Biological Molecules

This lesson covers the key biological molecules — carbohydrates, lipids, proteins and DNA — as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to know the structure, function and chemical tests for each type of molecule.

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Overview

All living organisms are made from a relatively small number of chemical elements, mainly carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), with smaller amounts of sulfur (S) and phosphorus (P).

These elements combine to form four main types of biological molecule:

1. Carbohydrates — made of carbon, hydrogen and oxygen
2. Lipids (fats and oils) — made of carbon, hydrogen and oxygen
3. Proteins — made of carbon, hydrogen, oxygen, nitrogen (and sometimes sulfur)
4. Nucleic acids (DNA) — made of carbon, hydrogen, oxygen, nitrogen and phosphorus

Exam Tip: You need to know the basic structure, function, and chemical test for each biological molecule. This is a common exam topic worth several marks.

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Carbohydrates

Carbohydrates are molecules made of carbon, hydrogen and oxygen. They are used primarily as a source of energy and for energy storage.

Types of Carbohydrate

Type	Examples	Role
Simple sugars (monosaccharides)	Glucose, fructose, galactose	Quick source of energy for respiration. Glucose is the main sugar used in aerobic and anaerobic respiration.
Double sugars (disaccharides)	Sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose)	Transported in plants (sucrose) and found in food sources.
Complex carbohydrates (polysaccharides)	Starch, glycogen, cellulose	Energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plant cell walls).

Starch vs Glycogen vs Cellulose

Polysaccharide	Found in	Function	Structure
Starch	Plants (e.g. potato tubers, seeds)	Energy storage	Made of long chains of glucose. Compact and insoluble — does not affect osmosis.
Glycogen	Animals (stored in liver and muscles)	Energy storage	Similar to starch but more branched — can be broken down quickly when energy is needed.
Cellulose	Plant cell walls	Structural support	Long, straight chains of glucose with cross-links forming strong microfibrils. Very tough and rigid.

Exam Tip: Starch and glycogen are both energy storage molecules made of glucose, but starch is found in plants and glycogen in animals. Cellulose is structural, not an energy store.

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Lipids (Fats and Oils)

Lipids include fats (solid at room temperature) and oils (liquid at room temperature). They are made of carbon, hydrogen and oxygen, but contain a higher proportion of hydrogen than carbohydrates.

Structure

Each lipid molecule is made of:

• One glycerol molecule
• Three fatty acid chains

This structure is called a triglyceride.

Functions of Lipids

Function	Explanation
Energy storage	Lipids store more than twice the energy per gram compared to carbohydrates, making them an efficient long-term energy store.
Insulation	A layer of fat beneath the skin (adipose tissue) reduces heat loss from the body.
Protection	Fat cushions and protects vital organs (e.g. kidneys).
Cell membranes	Phospholipids (a modified lipid) form the bilayer of all cell membranes.
Waterproofing	Waxy lipids on leaf surfaces reduce water loss.

Exam Tip: Remember that lipids release more energy per gram than carbohydrates. This is why the body stores excess energy as fat — it is more compact.

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Proteins

Proteins are large, complex molecules made of long chains of amino acids. There are 20 different amino acids that can be arranged in different sequences to produce thousands of different proteins.

Structure

• Amino acids are joined together by peptide bonds during protein synthesis at the ribosomes.
• The specific sequence of amino acids determines the shape of the protein.
• The protein folds into a specific 3D shape — this shape is critical for its function.
• If the shape changes (e.g. due to high temperature or extreme pH), the protein is denatured and can no longer function.

Functions of Proteins

Protein Type	Example	Function
Enzymes	Amylase, protease, lipase	Biological catalysts — speed up chemical reactions (e.g. digestion)
Structural	Collagen, keratin	Provide strength and support (e.g. in tendons, skin, hair)
Hormones	Insulin, growth hormone	Chemical messengers that travel in the blood to target organs
Antibodies	Immunoglobulins	Produced by white blood cells to defend against pathogens
Transport	Haemoglobin	Carries oxygen in red blood cells
Receptors	Receptor proteins in cell membranes	Detect chemical signals (e.g. hormones, neurotransmitters)
Contractile	Actin, myosin	Enable muscle contraction

Exam Tip: The key concept is that protein shape determines protein function. If the shape is altered (denatured), the protein cannot work. This is especially important for enzymes.

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DNA (Deoxyribonucleic Acid)

DNA is a nucleic acid that carries the genetic instructions for building and maintaining an organism. It is found in the nucleus of eukaryotic cells (and as a circular loop in prokaryotic cells).

Structure of DNA

• DNA is a double helix — two strands twisted around each other like a twisted ladder.
• Each strand is made of repeating units called nucleotides.
• Each nucleotide consists of:
  • A deoxyribose sugar
  • A phosphate group
  • A nitrogenous base (one of four types)

The Four Bases

Base	Abbreviation	Pairs with
Adenine	A	Thymine (T)
Thymine	T	Adenine (A)
Cytosine	C	Guanine (G)
Guanine	G	Cytosine (C)

Complementary Base Pairing

The two strands of DNA are held together by complementary base pairing:

• A always pairs with T (and vice versa)
• C always pairs with G (and vice versa)

The bases are joined by hydrogen bonds.

This means that if you know the sequence of one strand, you can work out the sequence of the other.

Worked Example:

If one strand of DNA has the sequence: A T C G G T A

The complementary strand will be: T A G C C A T

DNA and Genes

• A gene is a short section of DNA on a chromosome that codes for a specific protein (or polypeptide).
• The sequence of bases in a gene provides the code for assembling amino acids in the correct order during protein synthesis.
• The human genome contains approximately 20,000–25,000 genes.

Exam Tip: Remember the base pairing rules: A-T and C-G. A common exam question gives you one DNA strand and asks you to write the complementary strand. Always double-check your answer.

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Testing for Biological Molecules

The Edexcel specification requires you to know the standard food tests for each biological molecule.

Summary of Food Tests