Properties of Water Flashcards

Molecular Structure and Chemical Bonding of Water

• Liquid State Relevance: At the majority of temperatures found on Earth, water remains in a liquid state. This is biologically critical because nearly all metabolic reactions occurring within cells involve molecules dissolved in solution.

• Atomic Composition: A single water molecule consists of one oxygen atom and two hydrogen atoms ($H_2O$).

• Intramolecular Covalent Bonding: Within a single water molecule, the oxygen and hydrogen atoms are joined by covalent bonds. This bond involves the sharing of electrons between the atoms.

• Polarity and Unequal Electron Sharing:

  • Water exhibits an unequal sharing of electrons within its covalent bonds.
  • The electrons are pulled more closely toward the oxygen atom than the hydrogen atoms.
  • This result in partial charges, creating a polar molecule.
  • The oxygen atom carries a partially negative charge, symbolized as $\delta ^-$.
  • The hydrogen atoms carry partially positive charges, symbolized as $\delta ^+$.
  • In diagrams, these internal covalent bonds are represented by solid lines.

• Intermolecular Hydrogen Bonding:

  • Hydrogen bonds form between separate water molecules.
  • A hydrogen bond is defined as the attraction between the partially negative end ($\delta ^-$ oxygen) of one water molecule and the partially positive end ($\delta ^+$ hydrogen) of a different water molecule.
  • In scientific notation and diagrams, hydrogen bonds are represented using dashed or dotted lines to distinguish them from covalent bonds.

Cohesion and its Biological Implications

• Definition of Cohesion: Cohesion is the attraction between molecules of the same type. In this context, it refers to water molecules sticking to other water molecules due to hydrogen bonding.

• Transpiration in Plants:

  • The Xylem: Water and dissolved minerals travel upward through the plant via a specialized tube called the xylem.
  • The Process: Transpiration begins with the evaporation of water through the stomata (small openings in the plant leaf).
  • Tension Force: As water evaporates from the leaf, it pulls the next water molecule in the column upward because of cohesion.
  • Continuous Water Column: As long as there is a continuous column of water where each molecule is in contact with the one below it, the cohesion-driven tension force can pull water from the roots all the way to the top of the plant.

• Surface Tension as a Habitat:

  • Cohesion allows water molecules to hang onto each other so tightly that they create a strong surface.
  • Certain insects can walk on water without breaking the surface or getting wet.
  • At the microscopic level, the water surface "bends" under the weight of the insect's foot but does not break because the hydrogen bonds are strong enough to support the organism.
  • This provides a specialized habitat space on the surface of water bodies.

Adhesion and Capillary Action

• Definition of Adhesion: Adhesion is the attraction between water molecules and a solid surface.

• Capillary Action: This occurs when adhesion to the walls of a vessel (like a graduated cylinder or a xylem tube) pulls the water upward.

• The Meniscus: In a tube, water forms a "U-shaped" curve called a meniscus. This is a direct result of capillary action, where water adheres to the sides of the tube and climbs slightly.

• Role in Transpiration: Adhesion works alongside cohesion to support water transport in plants. While cohesion pulls the water column up, adhesion helps the water stick to the sides of the xylem walls, helping to pull water up against the force of gravity and maintaining moisture throughout the plant.

Water as a Solvent and Transport Medium

• Solvent Terminology:

  • Solute: The substance being dissolved.
  • Solvent: The liquid into which the solute is dissolved.
  • Solution: The resulting homogeneous mixture of the solute and solvent.

• Hydrophilic vs. Hydrophobic Substances:

  • Hydrophilic ("Water-loving"): These substances are soluble in water. They include polar molecules, such as glucose, and ions (charged particles).
  • Hydrophobic ("Water-hating"): These are non-polar substances that have very low solubility in water. Examples include lipids (fats), cholesterol, and oxygen ($O_2$) molecules.

• Metabolic Importance: Because water is a polar solvent, it can dissolve a wide range of substances, making it an ideal medium for the metabolic reactions required for life.

• Transport in Plants:

  • Xylem: Carries water and dissolved minerals.
  • Phloem: Carries sucrose and other carbohydrates, which must be in solution (dissolved in water) to flow through the plant.

• Transport in Animals (Blood System):

  • Blood Plasma: The liquid component of blood, primarily composed of water.
  • Soluble Components: Ions, glucose, and amino acids are hydrophilic and are easily transported dissolved in the plasma.
  • Insoluble Components: Oxygen, fats, and cholesterol are hydrophobic. Because they are not very soluble in water, they must be transported by being attached to specialized proteins or cell components.

Physical Properties: Buoyancy and Viscosity

• Buoyancy:

  • Buoyancy is the upward force exerted on an object by the water, opposing the downward force of gravity.
  • Objects less dense than water will float because the buoyant force is greater than the gravitational force acting on them. Denser objects sink.
  • Most aquatic organisms have a density similar to water.
  • Adaptation (Swim Bladder): Fish use a swim bladder to adjust their buoyancy. Filling the bladder with air decreases their density (allowing them to rise/float), while deflating it increases their density (causing them to sink).

• Viscosity:

  • Viscosity is a fluid's resistance to flow.
  • In water, viscosity provides resistance that allows organisms (like fish) to propel themselves forward using fins.
  • Solute Impact: As the concentration of dissolved solutes in water increases, the viscosity also increases. This creates biological limits on the concentrations of substances in fluids like blood; if viscosity becomes too high, the fluid becomes too resistant to flow through the vessels.

Thermal Properties of Water

• Thermal Conductivity:

  • Water is an excellent thermal conductor, meaning heat transfers through it easily.
  • Beneficial Applications: Blood plasma can pick up heat from metabolic activity in cells and transport it to the surface of the skin to be dissipated.
  • Negative Consequences: Organisms falling into cold water lose body heat rapidly to the environment because water conducts the heat away from the body efficiently.

• High Specific Heat Capacity:

  • Water can absorb a significant amount of heat energy without a large change in its own temperature.
  • It does not change temperature as easily as other substances (e.g., methane or alcohol).

• The Moderating Effect:

  • In aquatic environments, air temperatures may fluctuate significantly between day and night, but the water temperature remains relatively stable.
  • This provides a stable environment for aquatic organisms.
  • Body Temperature Maintenance: High specific heat capacity helps organisms maintain internal homeostatic temperatures. For example, metabolic heat enters the cytoplasm or blood plasma without causing a drastic, dangerous rise in temperature.
  • Case Study (African Elephant): Large ears expose blood (primarily water-based plasma) to the air. While water's thermal conductivity allows for heat exchange with the air to cool the animal, its specific heat capacity ensures the blood itself does not experience extreme temperature swings.

Comparative Adaptations: Seal vs. Loon

• Ringed Seal (Aquatic Environment):

  • Buoyancy: The seal can float easily because water is very buoyant, requiring less energy to stay at the surface.
  • Viscosity: Because water is viscous, the seal must expend significant energy to move through it compared to moving through air.
  • Thermal Conductivity: To counter water's high thermal conductivity (which would strip the seal's body heat), the seal has a thick layer of blubber (fat) for insulation.

• Arctic Loon (Aerial Environment):

  • Buoyancy: Air is much less buoyant than water, so the loon must expend high amounts of energy to stay aloft.
  • Viscosity: Air has much lower viscosity than water, making it easier (requiring less energy) to move through the medium.
  • Thermal Conductivity: Air is a poor thermal conductor compared to water, so the loon does not require the heavy blubber layers found in marine mammals like seals.