Week 5 - Oct 1

Lipids, Membranes & Membrane Transport

Introduction

Biological membranes are essential structures that define the boundaries of cells and organelles, regulate transport, and facilitate communication. Their unique properties arise from the diverse lipids and proteins that compose them.

Membrane Structure and the Fluid Mosaic Model

The Plasma Membrane as a Lipid Bilayer

• The plasma (cell) membrane is primarily composed of a lipid bilayer approximately 5–6 nm thick.

• Each layer of the bilayer is called a monolayer.

• Electron microscopy reveals the bilayer structure in cells such as red blood cells.

The Fluid Mosaic Model

• The Fluid Mosaic Model describes membranes as a dynamic arrangement of lipids, proteins, and carbohydrates.

• Proteins and lipids can move laterally within the layer, contributing to membrane fluidity.

• This model explains the diversity and flexibility of biological membranes.

Types and Properties of Lipids in Membranes

Major Classes of Lipids

• Fats (Triacylglycerols/Triglycerides): Composed of three fatty acids linked to glycerol. Main function is energy storage.

• Steroids: Lipids with a characteristic four-ring structure. Example: Cholesterol.

• Phospholipids: Major component of membranes, containing a phosphate group as part of a polar head and two hydrophobic tails.

Phospholipid Structure and Amphipathic Nature

• Phospholipids have a hydrophilic (polar) head and hydrophobic (non-polar) tails.

• This amphipathic nature causes them to spontaneously form micelles or bilayers in water.

Formation of Membrane Structures

• Micelles: Spherical structures with hydrophilic heads facing outward and hydrophobic tails inward.

• Lipid Bilayers: Two layers of phospholipids with hydrophobic tails facing each other and hydrophilic heads facing the aqueous environment.

Diversity of Phospholipids

• Phospholipids vary in:

  • Number of carbons in fatty acid tails

  • Degree of saturation (saturated vs. unsaturated; mono- vs. polyunsaturated)

  • Head group composition

  • Length of tails

  • Presence and type of sterols

• Different properties of phospholipids result in different membrane properties (e.g., fluidity, permeability).

Common Lipids in Mammals

Lipid Type	Head Group	Example
Phosphatidylethanolamine	Ethanolamine	Common in inner leaflet of plasma membrane
Phosphatidylserine	Serine	Involved in cell signaling
Phosphatidylcholine	Choline	Major component of eukaryotic membranes
Sphingomyelin	Phosphocholine	Abundant in myelin sheath of nerve cells
Sphingosine	Amine alcohol	Backbone for sphingolipids

Phospholipid Diversity Across Domains

• Bacteria and Eukarya: Phospholipids contain fatty acid tails.

• Archaea: Phospholipids contain isoprenoid tails, which provide stability in extreme environments.

Membrane Asymmetry

• Phospholipid composition differs between the two monolayers of the bilayer (extracellular vs. cytosolic sides).

• This asymmetry is important for membrane function and cell signaling.

Specialized Functions of Membrane Lipids

• Membrane lipids can provide specialized functions, such as forming myelin sheaths in neurons or participating in photosynthetic membranes in plants.

Sterols in Eukaryotic Membranes

• Cholesterol: Found in animal cell membranes; modulates fluidity and stability.

• Ergosterols: Present in fungi and some protozoa.

• Phytosterols: Found in plant membranes.

• Sterols are generally absent in prokaryotic cells.

Membrane Properties

Flexibility, Repair, and Expansion

• Flexible: Membranes allow cells to change shape.

• Repairable: Lipids can move to reform a continuous surface if the membrane is disrupted.

• Expandable: Cells can increase surface area by adding new membrane lipids.

How is this possible? The fluid nature of the lipid bilayer allows for lateral movement of lipids and proteins, enabling these properties.

Membrane Functions

Key Functions of Biological Membranes

1. Control of Transport: Regulate movement of substances into and out of cells and organelles; maintain a semi-permeable barrier; create ion gradients for energy and signaling.

2. Compartmentalization: Form organelles, increasing efficiency and surface area-to-volume ratio.

3. Signaling (Communication): Relay signals from outside to inside the cell and between cells (e.g., neurons).

4. Enzymatic Activity/Cell Metabolism: House enzymes for metabolic reactions.

5. Attachment/Recognition: Contain proteins for cell-cell recognition and attachment.

Regulation of Metabolic Pathways

Feedback Inhibition

• Many metabolic pathways are regulated by feedback inhibition.

• The end product of a pathway inhibits an enzyme that acts early in the pathway, preventing overproduction.

Example: In amino acid biosynthesis, the final product can inhibit the first enzyme in the pathway, maintaining homeostasis.

Key Terms and Concepts

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Saturated Fatty Acid: No double bonds; straight chains; pack tightly.

• Unsaturated Fatty Acid: One or more double bonds; kinks in chains; increase membrane fluidity.

• Micelle: Spherical structure formed by amphipathic molecules in water.

• Bilayer: Double layer of phospholipids forming the basic structure of membranes.

• Sterol: Lipid with a four-ring structure; modulates membrane properties.

Summary Table: Comparison of Major Membrane Lipids

Lipid Type	Structure	Main Function	Example
Phospholipid	Glycerol + 2 fatty acids + phosphate head	Membrane structure	Phosphatidylcholine
Sterol	Four-ring hydrocarbon	Membrane fluidity	Cholesterol
Glycolipid	Lipid + carbohydrate	Cell recognition	Ganglioside

Additional info: The notes above integrate and expand upon the provided slides and text, adding definitions, examples, and tables for clarity and completeness. For further study, see textbook chapters on membrane structure and function, and lipid diversity.