Unit C

Module 5 ~ Lesson 3

Calvin-Benson Cycle

Light-independent reactions do not directly depend on light energy in order to proceed, so they can occur during the daylight and during the night. The products of the light-dependent reactions, NADPH and ATP, move into the stroma of the chloroplast and provide energy for the Calvin-Benson Cycle. The energy contained in these two molecules is used in glucose synthesis.

Used with permission from Inquiry into Biology


The Calvin-Benson cycle has three stages.
  1. Carbon dioxide fixation is the process of carbon dioxide bonding to a larger carbon compound in the stroma, ribulose biphosphate (RuBP), to form a 6-carbon compound. The 6-carbon compound is so unstable, it immediately breaks into two 3-carbon molecules.
  2. The 3-carbon molecule is activated by energy from ATP and reduced by NADPH to form glyceride-3-phosphate (PGAL). Some PGAL molecules leave the cycle to form glucose.
  3. The remaining PGAL is recycled to form RuBP to continue the cycle. Again, energy from ATP is required for this step.

The following diagram shows where these chemicals are needed in the Calvin-Benson cycle.


Read

Read "The Light-Independent Reactions of Photosynthesis" on pages 176 and 177 of the textbook.

 Self-Check

Complete this self-check on the light-independent reactions.


Thinking Critically

Imagine that you are in charge of a greenhouse on a space station. The greenhouse has three purposes:

  • to produce food
  • to control air quality
  • to provide fresh water

How would each of the three listed purposes be performed? Consider what types of plants would be best to have in your greenhouse as well as the light conditions and the atmospheric composition.

 Activity

How are photosynthesis and cellular respiration related? Review this animation.

  • How are photosynthesis and cellular respiration related to each other?
  • Observe and identify each step in photosynthesis.