Water and Life: Properties, Functions, and Biological Importance

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Water and Life

Overview: The Molecule That Supports All of Life

Water is essential for all living organisms and is the only common substance that exists in the natural environment in all three physical states: solid, liquid, and gas. Most cells are surrounded by water and are composed of 70–95% water by mass. The abundance of water is a key reason why Earth is habitable.

Water is vital for cellular processes and supports life at molecular, cellular, and ecosystem levels.
Physical states: Water exists as ice, liquid, and vapor, allowing for diverse biological and environmental processes.
Cell composition: High water content in cells facilitates biochemical reactions and molecular transport.

Emergent Properties of Water

Four Properties That Facilitate Life

Water exhibits four emergent properties that make it uniquely suited to support life on Earth:

Cohesive behaviour: Water molecules stick together due to hydrogen bonding, aiding transport in plants.
Ability to moderate temperature: Water absorbs and releases heat with minimal temperature change, stabilizing environments.
Expansion upon freezing: Ice is less dense than liquid water, allowing it to float and insulate aquatic life.
Versatility as a solvent: Water dissolves a wide range of substances, facilitating chemical reactions and transport.

Structure and Bonding in Water

Polar Covalent Bonds and Hydrogen Bonding

Water is a polar molecule because oxygen is highly electronegative, causing an uneven distribution of charge. This polarity enables water molecules to form hydrogen bonds with each other.

Polar covalent bonds: Electrons are shared unequally between oxygen and hydrogen atoms.
Hydrogen bonds: Weak attractions between the partially positive hydrogen of one molecule and the partially negative oxygen of another.
Example: Hydrogen bonding is responsible for water’s high surface tension and its role in capillary action.

Cohesion and Adhesion

Cohesion of Water Molecules

Hydrogen bonds hold water molecules together, a phenomenon called cohesion. Cohesion helps transport water against gravity in plants, especially through xylem vessels.

Adhesion: Water molecules also stick to other surfaces, aiding movement through plant tissues.
Example: Water transport in plants relies on both cohesion and adhesion.

Temperature Moderation

Water’s Role in Heat Regulation

Water absorbs heat from warmer air and releases stored heat to cooler air. It can absorb or release large amounts of heat with little change in its own temperature due to its high specific heat capacity.

Specific heat: The amount of heat needed to raise the temperature of 1 gram of water by 1°C is high, stabilizing climates and organisms.
Example: Coastal regions experience milder climates due to water’s temperature moderation.

Expansion Upon Freezing

Floating of Ice on Liquid Water

Ice floats in liquid water because hydrogen bonds in ice are more ordered, making ice less dense than liquid water. This property insulates aquatic environments in cold climates.

Crystalline structure: Hydrogen bonds stabilize the structure of ice, increasing its volume and decreasing its density.
Example: Lakes and oceans do not freeze solid, allowing life to persist beneath the ice.

Water as a Solvent

Definitions and Properties

Water is known as the universal solvent due to its ability to dissolve many substances. Solutions, solvents, and solutes are key terms in understanding aqueous chemistry.

Solution: Homogeneous mixture of substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance dissolved.
Aqueous solution: Solution where water is the solvent.

Water’s Versatility as a Solvent

Water’s polarity allows it to dissolve ionic compounds and nonionic polar molecules. When ionic compounds dissolve, each ion is surrounded by a hydration shell of water molecules.

Hydration shell: Water molecules surround and stabilize ions in solution.
Example: Table salt (NaCl) dissolves in water, forming Na+ and Cl- ions surrounded by water.
Large polar molecules: Proteins and other macromolecules dissolve if they have polar or ionic regions.

Cellular Reactions and Water

Role of Water in Cells

Water comprises about 70% of most cells by mass and is essential for biochemical reactions. It supports hydrophilic molecules and influences the folding of hydrophobic molecules, such as proteins.

Hydrophilic molecules: Interact favorably with water, aiding in solubility and transport.
Hydrophobic molecules: Repelled by water, driving processes like protein folding and membrane formation.
Photosynthesis and respiration: Water is a reactant in photosynthesis and a product in cellular respiration.

Transport Across Membranes

Diffusion, Osmosis, and Aquaporins

Water crosses cell membranes by simple diffusion and facilitated diffusion through protein channels called aquaporins. Osmosis is the movement of water across a selectively permeable membrane.

Osmosis: Water moves from areas of low solute concentration to high solute concentration.
Aquaporins: Specialized membrane proteins that facilitate rapid water transport.
Example: Aquaporins are crucial in kidney function and plant water regulation.

Dehydration and Its Effects

Physiological Consequences

Dehydration can lead to overheating, reduced blood volume and pressure, impaired organ function, and ultimately organ failure. The body attempts to conserve water, but severe dehydration disrupts cellular and systemic processes.

Overheating: Reduced ability to regulate temperature.
Blood pressure: Kidneys conserve water, but blood flow to organs may decrease.
Organ failure: Severe dehydration can be fatal.

Hydrophilic and Hydrophobic Substances

Definitions and Biological Importance

Hydrophilic substances have an affinity for water and dissolve easily, while hydrophobic substances do not interact favorably with water. Hydrophobic molecules, such as oils, are major components of cell membranes.

Hydrophilic: Polar or charged molecules (e.g., salts, sugars).
Hydrophobic: Nonpolar molecules (e.g., lipids, oils).
Cell membranes: Composed of hydrophobic lipid bilayers, creating selective barriers.

Acids, Bases, and pH

Acidic and Basic Conditions Affect Living Organisms

The concentration of hydrogen ions (H+) and hydroxide ions (OH-) in water determines its acidity or basicity. The pH scale is used to measure these properties.

Acid: Substance that increases H+ concentration.
Base: Substance that reduces H+ concentration.
Strong acids/bases: Dissociate completely in water.
Weak acids/bases: Reversibly release and accept hydrogen ions.

The pH Scale

The pH of a solution is defined by the negative logarithm of the H+ concentration:

Equation:
For a neutral solution at 25°C: , so
Acidic solutions: pH < 7
Basic solutions: pH > 7
Most biological fluids: pH 6–8

Solution	pH Value
Stomach acid	2
Cola	3
Tomato juice	4
Human blood	7.4
Bleach	13
Additional info: Most biological fluids are maintained within a narrow pH range for proper cellular function.

Buffers

Maintaining pH Stability

Buffers are substances that minimize changes in concentrations of H+ and OH- in a solution. They are crucial for maintaining the internal pH of living cells near 7.

Buffer systems: Often consist of a weak acid and its corresponding base.
Example: The bicarbonate buffer system in blood:

When pH drops, bicarbonate accepts H+; when pH rises, carbonic acid donates H+.

Acidification: A Threat to Water Quality

Human Impact and Ocean Acidification

Human activities, such as burning fossil fuels, release CO2 into the atmosphere. About 25% of this CO2 is absorbed by oceans, where it forms carbonic acid and lowers pH—a process called ocean acidification.

CO2 + H2O → H2CO3: Carbonic acid formation.
As seawater acidifies, H+ ions combine with carbonate ions to produce bicarbonate, reducing carbonate availability.
Calcification: Marine organisms need carbonate to produce calcium carbonate (CaCO3) for shells and skeletons.
Environmental impact: Ocean acidification threatens coral reefs and other marine life.

Process	Equation
CO2 dissolves in water
Carbonic acid dissociates
Additional info: Reduced carbonate affects shell-building organisms.

Summary Table: Key Properties of Water

Property	Description	Biological Importance
Cohesion	Hydrogen bonds hold molecules together	Transport in plants
Temperature moderation	High specific heat	Climate stability
Expansion upon freezing	Ice less dense than liquid	Insulates aquatic life
Versatility as solvent	Dissolves many substances	Facilitates reactions