Carbohydrates

Free Carbohydrates revision notes for OCR A Level Biology – covering specification points 2.1.2(d), 2.1.2(e), 2.1.2(f) and 2.1.2(g).

Carbohydrates are a group of biological molecules that are a key source of energy and have structural roles in both animals and plants; glucose is one of the most important.

Glucose comes in two forms (isomers) called α (alpha) glucose and beta (β) glucose.

Alpha glucose and beta glucose - Carbohydrates OCR A Level Biology

Monosaccharides

Monosaccharides are individual sugar monomers, such as glucose.

The table below outlines some of the most common monomers used to build larger carbohydrates:

Monosaccharide	Molecular Formula	Type	Use
α-Glucose	C₆H₁₂O₆	Hexose	Energy source and primary respiratory substrate.
β-Glucose	C₆H₁₂O₆	Hexose	Energy source, a component of glycolipids and glycoproteins
Ribose	C₅H₁₀O₅	Pentose	A component of nucleotides (e.g. ATP, RNA)

All of these monosaccharide monomers are reducing, so they test positive in a Benedict’s test.

Disaccharide and Polysaccharide Formation

Carbohydrates like disaccharides and polysaccharides are made by joining monosaccharides using glycosidic bonds.

Glycosidic bonds form in condensation reactions between the hydroxyl groups of two monosaccharides, releasing a water molecule as waste.

Glycosidic bond in a condensation reaction - Carbohydrates OCR A Level Biology

When carbohydrates are hydrolysed (digested), enzymes (e.g. amylase or maltase) break the glycosidic bonds using water, releasing smaller sugars or monosaccharides.

Maltose hydrolysis of its glycosidic bond - Carbohydrates OCR A Level Biology

Disaccharides

Disaccharides are two sugar molecules joined together with a glycosidic bond.

The table below outlines the most common disaccharides formed from monosaccharides:

Disaccharide	Monomers joined	Use
Cellobiose	β-Glucose + β-Glucose	Intermediate in cellulose breakdown
Lactose	α-Glucose + β-Galactose	Sugar in milk and an energy store
Maltose	α-Glucose + α-Glucose	Intermediate in starch digestion
Sucrose	α-Glucose + Fructose	Transport sugar in plants

Of these disaccharides, only sucrose is non-reducing, so it gives a negative result in a Benedict’s test.

Polysaccharides

Polysaccharides are long chains of sugar monomers joined by glycosidic bonds.

The table below outlines the structure of the 4 main polysaccharides:

Polysaccharide	Monomers	Glycosidic Link	Structure
Cellulose	β-Glucose + β-Glucose	1-4	Straight chain with many hydrogen bonds between and within chains.
Glycogen	α-Glucose + α-Glucose	1-4 and 1-6	Coiled (less than starch) and highly branched
Amylose	α-Glucose + α-Glucose	1-4	Coiled into a spiral, held together by hydrogen bonds
Amylopectin	α-Glucose + α-Glucose	1-4 and 1-6	Coiled into a spiral held together by hydrogen bonds, but with branches (less than glycogen)

The table below outlines how structure relates to the function of each polysaccharide’s use:

Polysaccharide	Use(s)	How Structure Supports Function
Cellulose	Structural support in plant cell walls	Many hydrogen bonds between fibres provide tensile strength and rigidity.
Glycogen	Energy storage in animals	Compact spiral to store many glucose molecules. 1-6 glycosidic bonds create branches providing many access points for enzymes to release glucose molecules quickly.
Amylose	Long-term (slow-release) energy storage in plants	Compact spiral to store many glucose molecules.
Amylopectin	Energy storage in plants	Compact spiral to store many glucose molecules. 1-6 glycosidic bonds create branches providing many access points for enzymes to release glucose molecules quickly.

Polysaccharides are broken down into monomers and disaccharides during digestion by enzymes, typically to release monomers, which can then be used to release energy in respiration.

Carbohydrates

Monosaccharides

Disaccharide and Polysaccharide Formation

Disaccharides

Polysaccharides

Want to dive deeper?

Bundle: Carbohydrates

Textbook: Carbohydrates

MCQs: Carbohydrates

Flashcards: Carbohydrates