# Study Notes: Unity and Diversity of Life - The Role of Water ## Overview Life on Earth exhibits both unity and diversity. While all living organisms share common features due to a common ancestry, evolution has led to a rich biodiversity. A unifying factor across all life forms is the reliance on water, which plays a critical role in biological processes due to its unique physical and chemical properties. --- ## Key Concepts ### 1. **Water as the Medium for Life** - Life is thought to have originated in water, with the first cells forming in oceans. - Water provides a liquid medium where molecules can move, interact, and undergo chemical reactions essential for life. - The cytosol inside cells is a water-based medium, enabling metabolic processes. Water's liquid state allows molecules to move freely, facilitating the chemical reactions necessary for life. --- ### 2. **Hydrogen Bonds in Water** - Water molecules are polar due to the unequal sharing of electrons in covalent bonds between oxygen and hydrogen. - Oxygen has a partial negative charge ($\delta^-$). - Hydrogen has a partial positive charge ($\delta^+$). - The polarity of water molecules leads to hydrogen bonding: - A slightly positive hydrogen atom in one molecule is attracted to a slightly negative atom in another molecule. #### Representation of Hydrogen Bonds: $$ \text{H} \cdots \text{O} \cdots \text{H} $$ Hydrogen bonds are responsible for water's high boiling point and surface tension. --- ### 3. **Cohesion and Adhesion** #### Cohesion: - Water molecules stick to each other due to hydrogen bonding. - This property allows water to be transported under tension in plants (e.g., through xylem vessels). In tall trees like redwoods, cohesion enables water to travel from roots to leaves, even against gravity. #### Adhesion: - Water molecules stick to polar or charged surfaces. - Adhesion is critical for capillary action, where water moves through narrow spaces (e.g., in soil or plant cell walls). Adhesion helps keep plant cell walls moist, enabling photosynthesis by maintaining carbon dioxide absorption. --- ### 4. **Surface Tension** - Water molecules at the surface experience stronger cohesive forces, creating a "skin-like" effect. - Surface tension allows certain organisms, like water striders, to walk on water. Water striders use surface tension to stay afloat, supported by their water-repellent hairs. Students often confuse surface tension with adhesion. Remember, surface tension is due to cohesion among water molecules. --- ### 5. **Solvent Properties of Water** - Water is a versatile solvent due to its polarity: - It dissolves ionic compounds (e.g., sodium chloride) by surrounding ions with hydration shells. - It dissolves polar molecules (e.g., glucose) through hydrogen bonding. #### Hydrophilic vs. Hydrophobic: - **Hydrophilic substances**: Attracted to water (e.g., salts, sugars). - **Hydrophobic substances**: Not attracted to water (e.g., lipids, oils). In blood plasma, glucose dissolves easily due to its polarity, while fats require transport proteins like lipoproteins. Water is not a universal solvent; it cannot dissolve non-polar substances like oils. --- ### 6. **Water in Metabolism and Transport** #### Metabolism: - Cytoplasm is an aqueous solution where enzymes and substrates interact. - Water enables the movement and interaction of molecules, making it the medium for metabolism. #### Transport: - Water serves as a transport medium in both plants and animals: - **Plants**: Mineral ions in xylem sap; sugars in phloem sap. - **Animals**: Blood plasma carries dissolved substances like glucose, amino acids, and ions. Oxygen, though non-polar, is sparingly soluble in water. Hemoglobin in red blood cells increases oxygen transport efficiency. Water's solvent properties make it indispensable for nutrient transport and waste removal in organisms. --- ### 7. **Physical Properties of Water** #### High Specific Heat Capacity: - Water absorbs and retains heat, stabilizing temperatures in organisms and environments. #### High Latent Heat of Vaporization: - Water requires significant energy to evaporate, aiding in cooling mechanisms like sweating. #### Density and Ice Formation: - Ice is less dense than liquid water, allowing it to float and insulate aquatic ecosystems. In winter, ice on a lake's surface prevents the water below from freezing, protecting aquatic life. Many assume ice sinks because solids are denser than liquids. However, water's unique structure makes ice less dense. --- ### 8. **Water as a Habitat** - Aquatic environments provide unique challenges and opportunities: - **Buoyancy**: Blubber in marine mammals aids buoyancy and thermoregulation. - **Energy Efficiency**: Moving through water requires more energy than air due to water's higher density. Whales use blubber for insulation and buoyancy, adapting to cold aquatic habitats. Organisms in water often have streamlined bodies to reduce resistance and conserve energy. --- ### 9. **Water and Extraterrestrial Life** - Liquid water is considered essential for life. Space scientists search for water on other planets as a key indicator of potential life. Water vapor detected on exoplanet K2-18b suggests the possibility of life-supporting conditions. Water's unique properties make it a universal requirement for life as we know it. --- ## Practice Questions 1. **Explain why water is considered a polar molecule.** 2. **Describe how cohesion and adhesion contribute to water transport in plants.** 3. **Why is water's high specific heat capacity important for living organisms?** 4. **Compare the solvent properties of water for hydrophilic and hydrophobic substances.** 5. **How does surface tension benefit organisms like water striders?** --- ## Key Takeaways - Water's unique properties—cohesion, adhesion, solvent abilities, and thermal stability—are essential for life. - Hydrogen bonding underpins many of water's critical roles in biological systems. - Water serves as a medium for metabolism, transport, and habitat, making it indispensable for all known life forms. Understanding water's properties helps explain its central role in biology and its importance in the search for extraterrestrial life.