Chapter 15: Lipids

Introduction to Lipids

Lipids are a diverse group of naturally occurring organic compounds that are insoluble in water but soluble in nonpolar solvents. They play essential roles in biological systems, including energy storage, structural components of cell membranes, and signaling molecules.

• Hydrophobic: Lipids are nonpolar and do not dissolve in water.

• Biological importance: Lipids include fats, oils, waxes, phospholipids, and steroids.

• Examples: Cholesterol, triglycerides, phospholipids.

Types of Lipids

Main Classes of Lipids

Lipids are classified based on their structure and function:

• Fatty acids: Long hydrocarbon chains with a carboxylic acid group.

• Triacylglycerols (triglycerides): Esters formed from glycerol and three fatty acids.

• Phospholipids: Contain a phosphate group and are key components of cell membranes.

• Steroids: Characterized by a four-fused-ring structure (steroid nucleus).

• Waxes: Esters of long-chain fatty acids and long-chain alcohols.

General Structure: Lipids are mostly nonpolar, making them insoluble in water but soluble in organic solvents.

Fatty Acids

Definition and Structure

Fatty acids are long, unbranched hydrocarbon chains with a terminal carboxylic acid group. They are the building blocks of many complex lipids.

• Typically contain 12–18 carbon atoms.

• Can be saturated (no double bonds) or unsaturated (one or more double bonds).

• General formula:

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: Only single C–C bonds; straight chains; solid at room temperature.

• Monounsaturated fatty acids: One C=C double bond; kinked chain; liquid at room temperature.

• Polyunsaturated fatty acids: Two or more C=C double bonds; more kinks; lower melting points.

Examples of Fatty Acids

Properties of Fatty Acids

• Saturated fatty acids pack closely, resulting in higher melting points.

• Unsaturated fatty acids have kinks from double bonds, preventing tight packing and lowering melting points.

• Fatty acids are amphipathic: hydrophobic hydrocarbon tail and hydrophilic carboxyl head.

Essential Fatty Acids

Essential fatty acids cannot be synthesized by the human body and must be obtained from the diet. Examples include linoleic acid and linolenic acid.

• Required for normal growth and physiological functions.

• Deficiency can lead to health problems.

Prostaglandins

Definition and Function

Prostaglandins are hormone-like substances derived from fatty acids (arachidonic acid) and produced in most tissues. They act as local chemical messengers.

• Regulate inflammation, blood flow, and formation of blood clots.

• NSAIDs (Nonsteroidal Anti-Inflammatory Drugs) inhibit prostaglandin synthesis, reducing pain and inflammation.

Omega Fatty Acids

Omega-3 and Omega-6 Fatty Acids

Omega fatty acids are classified based on the position of the first double bond from the methyl (omega) end of the fatty acid chain.

• Omega-3 fatty acids: First double bond at the third carbon from the omega end (e.g., alpha-linolenic acid).

• Omega-6 fatty acids: First double bond at the sixth carbon from the omega end (e.g., linoleic acid).

• Both types are essential and play roles in cardiovascular health and inflammation regulation.

Waxes

Structure and Function

Waxes are esters of long-chain fatty acids and long-chain alcohols. They are hydrophobic and solid at room temperature.

• Protective coatings on leaves, fruits, feathers, and skin.

• Prevent water loss in plants and animals.

Triacylglycerols (Triglycerides)

Structure and Biological Role

Triacylglycerols are esters formed from glycerol and three fatty acids. They are the main form of energy storage in animals.

• Glycerol: A three-carbon alcohol with three hydroxyl groups.

• Fatty acids attach to glycerol via ester bonds.

• Can be simple (all three fatty acids the same) or mixed (different fatty acids).

Example: Glyceryl tristearate is a triacylglycerol with three stearic acid molecules attached to glycerol.

Key Concepts and Study Checks

• Lipids are hydrophobic and not soluble in water.

• Fatty acids can be saturated, monounsaturated, or polyunsaturated.

• Essential fatty acids must be obtained from the diet.

• Waxes are always saturated and solid at room temperature.

• Triacylglycerols are the main storage form of fat in the body.

Important Formulas

• General formula for fatty acids:

• Esterification reaction (formation of triacylglycerol):

Summary Table: Types of Fatty Acids

Additional info:

• Physiological pH is approximately 7.4 and body temperature is 37°C.

• Fatty acids in biological systems typically have an even number of carbon atoms, up to 20 or more.

• Hormones act as chemical messengers in the body.