin of photosynthesis.

  • Photosynthesis takes place in the chloroplasts and utilizes two types of chlorophyll in the process: chlorophyll A and chlorophyll B.

  • Photosynthesis occurs over two stages: the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle).

  • During the light-dependent reactions, sunlight is converted into ATP and NADPH in the thylakoid membranes of the chloroplast.

  • The light-dependent reactions take place in two photosystems: photosystem II and photosystem I, each of which contains a light-harvesting complex and a reaction center.

  • During the light-independent reactions, the ATP and NADPH synthesized in the light-dependent reactions are used to convert CO2 into carbohydrates in the stroma of the chloroplast.

  • The Calvin cycle occurs in three main stages: carbon fixation, reduction, and regeneration.

  • The light-dependent and light-independent reactions are mutually dependent on one another.


1. What is photosynthesis?

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to synthesize food.

2. Where does photosynthesis take place?

Photosynthesis takes place in chloroplasts, which contain the chlorophyll.

3. What are the reactants of photosynthesis?

During photosynthesis, six carbon dioxide molecules and six water molecules, are converted by light energy and water into a sugar molecule and six oxygen molecules. Thus, the reactants of photosynthesis are light energy, water, carbon dioxide and chlorophyll.

4. How are photosynthesis and cellular respiration related?

Photosynthesis converts carbon dioxide and water into oxygen and glucose. Cellular respiration converts oxygen and glucose into water and carbon dioxide.

Thus, photosynthesis requires the products of respiration (carbon dioxide and water), while respiration requires the products of photosynthesis (glucose and oxygen). Together, these two reactions allow cells to build and store energy. Also, atmospheric concentrations of carbon dioxide and oxygen are regulated by these two reactions.

5. What is the equation for photosynthesis?


We hope you enjoyed studying this lesson and learned something cool about Photosynthesis! Join our Discord community to get any questions you may have answered and to engage with other students just like you! Don’t forget to download our App to experience our fun VR classrooms – we promise, it makes studying much more fun 😎


  1. “Overview of Photosynthesis – Biology for AP® Courses – OpenStax.”, Accessed 25 Nov. 2021.
  2. “Calvin Cycle – an Overview | ScienceDirect Topics.”, Accessed 26 Nov. 2021.
  3. Kazilek. (2016, February 24). Endosymbiotic Theory | Ask A Biologist.
  4. Lumen Learning. (2019). The Calvin Cycle | Biology I.
  5. Martin, L. (2017). What Are the Roles of Chlorophyll A & B? Sciencing.
  6. Scitable. (2010). Photosynthesis, Chloroplast | Learn Science at Scitable.

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