Bio Chap 3
Chapter 3: Water and Life
This chapter focuses on the properties of water and its importance for life on Earth.
Water's Polarity and Hydrogen Bonds
Water is a polar molecule due to the unequal sharing of electrons in its polar covalent bonds. The oxygen atom is more electronegative than hydrogen, meaning it attracts electrons more strongly. Consequently, the oxygen atom has a partial negative charge (δ−), while the hydrogen atoms have partial positive charges (δ+).
The polarity of water molecules leads to hydrogen bonding. The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of another, forming a hydrogen bond. These bonds are weak individually but strong collectively, giving water its unique properties.
Emergent Properties of Water
Water has several emergent properties crucial for life, including:
Cohesion and Adhesion: Cohesion is the tendency of water molecules to stick together due to hydrogen bonding. Adhesion is the clinging of water to other substances.
Cohesion creates surface tension, which allows some insects to walk on water.
Cohesion and adhesion work together to transport water and nutrients against gravity in plants. Water evaporating from leaves pulls on the water column below, while adhesion helps the water stick to the cell walls, preventing it from falling back down.
Moderation of Temperature: Water moderates temperature by absorbing heat from warmer air and releasing it to cooler air.
This property stems from water's high specific heat, the amount of heat needed to raise the temperature of 1 gram of a substance by 1 degree Celsius.
Water's high specific heat is due to hydrogen bonding: a lot of heat energy is required to break these bonds before the water molecules can move faster and increase in temperature.
This property helps stabilize ocean temperatures and moderate coastal air temperatures, creating favorable environments for life.
Evaporative Cooling: Water has a high heat of vaporization, the amount of heat needed to convert 1 gram of liquid to gas. This is also due to hydrogen bonding, as the bonds must be broken before water molecules can evaporate.
Evaporative cooling occurs because the molecules with the highest kinetic energy (heat) evaporate first, leaving the remaining liquid cooler.
This process helps regulate the Earth's climate and prevents terrestrial organisms from overheating (e.g., sweating).
Floating of Ice: Ice is less dense than liquid water because of hydrogen bonding. As water freezes, the hydrogen bonds lock the molecules into a crystalline lattice, spacing them farther apart than in liquid water.
This unique property prevents bodies of water from freezing solid, allowing life to exist beneath the surface.
Versatility as a Solvent: Water is an excellent solvent due to its polarity.
Water dissolves ionic compounds by surrounding the ions with hydration shells, separating and shielding them from each other.
Water also dissolves polar molecules by forming hydrogen bonds with them.
Substances that readily dissolve in water are hydrophilic (water-loving), while those that do not are hydrophobic (water-fearing).
Acids, Bases, and pH
Occasionally, a hydrogen atom in a water molecule will shift to another water molecule, leaving its electron behind. This creates a hydronium ion (H3O+, often simplified as H+) and a hydroxide ion (OH−).
Acids are substances that increase the hydrogen ion concentration of a solution, while bases decrease it.
Strong acids and bases dissociate completely in water, while weak acids and bases only partially dissociate.
The pH scale measures the acidity or basicity of a solution.
pH is the negative logarithm (base 10) of the hydrogen ion concentration.
A pH of 7 is neutral, with equal concentrations of H+ and OH−.
A pH below 7 is acidic, with a higher concentration of H+.
A pH above 7 is basic, with a higher concentration of OH−.
Each pH unit represents a tenfold difference in H+ concentration.
Buffers help maintain a stable pH by accepting or donating hydrogen ions.
Buffers are typically weak acid-base pairs.
For example, the carbonic acid-bicarbonate buffering system helps maintain the pH of blood.
Ocean Acidification
Human activities, particularly the burning of fossil fuels, release carbon dioxide (CO2) into the atmosphere. Some of this CO2 is absorbed by the oceans, leading to ocean acidification.
Dissolved CO2 reacts with water to form carbonic acid, which lowers the pH of seawater.
Ocean acidification reduces the availability of carbonate ions, which are crucial for calcification, the process by which marine organisms build shells and skeletons.
This has severe consequences for marine ecosystems, particularly coral reefs, which are vital habitats for a wide variety of species.
This chapter highlights the importance of understanding the properties of water, as it plays a crucial role in maintaining life on Earth. The chapter emphasizes the threats posed by human activities, such as ocean acidification, and stresses the need for continued research and action to protect our water resources.