Lipids and Plasma Membrane

Learning outcomes for understanding biological membranes

Understanding the heterogeneity of biological membranes, the physical properties of phospholipids, properties of other membrane lipids, and the importance of membrane proteins.

Importance of the plasma membrane

It separates environments and controls solutes and molecule movement in and out of the cell, preventing harmful diffusion.

Key properties of biological membranes

Semi-permeable, 5-8 nm thick, and described by the fluid mosaic model.

Amphiphilic behavior in membrane lipids

Hydrophilic heads face aqueous environment, hydrophobic tails avoid water, forming micelles or bilayers.

Structure formed by glycerophospholipids

They form bilayers due to steric packing requirements of their two tails.

Types of lipid movement in bilayers

Lateral diffusion is common; transverse diffusion (flip-flop) is rare due to energy constraints.

Effect of temperature on membrane fluidity

Low temps cause a crystalline gel; normal temps allow a liquid crystal state, increasing fluidity.

Role of cholesterol in membrane fluidity

Acts as a fluid buffer—prevents rigidity at low temps and excess fluidity at high temps.

Effect of fatty acid chains on membrane fluidity

Saturated chains decrease fluidity; unsaturated chains with kinks increase fluidity.

Sphingolipids and their functions

Derived from ceramide; involved in cell signaling, myelin formation, and protein clustering.

Integral membrane proteins

Proteins tightly embedded in the membrane, often with hydrophobic transmembrane domains.

Structures formed by transmembrane protein domains

Alpha helices and beta barrels to shield polar backbones in the hydrophobic membrane core.

Peripheral membrane proteins

Temporarily attached proteins involved in cell signaling, often attaching to integral proteins.

Specialization of lipid and protein distribution in membranes

Different areas (e.g., apical vs. basolateral in epithelial cells) have distinct lipids and proteins for specific functions.