Light Independent Reactions

Step 1

Carbon dioxide (CO2) is obtained from the atmosphere and enters the leaf through pore called stomata.

Step 2

The carbon from carbon dioxide combines with a 5-carbon sugar called RuBP (ribulose biphosphate) through carbon fixation. This forms a very unstable 6-carbon compound.

Step 3

The unstable 6-carbon compound immediately break apart into 2 molecules of PGA, a three-carbon compound.

Step 4

ATP and NADPH go through a series of reactions to convert a molecule of PGA into PGAL (G3P). PGAL/G3P is also a 3-carbon compound.

Step 5

The Calvin cycle runs 6 times, which yields 12 molecules of PGAL/G3P.

Possibilities for the PGAL

1. 2 molecules of PGAL are combined together to form a molecule of glucose (C6H12O6) using energy from ATP.

2. The remaining 10 PGAL molecules are converted by a series of reactions into more RuBP so that the reaction can occur again

Alternate pathways for carbon fixation to deal with water loss

1. C4 pathway

2. CAM pathway

C4 pathway

Carbon dioxide is converted into a 4-carbon compound called oxaloacetic acid.the purpose of oxaloacetic acid is to store carbon dioxide and save it until it is needed for the Calvin cycle. The reaction is reversible. When the CO2 is needed, it is removed from oxaloacetic acid and sent to the Calvin cycle. They are able to maintain a high level of photosynthesis while conserving water by having their stomata’s closed. C4 plants include corn, sugar cane, and cran grass.

CAM Pathway

CAM plants open their stomata’s only at night. They take in CO2 at night and store it. Their stomates are closed during the day to conserve water. Their stored CO2 is used during the day for photosynthesis. This process is found in many plants that live in hot, dry areas. Such plants are cacti and pineapples.

Factors affecting the rate of photosynthesis

1. Water

   1. Water is required in the light dendent reactions. Water is obtained from the ground by the roots. A shortage of water in the ground can slow or stop photosynthesis. In order to prevent water loss from the plant, plants are covered with a waxy cuticle.

2. Temperature

   1. The process of photosynthesis depends upon the action of enzymes. Enzymes work the best at temperatures between 0 degrees celcius and 35 degrees celcius. Temperatures above or below this range may damage the enzymes and prevent them from functioning. At very low or high temperatures, photosynthesis may stop entirely.

3. Light intensity

   1. Increasing the light intensity increases the rate of photosynthesis