JW

Fluid mosaic

The Fluid-Mosaic Model: Key Concepts

  • The membrane is a mosaic composed of lipids, proteins, and sugars that together form a dynamic structure.

  • This mosaic concept emphasizes both diversity (different components) and organization (lipid bilayer with embedded proteins and carbohydrates).

  • The membrane is fluid, allowing components to move laterally within the plane of the membrane; some movements are rarer (e.g., flip-flop across the bilayer).

  • The model integrates structure, dynamics, and composition to predict how molecules interact with membranes in cells.

Learning objectives (from the transcript)

  • Describe how membrane components create the “mosaic” of the membrane.

  • Explain how membrane molecules can and cannot move.

  • Explain the types of data that demonstrate that the membrane is a fluid mosaic.

What does it mean to be a membrane mosaic?

  • A membrane mosaic consists of lipids, proteins, and sugars arranged in a way that forms a two-dimensional, continuously changing surface.

  • Key components visible in diagrams:

    • Lipid bilayer (approx. 5\ \text{nm} thick)

    • Lipid molecules embedded in or spanning the bilayer

    • Protein molecules embedded in or associated with the bilayer

  • Conceptual layout often shown with extracellular space on one side and cytosol on the other, separated by the lipid bilayer,

  • .

How do we know? (Methods and evidence sketches)

  • The idea of a mosaic is supported by several lines of evidence indicating varied composition and structure across membranes.

  • One technique highlighted is Thin-Layer Chromatography (TLC), which separates molecules according to polarity.

    • Principle: TLC separates membrane lipids (and other molecules) based on polarity differences; more polar molecules travel more slowly on the TLC plate.

    • This kind of separation helps identify the relative abundance of lipid components in different membranes.

    • .

Membrane composition: evidence of mosaicism across membranes

  • Even within the same organism, membrane composition is varied and mosaic.

  • Table 10-1: Major Lipid Components of Selected Biomembranes (MOL%)

    • The table lists lipid classes as percentages for different membranes:

    • PC = phosphatidylcholine

    • PE = phosphatidylethanolamine

    • PS = phosphatidylserine

    • SM = sphingomyelin

    • Membrane samples and their lipid compositions (MOL%):

    • Plasma membrane (human erythrocytes): PC 21\%, PE + PS 29\%, SM 21\%, Cholesterol 26\%.

    • Myelin membrane (human neurons): PC 16\%, PE + PS 37\%, SM 13\%, Cholesterol 34\%.

    • Plasma membrane (E. coli): PC 0\%, PE + PS 85\%, SM 0\%, Cholesterol 0\%.

    • Endoplasmic reticulum membrane (rat): PC 54\%, PE + PS 26\%, SM 5\%, Cholesterol 7\%.

    • Golgi membrane (rat): PC 45\%, PE + PS 20\%, SM 13\%, Cholesterol 13\%.

    • Inner mitochondrial membrane (rat): PC 45\%, PE + PS 45\%, SM 2\%, Cholesterol 7\%.

    • Outer mitochondrial membrane (rat): PC 34\%, PE + PS 46\%, SM 2\%, Cholesterol 7\%.

    • Primary leaflet location (directionality of lipids between exoplasmic and cytosolic faces) is indicated in the table: some membranes show exoplasmic localization, some cytosolic, and some both.

    • Note: The exact leaflet annotations vary by membrane type; the table provides a snapshot of localization patterns in addition to composition.

  • Takeaway: Lipid compositions vary widely across membranes, reinforcing the mosaic concept and suggesting functional specialization of membranes.

  • Source: Dowhan & Bogdanov (2002) in D. E. Vance & J. E. Vance, Biochemistry of Lipids, Lipoproteins, and Membranes; Alberts, ECB, 5th edition, Chapter 11.

Membranes are fluid

  • The membrane is held together by non-covalent interactions, allowing bonds to be readily broken and re-formed.

  • Implication: components can diffuse and rearrange without requiring dissolution of the membrane.

  • Mobility types usually discussed:

    • Lateral diffusion: movement of molecules within the same leaflet of the bilayer.

    • Flexion (tilting or bending of lipid tails).

    • Rotation: molecules rotate about their axes.

    • Flip-flop (rare): transverse movement from one leaflet to the other across the bilayer.

How do we know? (Real-time observation methods)

  • FRAP: Fluorescent Recovery After Photobleaching is a key method to observe membrane dynamics in real time.

    • Setup: membrane components (lipids or proteins) are labeled with fluorescence; a laser bleaches a patch of the membrane.

    • Observation: unbleached fluorescent molecules diffuse into the bleached area, and fluorescence in that patch recovers over time.

    • What FRAP measures: recovery time and the extent of recovery, which reflect lateral mobility and diffusion coefficients of membrane components.

    • Schematic elements in FRAP:

    • Bleached patch in the membrane

    • Labeled proteins or lipids around the patch

    • Diffusion of labeled molecules into the bleached region restoring fluorescence

  • FRAP data interpretation can reveal whether the membrane component is freely mobile, partially mobile, or immobile on the experimental timescale.

FRAP: What is it? (Concise definition from the slides)

  • FRAP stands for Fluorescent Recovery After Photobleaching.

  • It allows observation of real-time movement of membrane components by tracking the recovery of fluorescence in a bleached area.

  • It is used to quantify lateral diffusion and mobility of lipids and membrane proteins.

Supplementary evidence: membrane fluidity in humans (fish oil study)

  • A study mentioned (citation in the slides) reports that consuming supplemental fish oil for 21 or 42 days increased membrane fluidity in erythrocyte membranes of human volunteers.

  • Implication: dietary factors and fatty acid composition can modulate membrane fluidity, with potential effects on membrane function and cell signaling.

Synthesis: What this all means for membrane interactions

  • Membranes are amphipathic (having both hydrophobic and hydrophilic regions), mosaics (diverse components), and fluid (dynamic and adaptable).

  • This combination helps predict how other molecules interact with membranes (or don’t interact) depending on their polarity, size, and affinity for lipid environments.

  • Overall, the fluid-mosaic model provides a framework for understanding membrane organization, dynamics, and function in cellular contexts.

Quick reference: key terms and concepts

  • Amphipathic: molecules with both hydrophobic and hydrophilic regions, as seen in phospholipids.

  • Mosaic: a patchwork of different lipids, proteins, and sugars in the membrane.

  • Fluid: lipids and proteins can move within the plane of the membrane; some movements across the bilayer are rare.

  • Lateral diffusion: movement of membrane components sideways within the same leaflet.

  • Flip-flop: transverse movement from one leaflet to the opposite leaflet (rare).

  • TLC (Thin-Layer Chromatography): technique to separate molecules by polarity.

  • FRAP (Fluorescent Recovery After Photobleaching): method to study membrane component mobility in real time.

  • MOL%: mole percent composition used to describe lipid abundances.

  • Principal lipids involved: PC, PE, PS, SM, cholesterol; distributions vary by membrane type.

Connections to foundational principles and real-world relevance

  • The mosaic composition reflects evolutionary optimization: membranes tailor lipid and protein content to support specific organelle functions and cellular environments.

  • The fluid nature supports rapid signaling, trafficking, and membrane remodeling essential for processes like endocytosis, exocytosis, and vesicular transport.

  • Variation in lipid composition across membranes can influence membrane protein function, membrane curvature, and microdomain organization (e.g., lipid rafts concept).

  • Diet and fatty acid intake (e.g., fish oil) can alter membrane properties, affecting cell signaling and transport, illustrating the link between nutrition and cell physiology.

Notes on figures and sources from the transcript

  • Figure references include a diagram of the lipid bilayer showing extracellular space and cytosol, with labels for lipid bilayer (approx. 5 nm), lipids, and proteins.

  • TLC image and associated notes illustrate how polarity drives separation of membrane components.

  • The FRAP schematic explains the bleach-diffuse-recover cycle in membrane experiments.

  • Data tables and figures cited come from Alberts, ECB, 5th edition, Chapter 11, and Dowhan & Bogdanov (2002) in Biochemistry of Lipids, Lipoproteins, and Membranes.

Summary bullets for quick review

  • Membranes are dynamic mosaics of lipids, proteins, and sugars.

  • The lipid bilayer thickness is about 5\ \text{nm}, hosting both lipid and protein components.

  • Mobility in membranes includes lateral diffusion, flexion, rotation, and rarely flip-flop.

  • Evidence for fluid mosaicism comes from techniques like TLC and FRAP, among others.

  • Lipid compositions differ markedly among membranes (e.g., erythrocyte PM vs E. coli PM vs mitochondria membranes).

  • FRAP provides a real-time readout of lateral mobility and diffusion properties of membrane components.

  • Dietary fats (e.g., fish oil) can modulate membrane fluidity in humans.

  • The model supports predictions about interactions of other molecules with membranes and has broad implications for physiology and pharmacology.