Biomembranes and Cell Architecture

Prokaryotic and Eukaryotic Cell Architecture

  • Cell Classification: Cells are the fundamental units of life, broadly categorized into prokaryotic and eukaryotic cells.
  • Prokaryotic Cell Components:
    • Mesosome: An invagination of the plasma membrane.
    • Ribosomes: Sites of protein synthesis.
    • Cytoplasm: The cellular medium.
    • DNA: Genetic material.
    • Cell Wall: Protective outer layer.
    • Plasma Membrane: Regulation of transport.
  • Eukaryotic Cell Components:
    • Nucleus: Contains genetic material; enclosed by an inner and outer nuclear membrane with an intermembrane space.
    • Mitochondrion: Features include the outer mitochondrial membrane, inner mitochondrial membrane, matrix, and intermembrane space.
    • Endoplasmic Reticulum (ER): Continuous with the outer nuclear membrane.
    • Golgi Complex: Involved in protein processing.
    • Lysosome: Degrades cellular waste.
    • Vesicles: Transport compartments.
    • Plasma Membrane: Divided into the exoplasmic face and the cytosolic face relative to the cytosol and exterior.

Lipid Composition of Biomembranes

  • Basic Structure: The watery interior of cells is surrounded by the plasma membrane, a two-layered shell (bilayer) of phospholipids and proteins.
  • Amphipathic Nature: All membrane lipids are amphipathic, meaning they possess a water-seeking (hydrophilic) polar head group and a hydrophobic fatty chain tail.
  • Three Classes of Membrane Lipids:
    • Phosphoglycerides:
      • The most abundant class of lipids in biomembranes.
      • Structure: Consists of a hydrophobic tail of two fatty acyl chains esterified to the two hydroxyl groups in glycerol phosphate. The polar head group is attached to the phosphate group.
      • Variations: Fatty acyl chains commonly contain 1616 or 1818 carbons. Saturation levels vary, with 00, 11, or 22 double bonds.
      • Examples: Phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine.
    • Sphingolipids:
      • Structure: Consists of a hydrophobic tail made of a long-chain fatty acid attached to the amino group of sphingosine (an amino alcohol with a long hydrocarbon chain) and a polar head group.
      • Example: Sphingomyelins.
    • Cholesterol:
      • Structure: A steroid composed of a four-ring hydrocarbon and a hydroxyl substituent on one ring.
      • Physical Properties: Cholesterol is too hydrophobic to form a bilayer structure on its own and must be mixed with phospholipids (forming mixed bilayers).
      • Note: Cholesterol is notably absent in prokaryotic cells.

Physical and Biological Properties of the Lipid Bilayer

  • Spontaneous Formation: Lipid bilayers form spontaneously (e.g., liposomes).
  • Impermeability: The hydrophobic core acts as an impermeable barrier, preventing the diffusion of water-soluble (hydrophilic) solutes across the membrane. This barrier function is modulated by membrane proteins facilitating the transport of specific molecules.
  • Stability: The bilayer is maintained by hydrophobic and van der Waals interactions between lipid chains. It retains its characteristic architecture despite variations in external ionic strength and pH.

Membrane Proteins: Categories and Functions

  • Protein-to-Lipid Ratios: More proteins correlate with more functions.
    • Mitochondrial membrane: approx. 76%76\% protein.
    • Myelin: approx. 18%18\% protein.
  • Classification by Interaction:
    1. Integral Membrane Proteins (Transmembrane Proteins): Span the entire phospholipid bilayer and are composed of three segments (exoplasmic, cytosolic, and transmembrane). They can be removed using strong ion detergents.
    2. Lipid-anchored Membrane Proteins: Covalently bound to one or more lipid molecules. The hydrophobic carbon chain of the lipid is embedded in one leaflet of the membrane to anchor the protein.
    3. Peripheral Membrane Proteins: Do not interact with the hydrophobic core. They bind indirectly via integral proteins or directly with lipid head groups. They provide support and resist external tension and pressure.

Membrane Dynamics and lateral Mobility

  • Fluid Mosaic Model: Describes the membrane as a mosaic of components (lipids, glycoproteins, glycolipids, cholesterol, and proteins) that are free to move laterally.
  • Measuring Mobility:
    • FRAP (Fluorescence Recovery After Photobleaching): A technique using laser light to quantify the lateral movements of specific plasma membrane proteins and lipids labeled with GFP (Green Fluorescent Protein).
    • Cell Fusion: Demonstrated mobility by fusing human and mouse cells using Sendai Virus. After 40 min40\,\text{min} at 37 °C37\,\text{°C}, the membrane proteins were observed to have mixed thoroughly.

Mechanisms of Substance Movement Across Membranes

  • Net Flux: Indicates the imbalance between influx (movement into the cell) and efflux (movement out of the cell).
  • Four Basic Transport Mechanisms:
    1. Simple Diffusion (Passive): Through the lipid bilayer.
    2. Simple Diffusion (Passive) via Channels: Through aqueous protein channels. Channels have selective permeability and do not require binding with the transport material.
    3. Facilitated Diffusion (Passive): Mediated by protein transporters (carrier proteins) that bind the material and change shape to move it down its concentration gradient. Example: Glucose transporter (GluT1) in erythrocyte plasma membranes.
    4. Active Transport: Requires an energy-driven protein "pump" (using ATP) to move substances against a concentration gradient.
  • Endocytosis: Mechanism used by large polar molecules that cannot pass through the hydrophobic membrane by passive means.

Specialized Organelles of the Eukaryotic Cell

  • Plasma Membrane: Controls molecular movement and functions in cell-cell signaling and adhesion.
  • Mitochondria: Generates ATP via oxidation of glucose and fatty acids. The matrix contains enzymes, mitochondrial ribosomes, tRNA, and copies of the mitochondrial DNA genome.
  • Lysosomes: Possess an acidic lumen. They degrade internalized material and worn-out organelles.
    • Lysosomal Delivery Pathways:
      1. Phagocytic pathway: Delivery of insoluble particles from the cell surface.
      2. Endocytic pathway: Delivery of soluble macromolecules.
      3. Autophagic pathway: Delivery of worn-out organelles and bulk cytoplasm.
  • Nucleus and Nucleolus:
    • Nuclear Envelope: Double membrane; outer membrane is continuous with the rough ER.
    • Nucleolus: Site of most rRNA synthesis.
    • Nucleus: Contains chromatin; site of mRNA and tRNA synthesis.
  • Endoplasmic Reticulum (ER):
    • Smooth ER: Synthesizes lipids and detoxifies hydrophobic compounds.
    • Rough ER: Synthesizes, processes, and sorts secreted proteins, lysosomal proteins, and membrane proteins. Features membrane-bound ribosomes (comprising a Large subunit and a Small subunit) with specific A, P, and E sites.
  • Golgi Complex: Processes and sorts proteins synthesized on the rough ER.
  • Secretory Vesicles: Store secreted proteins and fuse with the plasma membrane for release.
  • Cytoskeleton: Filaments that provide structural integrity.
  • Plant-Specific Organelles:
    • Cell Wall: Made of cellulose; provides protection and shape.
    • Vacuole: Stores water, ions, and nutrients; aids in cell elongation.
    • Chloroplasts: Perform photosynthesis; contain internal membrane-bounded sacs.

Questions and Discussion

  • Question 1: Which type of Lipids can prevent a sudden decrease in cell membrane fluidity?
    • Answer: C (Cholesterol). Cholesterol helps maintain fluidity at low temperatures and stability at high temperatures.
  • Question 2: The ability to control substances entry and exit of cells is regulated by which components of the cell membrane?
    • Answer: C (Proteins). While the lipid bilayer is a barrier, protein channels and transporters regulate selective entry and exit.
  • In-Class Assignment:
    • Mitochondria: Generation of ATP; metabolic hub.
    • Nucleolus: rRNA synthesis and ribosome subunit assembly.
    • Golgi Complex: Post-translational modification, sorting, and packaging of proteins.