Photosynthesis is the conversion of electromagnetic radiation from the sun into biochemical energy
that supports nearly all life on Earth.
The physiology of photosynthesis determines where your food is grown today, and how locations
may shift with climate change.
The rest of this video segment presents a brief introduction to the secret life of plants.
All photosynthetic organisms follow the same biochemical pathway.
This is the Calvin Benson cycle or the C3 carbon fixation pathway.
In higher plants this pathway occurs in a special compartment called the chloroplast.
Rubisco: [RUBISCO sound] Ribulose-1,5-bisphosphate carboxylase oxygenase, the enzyme that comprises
up to 50% of all protein in leaves and thereby is the most prevalent protein on the planet,
facilitates the first part of this process.
>> Rubisco.
>> Rubisco exhibits conflicting tendencies and that it facilitates a combination of a
five carbon sugar RuBP, ribulose1,5-bisphosphate with either carbon dioxide or oxygen.
Number one: the reaction with carbon dioxide produces a six carbon compound that immediately
divides into two molecules of three carbon compounds.
Hence the name C3 Carbon fixation.
Six molecules of this compound pass through an elaborate chemical cycle that forms one
molecule fructose, 6 phosphate, a six carbon sugar, and regenerates six molecules of RUBP.
The reaction of the five carbon RUBP with oxygen splits it into a three carbon and a
two carbon compound and subsequently releases carbon dioxide, hence the name C2 pathway
or more commonly photo respiration.
In total photorespiration does not result in any net production of sugar thus photorespiration
has been viewed as a wasteful process, a vestige of the high co2 atmospheres under which plants
evolved.
The balance between C3 carbon fixation and photo respiration, depends on the relative
amounts of carbon dioxide and oxygen entering the portion of Rubisco [sound] involved in
the primary chemical reactions.
And the degree to which Rubisco [sound] attracts carbon dioxide, or oxygen.
Atmospheric concentrations of carbon dioxide and oxygen are currently 0.038% and 20.946%,
yeilding a carbon dioxide to oxygen ratio of 0.0018.
Gaseous carbon dioxide, however, is more soluble in water than oxygen, and so the carbon dioxide
to oxygen ratio near the chloroplast, the part of the cell where these reactions occur,
is about 0.026 at 25 degrees celcius.
Rubisco [sound] has a 50 to 100 fold greater affinity for carbon dioxide than oxygen.
All together, because of the relative concentrations and affinities, Rubisco [sound], catalyzes
about two or three cycles of C3 carbon fixation for every cycle of photorespiration.
Temperature influences the balance between C3 carbon fixation and photorespiration in
two ways.
The solubility of carbon dioxide in water decreases more with temperature than the solubility
of oxygen, resulting in a lower carbon dioxide to oxygen ratio.
In addition, the enzymatic properties of Rubisco [sound] shift with temperature stimulating
the reaction with oxygen to a greater degree than the one with carbon dioxide.
Higher temperatures therefore favor photorespiration over C3 carbon fixation and photosynthetic
conversion of light into sugars becomes less efficient.
With rising atmospheric carbon dioxide concentrations and falling oxygen concentrations the carbon
dioxide to oxygen ratio at the active side of Rubiso [sound] increases and thereby favors
RuBP carboxylation C3 carbon fixation over RuBP oxidation photorespiration.
Under such conditions, plants have relied solely on C3 carbon fixation.
C3 plants enjoy higher efficiencies in converting absorbed light into sugars.
Conversely when atmospheric carbon dioxide concentrations diminished to one half of current
levels during past ice ages RuBP carboxylation decreased and oxygenation increased.
These conditions spawned a CO2 pumping mechanization know as C4 carbon fixation.