|
|
 |
|
Calvin Cycle
|
|
|
In the second stage, the Calvin cycle,
carbon dioxide and the chemical energy stored in ATP and NADPH are used to form organic compounds. This process does not need
light to occur and is sometimes called the Dark reactions or the light independent reactions. The products of the light reactions
are used in the Calvin cycle and vice versa, making an ongoing cycle. Extra carbohydrates produced by plants can be stored
in roots and fruits to provide energy to autrophs and heterotroophs. Water is the main source of oxygen.
The fixation of carbon dioxide
is the first step in the light independent reactions. In this process carbon dioxide combines with a 5-carbon sugar, ribulosephosphate
(RUBP), to give two molecules of a 3-carbon compound, glycerate 3-phosphate (GP). In the presence of ATP and NADPH from the
light reactions is reduced to glyceraldehyde 3-phosphate (G3P). This is when carbohydrates are produced through photosynthesis.
|
|
|
|
|
|
|
|
|
|
|
|
There are three processes that exist for carbon fixation to occur. C3 carbon
fixation, C4 carbon fixation, and CAM. C3 plants (the most common carbon fixation) fix carbon exclusively through the Calvin
cycle and are called such because of the three-carbon compound that is initially formed. C4 plants fix carbon into four carbon
compounds and CAM is a water conserving pathway and is an abbreviation for crussulacean acid metabolism, because it was discovered
in plants of the family Crussulaceae first. C4 plants have their stomata partially closed. Certain cells in the plants have
an enzyme that can fix carbon dioxide into four-carbon compounds even when oxygen levels are high and carbon dioxide levels
are low. When producing the same amount of carbohydrates, C4 plantslose about half as much water as C3 plants. CAM plants
open their stomata at night, when the temperature is lower and close them during the day. Carbon dioxide is released from
these compounds during the day and enters the Calvin cycle. They grow fairly slowly. CAM plants lose less water than either
C3 or C4 plants.
 
|
|
|
|
