Lecture_07_Membrane Structure

Cell Membranes Overview

• Key Functions:

  • Acts as selective barriers to separate cell from surroundings.

  • Involved in cell communication, import/export of molecules, growth, and mobility.

  • Internal membranes in eukaryotic cells support compartmentation, selective transport, synthesis, and energy transduction.

Membrane Structure

• Composed of lipids and proteins arranged in a lipid bilayer.

• Types of Lipids:

  • Phospholipids: The most abundant, formed from two fatty acids, glycerol, and a phosphate group.

  • Amphipathic Nature:

    • Hydrophilic head (phosphate) and hydrophobic tails (fatty acids).

  • Other Lipids: Sterols, glycolipids, and triacylglycerols.

Lipid Bilayer Properties

• Forms spontaneously in water, creating sealed compartments (liposomes).

• Fluidity Factors:

  • Temperature: Higher temperatures increase fluidity.

  • Fatty acid chain length: Shorter tails increase fluidity.

  • Unsaturation: Double bonds in fatty acids enhance fluidity.

  • Cholesterol: Increases stability but can decrease overall fluidity.

Membrane Asymmetry

• Flippases and Floppases:

  • Important for maintaining lipid bilayer asymmetry during lipid movement.

  • Enzymatic Functions:

    • Flippases move specific lipids to the cytosolic side.

    • Floppases move specific lipids to the outer side of the bilayer.

  • Both processes require ATP.

Membrane Proteins

• Constitute roughly 50% of plasma membrane structure by mass.

• Types of Membrane Proteins:

  • Integral Proteins: Embedded within the lipid bilayer.

  • Peripheral Proteins: Non-covalently bound to the membrane; easier to detach.

• Membrane Protein Functions:

  • Transport (e.g., ion channels)

  • Reception (e.g., growth factor receptors)

  • Enzymatic activity (e.g., adenylyl cyclase)

  • Structural support (e.g., integrins)

Hydropathy Plots

• Graphical representations for predicting hydrophobic/hydrophilic regions in proteins.

• Useful for identifying potential transmembrane segments.

Special Membrane Protein Structures

• β-barrel Proteins: Form water-filled channels and found in the membranes of bacteria, mitochondria, and chloroplasts.

• Bacteriorhodopsin: An example of a membrane protein functioning as a proton pump, utilizing light to create proton gradients for ATP generation.

Chloroplast/Cell Cortex

• Underlies the plasma membrane, providing structural support via fibrous proteins like spectrin.

• Membrane protein movement can be restricted by tethering to the cytoskeleton or extracellular matrix, or diffusion barriers create distinct membrane domains.

FRAP Technique (Fluorescence Recovery After Photobleaching)

• Used to measure the lateral diffusion of membrane proteins by observing the recovery of fluorescence in bleached regions of the membrane.

Glycocalyx: Cell Surface Carbohydrates

• Composed of glycolipids and glycoproteins, contributing to cell protection, immune response, and cell-cell recognition, essential in processes like neutrophil migration during infections.