Water Properties and Their Biological Significance

Water Properties and Their Biological Significance

Hydrogen Bonding and Water Molecules

Water's unique properties arise from its molecular structure and the hydrogen bonds formed between molecules. These properties are essential for life on Earth.

• Polar Molecule: Water (H2O) is a polar molecule, meaning it has a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. This polarity allows water molecules to form hydrogen bonds with each other.

• Hydrogen Bonding: Hydrogen bonds are weak attractions between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another. These bonds are responsible for many of water's unique properties.

• Diagram: A typical diagram shows two water molecules with dashed lines representing hydrogen bonds between the hydrogen of one molecule and the oxygen of another.

Cohesion and Adhesion

Cohesion and adhesion are two important properties of water that result from hydrogen bonding and polarity.

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property allows water to form droplets and contributes to surface tension.

• Adhesion: The attraction between water molecules and other substances. This property helps water "stick" to surfaces, such as the walls of plant vessels.

• Biological Example: In plants, cohesion and adhesion work together to move water from roots to leaves through the xylem in a process called capillary action.

Heat and Temperature

Water has a high specific heat capacity, meaning it can absorb or release a large amount of heat with only a slight change in its own temperature.

• Specific Heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Biological Significance: Water's high specific heat helps stabilize temperatures in organisms and environments, protecting living things from rapid temperature changes.

• Example: Coastal areas have milder climates than inland areas due to the heat-absorbing capacity of large bodies of water.

Properties of Water: Observations and Explanations

Several observable phenomena can be explained by water's unique properties:

• Coastal vs. Inland Temperatures: Coastal areas have milder climates because water absorbs and releases heat more slowly than land.

• Insects Walking on Water: Surface tension, caused by cohesion, allows insects to walk on water without sinking.

• Water in a Glass: Adhesion causes water to climb the sides of a glass, forming a concave meniscus.

• Paper Towel Absorbing Water: Adhesion and capillary action allow water to move up a paper towel against gravity.

• Evaporative Cooling: When humans sweat, the evaporation of water from the skin removes heat, cooling the body.

Hydrophilic vs. Hydrophobic Substances

Substances interact with water in different ways based on their chemical properties.

• Hydrophilic: "Water-loving" substances that dissolve easily in water (e.g., salts, sugars).

• Hydrophobic: "Water-fearing" substances that do not dissolve in water (e.g., oils, fats).

Making a 1M Solution

To prepare a 1 molar (1M) solution of ethyl alcohol (C2H6O):

• Calculate the molar mass of ethyl alcohol: C (12.01) × 2 + H (1.01) × 6 + O (16.00) = 46.08 g/mol.

• Dissolve 46.08 grams of ethyl alcohol in enough water to make a total volume of 1 liter.

Dissociation of Water

Water can dissociate into ions, which is important for many biological processes.

• Equation:

$\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$

• In pure water, the concentration of both hydrogen ions ($[\mathrm{H^+}]$) and hydroxide ions ($[\mathrm{OH^-}]$) is $1 \times 10^{-7}$ M at 25°C.

Acids, Bases, and pH

The pH scale measures the concentration of hydrogen ions in a solution.

• Acid: A substance that increases the hydrogen ion concentration of a solution.

• Base: A substance that reduces the hydrogen ion concentration.

• pH Formula:

$\mathrm{pH} = -\log_{10}[\mathrm{H^+}]$

• Pure water has a pH of 7 (neutral).

Buffer Systems: The Bicarbonate Buffer

Buffers help maintain stable pH in biological systems. The bicarbonate buffer system is crucial in blood and other fluids.

• Equation:

$\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+}$

• This system helps resist changes in pH by absorbing excess H+ or OH- ions.

Ocean Acidification

Ocean acidification refers to the decrease in pH of ocean water due to increased absorption of atmospheric CO2.

• CO2 dissolves in seawater, forming carbonic acid, which lowers pH and affects marine life, especially organisms with calcium carbonate shells.

• Equation:

$\mathrm{CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+}$

• Effects: Reduced calcification in corals and shellfish, disruption of marine food webs.