2.1.3Inquiry Process
Carbohydrates: Definition, Functions and Examples — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 39 of 160
Part 1 of 5Sugars and Their Bonds
Carbohydrates contain carbon, hydrogen and oxygen, usually with hydrogen and oxygen in the ratio 2:1.
Monosaccharides
- Single sugar units such as glucose, fructose, galactose and ribose, which are soluble and provide a quick energy source.
Disaccharides
- Two monosaccharides joined by a glycosidic bond made in condensation: maltose, sucrose and lactose are common examples.
Polysaccharides
- Many monosaccharides joined into long chains, which are mostly insoluble and used for storage or structure.
Glycosidic bond
- The covalent link between two sugar units, broken during digestion by hydrolysis to release monosaccharides.
| Disaccharide | Made from | Found in |
|---|---|---|
| Maltose | Glucose + glucose | Germinating seeds |
| Sucrose | Glucose + fructose | Transport sugar in plants |
| Lactose | Glucose + galactose | Milk |
Part 2 of 5Storage Polysaccharides
Both plants and animals keep glucose in reserve in a compact form.
Storage Polysaccharides
Amylose
- An unbranched coiled chain of α-glucose joined by 1,4 bonds;
- the coil is compact, which suits storage in plants.
Amylopectin
- A branched form of starch with extra 1,6 bonds, so many ends are available for fast enzyme action.
Glycogen
- The animal storage carbohydrate, more highly branched than amylopectin, so glucose can be released very rapidly.
Why store as polymers
- Large insoluble polymers do not alter the water potential of the cell and cannot easily diffuse out.
Part 3 of 5Structural Polysaccharide: Cellulose
Monomer
- Cellulose is made of β-glucose, in which alternate glucose units are inverted so the chain is straight.
Hydrogen bonding
- Hydroxyl groups on neighbouring chains form many hydrogen bonds, binding chains into strong microfibrils.
Cell wall role
- Microfibrils in plant cell walls resist the pressure of water entering the cell, preventing bursting.
Digestion
- Most animals lack an enzyme to hydrolyse β-1,4 bonds, so cellulose passes through as fibre.
Part 4 of 5Functions of Carbohydrates
1
Glucose is oxidised in respiration to supply ATP for cell activities.
2
Excess glucose is converted into starch in plants or glycogen in animals for short-term storage.
3
Cellulose forms the plant cell wall and gives support to the plant.
4
Ribose and deoxyribose form part of nucleotides in RNA and DNA.
5
Carbohydrate chains attached to proteins and lipids take part in cell recognition.
Part 5 of 5Exam Checklist & Tips
2.1.3 – Key Points for the Exam
- Monosaccharide A single sugar unit, the monomer of carbohydrates.
- Glycosidic bond A covalent bond joining two sugar molecules by condensation.
- Starch A plant storage polysaccharide made of amylose and amylopectin.
- Glycogen A highly branched polysaccharide used as a short-term energy store in animals.
- Cellulose A structural polysaccharide of β-glucose forming plant cell walls.
Common Mistakes
- Mixing up α-glucose (starch, glycogen) and β-glucose (cellulose).
- Saying humans store starch; animals store glycogen.
- Stating that glycogen is unbranched; it is highly branched.
- Forgetting that formation of a disaccharide releases a water molecule.
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