Carbon Dioxide and Photosynthesis
Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic compounds (cellulose, lipids, and various proteins) by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to make 3-phosphoglycerate that is used in metabolism, to construct sugars that can be used as an energy source within the plant through respiration and as the raw material for the construction of more complex organic molecules, such as polysaccharides, nucleic acids and proteins during growth.
Even when greenhouses are vented, carbon dioxide must be introduced into them to maintain plant growth, as the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm (a limit of C3 carbon fixation photosynthesis). Plants can grow up to 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions.
Plants also emit CO2 during respiration, and so the majority of plants and algae, which use C3 photosynthesis, are only net absorbers during the day. A growing forest will absorb many tons of CO2 each year, but a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g. fallen branches) as is used in biosynthesis in growing plants. Nevertheless, mature forests are valuable carbon sinks, helping maintain balance in the Earth’s atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.