2.1.3Additional lessons
Carbohydrates: Definition, Functions and Examples — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 142
Part 1 of 5Monosaccharides and Disaccharides
Monosaccharide
- The single sugar unit, with general formula CₙH₂ₙOₙ, which is a soluble colourless crystalline compound.
Hexoses and pentoses
- Hexoses such as glucose contain six carbon atoms, whereas pentoses such as ribose contain five carbon atoms.
Glucose
- C₆H₁₂O₆ is the main respiratory substrate and is made by photosynthesis;
- it is polar and soluble, so it is easily transported.
Disaccharide
- Two monosaccharides joined by a glycosidic bond, such as maltose (glucose and glucose) or sucrose (glucose and fructose).
Isomers
- Alpha glucose and beta glucose have the same formula but the -OH on carbon 1 points in different directions.
| Disaccharide | Made from | Where found |
|---|---|---|
| Maltose | Glucose + glucose | Germinating seeds |
| Sucrose | Glucose + fructose | Sugar cane, transported in plants |
| Lactose | Glucose + galactose | Milk |
Part 2 of 5Polysaccharides
Polysaccharides
Starch
- Plant storage polysaccharide of alpha glucose, made of amylose (unbranched helix) and amylopectin (branched), stored as granules.
Glycogen
- Animal and fungal storage polysaccharide of alpha glucose, more branched than amylopectin so glucose is released quickly.
Cellulose
- Structural polysaccharide of beta glucose forming straight chains, whose hydrogen bonding creates strong fibres in plant cell walls.
Why store as polysaccharides
- They are compact, insoluble and do not change water potential, so they will not draw water into cells.
Part 3 of 5Functions of Carbohydrates
1
Glucose is oxidised in cell respiration to release energy that is transferred to ATP.
2
Excess glucose is joined by condensation into starch in plants or glycogen in animals for short-term energy storage.
3
Cellulose is laid down in cell walls, giving plant cells strength so that they resist bursting when water enters.
4
Carbohydrate chains attached to proteins and lipids on membranes act in cell recognition and signalling.
5
Ribose and deoxyribose, which are pentoses, form part of the backbone of RNA and DNA.
Part 4 of 5Worked Example
Question
- Maltose is formed by condensation of two glucose molecules (C₆H₁₂O₆).
- Deduce the molecular formula of maltose.
- Two glucose molecules contain C₁₂H₂₄O₁₂ in total.
- A condensation reaction releases one water molecule, H₂O.
- Subtract H₂ and O from the total: C₁₂H₂₂O₁₁.
Answer Maltose has the molecular formula C₁₂H₂₂O₁₁.
Part 5 of 5Exam Checklist & Tips
2.1.3 – Key Points for the Exam
- Monosaccharide A single sugar unit, such as glucose, fructose or galactose.
- Glycosidic bond The covalent bond that joins two sugar units after a condensation reaction.
- Amylose The unbranched helical component of starch.
- Amylopectin The branched component of starch.
- Cellulose A polysaccharide of beta glucose that strengthens plant cell walls.
Common Mistakes
- Saying starch is made of beta glucose; starch and glycogen use alpha glucose, cellulose uses beta glucose.
- Stating that animals store starch; animals store glycogen.
- Giving the formula C₆H₁₂O₆ for every monosaccharide; the general formula is CₙH₂ₙOₙ.
- Forgetting that cellulose is straight and unbranched while glycogen is highly branched.
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