Biology Lecture No. 9: Photosynthesis II
Monday February 6 , 2012
-The global amount of carbon fixation is equivalent to 105 Gigatons per year. Most of this value comes
from terrestrial photoautotrophs, but a great quantity derives from the oceans.
-Phytoplankton are the major photosynthetic organisms in the ocean. Such oceanic organisms that fix
carbon are typically found at the polar ice caps. They are known as ectotherms as their growth process is
-Although nutrients are a limiting factor for development at the poles, there is an abundance of iron, in
which the equatorial regions are lacking. Abundance of iron is what drives the greatest amount of
-Southern Ocean Iron Release experiment was an attempt to reduce the amount of CO in the 2
atmosphere by depositing iron in equatorial oceans to encourage phytoplankton activity. High levels of
chlorophyll were recorded where the iron was dropped.
The Calvin Cycle:
-Carbon fixation (the Calvin cycle) is an endergonic process that occurs in the stroma of the chloroplast.
RuBP (Ribulose biphosphate) is the major substrate involved in the process.
-Basically, 3RuBP react with 3CO t2 produce 6PGA (3-phosphoglycerate; 3 carbon molecule), which is
further converted to 1, 3-biophosphoglycerate (3 carbon molecule) and then into glyceraldehyde -3 –
phosphate (G3P; 3 carbon molecule), where some of this compound is used to regenerate RuBP and
rerun the cycle.
-The reduction of RuBP to G3P is driven by the energy in ATP and the reducing power of NADPH from
the light reaction.
-Every three rotations of the Calvin cycle yields one extra molecule of G3P, which is the reduced sugar
produced directly by the Calvin cycle. If the substrate is not regenerated each time, the cycle cannot
-The three major reactions of the Calvin cycle are: Carboxylation (addition of CO to2RuBP), Reduction
(From RuBP to G3P) and Regeneration (From G3P to RuBP). For every two molecules of G3P formed, one
molecule of glucose is produced.
Gas Exchange In Elysia:
-Every eukaryote has either mitochondria or chloroplast. When both of these organelles are present in
an organism, gas exchange is more complex. Respiration & Photosynthesis:
-Where photosynthesis makes ATP in order to only be used in the chloroplast and drive photosynthesis,
cellular respiration creates ATP in order to fuel the cell via the breakdown of glucose.
-Where photosynthesis can be measured by how fast CO is consumed (via the Calvin cycle) or how fast
oxygen is released (via the light reaction), cellular respiration can be measured by how fast organisms
are consuming oxygen or how fast they are producing CO . 2
Measuring Carbon Fixation:
-A greater rate of photosynthesis yields a decrease in CO as t2e carbon fixation rate (the amount of CO 2
used for the Calvin Cycle) increases. The carbon fixation rate is measured in mol C/Chlorophyll/sec.
Light Saturation Curve:
-CO 2ixation rate can be expressed as a function of light intensity in the photosystem. In the dark, gas
exchange is exclusively mitochondrial as plants undergo cellular respiration during the night. In this
instance, CO f2xation rate is negative as CO is 2ot being fixed, but rather released.
-The light saturation curve gives a net carbon fixation rate of zero as the rate of photosynthesis is
equivalent to the rate of cellular respiration.
-Plant growth is indicated by a greater carbon fixation rate than the light saturation curve. Even though
a plant can be under light, if that light is below the light saturation point, then the plant is mostly cellular
respiring and therefore losing energy.
Competition Between O & CO 2 2
-RuBisCO (Ribulose-biphosphate carboxylase) is a carboxylating enzyme that evolved in organisms 3
billion years ago and is the major carboxylating enzyme on the planet.