Biological Macromolecules: Lipids

Biological Macromolecules

Overview

Cells contain four major classes of biological macromolecules: carbohydrates, nucleic acids, proteins, and lipids. This section focuses on lipids, their structure, classification, and biological functions.

Lipids

General Properties and Functions

• Lipids are molecules with a high proportion of carbon-hydrogen (C-H) bonds, making them largely hydrophobic and insoluble in water.

• They differ from other biological macromolecules in structure and function.

• Main functions include:

  • Energy storage (e.g., fats and oils)

  • Membrane structure (e.g., phospholipids, glycolipids, cholesterol)

  • Signaling (e.g., steroid hormones, some terpenes)

• Major structural classes of lipids:

  • Fatty acids

  • Triacylglycerols

  • Phospholipids

  • Glycolipids

  • Steroids

  • Terpenes

Lipids

Fatty Acids

Fatty acids are long hydrocarbon chains with a carboxyl group at one end. They are the building blocks of many complex lipids.

• Saturated fatty acids: No double bonds between carbon atoms (e.g., palmitate, stearate).

• Unsaturated fatty acids: One or more double bonds (e.g., oleate, linoleate).

• Double bonds introduce kinks, affecting membrane fluidity.

Number of Carbons	Number of Double Bonds	Common Name
12	0	Laurate
14	0	Myristate
16	0	Palmitate
18	0	Stearate
18	1	Oleate
18	2	Linoleate
20	4	Arachidonate

Example: Oleic acid (18:1) is a common monounsaturated fatty acid found in olive oil.

Lipids

Triacylglycerols (Triglycerides)

Triacylglycerols are the main storage form of energy in animals. They are composed of three fatty acids esterified to a glycerol backbone.

• Formed by dehydration reactions between glycerol and fatty acids.

• Stored in adipose cells in humans and other mammals.

• Highly efficient for energy storage due to their reduced, hydrophobic nature.

Equation for Triacylglycerol Formation:

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

Example: Animal fats and vegetable oils are primarily composed of triacylglycerols.

Lipids

Phospholipids

Phospholipids are major components of cell membranes, forming the lipid bilayer. They are amphipathic, containing both hydrophobic and hydrophilic regions.

• Composed of a glycerol or sphingosine backbone, two fatty acids, and a phosphate group with a small polar or charged molecule (the "head group").

• Phosphoglycerides: Glycerol backbone (e.g., phosphatidylcholine).

• Sphingolipids: Sphingosine backbone (e.g., sphingomyelin).

Example: Phosphatidylcholine is a common phospholipid in eukaryotic membranes.

Lipids

Glycolipids

Glycolipids are lipids with one or more covalently attached carbohydrate groups. They are important for cell recognition and signaling.

• Often based on a sphingosine backbone with a carbohydrate chain of 1 to 6 monosaccharide units.

• Abundant in the outer leaflet of the plasma membrane.

Example: Glycolipids are key components of the cell surface in animal tissues.

Lipids

Steroids

Steroids are lipids with a characteristic four-ring hydrocarbon skeleton. They serve as membrane components and signaling molecules.

• Cholesterol is the most common steroid in animal cells, modulating membrane fluidity.

• Steroid hormones (e.g., estrogen, testosterone) are derived from cholesterol.

Example: Cholesterol is essential for animal cell membranes and as a precursor for steroid hormones.

Lipids

Terpenes

Terpenes are a diverse class of lipids built from isoprene units. They serve as pigments, vitamins, and precursors to other biomolecules.

• Examples include vitamin A, carotenoids, and some plant essential oils.

• Terpenes are synthesized by the repetitive joining of isoprene units ($C_5H_8$).

Example: Vitamin A (retinol) is a terpene essential for vision.

Additional info: This guide expands on the provided slides by including definitions, examples, and equations for clarity and completeness.