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1.5 Lipids

Syllabus
2025
Topic
1.5
Level
—

Learning objectives

1.5A—Describe the structure and function of lipidsDescribe the structure and function of lipids.• Lipids are typically nonpolar, hydrophobic molecules whose structure and function are derived from the way their subcomponents are assembled. Fatty acids can be described as either saturated or unsaturated.- i. Saturated fatty acids contain only single bonds between carbon atoms.- ii. Unsaturated fatty acids contain at least one double bond between carbon atoms, which causes the carbon chain to kink.- iii. The more double bonds in a fatty acid tail, the more unsaturated the lipid becomes.- iv. The more unsaturated a lipid is, the more liquid it is at room temperature.• Lipids provide a variety of functions for living organisms. Some examples of lipids are fats, steroids including cholesterol, and phospholipids.- i. Fats provide energy storage and support cell function. In some cases, they can also provide insulation to help keep mammals warm.- ii. Steroids are hormones that support physiological functions including growth and development, energy metabolism, and homeostasis.- iii. Cholesterol provides essential structural stability to animal cell membranes.- iv. Phospholipids group together to form the lipid bilayers found in plasma and cell membranes.- Exclusion: The molecular structure of specific lipids is beyond the scope of the AP Exam.

Lipid Assembly Links Shape to Function

Lipids are typically nonpolar and hydrophobic. Their properties depend on how their subcomponents are assembled. In fatty-acid tails, carbon–carbon bond type changes chain shape, which changes how the molecules associate with one another.

Fatty acid Carbon–carbon bonds Shape and trend
Saturated Only single bonds No double-bond kink
Unsaturated At least one double bond A double bond causes a kink
More unsaturated More double bonds More kinks and a greater tendency to be liquid at room temperature
Lipid class CED biological function
Fats Energy storage, support of cell function, and sometimes insulation in mammals
Steroids Hormonal support of growth and development, energy metabolism, and homeostasis
Cholesterol Structural stability in animal cell membranes
Phospholipids Assemble into lipid bilayers in plasma and cell membranes

A phospholipid contains a hydrophilic phosphate-containing head and hydrophobic lipid tails. In water, heads orient toward the aqueous environment while tails avoid water, so many phospholipids self-associate into a bilayer. Cholesterol sits within an animal-cell membrane and contributes structural stability.

Unsaturated means at least one carbon–carbon double bond; it does not mean the molecule contains no hydrogen. The molecular structures of specific lipids are explicitly outside AP Exam scope, so the required learning is the stated bond, shape, assembly, and function relationship.