2.1 Essential ideas

2.1.3 Carbohydrates and lipids

Both carbohydrates and lipids are composed primarily of carbon, hydrogen and oxygen. These molecules are used to store and supply energy.

The structure of carbohydrates

condensation reactionFigure 2.1.3a – Condensation reaction
Monosaccharides link to form glycosidic bonds by condensation (shown: formation of maltose). 

  • A disaccharide is made of two monosaccharide monomers. Some examples are shown below:

Monosaccharide monomers

Disaccharide

Structure

Glucose + fructose

Sucrose

sucrose

Figure 2.1.3b

Glucose + galactose

Lactose

lactoseFigure 2.1.3c

Glucose + glucose Maltose maltoseFigure 2.1.3d

 

Application: Structure and function of starch, cellulose and glycogen 

Monosaccharide units

Polysaccharide

Type of linkage/bond

Structure

Alpha-glucose

alpha glucoseFigure 2.1.3e

Starch (two forms: mylose and amylopectin

Amylose:
1-4 glycosidic linkages

Unbranched, helical chains

Amylopectin:
1-4 and 1-6 glycosidic linkages

Branched, helical chains

Glycogen

1-4 and 1-6 glycosidic linkages

Highly branched, helical chains

Beta-glucose

betaglucoseFigure 2.1.3f

Cellulose

1-4 glycosidic linkages

  • Straight chains
  • Hydrogen bonds form between -OH groups from adjacent chains to form strong fibres

polysaccharidesFigure 2.1.3g – Polysaccharides
Polysaccharide storage molecules in animals (left) and plants (centre and right) are made of alpha-glucose.

celluloseFigure 2.1.3h – Cellulose
Plant cell walls are made of straight cellulose fibres strengthened by hydrogen bonds.

Structure of triglycerides

  • Substances commonly called fats and oils are composed of triglycerides.
  • Triglycerides are formed by condensation reactions of three fatty acids and one glycerol molecule.
  • The resulting bond is an ester bond.

Figure 2.1.3i – TriglycerideFigure 2.1.3i – Triglyceride
Condensation reaction of one glycerol (C3H8O3) and three fatty acids (R-COOH) to form a triglyceride

Classification of fatty acids as saturated or unsaturated

  • Fatty acids are hydrophobic molecules that are composed of long hydrocarbon chains and a terminal carboxyl group (-COOH).
  • Fatty acids are classified by the presence of carbon-carbon double bonds in the chain.

Saturated

No double or triple bonds

Figure 2.1.3j

Figure 2.1.3j

Monounsaturated

Double bond at one location

Figure 2.1.3kFigure 2.1.3k

Polyunsaturated

Double bonds at more than one location

Figure 2.1.3lFigure 2.1.3l

Unsaturated fatty acids can be cis or trans isomers.

  • Isomers are molecules with the same chemical formula, but different molecular geometry.

Figure 2.1.3m – Cis and trans isomersFigure 2.1.3m – Cis and trans isomers
Cis and trans isomers of the fatty acid formula C18H34O2 (not all hydrogen atoms shown)

  • The cis form is bent at the position of the double bond, while the trans form is straight.
  • The arrangement of hydrogen atoms around the double bond is the reason for the difference in shape.
  • In the cis form, the hydrogen atoms are bonded on the same side of the carbon atoms.
  • In the trans form they are on opposite sides, as shown above.

Application: Lipids and long-term energy storage

  • In animals, glycogen is stored in the liver and muscles and used as a short-term energy store. It is easily broken down into glucose and used for cellular respiration (both aerobic and anaerobic).
  • Lipids are stored as adipose tissue under the skin and around the organs. The following properties make them more suitable for long-term energy storage than carbohydrates.

Property

Implication

High energy content

  • Lipids: 37kJ/g vs Carbohydrates: 16kJ/g.
  • Animals need to carry less fat to have the same energy reserves.

Insoluble and (mostly) hydrophobic

  • Will not affect the osmotic gradient of cells.
  • Does not need to be stored in association with water (unlike glycogen) which adds extra weight.

Insulation

  • Fat stored under the skin reduces heat loss.
  • Fat around organs absorbs shock.

Figure 2.1.3n – Long-term storage Hibernating animals use lipids for long-term energy storage. Carbohydrates are used for short-term energy storage.Figure 2.1.3n – Long-term storage
Hibernating animals use lipids for long-term energy storage. Carbohydrates are used for short-term energy storage.

glucoseFigure 2.1.3o – Glucose

Concept help

  • 1-4’ and ‘1-6’ refer to the position of carbon atoms in the glucose molecules involved in the glycosidic bond.
  • Saturated and unsaturated are concepts referring to the amount of hydrogen bonded to carbon. When carbon atoms in a fatty acid chain are bonded to the maximum number of hydrogen atoms, the acid is ‘saturated’ with hydrogen.

Skill

Visualising molecular structure: Hydrocarbon chains appear flat on the page. Use JMol to help visualise these structures in 3D. Download it here for free: http://www.jmol.org/

Study tips

Did you know?

Amylopectin makes up 80% of the starch content in potatoes. It is what makes potato starch sticky. Potatoes have been genetically modified to reduce the level of amylose and produce more amylopectin, which is added to many chemical adhesives.

phospholidFigure 2.1.3p – Phospholipid

Try it

Phospholipids are an important part of cell membranes because of their amphipathic properties. See 1.1.3. They are formed from a glycerol molecule, two fatty acids and a phosphate group. Can you locate the ester bonds on this diagram?