SPM Biology 4 Chemical Composition of the Cell Part 4 Organic Compounds in the Cell - Lipids

1. Contain carbon, hydrogen, oxygen.

2. Proportion of oxygen is lower than in carbohydrates. For example: stearic acid C₁₈H₃₆O₂.

3. Insoluble in water (non-polar molecule), but dissolve in other lipids and non-polar solvents (ether, ethanol, etc.).

4. Four main types of lipids:

• Fats and oils (triglycerides)
• Waxes
• Phospholipids
• Steroids

5. Importance:

• Store large amount of energy
• Sources of energy
• A major part of the structure of cell membranes

Fats and Oils (triglycerides)

Fats and oils

1. Fats are solid at room temperature (20ºC).

2. Oils are liquid.

3. Triglyceride is formed from a condensation reaction between 1 molecule of glycerol and 3 molecules of fatty acids. The bonds formed are called ester bonds.

Formation of triglyceride

4. Fats often contain only saturated fatty acid (single bond).

5. Oils usually contain unsaturated fatty acid (double bond).

Diagrammatic representation of fats

Structure of saturated and unsaturated fats

6. Importance of fats and oil:

• Function as energy reserve & storage materials. They provide 38kJ per gram, while carbohydrates provide only 17kJ per gram.
• Fats act as an insulator against the loss of heat. 

7. Types of fats

Similarities and differences of saturated fat and unsaturated fat

Waxes

1. Similar to triglycerides.

2. Produced by both plants & animals.

3. Usually hard solids at room temperature.

Waxes

4. Importance of waxes:

• Used to waterproof the external surfaces of plants & animals. E.g: cuticle of leaf, protective covering on an insect’s body.
• Also a constituent of the honeycomb of bees.

Phospholipids

• Major component of plasma membranes
• Made up of 1 glycerol, 2 fatty acid and 1 phosphate

See SPM Biology 3 Movement of Substances Across the Plasma Membrane Part 1 Structure of Plasma Membrane

Steroids

1. Complex ring structure. Do not contain fatty acids.

2. Occur in plants and animals.

3. Examples: 

Steroids