Carbohydrates: MDCAT Biology notes
Carbohydrates MDCAT notes: monosaccharides, ring forms, glycosidic bonds, disaccharides, oligosaccharides, starch, glycogen, cellulose, chitin and tests.
What carbohydrates are
Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give these units on hydrolysis. They contain C, H and O, usually with H and O in the ratio 2:1. The general formula is $\mathrm{C_x(H_2O)_y}$. For monosaccharides this becomes $\mathrm{C_n(H_2O)_n}$, or $\mathrm{(CH_2O)_n}$, and for polysaccharides it is $\mathrm{C_n(H_2O)_{n-1}}$, because one water is lost per bond. Carbohydrates are classified by the number of sugar units they contain.
Monosaccharides
- These are simple sugars with 3–7 carbons. They are sweet, soluble, crystalline and reducing. Monosaccharides are the sweetest class of carbohydrates.
- Aldoses carry an aldehyde group, for example glyceraldehyde (a triose), ribose (a pentose), glucose and galactose (hexoses).
- Ketoses carry a ketone group, for example dihydroxyacetone (a triose), ribulose (a pentose) and fructose (a hexose).
- Glucose ($\mathrm{C_6H_{12}O_6}$) is the main respiratory fuel. Glucose and fructose have the same molecular formula but different structures, so they are isomers.
- In water, pentoses and hexoses form rings. Glucose forms a six-membered glucopyranose ring and ribose forms a five-membered ribofuranose ring.
- Monosaccharides are parts of DNA and RNA (deoxyribose and ribose), ATP, NAD, NADP and ribulose bisphosphate.
Glycosidic bond
Two monosaccharides join by condensation, which removes a molecule of water and forms a glycosidic bond (a C–O–C link). This C–O bond holds complex carbohydrates together. Hydrolysis adds water and breaks the bond.
Oligosaccharides and disaccharides
Oligosaccharides give 2–10 monosaccharide units on hydrolysis. Some newer textbooks list disaccharides separately and define oligosaccharides as 3–10 units. Anything above 10 units, from 11 upward, is a polysaccharide. All three common disaccharides have the formula $\mathrm{C_{12}H_{22}O_{11}}$.
| Disaccharide | Units | Source | Reducing? |
|---|---|---|---|
| Sucrose | α-glucose + β-fructose (1→2 link) | Cane sugar, sugar beet | No, because the 1→2 link ties up both reducing groups |
| Lactose | Glucose + galactose | Milk sugar | Yes |
| Maltose | Glucose + glucose | First product of starch hydrolysis | Yes |
Diabetics limit sucrose and lactose because digesting them releases glucose into the blood.
Polysaccharides
Polysaccharides are large polymers, often of many thousands of units. They are tasteless, have a high molecular weight, are only sparingly soluble or insoluble, and are made by condensation.
| Polysaccharide | Monomer | Form | Role |
|---|---|---|---|
| Starch: amylose | α-glucose | Unbranched; soluble in hot water | Plant storage |
| Starch: amylopectin | α-glucose | Branched; insoluble in water | Plant storage |
| Glycogen | α-glucose | Highly branched | Animal starch, stored in liver and muscle |
| Cellulose | β-glucose | Unbranched fibres | Plant cell walls; dietary fibre that humans cannot digest |
| Chitin | N-acetylglucosamine | Unbranched | Fungal cell walls, arthropod exoskeleton |
Starch and cellulose are both polymers of glucose, and both are hexose polysaccharides. Iodine gives an intense blue colour with starch and a red colour with glycogen.
Common MDCAT traps
- Sucrose is non-reducing because of its α1→β2 glycosidic link; galactose, lactose and maltose are reducing.
- Glycogen, not cellulose, is the branched α-glucose polymer found in animals.
- Starch first breaks down to maltose, not directly to glucose.
- Only sucrose gives glucose and fructose on hydrolysis.
- Glucagon is a hormone and glycogen is the stored carbohydrate.
Quick revision
- Monosaccharides: $\mathrm{C_n(H_2O)_n}$.
- Glycosidic bonds form by removing water.
- Sugarcane contains sucrose; milk contains lactose.
- Chitin forms fungal cell walls and arthropod exoskeletons.
- Glycogen is the short-term carbohydrate store in animals.
- Polysaccharides have more than 10 units.