Lipids and Membrane Structure: Study Notes for BIOL 111

Lipids

Introduction to Lipids

Lipids are a diverse group of carbon-containing compounds that play essential roles in biological systems. They are characterized by their largely nonpolar and hydrophobic nature, which influences their function and behavior in cells.

• Lipids are organic molecules found in all living organisms, primarily composed of carbon (C), hydrogen (H), and sometimes oxygen (O).

• They are nonpolar, making them insoluble in water and ideal for forming biological membranes.

• Hydrocarbons are nonpolar molecules consisting only of carbon and hydrogen atoms.

• Lipids do not dissolve in water due to their hydrophobic hydrocarbon components, such as fatty acids.

• A fatty acid is a hydrocarbon chain bonded to a carboxyl (–COOH) functional group.

Example: Fatty acids and isoprene are key building blocks of many types of lipids.

Key Building Blocks of Lipids

• Fatty acids: Long hydrocarbon chains with a terminal carboxyl group.

• Isoprene: A five-carbon molecule that serves as a building block for certain lipids.

Example: The structure of a fatty acid consists of a carboxyl group attached to a long hydrocarbon chain, as shown in the diagram.

Types of Lipids in Cells

Major Classes of Lipids

Lipid structure varies widely, but the three most important types found in cells are:

• Fats (triacylglycerols or triglycerides)

• Steroids

• Phospholipids

Fats

Fats are energy-storage molecules composed of three fatty acids linked to a glycerol backbone.

• Also called triacylglycerols or triglycerides.

• Formed via dehydration reactions, resulting in ester linkages between glycerol and fatty acids.

Equation:

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

Example: Fats are the primary energy storage molecules in animals.

Steroids

Steroids are a family of lipids with a distinctive four-ring structure. They serve as signaling molecules and membrane components.

• Cholesterol is an important steroid in mammals, classified as a sterol (steroid alcohol).

• Other important steroids include estrogen, testosterone, and cortisol.

Example: Cholesterol is present in all living organisms and is a key component of animal cell membranes.

Phospholipids

Phospholipids are the primary component of all cell membranes. They consist of a polar head group and two nonpolar tails.

• Composed of a glycerol backbone linked to a phosphate group and two fatty acid tails.

• Phospholipids are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

Example: Phospholipids spontaneously form bilayers in water, which are the basis of biological membranes.

Phospholipids in Water

Behavior of Phospholipids in Aqueous Solutions

When placed in water, phospholipids organize themselves to minimize the exposure of their hydrophobic tails to water.

• Phospholipids can form micelles (small spherical structures) or phospholipid bilayers (sheet-like structures).

• In bilayers, hydrophilic heads face the surrounding water, while hydrophobic tails face each other inside the bilayer.

• Bilayer formation is spontaneous and does not require energy input.

Example: The plasma membrane of cells is a phospholipid bilayer that separates the cell's interior from the external environment.

Cell Membranes

Structure and Function of Cell Membranes

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that separates the inside of the cell from its environment.

• Primarily composed of a phospholipid bilayer.

• Functions include keeping damaging materials out, allowing entry of necessary materials, and facilitating chemical reactions.

Example: The plasma membrane controls the movement of substances into and out of the cell.

Membrane Fluidity and Permeability

Factors Affecting Membrane Fluidity

Membrane fluidity refers to how quickly molecules move within and across the lipid bilayer. Several factors influence fluidity:

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Hydrocarbon tail structure: Unsaturated tails (with double bonds) increase fluidity; saturated tails decrease it.

• Cholesterol content: Cholesterol decreases membrane fluidity and permeability by filling spaces between phospholipids.

Equation:

$\text{Fluidity} \propto \text{Unsaturation} + \text{Temperature} - \text{Cholesterol}$

Example: Membranes with more unsaturated fatty acids are more fluid and permeable than those with saturated fatty acids.

Membrane Permeability

Permeability is the tendency of a membrane to allow substances to pass through. Phospholipid bilayers exhibit selective permeability:

• Small, nonpolar molecules (e.g., O2, CO2, N2) cross quickly.

• Small, uncharged polar molecules (e.g., H2O) cross moderately.

• Large or charged molecules (e.g., glucose, ions) cross slowly or not at all.

Type of Molecule	Permeability	Examples
Small nonpolar	High	O2, CO2, N2
Small uncharged polar	Moderate	H2O
Large polar or charged	Low	Glucose, Na+, K+

Example: The phospholipid bilayer allows oxygen to diffuse rapidly but restricts the movement of ions.

Factors Influencing Membrane Permeability

• Number of double bonds in phospholipid tails: More double bonds (unsaturation) increase permeability.

• Length of hydrocarbon tails: Longer tails decrease permeability.

• Cholesterol content: More cholesterol decreases permeability.

• Temperature: Higher temperatures increase permeability.

Example: Plant cell membranes with high cholesterol content have lower permeability.

Key Terms

• Plasma membrane: The boundary separating the cell from its environment.

• Cell membrane: Another term for plasma membrane.

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions.

• Micelles: Spherical structures formed by amphipathic molecules in water.

• Phospholipid bilayer: Double layer of phospholipids forming the cell membrane.

• Lipid bilayer: General term for a double layer of lipids.

• Permeability: Ability of a membrane to allow substances to pass through.

• Selective permeability: Property of membranes to allow some substances to cross more easily than others.

• Saturated: Fatty acids with no double bonds.

• Unsaturated: Fatty acids with one or more double bonds.

• Liposome: Artificial vesicle formed from phospholipids.

• Cholesterol: Steroid molecule that modulates membrane fluidity and permeability.

• Hydrophobic tail: Nonpolar region of a phospholipid.

Summary of Learning Objectives

• Explain what a lipid is and how its structure affects its function.

• Describe the forms that phospholipids take when placed in a water solution and explain why.

• Interpret the role cholesterol plays in the membrane fluidity of a cell.

Additional info: Some diagrams and chemical structures were described in text for clarity. The table on permeability was inferred from standard textbook content and lecture notes.