Table of Contents
Water is so familiar in everyday life that it is easy to overlook how unusual and essential it is for living organisms. In this chapter, we focus on why water is the central medium of life: what makes it chemically special, how these properties arise from its structure, and why nearly all biological processes depend on it.
(Details of the specific properties and the autoprotolysis of water are discussed in the following subsections; here we focus on the overarching biological role of water as a medium.)
Why Life Needs a Medium
All living cells are sites of countless chemical reactions: nutrients must dissolve, molecules must move and collide, and products must be transported away. For this to happen efficiently, organisms need a medium in which:
- Substances can dissolve
- Molecules can move and react
- Heat can be distributed or buffered
- Structures (like membranes and proteins) can form and remain stable
On Earth, that medium is almost always water. Cells are mostly water by mass (often 60–90%), and even “dry” tissues usually contain significant water content.
Water as a Universal Biological Solvent
Water is often called the “universal solvent” of biology. This does not mean it dissolves everything, but that it dissolves more substances, and more effectively, than almost any other common liquid.
In living systems, this has several consequences:
- Transport of nutrients and wastes
Ions (like Na⁺, K⁺, Ca²⁺, Cl⁻) and many small organic molecules dissolve in water and can be transported within cells and through body fluids (blood, lymph, sap). - Environment for biochemical reactions
Most metabolic reactions occur in aqueous solution. Reactants must be dissolved to diffuse and encounter each other; enzymes and substrates interact in water. - Ionization and charge
Many biological molecules gain or lose protons (H⁺) in water, becoming charged. Their charge state influences: - Solubility
- Shape (conformation)
- Interactions with other molecules
This underlies acid–base balance and pH regulation in organisms.
Water as a Structural Environment
Water not only allows molecules to move; it also helps shape and stabilize biological structures.
- Protein folding
In water, nonpolar (water-repelling) parts of proteins tend to cluster away from the surrounding water, while polar parts remain exposed. This helps proteins fold into specific shapes that are essential for their function. - Membrane formation
Biological membranes are made of amphiphilic molecules (like phospholipids) with a water-attracting “head” and water-repelling “tail.” In water, they spontaneously arrange into bilayers and vesicles, forming the basic barrier of cells and organelles. - Macromolecular assemblies
Large complexes (such as ribosomes, cytoskeletal fibers, and chromatin) depend on a watery environment to assemble and maintain their structure. Water participates in many weak interactions (like hydrogen bonds) that hold these complexes together.
Water as a Medium for Temperature Regulation
Organisms constantly produce or absorb heat during metabolism and environmental change. Water plays a key role in stabilizing body and environmental temperatures because:
- A large proportion of an organism’s mass is water.
- Water absorbs or releases large amounts of heat with relatively small temperature changes.
Biological implications include:
- Thermal buffering inside organisms
Because of water’s high heat capacity, temperature changes within organisms are moderated, protecting sensitive biochemical processes. - Heat transport
In animals, water-based fluids (blood, lymph) transport heat from warmer to cooler regions, helping maintain internal temperature. - Evaporative cooling
The evaporation of water from surfaces (sweating in humans, transpiration in plants) removes heat, cooling the organism.
Water as a Participant in Chemical Reactions
Water is not just a background medium; it directly participates in many key reactions:
- Hydrolysis reactions
Large biological molecules (proteins, polysaccharides, nucleic acids) are broken down by adding water across specific bonds. This is central to digestion and turnover of macromolecules. - Condensation (dehydration) reactions
When cells build macromolecules, they often remove water in the process of forming new bonds. This is the reverse of hydrolysis and is crucial for biosynthesis. - Redox and energy conversions
In processes like photosynthesis and cellular respiration, water is either split or formed: - In photosynthesis, water is split to provide electrons and protons.
- In aerobic respiration, oxygen is reduced to water at the end of the electron transport chain.
These reactions are key to biological energy conversion.
Water and Cell Volume, Shape, and Pressure
Water movement into and out of cells influences their volume and mechanical properties:
- Osmosis
Water can move across semipermeable membranes depending on solute concentrations inside and outside the cell. This affects: - Cell volume (swelling or shrinking)
- Tension on the cell membrane
- Turgor in plants
In plant cells, water uptake into the central vacuole creates turgor pressure, which: - Keeps cells rigid
- Supports non-woody plant parts
Without sufficient water, turgor is lost and plants wilt. - Hydrostatic pressure in animals
In some animals (e.g., worms, jellyfish), internal water-filled cavities contribute to body shape and movement by acting as a hydrostatic skeleton.
Water as an Environmental Matrix for Life
Many organisms live directly in water (oceans, lakes, rivers), and even land-dwelling organisms evolved from aquatic ancestors. Water as an external medium provides:
- Support and buoyancy
Aquatic organisms are supported by the surrounding water, allowing very large body sizes and different body shapes than on land. - Medium for dispersal and reproduction
Many species release gametes, spores, or larvae into the water, which serve as a transport route and habitat. - Chemical communication
Signals like pheromones and other substances can disperse in water, enabling communication and orientation (e.g., following chemical gradients).
Water and the Origin of Life
While the origin of life is considered in detail elsewhere, water is central in most hypotheses:
- Early life is thought to have arisen in aquatic environments (oceans, lakes, or hydrothermal systems).
- Water provided:
- A medium where building blocks could concentrate and react
- Conditions for forming membranes and protocells
- Chemical flexibility for early metabolism
Thus, water is not only crucial for existing life, but likely played a key role in life’s emergence.
The Special Status of Water in Biology
Other liquids exist, but on Earth water is uniquely suited as life’s medium because:
- It is abundant and stable in liquid form across a wide temperature range compatible with complex chemistry.
- Its chemical and physical properties create conditions favorable for:
- Dissolving and transporting substances
- Stabilizing and shaping biological structures
- Moderating temperature
- Participating in energy and synthesis reactions
- Maintaining cell volume and pressure
For these reasons, when scientists search for life beyond Earth, they often first ask: Is there liquid water? Its presence is considered a key indicator of potential habitability, underlining the central idea of this chapter: water is not just helpful for life—it defines the conditions under which known life can exist.