Water: Properties and Molecular Structure

• Water constitutes 60-75% of human body weight.
• A 4% loss of body water leads to dehydration, and a 15% loss can be fatal.
• Survival without water is limited to 3 days, while survival without food can extend to a month.
• Water's molecular structure and special properties support life.

Molecular Structure of Water

• Water comprises two positively charged hydrogen atoms and one negatively charged oxygen atom.
• The arrangement of hydrogen and oxygen atoms creates an asymmetrical molecule.
• The asymmetrical molecule has a positive charge on one side and a negative charge on the other.
• This charge differential is called polarity and dictates water's interactions with other molecules.

Water as the “Universal Solvent”

• As a polar molecule, water interacts favorably with other polar molecules due to opposite charge attraction.
• Water molecules form relatively strong bonds with other polar molecules around it, including each other.
• The positive hydrogen of one water molecule bonds with the negative oxygen of an adjacent molecule, and so on.
• This bonding is called cohesion, and it helps plants take up water at their roots.
• Cohesion also contributes to water's high boiling point, which helps animals regulate body temperature.
• Water can form bonds with and surround the positive and negative regions of polar biological molecules.
• Water wriggles into the nooks and crannies between molecules of another substance, effectively breaking it apart or dissolving it.
• When sugar crystals are put in water, both water and sugar are polar, allowing individual water molecules to surround individual sugar molecules, breaking apart the sugar and dissolving it.
• Similarly, water breaks apart ionic molecules by interacting with both the positively and negatively charged particles.
• Salt is composed of sodium and chloride ions and water breaks these apart as well.
• Water's ability to dissolve a variety of molecules has earned it the designation of “universal solvent”.
• As a solvent, water helps cells transport and use substances like oxygen or nutrients and blood helps carry these molecules to the locations they are needed.
• Water facilitates the transport of molecules like oxygen for respiration and has a major impact on the ability of drugs to reach their targets in the body.

Water Supporting Cellular Structure

• Water fills cells to help maintain shape and structure.
• The water inside many cells creates pressure that opposes external forces, similar to putting air in a balloon.
• Water allows everything inside cells to have the right shape at the molecular level.
• Water contributes to the formation of membranes surrounding cells.
• Every cell on Earth is surrounded by a membrane composed of two layers of phospholipids,
• Phospholipids have a polar "head" and a nonpolar “tail.”
• The polar heads interact with water, while the nonpolar tails try to avoid water and interact with each other instead.
• Phospholipids spontaneously form bilayers with the heads facing outward towards the surrounding water and the tails facing inward, excluding water.
• The bilayer surrounds cells and selectively allows substances like salts and nutrients to enter and exit the cell.
• Without water, cell membranes would lack structure, and cells would be unable to keep important molecules inside and harmful molecules outside.
• Water also impacts some fundamental components of every cell: DNA and proteins.
• Amino acids are the building blocks of proteins and water drives the proteins to a specific shape to function correctly.
• Without the proper shape, proteins would be unable to perform these functions and a cell could not survive.
• Water molecules surround DNA in an ordered fashion to support its characteristic double-helix conformation.
• Without this shape, cells would be unable to follow the instructions encoded by DNA.

Chemical Reactions of Water

• Water is directly involved in chemical reactions to build and break down cell components, such as photosynthesis that creates sugars for all forms of life.
• Water participates in building larger molecules in cells and is required for the reverse reaction that breaks down these molecules.
• Water buffers cells from the dangerous effects of acids and bases.
• Water acts as both an acid and a base.
• A water molecule can give up a hydrogen and become OH− (acting as a base) or accept another hydrogen and become H3O+ (acting as an acid).
• This adaptability combats drastic changes of pH due to acidic or basic substances in the body in a process called buffering.
• Water supports life due to its important role with chemical reactions and its simple molecular structure helps maintain shapes for cells.
• There are still new properties of water being discovered.