# Water

> IB Biology SL · Theme A: Unity and Diversity
> Source: https://www.owlsprep.com/study/ib-biology-sl-u1-water/

This sub-topic explores the molecular structure of water, the hydrogen bonding arising from its polarity, and how these structural features produce properties that are essential for supporting life across all biological systems.

**Prerequisites:** [Basic covalent and polar bonding](https://www.owlsprep.com/study/ib-bio-sl-introduction-to-biomolecules/)

## Learning objectives

- Describe the molecular structure and polar nature of water
- Explain how hydrogen bonding produces water's key biological properties
- Link water's properties to their functional roles in living organisms
- Distinguish between hydrophilic and hydrophobic interactions and their biological significance

## Molecular Structure and Polarity

Water has the molecular formula $\text{H}_2\text{O}$, with one oxygen atom covalently bonded to two hydrogen atoms. Oxygen is more electronegative than hydrogen, so it pulls shared electrons closer to its nucleus, creating an uneven distribution of charge across the molecule.

**Polar molecule** — A molecule with unequal distribution of charge, resulting in a partially negative pole and one or more partially positive poles.

*Notation:* $\text{H}_2^{\delta^+} \text{O}^{\delta^-}$

*Example:* In water, oxygen carries a partial negative charge ($\delta^-$) and each hydrogen carries a partial positive charge ($\delta^+$).

Water has a bent molecular shape (bond angle ~104.5°), which means the partial charges do not cancel out. This makes the entire molecule polar, and allows the formation of hydrogen bonds between adjacent water molecules.

**Worked example:** Explain how water's polarity leads to hydrogen bond formation.

1. First, identify the partial charges on water molecules
2. $$\delta^+ H - \delta^- O - \delta^+ H$$
3. Opposite charges attract: the partially negative oxygen of one water molecule is attracted to the partially positive hydrogen of a neighboring water molecule.
4. This weak electrostatic attraction is called a hydrogen bond. Each water molecule can form up to four hydrogen bonds with adjacent water molecules.

> **Exam tip:** Always link water's bent shape and polarity to hydrogen bonding in exam answers — most questions expect this structural link to function.

## Biologically Important Properties of Water

All of water's key biological properties arise from hydrogen bonding. Hydrogen bonds between water molecules are weak individually, but they are constantly breaking and reforming, so their collective effect is very significant.

| Property | Definition | Key Biological Role |
| --- | --- | --- |
| Cohesion | Water molecules stick to other water molecules | Creates surface tension for aquatic organisms; holds water columns together in plants |
| Adhesion | Water molecules stick to other polar substances | Enables capillary action in plant xylem |
| High specific heat capacity | Large energy input needed to change temperature | Stabilizes temperature for organisms and aquatic habitats |
| High latent heat of vaporization | Large energy needed to turn liquid water to gas | Makes evaporative cooling (sweating/transpiration) efficient |

**Worked example:** How do cohesion and adhesion work together to move water up a tall tree's xylem?

1. Transpiration (evaporation of water from leaves) creates a pull on the water column extending from leaves to roots.
2. Cohesion holds the entire column of water molecules together: hydrogen bonds between adjacent water molecules prevent the column from breaking as it is pulled upwards.
3. Adhesion attracts water molecules to the polar cellulose molecules that make up xylem cell walls. This counteracts the downward pull of gravity.
4. Together, these properties allow water to move against gravity from roots to leaves, supporting photosynthesis in the leaves.

> **Exam tip:** Do not just list properties — always link each property to a specific biological function to get full marks.

## Hydrophobic and Hydrophilic Interactions

Water's polarity causes it to interact differently with polar versus non-polar substances. These interactions are critical for the structure of many biological molecules and assemblies, including cell membranes.

**Hydrophilic vs Hydrophobic** — Hydrophilic (water-loving) substances are attracted to water, dissolve in water, and are polar or ionic. Hydrophobic (water-fearing) substances repel water, do not dissolve in water, and are non-polar.

*Example:* Sugars and amino acids are hydrophilic; fats and steroids are hydrophobic.

**Worked example:** How do hydrophobic interactions drive cell membrane structure?

1. All cell membranes are made of phospholipids, which have two regions: a hydrophilic phosphate head and two hydrophobic fatty acid tails.
2. Cell membranes sit in an aqueous environment (water-based cytoplasm inside the cell and extracellular fluid outside).
3. Hydrophilic heads are attracted to water molecules, so they orient towards the aqueous environments on both sides of the membrane.
4. Hydrophobic tails avoid contact with water, so they cluster together in the interior of the membrane, forming a bilayer.
5. This spontaneous arrangement is the foundation of all cell membrane structure, enabling compartmentalization of cellular processes.

## Common pitfalls

- **Wrong:** Mixing up cohesion and adhesion
  - Why it fails: Examiners specifically test the distinction between these two properties, so mixing them up loses full marks
  - Correct: Remember root words: *co-* means same (cohesion = same molecules sticking together), *ad-* means toward (adhesion = toward other different molecules)
- **Wrong:** Calling hydrogen bonds covalent bonds
  - Why it fails: Hydrogen bonds are intermolecular attractions, not covalent bonds that share electrons between atoms
  - Correct: Describe hydrogen bonds as weak electrostatic attractions between partial charges on different molecules
- **Wrong:** Claiming ice is denser than liquid water
  - Why it fails: Hydrogen bonds in ice form a crystalline lattice that spaces molecules further apart than in liquid water
  - Correct: State that solid ice is less dense than liquid water, so ice floats on water
- **Wrong:** Calling all biological molecules hydrophilic
  - Why it fails: Many key biological molecules (e.g. lipids, cholesterol) are non-polar and hydrophobic
  - Correct: Classify polar/ionic molecules as hydrophilic and non-polar molecules as hydrophobic

## Cheatsheet

| Property | Key Biological Role |
| --- | --- |
| Polar, bent structure | Allows hydrogen bond formation between water molecules |
| Cohesion | Xylem water transport, surface tension for aquatic life |
| Adhesion | Capillary action in plants, counteracts gravity |
| High specific heat capacity | Stabilizes organism and habitat temperature |
| High latent heat of vaporization | Enables efficient evaporative cooling |
| Good solvent for polar/ionic solutes | Medium for metabolism, solute transport |
| Ice < liquid water density | Ice floats, insulates aquatic life in winter |
| Hydrophobic interactions | Drives phospholipid bilayer membrane formation |

## What's next

Water is the foundational molecule for all life on Earth, and understanding its properties sets the stage for learning about other biological molecules and how they interact in living systems. Next, we explore the structure and function of the major classes of macromolecules, starting with carbohydrates, whose hydrophilic properties make them ideal for energy storage and transport. The principles of hydrophilic and hydrophobic interactions you learned here will also be critical for understanding lipid structure and cell membrane assembly later.

- [Nucleic acids](https://www.owlsprep.com/study/ib-biology-sl-u1-nucleic-acids/)
- [Cell Structure](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-structure/)
- [Cell membranes and transport](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-membranes-and-transport/)

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