Through photosynthesis, plants transform sunlight into potential energy in the form of the chemical bonds of carbohydrate molecules. However, to use that stored energy to power their essential life processes – from growth and reproduction to healing damaged structures – plants must convert it into a usable form. That conversion takes place via cellular respiration, a major biochemical pathway also found in animals and other organisms.
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Respiration constitutes a series of enzyme-driven reactions that allow plants to turn the stored energy of carbohydrates made via photosynthesis into a form of energy they can use to power growth and metabolic processes.
Respiration allows plants and other living things to release the energy stored in the chemical bonds of carbohydrates such as sugars made from carbon dioxide and water during photosynthesis. While a variety of carbohydrates, as well as proteins and lipids, may be broken down in respiration, glucose typically serves as the model molecule for demonstrating the process, which can be expressed as the following chemical formula:
C6H12O6 (glucose) + 6O2 (oxygen) --> 6CO2 (carbon dioxide) + 6H2O (water) + 32 ATP (energy)
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Through a series of enzyme-facilitated reactions, respiration breaks the molecular bonds of carbohydrates to create usable energy in the form of the molecule adenosine triphosphate (ATP) as well as the byproducts of carbon dioxide and water. Heat energy is also released in the process.
Pathways of Plant Respiration
Glycolysis serves as the first step in respiration and doesn’t require oxygen. It takes place in the cell's cytoplasm and produces a small amount of ATP and pyruvic acid. This pyruvate then enters the inner membrane of the cell’s mitochondrion for the second phase of aerobic respiration – the Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) pathway, which encompasses a series of chemical reactions that release electrons and carbon dioxide. Finally, the electrons freed during the Krebs cycle enter the electron-transport chain, which releases energy used in a culminating oxidative-phosphorylation reaction to create ATP.
Respiration and Photosynthesis
In a general sense, respiration can be thought of as the reverse of photosynthesis: The inputs of photosynthesis – carbon dioxide, water and energy – are the outputs of respiration, although the chemical processes in between are not mirror images of one another. While photosynthesis only occurs in the presence of light and in chloroplast-containing leaves, respiration takes place both day and night in all living cells.
Respiration and Plant Productivity
The relative rates of photosynthesis, which produces food molecules, and respiration, which burns those food molecules for energy, influence overall plant productivity. Where photosynthesis activity exceeds respiration, plant growth proceeds at a high level. Where respiration exceeds photosynthesis, growth slows. Both photosynthesis and respiration increase with increasing temperature, but at a certain point, the rate of photosynthesis levels off while the respiration rate continues to escalate. This can lead to a depletion of stored energy. Net primary productivity – the amount of biomass created by green plants that is usable to the rest of the food chain – represents the balance of photosynthesis and respiration, calculated by subtracting the energy lost to power plant respiration from the total chemical energy produced by photosynthesis, aka the gross primary productivity.