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Cellular respiration

Breaking down glucose (food) without oxygen to provide available energy for the cells. The glucose reacts with oxygen to produce energy in the form of ATP with carbon dioxide and water as waste products


Single sugars units usually containing 3, 5 or 6 carbon atoms eg glucose

Glycosidic bond

The bond formed by a condensation reaction between two monosaccharide units.


Complex carbohydrates consisting of more than one sugar molecule

Cell membrane

The membrane which forms the boundary between the cytoplasm of a cell and the medium surrounding it and controls the movement of substances into and out of the cell.


Double sugars - two monosaccharide units joined together in a condensation reaction forming a glycosidic bond between them eg sucrose


Proteins that have a carbohydrate chain attached to them. The carbohydrate chain sticks out of the outside of the cell and is part of the cell recognition system.


Lipids that have a carbohydrate chain attached to them. The carbohydrate chain is attached to the outside of the cell and is part of the cell recognition system.


Ribose sugar which has lost one oxygen molecule involved in the structure of DNA.


A type of sugar: a mono saccharide with 6 carbon atoms (a hexose sugar).


Fungi (singular fungus) are either uni-cellular, as in yeasts, or multi-cellular, as in mushrooms, toadstools and moulds. Fungi have a nucleus, cytoplasm and a cell wall


A large organ in the upper abdomen which manufactures, stores and breaks down substances as required by the body


A common term for the digestive system.

Carbohydrates: monosaccharides, disaccharides and oligosaccharides

What have the cell walls of fungi, the tough outer coverings of insects, the cell walls of plants, the energy store in the human liver and the fuel for respiration in cells got in common? They are all carbohydrates, a group of organic compounds containing carbon, hydrogen and oxygen which range from very small individual molecules to very large macromolecules. They are usually classified as monosaccharides (single sugars), disaccharides (double sugars), oligosaccharides (several sugars: 3-9 units) or polysaccharides (complex carbohydrates often composed of hundreds or thousands of units which form macromolecules).

Different carbohydrates

Carbohydrates are everywhere!


These simple sugars contain carbon, hydrogen and oxygen and have a general formula (CH2O)n. In theory n can be any number but it is usually between 3 and 6. The monosaccharides most commonly studied include:

  • triose sugars (n=3) e.g. glyceraldehydes which play an important role in cellular respiration (e.g. C3H6O3)
  • pentose sugars (n=5) e.g. ribose and deoxyribose , which are important in the structure of DNA and RNA (e.g. C5H10O5)
  • hexose sugars (n=6) e.g. glucose, a key molecules in cellular respiration and photosynthesis (e.g. C6H12O6)

Two hexose sugars glucose (α glucose and β glucose) and fructose


Disaccharides are made up to two monosaccharide units joined together. Examples include the sucrose we eat (usually known as just sugar), and lactose, the sugar which sweetens milk. Disaccharides are joined using a simple condensation reaction which forms a bond between the two monosaccharide units known as a glycosidic bond. Glycosidic bonds are broken in a hydrolysis reaction to produce two monosaccharides.

Glucose and fructose

The formation and hydrolysis of a glycosidic bond

The arrangement of the atoms in monosaccharide molecules is complex due to the arrangement of the carbon bonds. As a result, several different isomers are common. This in turn affects the way bonds between the monosaccharide units are formed. Glycosidic bonds are named as a result of both the arrangement of the side groups (e.g. α and β) and by the numbers of the carbon atoms which are involved (e.g. 1,4 or 1,6). So, for example, you can have an α-1,4 glycosidic bond or a β -1,6 glycosidic bond.

Alpha and beta glycosidic bonds

α and β glycosidic bonds


These short chains of sugar molecules are usually found associated with another molecule including proteins (glycoproteins) and lipids (glycolipids). Oligosaccharides are very important as part of the cell recognition systems on cell membranes. Oligosaccharides also seem to be important for maintaining a healthy gut flora. They are found in certain fruits and vegetables including onions, leeks and asparagus.