Structure & Properties of Water (College Board AP® Biology)

Water as the Medium of Life

• Water is of the utmost biological importance

• As 71% of the Earth’s surface is covered in water, it is a major habitat for organisms

• The first cells evolved in a watery environment

  • This is believed to have been in the deep oceans, close to hydrothermal vents in the Earth's crust

• Some water and solutes got trapped within a membrane

  • Between 70% to 95% of the mass of a cell is water

• Chemical reactions began occurring within the membrane-bound structure

• This led to the evolution of cells

• Water in its liquid state allows dissolved molecules to move around, so they are easily able to collide and react with each other

• Most life processes occur in water, it is the medium in which all metabolic reactions take place in cells

• The link between water and life is so strong that scientists looking for life on other planets and moons look for evidence of water to suggest that life could have occurred there

Molecular Structure of Water

• Water is composed of atoms of hydrogen and oxygen

• One atom of oxygen combines with 2 atoms of hydrogen by sharing electrons (covalent bonding)

Water Molecule Diagram

• Although water as a whole is electrically neutral, the sharing of the electrons is uneven between the oxygen and hydrogen atoms

• The oxygen atom attracts the electrons more strongly than the hydrogen atoms

  • This is called the high electronegativity of oxygen compared to hydrogen

  • This results in a weak negatively charged region on the oxygen atom (δ-) and a weak positively charged region on the hydrogen atoms(δ+)

  • This also results in the molecule's asymmetrical shape (atoms not arranged in a straight line, the molecule is 'v-shaped')

• Think of the electrons spending more time around the oxygen atom than the hydrogen atoms

  • Due to oxygen's higher electronegativity

• When a molecule has one end that is negatively charged and one end that is positively charged it is also a polar molecule

• Water is therefore a polar molecule

Water Expands When it Freezes

• Most substances are more dense as solids than in their liquid state

  • However, water is an exception to this rule

  • Because there are fewer hydrogen bonds between molecules in ice than in liquid water

• Therefore, ice takes up more space than liquid water

• This explains why ice floats on water

• The consequence is that ice can form over liquid water in rivers, lakes and oceans

  • This protects the liquid water beneath from freezing itself

  • And so prevents bodies of water from freezing completely solid

  • So life can endure underneath the ice cap

    • The sea bed underneath the Arctic ice cap is one of the most biodiverse habitats on Earth

• The expansion of ice as it freezes causes weathering of rocks and is responsible for a wide range of geological changes that create a variety of habitats for organisms

pH Effects & Buffers

• Water can dissociate into H⁺ and OH^- ions

• The concentrations of H⁺ and OH^- ions affect the pH

  • A high concentration of H⁺ ions makes the pH low (acidic)

  • A high concentration of OH^- ions makes the pH high (alkaline)

• The pH in turn has an effect on living organisms 

  • For example, extreme pH can denature enzymes and other proteins

• It is in an organism's interests to stabilize pH where possible

• An example is the carbonic acid buffer system that helps to stabilize pH in the blood

carbonic acid ⇌ bicarbonate ions + hydrogen ions 
H₂CO₃ ⇌ HCO₃^- + H⁺

• Carbon dioxide can react with water to form carbonic acid, which can dissociate as shown, in an equilibrium reaction

• When conditions generate H⁺ ions, this causes the pH to fall (become acidic)

  • The equilibrium can shift to the right, removing some of the H⁺ ions from solution and stabilizing the pH

• Similarly, if the pH drifts upwards, the equilibrium can shift to the right, release more H⁺ ions into solution and cause the pH to move back down towards neutral