Lipids: Structure, Function, and Biological Importance (Chapter 34 Study Notes)

Lipids: Structure, Function, and Biological Importance

Key Terms and Definitions

• Lipid: A diverse group of hydrophobic or amphipathic biological molecules, including fats, oils, waxes, phospholipids, and steroids.

• Triglyceride: A lipid formed by the esterification of three fatty acids to a glycerol molecule; main form of energy storage in animals.

• Glycerol: A three-carbon alcohol that forms the backbone of triglycerides and phospholipids.

• Fatty Acid: A long hydrocarbon chain with a terminal carboxyl group; can be saturated or unsaturated.

• Saturated Fatty Acid: Contains no double bonds between carbon atoms; typically solid at room temperature.

• Unsaturated Fatty Acid: Contains one or more double bonds; usually liquid at room temperature.

• Amphipathic: Molecules that have both hydrophobic (nonpolar) and hydrophilic (polar) regions.

• Phospholipid: A lipid containing a phosphate group, two fatty acids, and a glycerol backbone; major component of cell membranes.

• Phosphate Group: A functional group consisting of a phosphorus atom bonded to four oxygen atoms; imparts polarity to molecules.

• Lipid Bilayer: A double-layered arrangement of phospholipids that forms the basic structure of cell membranes.

• Cell Membrane: The semipermeable membrane surrounding the cytoplasm of a cell, composed mainly of a lipid bilayer and proteins.

• Steroid: A type of lipid characterized by a carbon skeleton with four fused rings; includes cholesterol and hormones.

• Cholesterol: A steroid that is an essential component of animal cell membranes and a precursor for steroid hormones.

• Lipid Hormone: Hormones derived from lipids, such as steroid hormones (e.g., estrogen, testosterone).

Functions of Lipids

• Energy Storage: Triglycerides store energy efficiently in adipose tissue.

• Structural Components: Phospholipids and cholesterol are key components of cell membranes.

• Insulation and Protection: Fats provide thermal insulation and cushion vital organs.

• Signaling Molecules: Steroid hormones and some phospholipids act as signaling molecules in the body.

Example: Triglycerides are the main energy reserve in animals, while phospholipids form the structural basis of all biological membranes.

Structure of a Triglyceride

• Composed of one glycerol molecule and three fatty acids.

• Each fatty acid is attached to glycerol by an ester bond formed through a dehydration reaction.

Equation:

$\text{Glycerol} + 3\ \text{Fatty Acids} \rightarrow \text{Triglyceride} + 3\ \text{H}_2\text{O}$

Attachment of Fatty Acids to Glycerol

• Fatty acids are attached to the hydroxyl groups of glycerol via esterification.

• This process involves the removal of water (dehydration synthesis).

Types of Fatty Acids

• Saturated Fatty Acids: No double bonds; straight chains; pack tightly; solid at room temperature (e.g., butter).

• Unsaturated Fatty Acids: One or more double bonds; kinked chains; do not pack tightly; liquid at room temperature (e.g., olive oil).

Comparison Table:

Property	Saturated Fatty Acid	Unsaturated Fatty Acid
Double Bonds	None	One or more
Shape	Straight	Kinked (due to cis double bonds)
State at Room Temp	Solid	Liquid
Source	Animal fats	Plant oils

Fatty Acid Content in Fats

• Animal fats are typically high in saturated fatty acids.

• Plant fats (oils) are usually high in unsaturated fatty acids.

Phospholipids: Structure and Function

• Composed of a glycerol backbone, two fatty acids, and a phosphate group (often with an additional polar group attached).

• Major component of cell membranes, forming the lipid bilayer.

• Amphipathic nature: hydrophilic (polar) head and hydrophobic (nonpolar) tails.

Diagram Description: In a phospholipid, the phosphate-containing head faces outward (toward water), while the fatty acid tails face inward (away from water).

Hydrophilic and Hydrophobic Regions of Phospholipids

• Hydrophilic (polar) region: The phosphate group and attached polar molecules.

• Hydrophobic (nonpolar) region: The two fatty acid tails.

• This dual nature allows phospholipids to form bilayers in aqueous environments.

Term: Amphipathic describes molecules with both hydrophilic and hydrophobic regions.

Phospholipids in Cells

• Form the structural basis of all biological membranes.

• Enable compartmentalization and selective permeability in cells.

Cholesterol: Structure and Function

• Cholesterol is a steroid with a four-ring structure and a short hydrocarbon tail.

• It is amphipathic, with a small hydrophilic hydroxyl group and a large hydrophobic steroid ring system and tail.

• Cholesterol is interspersed within the phospholipid bilayer of animal cell membranes, modulating fluidity and stability.

Hydrophilic and Hydrophobic Regions of Cholesterol

• Hydrophilic region: The hydroxyl (-OH) group.

• Hydrophobic region: The steroid rings and hydrocarbon tail.

Term: Amphipathic is also used to describe cholesterol's dual nature.

Structural Role of Cholesterol in Cells

• Maintains membrane fluidity by preventing tight packing of phospholipids at low temperatures and restraining movement at high temperatures.

• Serves as a precursor for the synthesis of steroid hormones.

Hormones Produced from Cholesterol

• Steroid hormones are derived from cholesterol, including:

  • Estrogens

  • Testosterone

  • Cortisol

  • Aldosterone

Example: Cortisol is a steroid hormone involved in stress response, synthesized from cholesterol in the adrenal cortex.