Biological water refers to the water found within and surrounding living organisms, serving as the medium for life. The human body is composed of approximately 60 to 70 percent water by weight, a proportion that varies across different tissues and species. Water’s specific physical and chemical characteristics allow it to perform complex tasks necessary for survival. The unique molecular structure of water drives virtually every biological process, from protein folding to the regulation of body temperature.
The Unique Chemistry of Water
The importance of water in biology stems from its distinct molecular structure. A water molecule (\(\text{H}_2\text{O}\)) has a bent shape, where the single oxygen atom is bonded to two hydrogen atoms. Oxygen is significantly more electronegative than hydrogen, pulling the shared electrons closer to itself and creating an unequal distribution of charge. This polarity results in the oxygen side having a slight negative charge and the hydrogen sides having a slight positive charge, making water a dipole molecule.
This polarity allows individual water molecules to form weak attractions, known as hydrogen bonds, between the positive hydrogen of one molecule and the negative oxygen of another. The constant forming and breaking of these bonds give liquid water its properties, including high specific heat. Water can absorb or release large amounts of heat with only a slight change in temperature, which helps organisms stabilize their internal temperature despite environmental fluctuations.
Hydrogen bonding creates strong cohesive forces, causing water molecules to stick together, and adhesive forces, allowing them to cling to other polar surfaces. These forces enable capillary action, which moves water upward through the narrow vessels of plants. Water’s polarity makes it an excellent solvent, often called the “universal solvent,” because it can dissolve many polar or ionic substances, termed hydrophilic. Nonpolar substances, like fats and oils, are repelled by water (hydrophobic exclusion), a process essential for forming cellular membranes.
Classifying Water in Living Systems
The water within a living system is not uniform, but is broadly categorized based on its association with cellular components. “Free water,” or bulk water, represents the majority of an organism’s water content. This water is readily available to move and flow, acting as the primary medium for diffusion and the transport of nutrients and waste products within the cytoplasm and circulatory systems. Its properties are similar to those of pure water, allowing it to easily participate in metabolic reactions.
In contrast, “bound water” consists of molecules tightly associated with the surfaces of macromolecules, such as proteins, nucleic acids, and phospholipids. These water molecules are held in place by strong forces, including hydrogen bonds and electrostatic interactions with charged groups on the biomolecules. Bound water plays a structural role, forming a hydration shell necessary for maintaining the correct three-dimensional shape and function of these macromolecules. This water is so tightly held that it exhibits unique characteristics, such as a lowered freezing point, and cannot be easily extracted from the tissue.
Water’s Essential Roles in Cellular Function
Water performs several active functions inside the cell. As a solvent, it facilitates virtually all biochemical reactions by allowing reactants to collide and interact in an aqueous solution. The cytoplasm and nucleoplasm are complex water-based solutions, making the movement and concentration of molecules possible for metabolism.
Water is also a direct reactant or product in many metabolic pathways. In hydrolysis reactions, a water molecule is incorporated to break down large polymers, such as starches or proteins, into smaller subunits like sugars and amino acids. Conversely, in dehydration synthesis, two smaller molecules are joined together, releasing a water molecule as a product. This constant addition and removal of water is the basis for constructing and deconstructing the major molecules of life.
Water provides mechanical and structural support to cells and tissues. In plant cells, the osmotic flow of water into the central vacuole generates turgor pressure, a hydrostatic force that pushes the cell membrane against the rigid cell wall. This pressure makes plant tissue firm and upright; its loss causes wilting. In all organisms, water acts as a transport vehicle, moving substances over long distances, such as blood carrying oxygen and hormones, and within the cell, enabling the movement of organelles and molecules.
Maintaining Water Balance in Organisms
Organisms must actively manage their biological water content to ensure stable internal conditions, a process known as osmoregulation. Osmoregulation involves the precise control of the osmotic pressure of body fluids, maintaining the balance of water and dissolved solutes, or electrolytes. This balance is achieved by regulating water intake, retention, and excretion.
In humans and other mammals, the kidneys are the primary organs responsible for this regulation, filtering the blood and adjusting the amount of water reabsorbed based on the body’s needs. Hormones, such as antidiuretic hormone (ADH), control the permeability of kidney tubules to water, dictating how concentrated or dilute the urine will be. Specialized sensors in the brain, like osmoreceptors in the hypothalamus, detect changes in the concentration of solutes in the blood and trigger responses such as thirst or hormone release. Osmoregulators use these complex systems to survive in diverse environments, from freshwater to terrestrial habitats.