Light Independent Reactions

 

The second set of reactions goes by a number of names--light independent reactions, dark reactions, or the Calvin cycle. Basically, this is where the chemical energy (ATP and electrons) generated in the light reactions is used to "glue" carbon atoms together to make glucose. While they are called light independent because they don't depend directly on light, they do depend on the products of the light dependent reactions.  Carbon dioxide enters the cycle; that reaction is catalyzed by an enzyme called rubisco. When carbon dioxide levels are high and oxygen levels are low, rubisco will add carbon dioxide to RuBP. A series of reactions using the ATP and the electrons carried by NADPH will make 3-carbon molecules called phosphoglyceraldehyde (PGAL). Six turns of the cycle will produce twelve PGAL molecules; two of those will combine to form one glucose molecule and the rest regenerate molecules in the cycle so that it can continue to operate. To summarize the light independent reactions, let's look at the inputs and outputs:

 

 

INPUTS

OUTPUTS

CO2

Glucose

ATP

 

NADPH

 

 

Study the connection between the light dependent and light independent reactions. As well as how each fits into the overall process.

 

C4 and CAM plants are plants that have a modified form of photosynthesis as an adaptation to particular environmental conditions. Remember that we said rubisco would add carbon dioxide to the cycle when carbon dioxide levels were high and oxygen levels low. Plants cannot always maintain that situation. In particular, under hot and/or dry conditions, plants must close their stomata in order to conserve water. Dehydration is lethal. However, when stomata are closed, little carbon dioxide can diffuse into the leaf spaces and oxygen produced by photosynthesis cannot easily get out of the leaf. Under these conditions, rubisco catalyzes photorespiration instead--rubisco adds oxygen to RuBP instead of carbon dioxide. This slows down photosynthesis and results in much less photosynthesis. For normal plants, growth is clearly reduced. However C4 and CAM plants use alternate means to get carbon dioxide. C4 plants use a second cycle to bind carbon dioxide and keep carbon dioxide levels high enough to avoid photorespiration on hot days. The extra cycle does use ATP; as a result, the plants growth is slowed some but it definitely out competes plants that cannot use this step. Examples of C4 plants are crab grass, corn, and sugar cane. CAM plants, on the other hand, Stores carbon dioxide at night as organic acids and then releases it during the day. This process is often found in desert plants. Understand what those conditions are and how photosynthesis has been modified in each case.