Water (H₂O)

Description

Latin: aqua – water

Water is a simple but essential molecule composed of two hydrogen atoms and one oxygen atom (H₂O). It is a polar molecule, making it an excellent solvent for many substances, which is why it is often referred to as the "universal solvent." Water plays a critical role in all known forms of life. It regulates body temperature, transports nutrients and waste products, facilitates chemical reactions, lubricates joints, and helps maintain cell structure. Its unique properties, such as cohesion, adhesion, and high heat capacity, are vital for biological systems.

Summary

Water, derived from the Latin aqua, is a simple yet indispensable molecule composed of two hydrogen atoms covalently bonded to one oxygen atom (H₂O). It is a polar molecule due to the uneven distribution of charge, which allows it to form hydrogen bonds and act as a powerful solvent—earning it the title of the "universal solvent." Water makes up about 60% of the human body and is vital for virtually every physiological process. Its unique physical and chemical properties—such as high heat capacity, surface tension, and solvent capabilities—make it essential for maintaining homeostasis, facilitating biochemical reactions, and transporting substances throughout the body. Unlike other biomolecules, water is not a source of energy, but it is a medium in which all cellular processes occur.

Absorption of Water

Water absorption occurs primarily in the small and large intestines. After ingestion, water moves through the gastrointestinal tract and is absorbed via osmosis across the epithelial cells lining the intestines. Most water absorption (about 85–90%) happens in the small intestine, particularly the jejunum, while the remaining amount is absorbed in the colon. This process is driven by osmotic gradients created by active transport of electrolytes, especially sodium. Efficient water absorption is crucial for maintaining blood volume, pressure, and electrolyte balance. Any disruption in this process—due to illness, dehydration, or malabsorption—can lead to serious physiological consequences.

Functions

  • Solvent for biochemical reactions: Water dissolves ions and molecules, enabling enzymatic and metabolic processes to occur.

  • Temperature regulation: Due to its high heat capacity, water buffers the body against rapid temperature changes through sweat and blood flow.

  • Transport medium: Water is the primary component of blood and lymph, carrying nutrients, hormones, waste products, and gases.

  • Lubrication and cushioning: Water is a key component of synovial fluid, mucus, and cerebrospinal fluid, helping to protect and cushion organs and joints.

  • Cellular structure: Water maintains cell turgor and volume, influencing membrane integrity and intracellular environments.

  • Acid-base balance: Water participates in buffering systems and helps regulate the pH of bodily fluids.

Chemical Structure

Molecular formula: H₂O
Molecular mass: 18.015 atomic mass units (amu)
Atomic composition: Hydrogen (H), Oxygen (O)

Bond types:

  • O–H bonds: Single covalent (sigma, σ) bonds

  • Nonbonding pairs (lone pairs): Two lone pairs of electrons on the oxygen atom

  • Hydrogen bonds (intermolecular): Weak electrostatic attractions between the hydrogen of one water molecule and the oxygen of another

Molecular geometry:

  • Bent (angular) shape due to electron pair repulsion

  • Bond angle: ~104.5°

Bond order:

  • O–H bonds: Bond order 1 (single bonds)

Bond length:

  • O–H bond length: ~96 picometers (pm)

  • Hydrogen bond (intermolecular distance): ~180–200 pm (variable depending on temperature and phase)

Electron configuration (oxygen atom):

  • 1s² 2s² 2p⁴

Molecular polarity:

  • Strongly polar due to the electronegativity difference between oxygen and hydrogen and the bent geometry

  • Results in a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms

  • Dipole moment: ~1.85 Debye

Solubility:

  • Highly hydrophilic (miscible with itself and polar substances)

  • Excellent solvent for ionic and polar compounds

Functionality:

  • Solvent for biochemical reactions

  • Facilitates transport of nutrients and waste

  • Maintains temperature and pH balance

  • Forms hydrogen bonds critical for protein folding, DNA structure, and membrane integrity

Physiological Functions

Thermoregulation and Temperature Homeostasis

Water plays a critical role in maintaining body temperature through its high specific heat capacity. It absorbs and stores large amounts of heat with minimal temperature change, buffering the body against fluctuations. During heat stress, water is lost through sweat, which evaporates and cools the body. This evaporative cooling is essential for preventing overheating during exercise, fever, or exposure to hot environments.

Transport Medium for Nutrients and Waste

Water is the primary component of blood and lymph, serving as the medium through which nutrients, oxygen, hormones, and waste products are transported. It enables the distribution of glucose, amino acids, electrolytes, and other solutes throughout the body. Water also facilitates the removal of metabolic waste products—such as urea and carbon dioxide—via urine, sweat, and respiration.

Solvent for Biochemical Reactions

As the universal solvent, water dissolves a vast array of polar and ionic substances, enabling nearly all biochemical reactions to occur in aqueous environments. Enzymatic activity, substrate binding, and product formation all rely on water-based solutions. Cellular metabolism, including glycolysis, the citric acid cycle, and protein synthesis, occurs in water-rich intracellular fluids.

Maintenance of Cell Structure and Turgor

Water contributes to cell volume and structural integrity by generating osmotic pressure within cells. It maintains turgor pressure in plant and some animal cells, preventing cellular collapse or rupture. In animal cells, water balance across membranes is tightly regulated by osmosis to maintain homeostasis and support membrane function.

Lubrication and Protection of Organs and Tissues

Water is a key component of bodily fluids that reduce friction and protect tissues. Synovial fluid lubricates joints, cerebrospinal fluid cushions the brain and spinal cord, and mucus hydrates and protects epithelial surfaces. Saliva, tears, and other secretions also rely on water for functionality and protective roles.

Facilitation of Digestion and Nutrient Absorption

Water is essential in the digestive process, aiding in the breakdown, transport, and absorption of nutrients. It dissolves digestive enzymes and nutrients, helps form chyme in the stomach, and supports peristalsis throughout the gastrointestinal tract. Adequate hydration is critical for effective absorption of water-soluble vitamins and electrolytes.

Regulation of Electrolyte and pH Balance

Water maintains osmotic balance and fluid distribution across cells and tissues by interacting with electrolytes such as sodium, potassium, and chloride. It also plays a role in acid-base regulation, acting as a medium for buffer systems like bicarbonate, which help stabilize blood pH within a narrow, life-sustaining range.

Molecular and Cellular Communication

Water facilitates signal transmission within and between cells. It supports the movement of ions and molecules across membranes, enabling nerve impulses, muscle contractions, and hormone transport. Intracellular and extracellular water balance is also crucial for the function of signal transduction pathways and receptor activity.

References

Popkin, B.M., D’Anci, K.E. & Rosenberg, I.H. (2010) Water, hydration, and health, Nutrition Reviews, 68(8), pp. 439–458.

Stookey, J.D., Pieper, C., Cohen, H.J., Hooper, L. & Orr, P.T. (2014) Acute and chronic effects of hydration status on health, Nutrition Reviews, 72(Suppl 2), pp. 97–109.

Armstrong, L.E., Ganio, M.S., Casa, D.J., Lee, E.C., McDermott, B.P., Klau, J.F., Yamamoto, L.M. & Marzano, S. (2012) Hydration status and cardiovascular function, Current Opinion in Nephrology and Hypertension, 25(5), pp. 304–309.

Adrogué, H.J. & Madias, N.E. (2015) Molecular physiology of water balance, New England Journal of Medicine, 372(14), pp. 1349–1358.

Armstrong, L.E. (2012) Challenges of linking chronic dehydration and fluid consumption to health outcomes, Nutrition Reviews, 70(Suppl 2), pp. S121–S126.

Kenney, W.L. & Chiu, P. (2001) Water and electrolyte homeostasis brings balance to physiology, American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, 301(2), pp. R265–R274.

Benton, D. & Young, H.A. (2015) Effects of hydration status on cognition and mood, Nutrients, 7(11), pp. 9482–9499.