8.3 Water and Its Role in Biological Systems

Water is the unsung hero of life. Its unique properties, like high heat capacity and universal solvent abilities, make it crucial for biological processes. From regulating temperature to enabling chemical reactions, water's versatility is unmatched in sustaining life.

At the cellular level, water is indispensable. It forms the cytoplasm, facilitates osmosis, and participates in vital biochemical reactions. Understanding water's role in pH regulation and enzyme function is key to grasping the intricacies of cellular processes and overall organism health.

Water's Properties and Biological Significance

Properties of water for life

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• Molecular structure bent shape with polar covalent bonds facilitates hydrogen bonding (ice crystals)
• High specific heat capacity regulates temperature in organisms (mammals)
• High heat of vaporization produces cooling effect through evaporation (sweating)
• Cohesion and adhesion enable capillary action in plants and create surface tension (water striders)
• Universal solvent properties dissolve polar and ionic substances (salt)
• Density anomaly causes ice to float on liquid water providing thermal insulation for aquatic ecosystems (frozen lakes)

Water's role in cellular function

• Cytoplasmic medium facilitates movement of molecules and organelles within cells
• Hydrophobic and hydrophilic interactions form and stabilize cell membranes (phospholipid bilayer)
• Turgor pressure provides structural support in plant cells (crisp lettuce)
• Osmosis moves water across cell membranes regulating cell volume (red blood cells)
• Hydration shell around biomolecules stabilizes proteins and nucleic acids (DNA double helix)

Water in biochemical processes

• Hydrolysis reactions break down macromolecules (digestion of food)
• Condensation reactions form macromolecules (protein synthesis)
• Hydration and dehydration reactions catalyze enzyme processes (ATP hydrolysis)
• Proton transfer facilitates acid-base reactions (stomach acid neutralization)
• Redox reactions drive electron transport chain in cellular respiration (mitochondria)
• Photosynthesis uses water as electron donor (chloroplasts)
• Transport of nutrients and waste products throughout organism (blood circulation)

pH concept in biological systems

• pH defined as negative logarithm of hydrogen ion concentration measured on scale from 0 to 14
• Acid-base equilibrium: $\text{H}_2\text{O} \leftrightarrow \text{H}^+ + \text{OH}^-$
• Buffer systems maintain physiological pH (bicarbonate, phosphate buffers)
• Enzyme activity depends on pH for optimal catalytic function (pepsin in stomach)
• Protein structure and function affected by pH ionization of amino acid side chains
• Membrane transport processes utilize pH gradients in cellular compartments (lysosomes)
• Blood pH regulation crucial for oxygen transport (hemoglobin binding)
• Intracellular pH varies for optimal function in different organelles (acidic lysosomes, alkaline mitochondria)