The question of whether plants create their own water involves a subtle nuance of internal chemistry. Plants produce a small amount of water internally as a byproduct of their metabolic processes. However, this chemically generated supply is insignificant compared to the massive volumes of water a plant must acquire from its surroundings. The plant’s water economy balances this tiny internal creation with substantial external acquisition and considerable, unavoidable loss.
The Chemical Process That Creates Water
Plants continuously generate “metabolic water” within their cells through aerobic cellular respiration. This reaction occurs in the mitochondria as the plant converts stored sugars, primarily glucose, into usable energy. The purpose of this process is to produce adenosine triphosphate (ATP), the energy currency of the cell.
The final stage of cellular respiration involves the electron transport chain, where oxygen acts as the ultimate electron acceptor. This chemical action results in the formation of water. The reaction shows that the oxidation of one glucose molecule (\(\text{C}_6\text{H}_{12}\text{O}_6\)) combines with six molecules of oxygen (\(\text{O}_2\)) to produce six molecules of water (\(\text{H}_2\text{O}\)) and six molecules of carbon dioxide (\(\text{CO}_2\)). This water is immediately available for the plant’s internal cellular needs.
How Plants Acquire the Bulk of Their Water
The vast majority of a plant’s hydration comes directly from the soil. Water absorption begins at the roots, particularly through fine root hairs that vastly increase the surface area for contact with soil particles. This external acquisition is driven by osmosis.
Water moves from the soil into the root cells via osmosis. Once absorbed, this water and its dissolved mineral nutrients enter the plant’s vascular system. The water is channeled into the xylem, a specialized tissue that forms a continuous pathway from the roots to the leaves.
The upward movement of water through the xylem, often against gravity, is explained by the cohesion-tension theory. Water molecules are strongly cohesive, allowing them to form an unbroken column within the narrow xylem vessels. This column is pulled upward by the negative pressure, or tension, created by the evaporation of water from the leaves.
The Unavoidable Loss of Water Through Transpiration
The continuous stream of water drawn up from the roots is driven by transpiration, the evaporation of water from the plant’s aerial parts, mainly the leaves. Plants must exchange gases with the atmosphere to sustain photosynthesis, requiring the intake of carbon dioxide (\(\text{CO}_2\)). This gas exchange occurs through tiny pores on the leaf surface called stomata.
When the stomata open to allow \(\text{CO}_2\) to enter, water vapor simultaneously escapes into the drier outside air. This loss is an unavoidable trade-off, as the plant must open its stomata to grow. Stomatal transpiration accounts for the majority, typically 80 to 90%, of all water lost by the plant.
The magnitude of this water loss is considerable; a plant can lose up to 99% of the water absorbed by its roots. This high rate of loss creates a constant deficit within the plant. This deficit generates the powerful suction force that pulls the water column up the xylem from the roots. The plant must constantly replenish this lost water to prevent dehydration.
Why Metabolic Water is Not Enough
The internal water produced during cellular respiration is a simple metabolic byproduct, not a primary source of hydration. The volume is negligible when compared to the plant’s daily needs. Only a tiny fraction, less than 1%, of the water absorbed is used in metabolic activities or produced by respiration.
The plant’s survival requires replacing the vast quantities of water lost through transpiration. The water absorbed from the soil is magnitudes greater than the water generated internally. External water acquisition is necessary to maintain the turgor pressure that keeps the plant structurally upright. It also sustains the continuous flow that delivers essential mineral nutrients from the roots to the shoots.