4.4 - Lipids

Types of Lipids

• Lipids are naturally occurring hydrophobic compounds found in plant and animal tissues.
• Lipids consist of carbon (C), hydrogen (H), and oxygen (O), with a much higher ratio of hydrogen atoms to oxygen atoms than carbohydrates.
• Lipids are insoluble in water but soluble in organic solvents such as alcohol, ether, and chloroform.
• The main types are fats, waxes, phospholipids, and steroids.

Fats and Oils (Triglycerides)

• Fats and oils are triglycerides; a triglyceride is a type of ester.
• One glycerol molecule and three fatty acid molecules combine by condensation to form one triglyceride molecule and three water molecules.
• $\text{Glycerol}+3\text{ fatty acids}\stackrel{\text{condensation}}{\to }\text{triglyceride}+3\text{ water}$
• A triglyceride is broken down by hydrolysis into glycerol and three fatty acids.
• $\text{Triglyceride}+3\text{ water}\stackrel{\text{hydrolysis}}{\to }\text{glycerol}+3\text{ fatty acids}$
• Glycerol is a three-carbon alcohol containing three hydroxyl groups ($-OH$).
• Saturated and unsaturated fats both contain carbon, hydrogen, and oxygen; both contain glycerol and fatty acids; both are nonpolar molecules.

Characteristic	Saturated fats	Unsaturated fats
Carbon bonds	Single bonds only	At least one double bond
Additional hydrogen	Cannot be added	Can be added at double bonds
State at room temperature	Solid	Liquid
Sources	Butter, animal fat	Olive oil, fish oil

Waxes

• One alcohol molecule combines with one fatty acid molecule.
• Waxes are waterproof.

Phospholipids

• A major component of plasma membranes.
• One glycerol molecule combines with two fatty acid molecules and one phosphate group.

Steroids

• Lipids that do not contain fatty acids.
• Examples: cholesterol, testosterone, estrogen, and progesterone.

Importance of Lipids in a Cell

• Fats store energy in animals, protect internal organs as a lining, and insulate animals against heat loss.
• Waxes are important components of leaf cuticles and sebum secreted by skin.
• Glycolipids maintain plasma-membrane stability and aid cell identification.
• Cholesterol is required to synthesise steroid hormones.