Membrane Structure & Components

• Lipid Bilayers as the Basis of Cell Membranes


  Cell membranes are fundamental structures in biology, acting as the boundary between the cell's internal environment and the outside world. The key to understanding their structure and function lies in the unique properties of phospholipids and other amphipathic lipids.
• Phospholipids: The Building Blocks


  Phospholipids are the primary components of cell membranes. They have a unique structure that makes them perfect for this role:

  1. Hydrophilic head: Contains a phosphate group
  2. Hydrophobic tails: Two fatty acid chains

  This dual nature (hydrophilic and hydrophobic) is why phospholipids are called amphipathic molecules.
• Formation of Lipid Bilayers

  When phospholipids are placed in water, they naturally arrange themselves into a bilayer structure:

  1. The hydrophilic heads face outwards, interacting with water on both sides of the membrane
  2. The hydrophobic tails face inwards, away from water

  This arrangement is energetically favorable and occurs spontaneously due to the hydrophobic effect.
• Lipid Bilayers as Barriers


  The unique structure of the lipid bilayer makes it an effective barrier between aqueous solutions. Here's why:

  1. Hydrophobic core
  2. Low permeability
• Hydrophobic core: 


  The inner part of the membrane is composed of the fatty acid tails, creating a hydrophobic environment.
• Low permeability: 


  This hydrophobic core has low permeability to:

  • Large molecules
  • Hydrophilic particles
  • Ions
  • Polar molecules
• Why is the membrane an effective barrier?

  The effectiveness of the membrane as a barrier can be explained by the following:

  1. Energetic barrier
  2. Size exclusion
  3. Charge repulsion
• Energetic barrier: 


  For a hydrophilic molecule to pass through the membrane, it would need to break its hydrogen bonds with water and enter the hydrophobic core. This is energetically unfavorable.
• Size exclusion: 


  Large molecules, regardless of their polarity, are generally unable to pass through the tightly packed lipid bilayer.
• Charge repulsion:

  The slight negative charge of the phosphate groups in the membrane heads can repel negatively charged ions.
• The selective permeability of membranes is crucial for maintaining cellular homeostasis and allowing for controlled transport of substances in and out of the cell.
• Integral and Peripheral Proteins in Membranes


  Membrane proteins are crucial components of cell membranes, performing various functions. They can be classified into two main categories: integral and peripheral proteins.
• Integral Proteins


  • Embedded in one or both lipid layers of the membrane
  • Have hydrophobic regions that interact with the membrane's hydrophobic core
  • Difficult to extract from the membrane without disrupting its structure
• Functions of Integral Proteins may include:


  • Channels or transporters for specific molecules
  • Receptors for cell signaling
  • Enzymes
  • Cell adhesion molecules
• Peripheral Proteins


  • Attached to one or other surface of the bilayer
  • Do not penetrate the hydrophobic core of the membrane
  • Can be easily removed without disrupting the membrane structure
• Functions of Peripheral Proteins may include:


  • Enzymes
  • Cell surface markers
  • Cytoskeleton attachments
  • Regulatory proteins
• Diversity of Membrane Proteins

  1. Diverse structures: They can vary greatly in size, shape, and composition
  2. Various locations: They can span the entire membrane, be partially embedded, or attached to the surface
  3. Multiple functions: They play roles in transport, signaling, enzymatic activity, and more
• The fluid mosaic model describes how these proteins are distributed throughout the membrane, able to move laterally unless restricted.
• Fluid Mosaic Model of Membrane Structure


  The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the structure of biological membranes.

  
• Key Components of Fluid Mosaic Model of Membrane Structure

  1. Phospholipid bilayer: Forms the basic structure of the membrane
  2. Integral proteins: Embedded in the membrane, often spanning its entire width
  3. Peripheral proteins: Attached to the membrane surface
  4. Glycoproteins: Proteins with attached carbohydrates on the extracellular side
  5. Cholesterol: Interspersed among phospholipids, affecting membrane fluidity
• Hydrophobic and Hydrophilic Regions:


  • Hydrophilic: Phospholipid heads, outer portions of integral proteins, peripheral proteins, carbohydrate chains
  • Hydrophobic: Phospholipid tails, inner portions of integral proteins, cholesterol
• Key Features of the Model:

  1. Fluidity: Lipids and some proteins can move laterally in the plane of the membrane
  2. Asymmetry: The inner and outer faces of the membrane are different
  3. Selective permeability: The membrane controls what enters and exits the cell
• Structure and Function of Glycoproteins and Glycolipids

  Glycoproteins and glycolipids are important components of cell membranes, characterized by carbohydrate chains attached to proteins or lipids, respectively.
• Structure of Glycoproteins:


  • Protein core with attached carbohydrate chains
  • Carbohydrates typically branched and complex
• Structure of Glycolipids:

  • Lipid (usually sphingolipid) with attached carbohydrate chain
  • Carbohydrates often simpler than in glycoproteins
• Location: 

  Both glycoproteins and glycolipids are found on the extracellular side of the membrane.
• Cell Adhesion:

  • Glycoproteins and glycolipids can bind to complementary molecules on other cells
  • Important for tissue formation and maintenance
• Cell Recognition:

  • Act as "cellular ID tags"
  • Allow cells to identify and interact with specific other cells
  • Crucial in immune responses, hormone binding, and other cellular interactions
• The specific arrangement of sugars in these molecules provides a vast array of potential "codes" for cellular recognition.