Membrane Biology

CHAPTER 11: Membranes

The Function of Biological Membranes

  • Biological membranes serve critical functions in cellular management and material transport.

The Structure and Composition of Membranes

  • Membranes are complex structures primarily composed of lipids and proteins.

Key Topics

  • Membrane dynamics

  • Structure and function of membrane proteins

  • Transport mechanisms across biological membranes

Dynamics of Membranes

  • Membranes are dynamic and fluid, allowing for movement and interaction of various components.

Structure and Composition of Membranes

Lipid Aggregation

  • Lipids aggregate into various structures in water, with their formation depending on:

    • Type of lipid

    • Concentration of lipids

Types of Structures Formed

  • Micelles

  • Liposomes

  • Bilayers

Micelle Formation

  • Micelles are formed in solutions of amphipathic molecules that possess larger heads than tails, such as:

    • Fatty acids

    • Sodium dodecyl sulfate

  • A micelle consists of a few dozen to thousands of lipid molecules, with aggregation occurring when concentrations exceed a threshold.

Vesicles (Liposomes)

  • Small bilayers can spontaneously seal into spherical vesicles (liposomes):

    • Vesicle membranes can incorporate artificially inserted proteins.

    • Central aqueous cavity can enclose dissolved molecules.

    • Useful for transport (e.g., drug carriers) due to their ability to fuse with cell membranes or each other.

Membrane Bilayer Structure

  • Composed of two leaflets of lipid monolayers:

    • Hydrophilic head groups interact with water.

    • Hydrophobic fatty acid tails are embedded inside.

    • One leaflet faces the cytoplasm; the other faces the extracellular space or organelle interior.

What Are Membranes?

  • Membranes are complex lipid-based structures forming pliable sheets, composed of various lipids and proteins, and can be glycosylated.

  • All cells possess a cell membrane that segregates the cell from its surroundings; eukaryotic cells possess various internal membranes dividing cellular space into compartments.

Functions of Membranes

  • Define cell boundaries.

  • Facilitate selective import and export of materials:

    • Import of nutrients (e.g., lactose).

    • Export of waste and toxins (e.g., antibiotics).

  • Retain metabolites and ions within cells.

  • Sense external signals and transmit information intracellularly.

  • Provide compartmentalization:

    • Separate energy-producing reactions from energy-consuming ones.

    • Keep proteolytic enzymes away from vital cellular proteins.

  • Support nerve signal transmission.

  • Store energy via proton gradients.

  • Participate in ATP synthesis.

Common Features of Membranes

  • Sheet-like, flexible structure, thickness of 30–100 Å (3–10 nm).

  • Composed mainly of bilayered lipids (with the exception of archaebacteria, which employ a monolayer of bifunctional lipids).

  • Form spontaneously in aqueous solutions, stabilized by noncovalent forces, primarily the hydrophobic effect.

  • Protein molecules span and associate with the lipid bilayer:

    • Asymmetric distribution; certain lipids prefer the inner leaflet, others the outer.

    • Carbohydrate moieties are uniformly located outside the cell.

    • Electrically polarized, with the inside of the membrane exhibiting a negative charge (approximately -60mV).

    • Fluid structures consisting of a two-dimensional solution of oriented lipids.

Fluid Mosaic Model of Membranes

  • Proposed by Singer and Nicholson in 1972, which suggests:

    • Lipids create a viscous, two-dimensional solvent into which proteins are embedded.

    • Integral proteins are deeper within the bilayer, often spanning it.

    • Peripheral proteins are loosely associated and can be easily removed.

    • Examples of membrane components:

    • GPI-anchored proteins, glycolipids, sterols, and lipids in bilayer configuration.

Membrane Composition

Variation in Composition

  • Membrane lipid composition varies among organisms, tissues, and organelles:

    • Different ratios of lipid to protein, phospholipid types, and sterol abundance across various species.

    • Prokaryotes generally lack sterols.

    • Cholesterol is prevalent in animal plasma membranes but is virtually absent in mitochondria.

    • Specific lipids (e.g., galactolipids) are abundant in certain plant organelles like chloroplasts.

Composition Data

  • TABLE 11-1 illustrates the major components of plasma membranes in various organisms, showcasing the percentage composition of proteins, phospholipids, and sterols.

Membrane Structure in Archaea

  • Phospholipids in archaea possess unique glycerol chirality:

    • Archaea have L-glycerol, bacteria have D-glycerol.

  • Archaea exhibit unique fatty acids:

    • Use branched isoprene chains instead of the unbranched fatty acid chains seen in bacteria.

  • Key linkages differ:

    • Ether linkages in archaea vs. ester linkages in bacteria.

Monolayer in Archaea

  • Some archaea, such as Sulfolobus solfataricus, thrive in harsh environments (75–80 °C, pH 2–3) and utilize isoprenoid tetraethers to enhance membrane stability.

Membrane Asymmetry

  • Membrane bilayers demonstrate significant asymmetry:

    • Each leaflet possesses different lipid compositions, with the outer layer often more positively charged.

    • The presence of phosphatidylserine on the outside signals protein interactions:

    • In platelets, it activates blood clotting; in other cells, it marks cells for destruction.

Percent Distribution of Phospholipids in Erythrocytes

  • Illustrated data shows the percentage distribution of various phospholipids (e.g., Phosphatidylethanolamine, Phosphatidylcholine) across the inner and outer leaflets, indicating their asymmetrical distribution in membranes.

Functions of Proteins in Membranes

  • Membrane proteins serve various functions:

    1. Receptors: Detect signals from outside the cell, such as hormones and neurotransmitters.

    2. Channels, Gates, and Pumps: Facilitate the transport of nutrients, ions, and neurotransmitters.

    3. Enzymes: Involves in processes like lipid biosynthesis and ATP synthesis (e.g., F0F1-ATPase).

Types of Membrane Proteins

  • Three main categories:

    • Peripheral proteins: Associated with the membrane surface, easily removed by ionic interactions.

    • Integral proteins: Span the entire membrane, are hydrophobic, and removed with detergents, retaining associated lipids.

Six Types of Integral Membrane Proteins

  • Classification based on structure and function:

    • Includes various domains, with each type having unique biochemical properties.

Hydropathy Plots

  • Used to predict transmembrane domains based on the hydrophobicity of sequences.

  • Glycophorin and Bacteriorhodopsin are examples of proteins analyzed through these plots, showcasing hydrophobic and hydrophilic regions.

Amino Acids in Membrane Proteins

  • Transmembrane segments are predominantly hydrophobic, while specific charged amino acids are localized in aqueous domains, ensuring appropriate protein function within the lipid environment.

Lipid Anchors in Membrane Proteins

  • Some proteins are anchored by covalent links to lipids, enhancing targeting and membrane association—common anchors include:

    • Long-chain fatty acids, isoprenoids, and GPI anchors found only on the outer face of the plasma membrane.

Farnesylation of Proteins

  • Targets proteins to the inner leaflet of membranes via a CaaX signature, influencing cellular activity and stability.

Membrane Physical Properties

  • Membranes are dynamic and can transition between various phases, affecting their permeation properties:

    • Generally impermeable to large polar solutes but allow small polar and nonpolar molecules.

    • Membrane fluidity is influenced by fatty acid composition; unsaturation and chain length affect interaction stability.

Membrane Composition Variation with Temperature

  • E. coli adjusts fatty acid composition according to growth temperature to maintain membrane integrity and fluidity, demonstrated in TABLE 11-2 showing varying percentages of different fatty acids under different growth conditions.

Membrane Rigidity and Permeability

  • Sterols (e.g., cholesterol, ergosterol) and hopanols enhance the rigidity and permeability of membranes across various organisms.

Study of Membrane Dynamics: FRAP

  • Fluorescence Recovery After Photobleaching (FRAP) measures lateral lipid diffusion:

    • Allows calculation of the diffusion coefficient, indicating rapid lateral movement.

FRAP Methodology

  • Involves three steps:

    1. Labeling the outer cell surface.

    2. Photobleaching a specific area with intense laser.

    3. Measuring fluorescence recovery over time to assess lipid mobility.

Membrane Rafts and Curvature

  • Lipid rafts: Specialized microdomains within membranes, conducive to protein segregation and signaling, rich in glycosphingolipids.

  • Certain proteins (e.g., caveolin) induce membrane curvature, influencing the biophysical properties of membranes and enabling functional specialization.

Membrane Fusion Mechanisms

  • Membranes can fuse through spontaneous or protein-mediated processes; notable instances include:

    • Fusion during neurotransmitter release and viral entry.

Transport Across Membranes

  • Transport mechanisms consist of passive diffusion for small nonpolar molecules and facilitated pathways for polar molecules or ions using specific proteins.

Types of Transport Mechanisms

  • Passive Transport: Includes simple diffusion and facilitated diffusion along electrochemical gradients.

  • Active Transport: Can be primary, secondary, or mediated by specific cotransport mechanisms.

Classes of Transport Systems

  • Systems can be classified into:

    • Uniport: Transport of a single substance.

    • Symport: Transport of two substances in the same direction.

    • Antiport: Transport of two substances in opposite directions.

Glucose Transport Mechanisms

  • Glucose transport can be mediated by various protein types, with distinct configurations for different membrane satisfactions:

    • Includes Na+-glucose symporter and glucose uniporters operating in epithelial cells.

Bicarbonate Transport Observations

  • The bicarbonate transporter functions in maintaining membrane potential essential for respiratory processes, involving exchanges with chloride ions in erythrocytes.

Active Transport Mechanisms

  • Classifications of primary and secondary active transport depict how prolings energy (ATP or ion gradients) facilitates solute movement against gradients.

Energetics of Transport Proteins

  • Proton-driven ATPases are crucial for pH regulation and ATP synthesis in mitochondrial and chloroplast membranes, depicting reversible actions based on proton gradient dynamics.

Specialized Protein Structures

  • Structures such as aquaporins ensure facilitated passage of water across membranes, while ion channels regulate specific ion flows, maintaining concentration gradients essential for cellular functions.

Clinical Implications of Ion Channel Defects

  • Disorders resulting from ion channel mutations lead to various pathologies; specific channels (NA+, Ca2+, etc.) have clear links to diseases.

Example: Cystic Fibrosis

  • Cystic fibrosis results from mutations in epithelial cell ion channels, highlighting the significance of membrane protein integrity in health and disease.

Chapter 11: Summary

  • Membranes comprise diverse lipids and proteins, forming selectively permeable bilayers.

  • The properties of these bilayers may vary significantly among organisms, contributing to cellular functions such as transport, energy storage, and compartmentalization.

  • Active transport mechanisms depend on ATP and exhibit a range of forms to accommodate various cellular needs.

This chapter presented a comprehensive overview of the structural and functional complexities of biological membranes, emphasizing their essential roles in cellular life.