# Water and inorganic ions

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u2-water-and-inorganic-ions/

This module covers the key properties of water that make it essential for all life, and the core biological roles of common inorganic ions. It is a foundational topic for all further study of biological molecules and processes, and frequently appears in early exam questions.

**Prerequisites:** Basic understanding of covalent bonding and molecular polarity

## Learning objectives

- Describe the structure and polarity of water, and how this leads to hydrogen bonding
- Explain how each key property of water supports biological processes in living organisms
- Recall the key biological roles of common inorganic ions required by CIE syllabus
- Answer exam questions linking structure/property to function for water and ions

## Structure and Polarity of Water

**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 ($\delta^-$) and each hydrogen atom with a partial positive charge ($\delta^+$). This unequal charge distribution makes water polar.

**Check your understanding**

Check your understanding before proceeding:

1. 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

   *Answer:* Oxygen is partially negative, hydrogens are partially positive

   *Why:* Correct! Differences in electronegativity create the partial charge distribution that makes water polar.

## Biological Properties of Water

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.

> **tip**
>
> In CIE exams, always explicitly link a property of water to its biological role. You will not get full marks for only listing properties.

## Inorganic Ions and Their Biological Roles

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

*Notation:* Cations (positive charge) / Anions (negative charge)

| 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 ($Mg^{2+}$): Magnesium forms the central part of the chlorophyll molecule, which absorbs light energy for photosynthesis in plant cells.
3. 2. Phosphate ions ($PO_4^{3-}$): Phosphate groups form the sugar-phosphate backbone of DNA and RNA, which store genetic information in all living organisms.

## Common pitfalls

- **Wrong:** Confusing covalent bonds within water with hydrogen bonds between water molecules.
  - Why it fails: Markers specifically test this distinction, and will deduct marks for mixing up intramolecular and intermolecular bonds.
  - Correct: Covalent bonds hold atoms together within a single water molecule; hydrogen bonds are weak attractions between separate water molecules.
- **Wrong:** Claiming ice is denser than liquid water.
  - Why it fails: Hydrogen bonding spaces molecules further apart in ice, not closer.
  - Correct: Ice is less dense than liquid water, so it floats and insulates aquatic organisms below ice in cold climates.
- **Wrong:** Saying magnesium ions are a component of hemoglobin.
  - Why it fails: Confuses the ion cofactor for chlorophyll (plants) with hemoglobin (animals).
  - Correct: Magnesium is the core ion of chlorophyll; iron is the ion found in mammalian hemoglobin.
- **Wrong:** Only listing properties of water in an explain question, without linking to a role.
  - Why it fails: CIE mark schemes require an explicit link between property and function to award full marks.
  - Correct: Structure your answer as: Property → how it arises from water structure → biological role.
- **Wrong:** Claiming all inorganic ions do not contain carbon.
  - Why it fails: Some simple inorganic ions (like hydrogen carbonate) contain carbon, but are still classified as inorganic.
  - Correct: Inorganic ions are defined by not having a carbon-carbon backbone, not by the complete absence of carbon.

## 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 |

## 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.

- [Carbohydrates](https://www.owlsprep.com/study/cie-9700-u2-carbohydrates/)
- [Lipids](https://www.owlsprep.com/study/cie-9700-u2-lipids/)
- [Amino acids and proteins](https://www.owlsprep.com/study/cie-9700-u2-amino-acids-and-proteins/)

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