Membranes and Lipids: Structure and Function (Chapter 5 Study Notes)

Membranes and Lipids

Introduction

Biological membranes are essential structures that define the boundaries of cells and organelles, regulate the movement of substances, and facilitate communication and signaling. The study of membranes and lipids is fundamental to understanding cell biology and physiology.

Membrane Structure

Overview of Lipid Membranes

Cell membranes are primarily composed of lipids and proteins, forming a dynamic and selectively permeable barrier. The structure and composition of membranes allow them to maintain integrity and function under various conditions.

• Phospholipid Bilayer: The basic framework of all biological membranes, consisting of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Cholesterol: Present in animal cell membranes, cholesterol modulates membrane fluidity and stability.

• Cell Surface Markers: Molecules such as glycoproteins and glycolipids on the outer leaflet of the membrane serve as identification tags and participate in cell signaling.

• Transmembrane Proteins: Proteins that span the membrane and are involved in transport, signaling, and structural support.

• Interior Membrane-Associated Proteins: Proteins attached to the inner surface of the membrane, often involved in cytoskeletal attachment and signaling.

Notable Properties:

• Remain fluid at low temperatures (do not freeze easily).

• Remain intact at high temperatures.

• Impermeable to most biological molecules and ions, allowing selective transport.

Example: The scanning electron micrograph of erythrocytes (red blood cells) demonstrates the importance of membrane integrity and flexibility in cell shape and function.

Fluidity of Membranes

Experimental Evidence: Frye and Edidin (1970)

Experiments by Frye and Edidin demonstrated that proteins can move laterally within the membrane, supporting the concept of membrane fluidity.

• Lateral Movement: Membrane proteins and lipids are not static; they can diffuse laterally within the plane of the membrane.

• Fluid Mosaic Model: Proposed by Singer and Nicolson, this model describes the membrane as a mosaic of proteins floating in or on a fluid lipid bilayer.

Key Hypotheses:

• Membranes are laterally fluid, allowing for the movement and intermixing of proteins and lipids.

• Experimental fusion of human and mouse cells showed that membrane proteins intermixed over time, confirming lateral mobility.

Example: Graphs from cell fusion experiments illustrate the increase in mosaic (mixed) cells over time, supporting the fluid mosaic model.

Main Components of Membranes

Phospholipids

Phospholipids are amphipathic molecules with hydrophilic heads and hydrophobic tails, forming the bilayer structure of membranes.

• Structure: Glycerol backbone, two fatty acid tails, and a phosphate group.

• Function: Provide the basic barrier to water-soluble substances.

Cholesterol

Cholesterol is interspersed within the phospholipid bilayer of animal cells.

• Role: Modulates membrane fluidity by preventing tight packing of phospholipids at low temperatures and restricting movement at high temperatures.

• Effect: Maintains membrane integrity and flexibility.

Glycolipids and Glycoproteins

Glycolipids and glycoproteins are found on the outer leaflet of the membrane and serve as cell surface markers.

• Function: Cell recognition, signaling, and adhesion.

• Example: Blood group antigens are glycolipids and glycoproteins on erythrocyte membranes.

Membrane Proteins

Proteins embedded in or associated with the membrane perform a variety of functions.

• Integral (Transmembrane) Proteins: Span the membrane and are involved in transport and signaling.

• Peripheral Proteins: Attached to the membrane surface, often involved in signaling or structural support.

Summary Table: Membrane Components and Functions

Component	Structure	Function
Phospholipid	Bilayer with hydrophilic heads and hydrophobic tails	Barrier to water-soluble substances
Cholesterol	Interspersed within bilayer	Modulates fluidity and stability
Glycolipid/Glycoprotein	Lipid/protein with carbohydrate chain	Cell recognition and signaling
Integral Protein	Spans membrane	Transport, signaling
Peripheral Protein	Attached to membrane surface	Signaling, structural support

Key Terms and Definitions

• Phospholipid Bilayer: Double layer of phospholipids forming the fundamental structure of cell membranes.

• Fluid Mosaic Model: Describes the membrane as a fluid structure with proteins embedded in or associated with a sea of lipids.

• Integral Protein: Protein that spans the membrane and is permanently attached.

• Peripheral Protein: Protein attached to the surface of the membrane.

• Cholesterol: Steroid molecule that modulates membrane fluidity.

• Glycolipid/Glycoprotein: Lipid or protein with carbohydrate attached, involved in cell recognition.

Additional info:

• The images provided illustrate the structure of the phospholipid bilayer and the appearance of red blood cells, emphasizing the importance of membrane composition in cell shape and function.

• Experimental evidence from cell fusion studies supports the fluid mosaic model, a foundational concept in cell biology.