4.4 - Lipids

Lipids are water-insoluble, hydrophobic structurally diverse compounds that can be found in both animals and plants. They are soluble in organic solvents such as ether, chloroform, acetone, alcohol and benzene. Lipids contain elements such as carbon, hydrogen and oxygen where the ratio of hydrogen atoms to oxygen atoms is higher compared to carbohydrate.

Figure 13 - Butter, cheese, fried food, nuts and oil are examples of food that are high in lipid content.

Types of Lipids

Lipids can be classified into fats, phospholipids, steroids and waxes. Triglycerides are the main component in fats and oils. A triglyceride consists of a glycerol and three fatty acid chains. Fatty acids can be divided into saturated and unsaturated fatty acids. Comparison between saturated and unsaturated fats can be seen in the table below.

Similarities and Differences Between Saturated Fatty Acids and Unsaturated Fatty Acids

• Similarities

  • Both contain carbon, hydrogen and oxygen elements.
  • Both have glycerol and fatty acids.
  • Both are non-polar molecules.

• Differences

  • Saturated Fatty Acids：

    • Fatty acids: Only contain single bonds (saturated)
    • Bonding: No double bonds
    • State at room temperature: Solid
    • Melting point: High
    • Sources: Butter, animal fat

  • Unsaturated Fatty Acids：

    • Fatty acids: Contain at least one double bond (unsaturated)
    • Bonding: Have double bond (can still receive hydrogen atoms)
    • State at room temperature: Liquid
    • Melting point: Low
    • Sources: Olive oil, fish oil

Wax is a simple lipid that contains a long-chain of alcohol and a fatty acid (ester organic compound).

Figure 14 - Natural waxes are esters. There are also synthetic waxes lack functional groups. Synthetic waxes are used in food production, cosmetics and adhesives.

Phospholipids are molecules that make up the plasma membrane. The phospholipids consist of a hydrophilic head and a hydrophobic tail. They contain one molecule of glycerol combined with two fatty acid chains.

Figure 15 - In Chapter 3, we have learnt about the composition of plasma membrane. One of which is the phospholipid bilayer.

Steroids are another class of lipid molecule. They have a distinctly different structure from other lipids (structure of four fused rings). Fatty acids are not found in steroids. Some examples of steroids are cholesterol and steroid hormones (testosterone, estrogen, progesterone)

Figure 16 - The most common steroid is cholesterol. Cholesterol is used in the synthesis of Vitamin D, steroid hormones, cortisol and bile salts.

Importance of Lipids in Cells

• Fats function as reserved energy
• Waxes found in cuticles on the surface of leaves and in sebum from skin secretions (protection)
• Fats act as heat insulator
• Fats protects internal organs (as cushion)
• Glycolipids stabilize plasma membrane and help in cell-cell recognition
• Cholesterol involved in steroid hormones synthesis