Respiration in plants is the fundamental metabolic process by which they convert stored chemical energy into a form immediately usable for all life functions. This process involves the breakdown of sugars, the plant’s food source, to release energy for essential activities like growth, nutrient uptake, and cell maintenance. Although plants produce their own food through photosynthesis, they must continuously break that food down to power their cells.
The Cellular Process of Energy Release
The mechanism for extracting usable energy from stored sugars is aerobic cellular respiration, a chemical process requiring oxygen. The reaction uses glucose and oxygen as inputs, yielding carbon dioxide, water, and chemical energy.
The process is summarized by the chemical equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy. This energy is captured in Adenosine Triphosphate (ATP), the cell’s “molecular unit of currency” that powers nearly all cellular work.
Cellular respiration begins in the cytoplasm with glycolysis, where the six-carbon glucose molecule is broken down into smaller, three-carbon molecules. This initial stage generates a small amount of ATP and does not require oxygen.
The remaining steps are oxygen-dependent and occur within the mitochondria, the cell’s powerhouses. The products from the initial breakdown enter the mitochondria where they are disassembled, extracting a much greater yield of energy.
This energy synthesizes large quantities of ATP from its precursor, Adenosine Diphosphate (ADP). When a plant cell needs to perform work, such as transporting nutrients or synthesizing new proteins, the terminal phosphate bond of ATP is broken, releasing the stored energy.
How Plants Exchange Gases
For respiration to occur, plants must constantly take in oxygen and release the carbon dioxide byproduct, a function known as gas exchange. This exchange primarily happens through specialized, microscopic pores called stomata, found on the surfaces of leaves and young stems. Each stoma is flanked by a pair of guard cells that regulate the pore’s opening and closing.
Stomata typically open during the day to allow carbon dioxide for photosynthesis, which also allows oxygen to enter for respiration and water vapor to escape. The opening mechanism involves water moving into the guard cells, causing them to swell and become turgid. Their unique structure causes them to bow outward when turgid, effectively opening the pore.
When a plant is under water stress, the guard cells lose turgor, causing the stomata to close and conserve moisture. In woody stems and older roots, gas exchange occurs through structures called lenticels.
Lenticels are small, raised areas of loosely packed cells that create pores in the bark. These pores allow oxygen to diffuse into the inner tissues and carbon dioxide to exit. The combined action of stomata and lenticels manages the flow of gases, ensuring all living cells receive the necessary oxygen.
Respiration and Photosynthesis: A Necessary Partnership
A common misconception is that plants only breathe at night, but respiration is a continuous, round-the-clock requirement for all living plant cells. The need for usable energy (ATP) never stops, so cellular respiration occurs day and night. Photosynthesis, however, is light-dependent and only occurs during daylight hours.
These two processes are inextricably linked, forming a cyclical partnership. The products of photosynthesis—glucose and oxygen—serve as the necessary inputs for cellular respiration. In return, the carbon dioxide and water produced by respiration are the molecules needed as reactants for photosynthesis.
This internal cycling is efficient, but the plant must produce a net gain of sugars to grow. The compensation point describes the environmental condition, typically low light intensity, at which the rate of oxygen consumed by respiration is precisely balanced by the oxygen released through photosynthesis.
At the compensation point, the plant is neither gaining nor losing carbon, meaning its net carbon dioxide assimilation is zero. This point is often reached twice daily, at dawn and dusk, when light intensity is low. For the plant to achieve growth, the rate of photosynthesis must exceed the rate of respiration throughout the daylight hours.