Continue to use the Photosynthesis Notes Guide from the previous lesson while learning the material in this lesson. It is located in the sidebar if you did not print it in the previous lesson.


Photosynthesis is the process that provides energy for almost all life. During photosynthesis, autotrophs use the sun’s energy to make carbohydrate molecules from water and carbon dioxide, releasing oxygen as a byproduct. Photosynthesis occurs in the chloroplasts of plant cells and algae and in the cell membranes of certain bacteria.

The process of photosynthesis can be summarized by the following equation:


Watch the following video clip from NOVA entitled, ‘Earth from Space’ to see the impact of photosynthesis on a global scale. While watching, think about why it is that oxygen and carbon cycle from day to night on the planet. (Hint: Both Photosynthesis and Cellular Respiration are reverse processes.)



There are two stages to photosynthesis:

  1. Light Dependent reactions
  2. Light Independent reactions (Calvin Cycle)

While you read through the lesson, use the Photosynthesis Graphic Organizer in the sidebar (and below) to keep track of each stage:

Stage Where It Occurs What Goes In What Comes Out
Light Dependent Reactions thylakoid
Light Independent Reactions (Calvin Cycle) stroma

Stage 1: Light Dependent Reactions

In the Light Dependent Reactions, chlorophyll in the thylakoids captures light energy.

Look at the diagram below that shows the Light Dependent Reactions. The chlorophyll is grouped in areas of the thylakoids called Photosystems I and II (see the diagram below). As the light hits each group of chlorophyll, each system loses an electron.


Both electrons are passed down an electron transport chain. One generates ATP and the other NADPH, an electron carrier. Chemical energy has been generated from sunlight energy.

To replace this initial electron loss in Photosystem II, water is split into hydrogen ions, electrons, and oxygen (O2). Photolysis is the splitting of a water molecule. The missing electron in Photosystem I is replaced by Photosystem II.

The light reaction traps energy from sunlight and does not involve CO2 and does not make sugars. The O2 diffuses out of the chloroplast as a byproduct.

Stop and Think


Where did the oxygen gas we breathe originate? We know it is a byproduct of the light reactions – where did it come from?

You are not required to know the detail in this video, but rather get a feel for the overall process and the roles of the following molecules:

  1. water
  2. photons of light
  3. chlorophyll
  4. ATP
  5. NADPH
  6. Oxygen


Stage 2: Light Independent Reactions (or the Calvin Cycle)

The Light Independent Reactions are a series of reactions referred to as the Calvin Cycle. They all occur in the stroma of the chloroplast. The chemical energy stored in ATP and NADPH powers the formation of glucose, using carbon dioxide (CO2) and the H+ from the NADPH+. The light independent reactions can occur in the light or in the dark.

This step of photosynthesis is important, because it “fixes” carbon from a gas to an edible form that other organisms on the planet can use. This is called carbon fixation.   Do you recall that life on earth is carbon based? Autotrophs are the only organisms on earth that can create high-energy organic compounds from low energy gaseous carbon. Carbon fixation in the Calvin Cycle is a unique and life-sustaining event.

Take a look at the following diagram of the Calvin Cycle and then read through the summary of it’s events below:


Steps of the Calvin Cycle: Locate each of these events in the picture.

  1. Carbon dioxide enters the leaves of plants and diffuses into the cells that contain chloroplasts.
  2. A molecule of CO2 is fixed to an existing 5 carbon sugar by the enzyme RuBisCO. This enzyme is one of the most important and abundant enzyme on Earth!
  3. The 6 carbon sugar that results is unstable. It splits in two.
  4. A series of reactions occur where ATP and NADPH donate energy and matter to create a molecule of glucose.
  5. The 5 carbon sugar originally used is regenerated and the cycle turns again with another CO2.
  6. It takes 6 turns of the cycle to make one molecule of glucose.

Stop and Think

During the last “Stop and Think” above we discovered that water is the source of the oxygen we breathe.  What makes up the mass of plants then, especially as they grow in mass over time? Is it the nutrients in the soil, or the carbon dioxide that add to their bodies?


Factors Affecting the Rate of Photosynthesis

Photosynthesis doesn’t always work at maximum speed; the environment can influence the rate at which a plant can carry out photosynthesis.


Three things in the plant’s environment affect the rate of photosynthesis:

  • Light Intensity: this is one of the most important factors. As light intensity increases, the rate of photosynthesis initially increases and then levels off to a plateau.
  • CO2 Levels: Increasing the level of CO2 stimulates photosynthesis until the rate reaches a plateau.
  • photosynthesisstoma
  • Temperature: Raising the temperature accelerates the chemical reactions involved in photosynthesis. The rate of photosynthesis increases as temperature increases. The rate of photosynthesis generally peaks at a certain temperature, and photosynthesis begins to decrease steeply.

Water could also be included in this list to some extent. Plants can lose water to evaporation through small pores on their leaves called stomata.

The stomata are openings on leaves that allow CO2 and O2 enter and exit the plant. Closing stomata means they reduce water loss, but also means that less CO2 will enter the leaf, reducing photosynthesis.