Water and inorganic ions

CIE A-Level Biology· Unit 2: Biological Molecules, Topic 1· 15 min read

1. Structure and Polarity of Water★☆☆☆☆⏱ 5 min

📘 Definition

Polar Molecule

A molecule with unequal electron distribution, creating a partially positive end and a partially negative end due to differences in electronegativity between bonded atoms

Example:

Water is a classic example of a small polar molecule

A single water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms. Oxygen is significantly more electronegative than hydrogen, so it pulls shared electrons closer to its nucleus.

📐 Worked Example

Explain why water is classified as a polar molecule.

1
Polarity occurs when bonded atoms have large differences in electronegativity (ability to attract shared electrons).
2
Oxygen has a much higher electronegativity than hydrogen, so it pulls the electrons in O-H covalent bonds towards itself.
3
This leaves the oxygen atom with a partial negative charge ( $δ^{-}$ ) and each hydrogen atom with a partial positive charge ( $δ^{+}$ ). This unequal charge distribution makes water polar.

✓ Quick check

Check your understanding before proceeding:

Which statement correctly describes the charge distribution in water?
- Oxygen is partially positive, hydrogens are partially negative
- Oxygen is partially negative, hydrogens are partially positive
- All atoms have no net charge
- Only oxygen has a full negative charge
Reveal answer
1 —
Correct! Differences in electronegativity create the partial charge distribution that makes water polar.

2. Biological Properties of Water★★☆☆☆⏱ 7 min

The polarity of water allows it to form weak hydrogen bonds between adjacent water molecules. These hydrogen bonds give water a unique set of properties that are critical for life:

Cohesion: Attraction between water molecules, due to hydrogen bonding
Adhesion: Attraction between water and other polar molecules
Good solvent: Dissolves polar and ionic substances
High specific heat capacity: Resists temperature change
High latent heat of vaporization: Requires lots of energy to evaporate
Lower density of ice than liquid water: Ice floats on liquid water

📐 Worked Example

Explain how two properties of water make it suitable for transport in plant xylem.

1
Xylem transports water and dissolved ions from roots to leaves. The first key property is cohesion:
2
Cohesion is the attraction between water molecules, which arises from hydrogen bonding between polar water molecules.
3
As water evaporates from the leaves, cohesion pulls a continuous column of water up the narrow xylem vessels.
4
The second key property is adhesion, the attraction between water molecules and the cellulose in xylem walls.
5
Adhesion prevents the water column from breaking under gravity, supporting continuous upward transport of water to all parts of the plant.

3. Inorganic Ions and Their Biological Roles★★☆☆☆⏱ 8 min

📘 Definition

Inorganic Ion

Cations (positive charge) / Anions (negative charge)

Charged particles that do not contain carbon-carbon backbones, dissolved in aqueous solution in cells and required for biological function.

Ion	Type	Key Biological Role	Example Organism
Ca $^{2 +}$	Cation	Bone/teeth structure, blood clotting, nerve function	Mammals
Na $^{+}$	Cation	Nerve impulse transmission, glucose co-transport	Animals
K $^{+}$	Cation	Stomata regulation, resting nerve potential	Plants & Animals
Mg $^{2 +}$	Cation	Central component of chlorophyll	Plants
NO $_{3}^{-}$	Anion	Nitrogen source for amino acid synthesis	Plants
PO $_{4}^{3 -}$	Anion	Component of DNA, ATP, and phospholipids	All organisms

📐 Worked Example

Name two inorganic ions and explain one biological role of each. [4 marks]

1
For 4 marks, you get 1 mark for each correctly named ion and 1 mark for each correctly linked role. A full mark answer is:
2
1. Magnesium ions ( $M g^{2 +}$ ): Magnesium forms the central part of the chlorophyll molecule, which absorbs light energy for photosynthesis in plant cells.
3
1. Phosphate ions ( $P O_{4}^{3 -}$ ): Phosphate groups form the sugar-phosphate backbone of DNA and RNA, which store genetic information in all living organisms.

4. Common Pitfalls

Wrong move:

Confusing covalent bonds within water with hydrogen bonds between water molecules.

Why:

Markers specifically test this distinction, and will deduct marks for mixing up intramolecular and intermolecular bonds.

Correct move:

Covalent bonds hold atoms together within a single water molecule; hydrogen bonds are weak attractions between separate water molecules.

Wrong move:

Claiming ice is denser than liquid water.

Why:

Hydrogen bonding spaces molecules further apart in ice, not closer.

Correct move:

Ice is less dense than liquid water, so it floats and insulates aquatic organisms below ice in cold climates.

Wrong move:

Saying magnesium ions are a component of hemoglobin.

Why:

Confuses the ion cofactor for chlorophyll (plants) with hemoglobin (animals).

Correct move:

Magnesium is the core ion of chlorophyll; iron is the ion found in mammalian hemoglobin.

Wrong move:

Only listing properties of water in an explain question, without linking to a role.

Why:

CIE mark schemes require an explicit link between property and function to award full marks.

Correct move:

Structure your answer as: Property → how it arises from water structure → biological role.

Wrong move:

Claiming all inorganic ions do not contain carbon.

Why:

Some simple inorganic ions (like hydrogen carbonate) contain carbon, but are still classified as inorganic.

Correct move:

Inorganic ions are defined by not having a carbon-carbon backbone, not by the complete absence of carbon.

5. Quick Reference Cheatsheet

Water Property / Ion	Key Biological Role
Cohesion	Continuous water column in xylem
Adhesion	Prevents water column breaking
High specific heat capacity	Stable temperature for enzymes/habitats
High latent heat of vaporization	Cooling via sweating/transpiration
Good polar solvent	Transport + metabolic reaction medium
Ice less dense than water	Insulation for aquatic life
Ca²⁺	Bones, blood clotting, nerve function
Na⁺	Nerve impulses, co-transport
Mg²⁺	Chlorophyll component
NO₃⁻	Nitrogen source for plants
PO₄³⁻	Nucleic acids, ATP, phospholipids

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.

2022 · 1
MCQ on water properties
2021 · 2
Describe roles of inorganic ions
2023 · 1
Cohesion in xylem transport

Going deeper

syllabusCIE 9700 Unit 2 Syllabus ExtractOfficial syllabus for this topic

What's Next

This topic provides the foundational inorganic context for all biological processes. All organic biological molecules function in aqueous solution, and depend on water polarity and inorganic ion cofactors to work correctly. This topic is frequently tested as multiple choice or short answer questions in CIE exams, so memorizing the ion roles and property-function links is an easy way to secure early marks. Next, you will study the major classes of organic biological molecules that make up living organisms, starting with carbohydrates, then lipids, proteins, and nucleic acids.