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The University of the West Indies Department of Biological and Chemical Sciences |
BL 05B - Preliminary Biology II |
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The University of the West Indies Department of Biological and Chemical Sciences |
BL 05B - Preliminary Biology II |
The Light-independent Reactions
What happens to CO2? So far, the process has not involved the CO2 seen in the equations.
In 1945 the isotope of carbon, 14C, was used to answer this question. By separating chloroplasts from other cell organelles, then separating the grana and stroma and testing the stroma and grana with different chemicals including 14CO2, scientists found the following:
| Materials tested | 14C in compounds in counts per minute |
| stroma | 4 000 |
| stroma + grana | 96 000 |
| stroma + ATP | 43 000 |
| stroma + ATP + NADPH+H+ | 97 000 |
Note that the ATP and NADPH+H+ used together give the same result as the grana.
In other experiments (1946-53), cells of an alga were exposed to 14CO2 for different periods of illumination (from seconds to minutes) and the compounds that contained 14C were analysed. After 60 s of illumination, 14C was found in sugars, amino acids and other compounds. In order to determine the first compound in which 14C was incorporated, the time of illumination was reduced to a few seconds. At 4 s, a 3-carbon compound
phosphoglygerate (PGA) was detected with radioactivity.They then looked for the 2-carbon compound combined with the CO2 to produce this 3-carbon compound. None was found. Instead, it was found that a 5-carbon sugar with two phosphate groups,
ribulose bisphosphate (RuBP), was the acceptor for the CO2. For this discovery and the remainder of the reactions, Melvin Calvin was awarded a Nobel Prize in 1961.What happens is that RuBP (a 5-carbon compound) accepts CO2 (a 1-carbon compound) and the 6-carbon product immediately splits in to 2 molecules of PGA (a 3-carbon compound). The PGA goes through a series of enzymatic reactions, which end in the formation of 6-carbon sugars, amino acids and other compounds AND RuBP is regenerated. This series of reactions in the light-independent phase of photosynthesis is also called the
Calvin cycle.The Calvin cycle can be divided into four stages:
carboxylation, reduction, regeneration and product synthesis.Carboxylation
CO2 is enzymatically added to RuBP to form 2 molecules of PGA. This is also called CO2 fixation and is catalysed by the enzyme
ribulose bisphosphate carboxylase oxygenase (rubisco).CO2 + RuBP
º 2 PGAReduction
PGA is then reduced by NADPH + H+ to form triose phosphate (TP, also called glyceraldehyde phosphate). Energy is required for this reaction and it is provided by ATP. GP is a sugar, a high-energy compound.
PGA + ATP + (NADPH+H+)
º TP + ADP + NADP+Regeneration
In order for photosynthesis - the dark phase - to continue, RuBP has to be replenished to react with the CO2 that is constantly diffusing into the chloroplasts. Further investigation of the compounds containing 14C during photosynthesis showed that RuBP is formed from GP through a series of complex enzymatic reactions. RuBP has to be regenerated for continued CO2 fixation.
The general equation can be represented as:
18 ATP
6 CO2 + 6 H2O ???? 2 PGA + 6 RuBP
12 NADPH+H+
Product synthesis
Some of the TP is used to form 6-carbon sugars by condensation i.e. 2 molecules of TP combine to form one molecule of 6-carbon (hexose) sugar. The hexoses are converted into sucrose like starch. Some PGA molecules are also used to form amino acids and lipids. Starch is therefore not an immediate product of photosynthesis but a storage product.
PGA is also important in the synthesis of amino acids and lipids. PGA is used to form fatty acids while glycerol is made form TP.
For building up carbohydrates:
TP
º hexose sugars º starch, celluloseFor lipids:
PGA
º Acetyl CoA º fatty acidsTP
º glycerolFor proteins:
PGA
º Acetyl CoA º amino acids º proteinsLimiting factors
When a reaction is proceeding, the limiting factor is that factor which is required for the reaction and is in shortest supply
. It is that factor which most inhibits or limits the rate of the reaction.Several factors influence the rate of photosynthesis: the intensity and frequency of light, temperature, carbon dioxide concentration and water. The effect of each of these factors gan be gauged by having all of the other factors in excess, having one factor in limited supply and measuring the rate of photosynthesis as the supply of the limiting factor is varied.
The graphs can be summarised below:
Figure: Effect of light intensity, temperature and carbon dioxide concentration on the rate of photosynthesis.
For all curves, as light intensity increases, the rate of photosynthesis increases. So, in the initial stages when light intensity is low, this is the limiting factor. When light intensity is no longer limiting, other factors can be limiting.
The effect of temperature is seen by comparing graphs A and B, or graphs C and D, where as temperature increases, the rate of photosynthesis increases. This is because of the effect of temperature on the enzyme-catalysed reactions in the light-independent phase.
To observe the effect of CO2, compare graphs B and D, or graphs A and C.
Under normal conditions, CO2 is the limiting factor in photosynthesis.
On the Web:
The Calvin cycle
http://www.blc.arizona.edu/courses/181gh/rick/photosynthesis/Calvin.html
C4 Plants:
http://www.blc.arizona.edu/courses/181gh/rick/photosynthesis/C4.html
Last modified: August 27, 2004