Water is the fundamental component of all known biological systems. This simple molecule, H₂O, acts as the omnipresent matrix where the processes defining life occur, from the smallest bacterium to the largest whale. Its unique chemical and physical properties make it the only substance capable of supporting the complex, highly ordered reactions required for metabolism, growth, and reproduction. Water is the universal solvent and medium essential for maintaining the structure and function of all organisms.
Water Content Across Life Forms
The proportion of water in an organism’s mass varies widely depending on the species, age, and type of tissue. The human body typically consists of 55 to 70 percent water, with adult males generally having a higher percentage than adult females due to differences in lean tissue mass. Within a single organism, water content is not uniform; the lungs are 83 percent water, the heart and brain are around 73 percent, and denser tissues like bone may only contain about 31 percent.
Organisms such as jellyfish can be composed of over 95 percent water by mass, existing almost entirely as a water-based gel. Plants also exhibit high water content, generally ranging between 80 and 90 percent, which is necessary for maintaining rigidity and facilitating nutrient transport. This volume underscores water’s role as the dominant substance in all active biological matter.
The Role of Water as the Medium for Life’s Chemistry
Water’s molecular structure, specifically its polarity, establishes it as the universal solvent for biological processes. Each water molecule has a slight negative charge near the oxygen atom and slight positive charges near the two hydrogen atoms. This polarity enables water to form hydrogen bonds and interact with a vast array of charged or polar molecules, allowing it to effectively dissolve nutrients, gases, and waste products. This makes water the essential transport fluid within and between cells.
This solvent capability creates the aqueous environment necessary for all metabolic reactions. Water is an active participant in life’s chemical processes. For example, in hydrolysis reactions, a water molecule is inserted to break down complex macromolecules like proteins and carbohydrates into smaller subunits. Conversely, in dehydration synthesis reactions, two smaller molecules join to form a larger one, resulting in the removal of a water molecule. These opposing, water-mediated reactions allow cells to constantly build and break down biological structures for energy and function.
How Water Maintains Physical Stability
Beyond its chemical roles, water’s physical properties maintain the structural and thermal stability of living systems. Water has a high specific heat capacity, meaning it requires a large amount of energy to change its temperature. This property acts as a thermal buffer, allowing organisms, particularly warm-blooded animals, to absorb or release significant heat generated by metabolism without experiencing drastic internal temperature fluctuations.
Water also exhibits powerful cohesive and adhesive forces due to hydrogen bonding, allowing it to stick to itself and to other surfaces. These forces are fundamental to plant physiology, enabling the movement of water against gravity from the roots to the highest leaves through narrow tubes via the cohesion-tension theory. Furthermore, the pressure exerted by water inside plant cells, called turgor pressure, provides the rigidity necessary to support non-woody structures and prevent wilting.
Life in Low-Water States
Some organisms appear to contradict water’s necessity by entering anhydrobiosis, or life without water. Organisms like tardigrades, certain nematode worms, and the spores of bacteria and fungi can survive extreme desiccation, losing almost all measurable water content. During this dormant phase, metabolic activity comes to a reversible standstill, and the organism exists in a state of suspended animation.
These survival states are not completely devoid of water, but contain a small fraction known as “bound water.” This bound water is tightly associated with cellular macromolecules, such as proteins and membranes, helping to stabilize their structure against damage from drying. The ability of the organism to resume life upon rehydration relies entirely on this maintenance of structural integrity.