Biology Last Class: Membrane Structure

Membrane Structure and Composition

• Acyl chain angularity contributes to membrane fluidity.
• Cholesterol plays a significant role in modulating membrane properties.
• The fluid mosaic model describes the dynamic nature of membranes as first proposed by Singer and Nicolson in 1972.

Lipid Composition in Hepatocytes

• Lipid proportions vary across different organelles within hepatocytes.
• The most common lipid in plasma membranes is phospolipid, essential for membrane integrity.

Trilaminar Structure of Membrane

• Membrane appears trilaminar under electron microscopy, characterized by two electron-dense lines.
• Experimental evidence includes chemical permeability studies and physical analyses via X-ray diffraction.

Protein and Lipid Interaction

• Integral proteins reside in a lipid bilayer, held by hydrophobic interactions with non-polar amino acids.
• Carbohydrate motifs on proteins extend into the extracellular space, forming glycoproteins and glycolipids.
• Protein orientation within the membrane is asymmetric, influenced by functional requirements.

Lipid Mobility and Dynamics

• Lipids exhibit constant movement within the membrane, including flip-flop and lateral diffusion.
• Flip-flop movement between bilayer sides is slow, accelerated by the enzyme flippase.
• Factors affecting lipid mobility include:
  • Thermal movement of fatty acid groups.
  • Lateral diffusion influenced by temperature.
  • Transition temperature marks the shift from a solid to a fluid state.

Transition Temperature

• Critical temperature where membranes shift from paracrystalline to liquid state, dependent on lipid composition.
• Saturated fatty acids promote tighter packing and higher transition temperatures, whereas unsaturated fatty acids introduce fluidity through double bonds.
• Sterols modulate transition temperatures by restricting fatty acid movement.

Experimental Evidence of Membrane Dynamics

• FRAP (Fluorescence Recovery After Photobleaching) allows measurement of lateral diffusion.
• Fusion experiments using different species (mouse and human cells) showcase the mobility of membrane components.
• The study by Frye and Eddidin demonstrated protein mixing via fluorescence labeling.

Characteristics of Biological Membranes

• Flexible structures capable of undergo shape change and self-sealing.
• Participate in membrane fusion processes relevant to exocytosis, phagocytosis, fertilization, and infection routes.

Physical Properties of Membrane Structure

• Phospholipid membranes closely mimic natural membranes.
• Mixtures achieving comparable proportions lead to spontaneously forming bilayers or liposomes.
• X-ray diffraction supports bilayer structure evidence.
• Membranes possess selective permeability, particularly to polar solutes.

Membrane Thickness and Composition Specifics

• Membranes typically measure 5 to 8 nm in thickness and exhibit a trilaminar appearance.
• Composed primarily of lipids and proteins, with specific lipid-protein ratios varying by membrane type.
• Contains receptors for extracellular signals and molecules facilitating cell adhesion.

This quiz covers the architecture of cell membranes, emphasizing the fluid mosaic model and the role of lipids and cholesterol in membrane fluidity. Additionally, it explores the lipid composition of different organelles in a hepatocyte and discusses experimental evidence supporting the trilaminar structure. Test your understanding of these key concepts in cell biology!