The Stages of Photosynthesis
Sadhela. (2009, October 24). Light Dependant Reactions of Photosynthesis Animation.YouTube. Retrieved December 4, 2012, from http://www.youtube.com/watch?v=BK_cjd6Evcw
- The process of photosynthesis involves 2 primary stages: light reactions (or light dependent reactions) and dark reactions (or light independent reactions).
- Light Reactions:
- Light (dependent) reactions occur in the thylakoids of the chloroplast.
- During the light reaction, sunlight is converted into chemical energy in the form of ATP, (or adenosine triphosphate, a compound used to store and release energy) and in the form of NADPH (a high energy electron carrier molecule).
- Light reactions produce oxygen gas and convert ADP (adenosine diphosphate, like ATP, except ADP has one less phosphate group) and NADP+ (like NADPH, but in the low-energy state) into energy carriers ATP and NADPH. The ATP and NADPH will then provide the energy needed to build high-energy sugars from low-energy carbon dioxide.
- What the cell takes in during Light (dependent) reactions: 1. Light (sunlight) 2. H2O 3. ADP 4. NADP+
- What the cell releases during the Light (dependent) reactions: 1. Oxygen 2. ATP 3. NADPH
- Dark Reactions:
- Dark reactions, which are also often referred to as the Calvin Cycle, occur in the stroma of the chloroplast.
- During Dark reactions (as dark implies...), no light is required to power the reaction.
- In the Calvin Cycle, the ATP and NADPH that have been produced from the light (dependent) reactions are used to produce high-energy sugars, such as glucose.
- The Calvin Cycle begins when CO2 enters the chloroplast from the atmosphere. Six CO2 molecules from the atmosphere are combined with six 5-carbon molecules to produce twelve 3-carbon compounds.
- "To produce sugar, the energy from the ATP and NADPH is used to convert the 3-carbon molecules to higher energy forms. Two of the 3-carbon molecules are removed from the cycle to produce sugars, lipids, amino acids, and other compounds. The remaining 3-carbon molecules are converted back into 5-carbon forms that are used to start the cycle again." (Miller & Levine Biology, 2010).
- To summarize dark reactions, the Calvin cycle uses 6 molecules of CO2 to produce a single 6-carbon sugar molecule.
- What the cell takes in during Light (independent) reactions: 1. ATP 2. NADPH 3. CO2
- What the cell releases during Light (independent) reactions: 1. NADP+ 2.
(Krueger, 2012)