Question: When is CO2 used by plants?
About the article "Yeast", section "Aquarium hobby".

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Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to nourish plants in planted aquaria.
A homemade setup is widely used as a cheap and simple alternative to pressurized CO2 systems.
While not as effective as these, the homemade setup is considerably cheaper for less-demanding hobbyists.
Several recipes for homemade CO2 are available, but they are variations of the basic recipe: Baker's yeast, with sugar, baking soda, and water, are added to a plastic bottle.
A few drops of vegetable oil at the start reduces surface tension and speeds the release of CO2.
This will produce CO2 for about two or three weeks; the use of a bubble counter determines production.
The CO2 is injected in the aquarium through a narrow hose and released through a diffuser that helps dissolve the gas in the water.
The CO2 is used by plants in the photosynthesis process.
The lime industry is a significant carbon dioxide emitter.
The manufacture of one tonne of calcium oxide involves decomposing calcium carbonate, with the formation of 785 kg of CO2 in some applications, such as when used as mortar; this CO2 is later re-absorbed as the mortar goes off.
Additionally, if the heat supplied to form the lime (3.75 MJ/kg in an efficient kiln) is obtained by burning fossil fuel it will release CO2: in the case of coal fuel 295 kg/t; in the case of natural gas fuel 206 kg/t.
The electric power consumption of an efficient plant is around 20 kWh per tonne of lime.
This additional input is the equivalent of around 20 kg CO2 per ton if the electricity is coal-generated.
Thus, total emission may be around 1 tonne of CO2 for every tonne of lime even in efficient industrial plants, but is typically 1.3 t/t.
However, if the source of heat energy used in its manufacture is a fully renewable source, such as solar, wind, hydro or even nuclear, and depending on the ultimate use of the lime there may be no net emission of CO2.
Less energy is required in production per weight than portland cement, primarily because a lower temperature is required. 
Gaia scientists see the participation of living organisms in the carbon cycle as one of the complex processes that maintain conditions suitable for life.
The only significant natural source of atmospheric carbon dioxide (CO2) is volcanic activity, while the only significant removal is through the precipitation of carbonate rocks.
Carbon precipitation, solution and fixation are influenced by the bacteria and plant roots in soils, where they improve gaseous circulation, or in coral reefs, where calcium carbonate is deposited as a solid on the sea floor.
Calcium carbonate is used by living organisms to manufacture carbonaceous tests and shells.
Once dead, the living organisms' shells fall to the bottom of the oceans where they generate deposits of chalk and limestone.
One of these organisms is Emiliania huxleyi, an abundant coccolithophore algae which also has a role in the formation of clouds.
CO2 excess is compensated by an increase of coccolithophoride life, increasing the amount of CO2 locked in the ocean floor.
Coccolithophorides increase the cloud cover, hence control the surface temperature, help cool the whole planet and favor precipitations necessary for terrestrial plants.
Lately the atmospheric CO2 concentration has increased and there is some evidence that concentrations of ocean algal blooms are also increasing.
Lichen and other organisms accelerate the weathering of rocks in the surface, while the decomposition of rocks also happens faster in the soil, thanks to the activity of roots, fungi, bacteria and subterranean animals.
The flow of carbon dioxide from the atmosphere to the soil is therefore regulated with the help of living beings.
When CO2 levels rise in the atmosphere the temperature increases and plants grow.
This growth brings higher consumption of CO2 by the plants, who process it into the soil, removing it from the atmosphere. 
Integrated fluorometers are being used by leading plant physiologists to probe the photosynthetic process in plants.
The results of this research will enable scientists to improve the efficiency of CO2 utilization by crops and estimate the capacity of grasslands, forests and ecosystems to mitigate climate change.
It is hoped that these devices will enable us to screen genetic mutants to identify plants with an increased capacity to assimilate CO2. 
PPDK is used in the C4 pathway, to improve the efficiency of carbon dioxide fixation.
In environments where there is a lot of light, the rate of photosynthesis in plants is limited by the rate of carbon dioxide (CO2) uptake.
This can be improved by using a series of chemical reactions to transport CO2 from mesophyll cells (which are located on the outside of a leaf) to bundle sheath cells (which are located inside the cells).
PPDK converts pyruvate to PEP, which reacts with CO2 to produce oxaloacetate.
When CO2 is released in the bundle sheath cells, pyruvate is regenerated, and the cycle continues.
Though the reaction catalysed by PPDK is reversible, PEP is favoured as the product in biological conditions.
This is due to the basic pH in the stroma, where the reaction occurs, as well as high concentrations of adenylate kinase and pyrophosphatase.
Because these two enzymes catalyse exergonic reactions involving AMP, and disphosphate, respectively, they drive the PPDK-catalysed reaction forward.
Because PPDK consumes ATP, the C4 pathway is unfavourable for plants in environments with little access to light, as they are unable to produce large quantities of ATP.PPDK is highly abundant in C4 leaves, comprising up to 10% of total protein.
Research has shown that the enzyme is about 96% identical in different species of plants.
Hybridization experiments revealed that the genetic differences correlate with the extent to which the plants perform the C4 pathway – the uncommon sequences exist in plants which also display C3 characteristics.
Interestingly, PPDK is also found in small quantities in C3 plants.
Evolutionary history suggests that it once had a role in glycolysis, and eventually evolved into the C4 pathway.