14_A1.1 WaterHodder 2025 Biology for the IB Diploma Third Edition

A1.1 Water Concept: Unity

• Guiding Questions and Diversity

  • Common ancestry gives living organisms shared features, while evolution leads to biodiversity of life.

  • All living organisms require water to exist.

• Role of Water

  • Enzymes need water to function as they work in a chemically stable medium.

Essential Properties of Water

• Physical and Chemical Properties

  • What makes water essential for life?

  • Challenges and opportunities of water as a habitat.

Syllabus Content Overview

• Subtopics Covered

  • A1.1.1: Water as the medium for life

  • A1.1.2: Hydrogen bonds from polar covalent bonds in water

  • A1.1.3: Cohesion due to hydrogen bonding

  • A1.1.4: Adhesion impacts and organisms

  • A1.1.5: Solvent properties in metabolism and transport

  • A1.1.6: Properties' consequences in aquatic habitats

  • A1.1.7: Extraplanetary origin of water on Earth (HL only)

  • A1.1.8: Water's presence and search for extraterrestrial life (HL only)

Water: The Medium for Life

• Water Coverage on Earth

  • 71% of Earth's surface is water; 97% is oceanic, 3% is freshwater.

  • Evidence suggests water has existed for 3.8 billion years.

• Origin of Life In Water

  • First cells originated in water, with oceans blocking harmful UV radiation.

  • Water comprises 65%-95% of mass in most multicellular organisms.

• Consequences of Water Properties

  • Unique properties allow life at various scales, from bacteria to trees.

  • Water is essential for survival; its properties often taken for granted.

Freshwater as a Limited Resource

• Activity

  • Group work: Create a poster on threats to freshwater sources and solutions for clean drinking water.

Theme A: Unity and Diversity – Molecules

• Chemical Bond Definitions

  • Covalent Bond: Between atoms sharing pairs of electrons.

  • Polar Molecule: Unequal charge distribution in a molecule.

  • Hydrogen Bond: Weak attractions between hydrogen (in one molecule) and electronegative atoms (like oxygen) in another.

Structure of Water Molecules

• Water Molecule Description

  • Consists of one oxygen and two hydrogen atoms, covalently bonded.

  • Polar nature leads to hydrogen bonding, forming a V-shaped molecule.

  • Causes water's unique properties due to the charge distribution (net negative on oxygen, net positive on hydrogen).

Properties of Water

• Hydrogen Bonds

  • Responsible for unique water properties.

  • Intermolecular forces impact biological systems heavily.

• Cohesion and Adhesion

  • Cohesion: Attraction between water molecules leads to surface tension.

  • Adhesion: Attraction to other polar surfaces allows water to move through substances like xylem in plants.

Water Transport in Plants

• Transpiration Process

  • Water drawn through xylem by evaporative loss from leaves.

  • Cohesion allows the water column to remain unbroken despite tension.

Viscosity and Flow of Water

• Low Viscosity

  • Water flows easily due to lower viscosity compared to other liquids.

  • The property is crucial for nutrient transport in organisms.

• Surface Tension Exploitation

  • Allows organisms like the pond skater to move on water surfaces without sinking.

Thermal Properties of Water

• Specific Heat Capacity

  • Requires significant energy to change temperature due to hydrogen bonding.

  • Coral water helps maintain stable aquatic environments compared to terrestrial temperature fluctuations.

Biochemical Relevance of Water

• Role in Enzymatic Reactions

  • Most enzymes require aqueous environments for effective function.

  • Hydrogen bonds aid enzyme-substrate binding relationship during reactions.

Extraplanetary Water Origin (HL only)

• Formation Theories

  • Origin linked to asteroids and their content of ice and organic material.

  • Carbonaceous chondrites may have delivered water to Earth, indicating a potential life-supporting origin.

Goldilocks Zone and Extraterrestrial Life

• Definition and Importance

  • Goldilocks zone: Regions where conditions are just right for liquid water.

  • Essential in the search for life-supporting planets beyond Earth.

• Current Investigations

  • NASA's quest for water on Mars, using transit spectroscopy for discovery.

  • Identifying 'water signatures' in exoplanets to ascertain potential for life.