Carbohydrates are one of the many different biological molecules you must know about.
Monosaccharides
Monosaccharides are the monomer unit of carbohydrates. They join together during condensation reactions to form carbohydrates - held together by glycosidic bonds.
Examples of monosaccharides include: glucose, galactose and fructose.
Glucose
Glucose has two isomers, these are: alpha-glucose and beta-glucose. This means that they have the same chemical formula, but the arrangement of their atoms is different.
Alpha-glucose forms straight-chain bonds
Beta-glucose forms branched bonds
Disaccharides
Disaccharides are formed when two monosaccharides are joined together during a condensation reaction and are held together with a glycosidic bond.
| Molecule | Description |
|---|---|
| Maltose | composed of two glucose molecules (held by an alpha 1, 4 glycosidic bond) |
| Sucrose | composed of a glucose molecule and a fructose molecule (held by an alpha 1, 4 glycosidic bond) |
| Lactose | composed of a glucose molecule and a galactose molecule (held by a beta 1, 4 glycosidic bond) |
You'll also need to remember which monosaccharide forms which disaccharide
| Monosaccharides | Disaccharide Product |
|---|---|
| glucose + glucose | maltose |
| glucose + fructose | sucrose |
| glucose + galactose | lactose |
Polysaccharides
Polysaccharides are formed when multiple monosaccharides are joined together during condensation reactions - this results in glycosidic bonds.
| Molecule | Description |
|---|---|
| Glycogen | polymer composed of alpha-glucose |
| Starch | polymer composed of alpha-glucose |
| Cellulose | polymer composed of beta-glucose |
Starch
Starch is a large molecule used to store glucose, it is a highly compact molecule and can store lots of glucose molecules in a small space. Therefore, it is considered energy dense.
It is also insoluble which means that it cannot affect the osmotic balance of the cell - water cannot eneter or leave the cell via osmosis.
Starch is composed of two different types: amylose and amylopectin
Amylose
- It is used for the storage of glucose
- It is an alpha helice straight chain
- Makes up around 20% of starch
- Alpha-glucose molecules joined by alpha 1, 4 glycosidic bond
- It forms a helix structure
- There are few branch ends
- It is highly compact
- It is hydrolysed by amylase
Amylopectin
- It is highly branched
- Makes up around 80% of starch
- It is composed of alpha-glucose molecules joined by alpha 1, 4 glycosidic bonds with some alpha 1, 6 branches
- It has more branch endings than amylose
- It is hydrolysed by amylase quicker than amylose
Glycogen
Glycogen is used for the storage of glucose, and is more efficient than amylopectin. It is highly branched and compact - therefore it is considered energy dense.
It is composed of alpha-glucose molecules joined by alpha 1, 4 glycosidic bonds with alpha 1, 6 branches that are found every 20 to 30 monomers.
Glycogen is also insoluble, therefore it does not affect the osmotic balance of the cell. Also, it is a large molecule and cannot leave the cell.
Cellulose
Cellulose is the main component in cell walls - therefore it is very strong as thousands of chains link together. This provides the plant with structure and stability.
It is an unbranched chain composed of beta-glucose molecules joined by beta 1, 4 glycosidic bonds and the alternate glucose molecules are inverted. The hydrogen bonds between the chains produce microfibrils.
This carbohydrate is also fully permeable - meaning that water and numerous other substances can move through it e.g. water through a plant's membrane.
The enzyme which breaks cellulose down is cellulase.
Glucose is a heptose sugar. This means that each molecule of glucose has six carbon atoms.