IBDP Biology 2025 SL&HL: A1.1 Water Study Notes

A1.1.1 Water as the Medium for Life

Key Points:

• Water as the Origin of Life: The first cells originated in water, and it remains the medium for most biological processes.
• Properties of Water: Due to its structure, water has many useful properties, making it crucial for life on Earth.
• Essential Idea: Water is fundamental as the medium of life.

Nature of Science:

• Theories in Science: Use of theories to explain phenomena, such as hydrogen bonds between water molecules explaining water's properties.
• Hydrogen Bonds: While scientists hypothesize hydrogen bonds exist between water molecules, absolute proof is challenging.

Importance of Water for Life:

• Universal solvent: Water dissolves and transports molecules.
• Metabolite role: Participates in condensation and hydrolysis reactions.
• Temperature buffer: Helps maintain temperature in organisms and environments.
• Structural maintenance: Supports structures like cell membranes, proteins, and DNA.

A1.1.2 Hydrogen Bonds and Polar Covalent Bonds

Key Concepts:

• Atomic Structure: Understanding atoms (protons, neutrons, electrons) and their differences from ions.
• Ion Types: Anion (negative ion) and cation (positive ion).
• Bond Types: Differences among covalent, ionic, and hydrogen bonds.

Water Molecule Structure:

• Molecular Formula: H₂O, with a bent shape due to polar covalent bonds.
• Polar Nature: Unequal sharing of electrons between O and H leads to partial charges.
• Hydrogen Bond Formation: Attraction between slightly positive H and slightly negative O in adjacent molecules.

Impact of Hydrogen Bonds:

• Hydrogen bonds, despite being weak individually, collectively give water unique properties essential for life.

Location of Polar Covalent Bonds:

• Two polar covalent bonds exist in a water molecule, between the O atom and each H atom.