# Biological Molecules

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u2-overview/
> Weight: n/a

This unit introduces all core inorganic and organic molecules that form living organisms, covering their structures, functions, and identification methods. It is the foundational biochemistry for all subsequent A-Level Biology topics.

**Prerequisites:** Basic understanding of atomic structure and chemical bonding

## Learning objectives

- Describe the structure and biological function of all key organic and inorganic biological molecules
- Relate molecular structure to specific functional roles in cells and organisms
- Recall the methodology and positive results of standard biochemical tests for biological molecules
- Apply knowledge of molecular properties to explain core biological processes

## Unit at a Glance

This unit follows a logical learning arc, starting with small inorganic molecules (water and ions) that are critical for life, then moving through each of the four major classes of biological macromolecules. We end with a dedicated sub-topic on practical biochemical testing, a frequent exam assessment area.

A core unifying theme across this unit is that a molecule's structure directly determines its function. This connection is tested repeatedly in CIE exams, so we will emphasize it across all sub-topics.

This unit is split into the following sub-topics:
- [Water and inorganic ions](https://www.owlsprep.com/study/cie-9700-u2-water-and-inorganic-ions/) — Explores the unique biologically important properties of water and functional roles of common inorganic ions.
- [Carbohydrates](https://www.owlsprep.com/study/cie-9700-u2-carbohydrates/) — Covers structure and function of monosaccharides, disaccharides, and polysaccharides.
- [Lipids](https://www.owlsprep.com/study/cie-9700-u2-lipids/) — Explores triglyceride, phospholipid, and steroid structure and their biological roles.
- [Amino acids and proteins](https://www.owlsprep.com/study/cie-9700-u2-amino-acids-and-proteins/) — Introduces general amino acid structure and the formation of peptide bonds.
- [Protein structure](https://www.owlsprep.com/study/cie-9700-u2-protein-structure/) — Covers the four levels of protein structure, bonding, and the process of denaturation.
- [Nucleotides](https://www.owlsprep.com/study/cie-9700-u2-nucleotides/) — Explores nucleotide structure, and the structure and roles of DNA and RNA nucleic acids.
- [Biochemical tests for biological molecules](https://www.owlsprep.com/study/cie-9700-u2-biochemical-tests-for-biological-molecules/) — Details procedures and positive/negative results for all standard biological molecule tests.

## Common pitfalls

- **Wrong:** Confusing alpha-glucose and beta-glucose structure
  - Why it fails: This leads to incorrect descriptions of starch, glycogen, and cellulose function
  - Correct: Remember alpha-glucose has -OH below carbon 1; beta-glucose has -OH above carbon 1
- **Wrong:** Mixing up the definitions of protein structure levels
  - Why it fails: Examiners regularly ask to identify each level from description, leading to easy lost marks
  - Correct: Primary = amino acid order, secondary = local folding, tertiary = 3D shape, quaternary = multiple polypeptide chains
- **Wrong:** Forgetting the hydrolysis step for non-reducing sugar tests
  - Why it fails: Students often report a negative result as confirming no sugar, rather than confirming non-reducing
  - Correct: Always add the acid hydrolysis and neutralization step after a negative initial Benedict's test

## Cheatsheet

| Key Concept | Core Fact |
| --- | --- |
| Water polarity | Polarity allows hydrogen bonding, creating cohesion, thermal stability, and solvent properties |
| Glycosidic bond | Formed by condensation between two monosaccharides, broken by hydrolysis |
| Phospholipid structure | Has a hydrophilic phosphate head and two hydrophobic fatty acid tails |
| Peptide bond | Covalent bond between carboxyl group of one amino acid and amino group of another |
| Protein denaturation | Breaks bonds holding tertiary structure together, changing shape and function |
| Nucleotide structure | Consists of a pentose sugar, nitrogenous base, and phosphate group |
| Biuret test | Tests for peptide bonds (proteins); purple colour = positive result |

## What's next

Start this unit with the first sub-topic on water and inorganic ions to build your foundation. Once you complete all sub-topics in this unit, you will move on to the next unit on enzymes, which builds directly on your knowledge of protein structure from this unit.

- [Water and inorganic ions](https://www.owlsprep.com/study/cie-9700-u2-water-and-inorganic-ions/)
- [Carbohydrates](https://www.owlsprep.com/study/cie-9700-u2-carbohydrates/)
- [Lipids](https://www.owlsprep.com/study/cie-9700-u2-lipids/)

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