Question: How do plants use CO2?
About the article "Yeast", section "Aquarium hobby".

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Increases in atmospheric CO2 concentration for affect how plants photosynthesise, resulting in increases in plant water use efficiency, enhanced photosynthetic capacity and increased growth.
Increased CO2 has been implicated in ‘vegetation thickening’ which affects plant community structure and function.
Depending on environment, there are differential responses to elevated atmospheric CO2 between major ‘functional types’ of plant, such as C3 plant and C4 carbon fixation plants, or more or less woody species; which has the potential among other things to alter competition between these groups.
Increased CO2 can also lead to increased Carbon : Nitrogen ratios in the leaves of plants or in other aspects of leaf chemistry, possibly changing herbivore nutrition. 
CAM plants that use Crassulacean acid metabolism as an adaptation for arid conditions.
CO2 enters through the stomata during the night and is converted into the 4-carbon compound, malic acid, which releases CO2 for use in the Calvin cycle during the day, when the stomata are closed.
The jade plant ("Crassula ovata") and cacti are typical of CAM plants.
Sixteen thousand species of plants use CAM.
These plants have a carbon isotope signature of -20 to -10 ‰. 
CO2 rockets are a very safe and simple way to experiment with model rockets.
Most CO2 rockets are very small and do not travel very high or fast.
The common method of construction for a CO2 rocket is to take a CO2 canister and use it as the motor to propel the projectile, or load into the air. 
In the face of ecological contingencies such as increasing temperatures, changes in rainfall patterns, long term climate change, and biotic influences of human management interventions, it is expected to reduce the production and quality of food and have a negative impact on agricultural production.
The CO2 fertiliser effect has been greatly overestimated during Free-Air Carbon dioxide Enrichment (FACE) experiments where results show increased CO2 levels in the atmosphere enhances photosynthesis, reduce transpiration, and increase water use efficiency (WUE).
Increased biomass is one of the effects with simulations from experiments predicting a 5–20% increase in crop yields at 550 ppm of CO2.
Rates of leaf photosynthesis were shown to increase by 30-50% in C3 plants, and 10-25% in C4 under doubled CO2 levels.
The existence of a feedback mechanism results a phenotypic plasticity in response to [CO2]atm that may have been an adaptive trait in the evolution of plant respiration and function.
Predicting how stomata performs during adaptation is useful for understanding the productivity of plant systems for both natural and agricultural systems.
Plant breeders and farmers are beginning to work together using evolutionary and participatory plant breeding to find the best suited species such as heat and drought resistant crop varieties that could naturally evolve to the change in the face of food security challenges. 
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.