Water

IB Biology SL· Theme A: Unity and Diversity, Unit 1, Water· 20 min read

1. Molecular Structure and Polarity★☆☆☆☆⏱ 6 min

Water has the molecular formula $e x t H_{2} e x t O$ , 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.

📘 Definition

Polar molecule

$H_{2}^{δ^{+}} O^{δ^{-}}$

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

Example:

In water, oxygen carries a partial negative charge ( $δ^{-}$ ) and each hydrogen carries a partial positive charge ( $δ^{+}$ ).

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.

2. Biologically Important Properties of Water★★☆☆☆⏱ 8 min

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
G	o	o	d		s	o	l	v	e	n	t		f	o	r		p	o	l	a	r	/	i	o	n	i	c		s	o	l	u	t	e	s
D	i	s	s	o	l	v	e	s		m	o	s	t		c	h	a	r	g	e	d	/	p	o	l	a	r		m	o	l	e	c	u	l	e	s
A	c	t	s		a	s		a		m	e	d	i	u	m		f	o	r		m	e	t	a	b	o	l	i	c		r	e	a	c	t	i	o	n	s	a	n	d	t	r	a	n	s	p	o	r	t

📐 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.

3. Hydrophobic and Hydrophilic Interactions★★☆☆☆⏱ 6 min

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.

📘 Definition

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.

4. Common Pitfalls

Wrong move:

Mixing up cohesion and adhesion

Why:

Examiners specifically test the distinction between these two properties, so mixing them up loses full marks

Correct move:

Remember root words: co- means same (cohesion = same molecules sticking together), ad- means toward (adhesion = toward other different molecules)

Wrong move:

Calling hydrogen bonds covalent bonds

Why:

Hydrogen bonds are intermolecular attractions, not covalent bonds that share electrons between atoms

Correct move:

Describe hydrogen bonds as weak electrostatic attractions between partial charges on different molecules

Wrong move:

Claiming ice is denser than liquid water

Why:

Hydrogen bonds in ice form a crystalline lattice that spaces molecules further apart than in liquid water

Correct move:

State that solid ice is less dense than liquid water, so ice floats on water

Wrong move:

Calling all biological molecules hydrophilic

Why:

Many key biological molecules (e.g. lipids, cholesterol) are non-polar and hydrophobic

Correct move:

Classify polar/ionic molecules as hydrophilic and non-polar molecules as hydrophobic

5. Quick Reference 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

6. Frequently Asked

Why is water called the universal solvent?

Water dissolves almost all polar and ionic substances, which makes it an excellent medium for metabolic reactions and transport of solutes in living organisms. It cannot dissolve non-polar substances like lipids, so the 'universal' label is not technically accurate.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 1
Water properties in xylem transport
2023 · 2
Structure of water linked to function

Going deeper

syllabusIB Biology SL Official SyllabusFull Theme A reference

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.