Key Concepts in IB Biology: Water

Overview

This lecture covers the essential concepts related to water in the context of IB Biology. It focuses on the molecular structure of water, its physical properties, and its role in biological processes and as an aquatic habitat.

Molecular Structure of Water

• Composition: Water is made up of two hydrogen atoms covalently bonded to one oxygen atom.
  • Covalent bond involves sharing of electrons.
• Polarity: Oxygen is more electronegative than hydrogen, pulling shared electrons closer and creating a partial negative charge on oxygen and a partial positive charge on hydrogen.
• Shape: Bent shape with a 105° angle between the hydrogen-oxygen-hydrogen bonds.
• Dipole Moment: Water's polarity results in a dipole moment, making it polar.

Hydrogen Bonding

• Water molecules are attracted to each other through hydrogen bonds.
• Hydrogen bonds occur between the negative oxygen atom of one water molecule and the positive hydrogen atom of another.
• These bonds give water its unique properties, such as high boiling point and strong solvent ability.

Solvent Ability

• Water's polarity attracts other polar molecules, making it a powerful solvent.
• Hydrophilic substances dissolve in water due to hydrogen bonding.
• Biological Importance:
  • Early Earth: Primordial oceans as medium for formation of polymers building cells.
  • Modern Cells: Cytosol in cells is 70% water, facilitating chemical reactions.
  • Transport in Blood: Hydrophobic molecules like cholesterol require packaging (e.g., lipoproteins) due to water's solvent properties.

Physical Properties of Water

Specific Heat Capacity

• Water has high specific heat capacity, absorbing and storing heat, then releasing it slowly.
• Importance for marine environments: Creates warmer microclimates.

Thermal Conductivity

• Water has high thermal conductivity due to rapid energy transfer through hydrogen bonding.
• Example: Ringed seal's blubber insulates against heat loss in water.

Buoyancy and Viscosity

• Buoyancy: Water's density provides upward force, aiding organisms in floating.
• Viscosity: Water's viscosity provides resistance, aiding swimming efficiency while not requiring excessive energy.

Cohesion and Adhesion

Cohesion

• Polar molecules with hydrogen bonds create attraction forces, causing surface tension.
• Example: Insects can move across water surface due to surface tension.

Adhesion

• Adhesion: Attraction between different substances.
• Examples:
  • Plant Xylem: Water adheres to cell walls of xylem, aiding upward movement against gravity.
  • Capillary Action: Water moves up thin tubes, driven by adhesion and cohesion.

Capillary Action

• Involves adhesion and cohesion, allowing water to move upwards against gravity.
• Seen in plant xylem and soil.
• Soil: Adhesion to soil particles, moving water through tiny spaces, essential for plant hydration.

Summary

• Water's polarity and hydrogen bonding play crucial roles in its high solvent ability, specific heat capacity, buoyancy, viscosity, and thermal conductivity.
• These properties make water fundamental for biological systems and natural environments, facilitating life-supporting functions in cells, organisms, and ecosystems.