Plasma Membrane

Organization of Cells

  • Cells are organized by membranes, defining structural boundaries.

    • Plasma Membrane:

    • Composed of lipids, proteins, and carbohydrates.

    • Functions as a gateway to the cell.

Membrane Functionality

  • Quote: "Good fences make good neighbors.” - Robert Frost, Mending Wall, 1914

  • Core Functions of the Plasma Membrane:

    1. Protection: Offers a protective barrier from the external environment.

    2. Selective Permeability: Regulates what enters and leaves the cell; described as SEMI-PERMEABLE.

Learning Objectives

  1. Membrane System Organization:

    • Understand the arrangement and function.

  2. Chemical Structure and Properties:

    • Phospholipids (pages 6-13)

    • Glycolipids (page 14)

    • Sterols (pages 15-16)

  3. Fluid Mosaic Model Characteristics:

    • Lipid Bilayer (pages 33-37)

    • Fluidity (pages 38-43)

    • Asymmetry (pages 44-46)

    • Selective Permeability (page 47)

  4. Membrane Proteins: (pages 49-55)

  5. Macromolecular Complexes of the Plasma Membrane: (pages 71-76)

Membrane Composition

  • Biological Membrane System:

    • Membranes are formed as two layers, a sheath-like structure.

    • Main Components:

    • Lipids

    • Proteins

    • Carbohydrates

    • Membrane Thickness: Ranges from 60 Å (6 nm) to 100 Å (10 nm).

    • Formed by non-covalent interactions of lipids and proteins.

    • Acts as a barrier, limiting the passage of polar molecules and ions (due to lipid bilayer).

Structure and Classification of Membrane Lipids

  • Classification based on Chemical Composition:

    • Three Major Classes:

    1. Phospholipids

    2. Glycolipids

    3. Sterols

    • Phospholipid Characteristics:

    • Hydrophilic (Polar) end and Hydrophobic (Non-Polar) end.

    • Amphipathic nature:

      • One end is hydrophilic, the other is hydrophobic, containing hydrocarbon chains.

Lipids: Phospholipid Structure

  • Chemical Components:

    1. Polar Head Group

    2. Phosphate

    3. Glycerol Backbone

    4. Fatty Acid Tails

    • Most abundant class of membrane lipids.

Phospholipid Fatty Acid Tails

  • Tail Composition:

    • Saturated Fatty Acids: No double bonds between carbon; max hydrogen attachment.

    • Unsaturated Fatty Acids: Contains double bonds; fewer hydrogen atoms.

  • Visualization:

    • C3, C2, C1 of Glycerol linked with fatty acids R1 and R2.

  • Key Functionality:

    • Regulate structural integrity and mobility of membranes.

Glycerol Backbone in Phospholipids

  • Glycerol Structure:

    • Glycerol (C3) links to Polar Head Group through a phosphodiester bond.

    • Fatty acids esterified to positions C1 and C2.

Polar Head Groups of Phospholipids

  • Functions of Polar Head Groups:

    • Contribute to membrane curvature, cell signaling, and substrate transport.

  • Examples of Polar Head Groups:

    • Phosphatidylcholine, Phosphatidylethanolamine, Phosphatidylserine, etc.

  • Each phospholipid is named based on its polar head group.

Glycolipids

  • Composition:

    • Conjugated lipids with sugar residues (usually glucose or galactose).

    • Connected via glycosidic bonds and function mainly in cell recognition, adhesion, and membrane stability.

  • Structure Unique from Phospholipids:

    • Polar head group replaced by sugar residues, with a similar backbone structure.

Sterols (Cholesterol)

  • Sterol Structure:

    • Composed of four fused carbon rings with hydrocarbon alkyl side chains.

  • Functionality in Animal Membranes:

    • Constitutes 30-40% of membrane lipids and presents an amphipathic nature that impacts fluidity.

Fluid Mosaic Model

  • Developed by Singer and Nicolson (1972) to understand membrane organization.

    • Fluid: Indicates movement within the membrane.

    • Mosaic: Illustrates diversity in molecules within the membrane.

  • Key Characteristics:

    • Lipid Bilayer: Two layers of lipids arranged with hydrophobic tails facing inward and hydrophilic heads outward.

    • Asymmetry and Selective Permeability are also essential attributes.

Membrane Fluidity

  • Lateral diffusion of lipids occurs rapidly, with a frequency of $10^7$ times per second.

  • Flip-flop of lipids across bilayer is rare due to energetics.

  • Cholesterol’s Role:

    • Acts as a buffer, influencing the fluidity based on saturation of fatty acids present.

Membrane Asymmetry

  • Different lipid compositions in cytosolic and extracellular leaflets.

    • Cytosolic is richer in phosphatidylserine and phosphatidylethanolamine.

    • Extracellular is rich in sphingomyelin and phosphatidylcholine.

Functions of Plasma Membrane Proteins

  • Membrane proteins constitute about 20-30% of the genome and roughly 50% of membrane volume.

  • Types of Proteins and their Functions:

    1. Transporters: E.g., Na+ pump

    2. Anchors/Linkers: E.g., integrins

    3. Receptors: E.g., PDGF receptor

    4. Enzymes: E.g., adenylyl cyclase

Summary and Key Terms

  • Cell Membrane: A specialized structure controlling movement of substances in/out of the cell.

  • Hydrophobic/Hydrophilic: Terms defining molecule interactions with water.

  • Amphipathic: Molecules with both hydrophobic and hydrophilic properties.

  • Phospholipid: Key lipid type forming bilayer, characterized by a polar head and hydrophobic tails.

  • Fluid Mosaic Model: Describes the dynamic nature of membrane structure and function.