Photosynthesis: The Light Independent Stage in a Snap! Unlock the full A-level Biology course at created by Adam Tildesley, Biology expert at SnapRevise and graduate of Cambridge University.
The key points covered of this video include:
1. Overview of the Light Independent Stage
2. Fixation of Carbon Dioxide
3. Formation of Organic Molecules in the Calvin Cycle
4. The Regeneration of RuBP
Overview of the Light Independent Stage
The light independent stage is the second stage of photosynthesis and it occurs in the stroma of chloroplasts. In the light independent stage the products of the light independent stage and carbon dioxide are used to synthesize organic molecules. The main metabolic pathway of the light independent stage is called the Calvin Cycle. Although the Calvin Cycle does not require light energy, it does depend on the products of the light dependent stage. Therefore if there is no light, the products of the light dependent stage do not form and thus the Calvin Cycle cannot take place. The pumping of the protons into thylakoid space during the light dependent stage increases the pH of the stroma to 8. A pH of 8 is the optimum pH for the enzymes involved in the Calvin Cycle.
Fixation of Carbon Dioxide in the Calvin Cycle
Carbon dioxide from the atmosphere diffuses into the leaf via the stomata. This carbon dioxide then diffuses across the plant cell wall and chloroplast envelope and into the stroma. The first step of the Calvin cycle involves carbon dioxide combining with the 5 carbon compound ribulose biphosphate (RuBP). This reaction is catalysed by the enzyme RuBisCo (ribulose biphosphate carboxylase-oxygenase). This reaction causes the RuBP to be carboxylated which forms an unstable intermediate compound with six carbons. This unstable six carbon compound immediately breaks down to form two molecules of glycerate-3-phosphate (GP). This whole process is referred to as the fixation of carbon dioxide.
Formation of Organic Molecules in the Calvin Cycle
After carbon dioxide has been fixed, GP is reduced to form the organic molecule triose phosphate (TP). GP is reduced to TP using hydrogen atoms from the reduced NADP that was formed in the light dependent reaction. The ATP that was made in the light dependent reaction is then also used at this stage to make two molecules of ADP. Therefore for each molecule of carbon dioxide, two molecules of ADP are produced. From the Calvin Cycle, two molecules of TP are needed to synthesize the hexose sugare glucose. This glucose can then be converted to sucrose, starch or cellulose for use in the plant cell. TP can also be used to synthesize amino acids, fatty acids and glycerol.
The Regeneration of RuBP
Chloroplasts contain only low levels of RuBP because it is continuously being converted to GP. In order for the Calvin cycle to continue, RuBP has to be regenerated. Therefore 10 TP molecules out of every 12 TP molecules are used to regenerate six molecules of RuBP. The remaining 2 molecules out of 12 molecules of TP are the product and these are used to synthesize organic compounds like glucose.
Summary
The light independent stage of photosynthesis takes place in the stroma of the chloroplasts
The main metabolic pathway in the light independent stage is the Calvin cycle
Although the Calvin cycle does not directly need light energy, it is reliant on the products of the light dependent stage therefore it cannot occur without light
The Calvin cycle involves the fixation of carbon dioxide where carbon dioxide is combined with RuBP to form GP
GP is then reduced using the hydrogen atoms from reduced NADP formed in the light dependent reaction and this process produces the organic molecule TP
TP can then be used to synthesise more complex organic molecules like glucose, amino acids, fatty acids and glycerol
RuBP has to be regenerated so 10 TPs out of every 12 TP molecules are used to regenerate RuBP
The remaining 2 TP molecules out of the 12 are the product and are used to synthesise complex organic molecules
