A1.1 Water and its properties
A1.1 Water
Overview: Water’s physical and chemical properties are essential for life; it acts as a medium for chemical reactions, transport, and habitat. The content covers water’s polarity, hydrogen bonding, cohesion, adhesion, solubility, and the implications of these properties in biology and ecology (e.g., plants, animals, and aquatic environments). It also includes related topics such as extraterrestrial presence of water, and data-based questions about tree height and xylem pressure.
A1.1.1 Water as the medium for life
Life is believed to have begun in water; Darwin noted life arising in a "warm little pond" (1871), but most hypotheses place the origin in oceans rather than a pond.
During the origin of life, a small volume of water became enclosed in a membrane; solutes dissolved in this water allowed chemical reactions to occur between solutes.
After billions of years, most molecules of life remain dissolved in water; in a liquid state, molecules can move and interact, enabling life processes.
Water in a liquid state enables interactions and reactions; it serves as the medium in which metabolic processes occur.
Figure references: Figure 3 indicates water vapor detection on exoplanet K2-18b; Figure 4 shows polarity of water molecules.
A1.1.2 Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
Water molecules have covalent bonds between oxygen and hydrogen; sharing electrons is unequal, making the bonds polar covalent.
Oxygen is more electronegative, pulling electrons toward itself, creating partial negative charge on O and partial positive charges on H.
This polarity makes water molecules bent, with hydrogens on the same side (poles: partially positive H and partially negative O).
The unequal sharing yields polar covalent bonds, giving rise to hydrogen bonding between water molecules (intermolecular force, not a true covalent bond).
Hydrogen bonds form when a slightly positive hydrogen in one polar molecule is attracted to a slightly negative atom in another polar molecule.
Although hydrogen bonds are relatively weak, the small size and high number of water molecules lead to a large cumulative effect, giving water its unique properties vital for life.
Figure 5 depicts hydrogen bonds between water molecules; Figure 6 describes a demonstration of hydrogen-bond strength using a syringe apparatus.
A1.1.3 Cohesion of water molecules due to hydrogen bonding and consequences for organisms
Cohesion: mutual attraction between water molecules via hydrogen bonds; energy is required to break these bonds.
Consequences: enables water transport in plants (xylem) and sustains surface habitats (surface tension).
In plants, cohesion supports the ascent of water in xylem under tension (water columns pulled from leaves due to evaporation and soil-water attractions).
Hydrogen bonds allow water columns to withstand large tensile forces; breaking a column would require simultaneous breaking of many hydrogen bonds.
If hydrogen bonding were weaker, water columns would break more easily, limiting tall plant growth.
Figure 7 (tug-of-war rope) illustrates tension concepts; Figure 8 shows water surface habitats (e.g., water striders, mosquito larvae, Dolomedes fimbriatus).
A1.1.4 Adhesion of water to materials that are polar or charged and impacts for organisms
Adhesion: water forms bonds with polar/charged surfaces, causing water to stick to surfaces and enabling movement through capillary action.
Capillary action occurs as water moves through narrow tubes due to formation of hydrogen bonds between water and tube surfaces (e.g., glass).
Replacing air with water in capillary spaces releases energy due to new hydrogen bonds formed between glass and water.
Porous solids (e.g., paper) have large surface areas that attract water, enabling suction and capillary action (wetting and drawing water through gaps in cellulose).
Soil-water interactions: water in soils is attracted to soil surfaces and can be drawn through capillary action, enabling rise from underground sources.
In xylem, adhesion helps refill air-filled vessels; capillary action assists sap rise in spring as water refills conduits in deciduous trees.
Figure 10 shows moss paraphyllia with water-attracting cellulose walls; Figure 11–12 discuss natural sponge structure and water absorption.
A1.1.5 Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals
Water as solvent: polar nature allows formation of hydration shells around charged/polar solutes, preventing clumping and keeping solutes in solution.
Water’s partially negative O and partially positive H poles facilitate dissolution of ions and polar molecules; water forms hydrogen bonds with polar molecules.
Hydrophilic vs hydrophobic: substances that dissolve or adhere to water are hydrophilic; non-polar substances with little attraction to water are hydrophobic.
Hydrophilic substances include polar molecules (glucose) and ions (Na+, Cl−) and amino acids; hydrophobic substances include non-polar lipids.
Water acts as a solvent in metabolism: cytoplasm is an aqueous solution where dissolved enzymes catalyze reactions; reactions enabled by mobility and proper collision frequencies.
Transport systems relying on aqueous transport: in plants (xylem sap and phloem sap) and in animals (blood plasma).
Blood plasma and dissolved solutes: Na+, Cl−; amino acids; glucose; oxygen solubility is limited in water and decreases with temperature; this necessitates hemoglobin to boost oxygen transport.
Fat transport: fat molecules are non-polar and largely water-insoluble; coated with a phospholipid monolayer to form droplets that stay suspended in blood plasma.
Oxygen solubility: dissolved oxygen in water is limited; as temperature rises, oxygen solubility decreases; high oxygen transport requires hemoglobin with binding sites to increase carrying capacity.
Relevance to humans: intravenous fluids (saline) and total parenteral nutrition (TPN) demonstrate water’s central role in physiology and treatment.
See Fig. 13–14 for transport and solvent roles in plants and animals.
A1.1.6 Physical properties of water and the consequences for animals in aquatic habitats
Buoyancy (Archimedes’ principle): Upward buoyant force equals the weight of displaced fluid. If object density < fluid density, it floats; if denser, it sinks.
Equation: F{b} = ho{ ext{fluid}} \, g \, V_{ ext{displaced}}
Living tissues have densities near that of water; swim bladders in fish and gas vesicles in cyanobacteria adjust overall density for buoyancy.
Viscosity: Water is more viscous than many solvents; viscosity arises from internal friction as parts of the fluid move at different speeds; higher viscosity means greater resistance to flow.
Seawater is more viscous than freshwater due to dissolved salts.
Thermal conductivity: Water conducts heat relatively well; fats/oils conduct ~25% as fast as water; air ~5% of water’s conductivity.
Specific heat capacity: Water has a high specific heat due to hydrogen bonding restricting molecular motion.
Value: c{ ext{water}} = 4.18\ \, \text{J g}^{-1} \text{K}^{-1}; for air, c{ ext{air}} = 1.01\ \, \text{J g}^{-1} \text{K}^{-1}
This high heat capacity stabilizes temperatures in aquatic environments and helps homeotherms maintain body temperature.
Implications for aquatic animals vs. air environments:
Drag: There is about 800 times more drag on a body moving through water than through air at the same velocity, affecting energy expenditure for locomotion.
Thermal stability: Water’s high thermal conductivity and high specific heat provide thermally stable environments for aquatic organisms, while air acts as an insulator.
Heat transfer in organisms: The water content of blood allows efficient transport of heat from muscles to other parts of the body or to the environment to dissipate heat.
Practical contrast: Loon (bird) vs Ringed seal (mammal) related to buoyancy, energy costs, and heat exchange; water’s properties influence energy budgets differently for flying vs swimming animals.
A1.1.7 Extraplanetary origin of water on Earth and reasons for its retention
Concept: Water may have extraterrestrial origins; water presence on Earth is linked to retention and delivery mechanisms (as inferred from discussions and figures like Figure 3).
The presence of liquid water is considered critical for the evolution of life on other planets as well (e.g., detection of water vapor on exoplanets like K2-18b).
Retention factors for water on Earth relate to gravity, atmospheric composition, and possibly delivery by comets/asteroids; these topics connect to the search for life beyond Earth (A1.1.8).
A1.1.8 Relationship between the search for extraterrestrial life and the presence of water
Liquid water is considered a key criterion when assessing habitability and the potential for life on other worlds.
The search for extraterrestrial life often begins with assessing the presence of liquid water due to its essential role as a solvent, medium for metabolism, and facilitator of chemical reactions.
Figure 3 indicates water vapor detection on a distant planet, highlighting the relevance of water in astrobiology discussions.
Data-based questions: Tall trees
Description: The tallest trees are redwoods (Sequoia sempervirens); Hyperion is approximately 116 m tall. Xylem pressures in small branch segments were recorded at different heights during the dry season (late September to early October).
Figure 9 presents data with two data groups: pre-dawn vs midday pressures.
Questions (data interpretation and reasoning):
1. Independent variable and dependent variable:
a) Height above ground is the independent variable.
b) Xylem pressure is the dependent variable.
2. Using data, predict a maximum height for redwoods given a pressure below -2.0 MPa leads to column failure; explain your reasoning.
2a. State the relationship between height above ground and xylem pressure before dawn.
2b. Suggest reasons for this relationship.
3. Compare pre-dawn vs midday pressures (pre-dawn vs midday) and propose reasons for any differences.
3a. Pre-dawn vs midday comparison data interpretation (refer to Figure 9 data points).
3b. Explanation for observed differences (e.g., evaporation, soil moisture, atmospheric demand).
The data illustrate how xylem tension increases with height and how maximum tensile strength limits tree height.
Measuring variables: Determining wet and dry mass
Natural sponge vs synthetic sponge experiment (Hooke’s sponge context, spongin and iodine content):
Sponge structure: natural sponge is the soft skeleton of sponges (phylum Porifera) with a porous skeleton made of spongin.
Spongin contains iodine and is resistant to proteases; it readily adsorbs water due to porosity and large surface area.
Synthetic sponges replaced natural sponges; similar properties but reduce pressure on wild sponge populations; however, plastic sponges can harm ecosystems.
Practical activity: If possible, compare natural vs synthetic sponge sources and measure water absorption.
1) Obtain samples from sustainable sources.
2) Examine structures under a microscope.
3) Dry sponges (e.g., in an oven at 80°C for 24 h) and weigh each dry sponge.
4) Allow sponges to soak up water to their maximum capacity.
5) Weigh each saturated sponge.
6) Calculate the amount of water retained as a percentage of the dry mass.
A1.1.5 (continued) Additional notes on the solvent role and metabolism
Substances dissolved in water become part of a solution; water is the solvent and dissolved substances are solutes.
Metabolism relies on aqueous cytoplasmic solutions; dissolved enzymes catalyze reactions; mobility of solutes allows collisions at enzyme active sites.
Transport systems:
In plants: xylem sap (mineral ions) and phloem sap (sucrose and photosynthetic products).
In animals: blood plasma carries ions (Na+, Cl−), amino acids, glucose; oxygen solubility is limited and regulated by hemoglobin; fats are carried as small droplets coated with phospholipids to prevent coalescence.
Oxygen transport limitation: dissolved oxygen in plasma decreases with temperature; at 37°C, carrying capacity is reduced compared to cooler temperatures, necessitating hemoglobin.
Connections, implications, and examples
Real-world relevance: Water’s properties explain ecological patterns (e.g., surface habitats, water transport in plants, buoyancy in aquatic animals).
Ethical/philosophical: Understanding water’s role highlights the importance of water conservation and sustainable resource management (e.g., sponges and marine ecosystems; sustainable sponge harvesting).
Practical: Knowledge of water properties informs medical treatments (intravenous saline, TPN) and the design of biomimetic materials and irrigation strategies.
Formulas and constants referenced:
Buoyancy (Archimedes): F{b} = \rho{\text{fluid}} \, g \, V_{\text{displaced}}
Maximum xylem tension observed: p_{\text{xylem,min}} \approx -2.0 \text{ MPa}
Specific heat capacity of water: c_{\text{water}} = 4.18\ \text{J g}^{-1}\ \text{K}^{-1}
Specific heat capacity of air: c_{\text{air}} = 1.01\ \text{J g}^{-1}\ \text{K}^{-1}
Relative conductivities: water > fats/oils (≈ 0.25 of water) > air (≈ 0.05 of water)
Notable figures: Figure 3 (exoplanet water presence), Figure 4 (water polarity), Figure 5 (hydrogen bonds), Figure 6 (demonstration apparatus), Figure 7 (tug-of-war analogy for tension), Figure 8 (water-surface habitats and organisms), Figure 9 (data on tall tree heights and xylem pressures), Figure 10 (moss paraphyllia), Figure 11–12 (sponge structure and images), Figure 13–14 (transport roles and hospital fluids).
Quick glossary reminders
Hydrophilic: substances attracted to water and soluble in water.
Hydrophobic: non-polar substances not attracted to water and poorly soluble in water.
Cohesion: water–water attraction via hydrogen bonds allowing continuous columns in xylem.
Adhesion: water–surface attraction enabling capillary rise.
Capillary action: movement of water within small-diameter tubes due to cohesive and adhesive forces.
Hydration shell: water molecules surrounding ions/polar molecules to stabilize them in solution.
Hemoglobin: protein that binds oxygen, increasing blood's oxygen-carrying capacity.
Spongin: iodine-containing protein in natural sponges contributing to water uptake and resilience.