Chapter 7: Membranes: Their Structure, Function & Chemistry

ABS311: Molecular and Cellular Biology

Chapter 7: Membranes: Their Structure, Function, and Chemistry

General Overview of Biological Membranes

  • Membranes play crucial roles in cellular biology, including:

    • Separating cells from their external environment.

    • Forming biochemically distinct compartments in eukaryotic cells.

    • Growing by the expansion of preexisting membranes.

Learning Outcomes

  • Upon completion of this chapter, students should be able to:

    1. Differentiate the five functional roles that biological membranes play in cell biology.

    2. Investigate how the fluid mosaic model supports the structure of cellular membranes.

    3. Explain how the structure of biological membranes defines their selective permeability.

Membrane Structure and Components

Composition of Biological Membranes

  • Biological membranes are complex structures made up of:

    • Lipids (~50% of membrane composition)

    • Phospholipids are the primary component.

    • Sterols such as cholesterol contribute to membrane fluidity.

    • Proteins (~50% of membrane composition)

    • Integral proteins: Span the membrane and require detergents for removal.

    • Peripheral proteins: Bound to the surface of the membrane, removed by changes in salt concentrations or pH.

    • Carbohydrates:

    • Glycolipids (5% of lipids)

    • Glycoproteins (1% of proteins)

Lipid Bilayer Structure

  • Lipid bilayer:

    • Composed of amphipathic phospholipids.

    • Polar (hydrophilic) head groups face water.

    • Nonpolar (hydrophobic) tails are oriented inward, away from water.

    • Stable yet fluid dynamic structure allowing for motion and rearrangement.

Fluidity of Biological Membranes

  • Membrane fluidity is essential for proper function, influenced by:

    • Temperature: affects the state of the membrane (fluid vs solid).

    • Fatty acid composition: saturation levels influence packing.

    • Saturated fatty acids lead to tightly packed, more viscous membranes.

    • Unsaturated fatty acids introduce kinks, promoting fluidity.

    • Sterol content: Cholesterol acts as a fluidity buffer:

    • At high temperatures, restricts lipid movement.

    • At low temperatures, prevents formation of rigid gel states.

Transition Temperature (Tm)

  • Each membrane has a characteristic temperature at which it transitions from a fluid state to a solid state, referred to as transition or melting temperature (Tm).

  • Tm depends on the chemical nature of the phospholipids in the membrane, and organisms can adapt their membranes to accommodate growth temperatures by modifying hydrocarbon chains.

Homeoviscous Adaptation

  • Organisms can regulate membrane fluidity through homeoviscous adaptation by altering the lipid composition in response to environmental temperature changes.

Membrane Protein Functions

  • Membrane proteins serve various critical functions:

    • Enzymes: Catalyze reactions on the membrane.

    • Transporters and channels: Facilitate selective movement of compounds across the membrane.

    • Structural proteins: Help maintain membrane shape and stability.

    • Endocytosis and Exocytosis: Involved in the acquisition and release of materials.

    • Receptors: Mediate signal acquisition from external stimuli.

    • Cell Communication: Enable signaling between cells.

Types of Membrane Proteins

  • Integral proteins:

    • Can be transmembrane (cross the membrane) or anchored to one membrane side; require detergents for extraction.

    • Some are permanently anchored on one side.

  • Peripheral proteins: Bound to the membrane surfaces and can be removed by perturbation of the ionic environment.

  • Lipid-anchored proteins: Covalently bonded to lipids within the bilayer.

  • Glycoproteins: Result from the binding of sugars (oligosaccharides) to proteins.

Membrane Dynamics and Asymmetry

Movement of Phospholipids in Bilayers

  • Phospholipids can undergo lateral diffusion (rotation and translocation), which confirms membrane fluidity.

  • Movement between the two membrane faces (flip-flop) is typically slow without the action of specific enzymes (flippases, floppases, and scramblases), resulting in lipid bilayer asymmetry.

Membrane Asymmetry

  • Most lipids are unequally distributed between the two monolayers, a state that is established during membrane biogenesis and maintained thereafter.

  • Glycolipids are predominantly found in the outer leaflet, playing roles in signaling and recognition.

Techniques for Studying Membranes

Freeze-Fracture Electron Microscopy

  • Used to visualize membrane proteins by fracturing the bilayer using a diamond knife to observe exposed surfaces of membrane proteins under Scanning Electron Microscopy (SEM).

Atomic Force Microscopy (AFM)

  • A technique where a probe scans the membrane surface, allowing detailed analysis of membrane domains.

Fluorescence Recovery After Photobleaching (FRAP)

  • Involves photobleaching a specific area of the membrane and observing the recovery of fluorescence as labeled molecules diffuse back in, demonstrating membrane fluidity.

SDS-PAGE Gel Electrophoresis

  • Membrane proteins can be isolated by lysing membranes with detergents like SDS, followed by electrophoresis for separation in polyacrylamide gels, allowing analysis of protein composition.

Summary: Importance of Membrane Fluidity

  • Membrane fluidity is vital for enabling the diffusion of proteins and lipids necessary for cellular function.

  • Maintaining the correct balance of fluidity is essential for preventing membrane leakage or rigidity that could impede cellular processes.

  • Cholesterol is essential for buffering membrane fluidity, ensuring membranes maintain optimal functionality under varying temperatures.