Cell Membranes: Structure and Composition

Roles and Importance of Cell Membranes

  • Barrier:

    • Separates the intracellular space from the external environment.

  • Control of Movement:

    • Determines the molecules that can cross in and out of the cell.

  • Cell Signalling:

    • Allows outside messages, such as hormones and growth factors, to modulate cellular functions.

  • Cell Recognition:

    • Facilitates cells in recognizing and interacting with other cells and the extracellular matrix (ECM).

  • Homeostasis:

    • The cell membrane dynamically maintains a controlled and stable internal environment within the cell.

Phospholipids (Recap)

  • Structure:

    • Phospholipids have a polar, hydrophilic "head" and hydrophobic fatty acid "tails."

  • Variation:

    • Phospholipids differ in fatty acid chain length, degree of saturation, and the nature of the polar head.

Membranes, Micelles, Liposomes

  • Amphipathic Molecules:

    • Structures such as phospholipids can form different structures in aqueous solutions.

  • Types of Structures:

    • Micelles: Formed by a single layer of phospholipids.

    • Bilayers: Form structures such as membranes and liposomes.

Phospholipid Bilayer

  • Formation:

    • A lipid bilayer forms through:

    • Hydrophobic Interactions: Between nonpolar tails of phospholipids.

    • Hydrophilic Interactions: Between polar heads of phospholipids and water molecules.

The Fluid Mosaic Model of Biological Membranes

  • Model Overview:

    • Proposed by Singer and Nicolson, describes the structure of biological membranes.

  • Structure:

    • Phospholipid bilayers act like a "lake" in which various proteins are either "floating" or immersed.

Membrane Fluidity

  • Movement of Phospholipids:

    • Can move within the bilayer; most lipids and some proteins drift laterally.

  • Transverse Movement:

    • Rare flip-flop movement across the membrane.

  • Flippases:

    • Transmembrane proteins that assist in moving lipids from the extracellular to the intracellular layer.

  • Floppases:

    • Perform the opposite action of flippases (facilitate movement from intracellular to extracellular).

Role of Cholesterol in Membrane Fluidity

  • Effects at Different Temperatures:

    • Warm Temperatures (e.g., 37°C): Cholesterol restrains the movement of phospholipids.

    • Cool Temperatures: Prevents tight packing, thereby maintaining fluidity.

  • Overall Function:

    • Stabilizes membrane fluidity overall.

Membrane Glycolipids

  • Glycolipids:

    • Some membrane lipids are linked to carbohydrates, generally found on the extracellular side of the membrane.

    • They cluster with cholesterol to form lipid rafts.

  • Function:

    • Play an important role in cell signalling and protein trafficking.

Membrane Asymmetry

  • Structure:

    • Cell membranes have distinct inside and outside faces, exhibiting structural and functional asymmetry.

  • Determining Factors:

    • The asymmetrical distribution of proteins, lipids, and associated carbohydrates is determined during membrane synthesis in the endoplasmic reticulum (ER) and Golgi apparatus.

Overview of Membrane Proteins

  • Functionality:

    • Biological membranes contain various proteins embedded in the lipid bilayer, determining the membrane's specific functions.

  • Classification:

    • Based on their position and/or function within the membrane.

Classification of Membrane Proteins by Location

  • Peripheral Proteins:

    • Bound to the surface of the membrane.

  • Integral Proteins:

    • Penetrate the hydrophobic core of the membrane.

    • Transmembrane Proteins:

    • Integral proteins that span the membrane entirely.

Integral Membrane Proteins

  • Hydrophobic Domains:

    • Composed of one or more stretches of nonpolar amino acids, typically coiled into alpha helices.

Glycoproteins

  • Definition:

    • Membrane proteins that have carbohydrate residues covalently linked to them, located on the extracellular side of the membrane.

  • Functions:

    • Involved in cell signaling, cell-cell recognition, and cell adhesion.

  • Note:

    • Do not confuse with glycolipids.

Membrane Channels, Carriers, and Pumps

  • Specialized Proteins:

    • Regulate the traffic of molecules in and out of the cell across the plasma membrane, with detailed functions to be explored in subsequent lectures.

Receptors and Signal Transduction

  • Definition:

    • Specialized membrane proteins that bind specific molecules (ligands) at the cell surface and transmit messages into the cell to activate responses.

  • Further Learning:

    • Detailed mechanisms will be covered in future lectures.

Membranes Interface with Cytoskeleton and Extracellular Matrix (ECM)

  • Future Discussion:

    • Later lecture will focus on the cytoskeleton and extracellular matrix interactions.

Techniques for Studying Cell Membranes

  • Freeze-Fracture Technique:

    • Specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer, supportive of the fluid mosaic model.

  • Fluorescence Microscopy:

    • Utilized to visualize various structures and components of the cell membranes.

    • Staining Techniques:

    • Lipid rafts visualized using red-fluorescent cholera toxin B (Lin et al., 2019, DOI: 10.3389/fimmu.2019.00109).

    • Membrane glycoproteins stained with green-fluorescent Concanavalin A.

    • Beta 1 Adrenergic Receptors immunostained using green-fluorescent antibodies.