Cell Membrane Structure and Function

Cell Membrane

• The cell membrane is a thin structure that encloses the cellular contents and regulates the transport of substances.
• It is visible by electron microscopy (EM), appearing as two electron-dense lines separated by an electron-lucent intermediate zone.
• The cell membrane is composed of lipids, proteins, and carbohydrates
  • Lipids (30%): Phospholipids and cholesterol
  • Proteins (60%): Integral and peripheral proteins.
  • Carbohydrates (10%): Short chains of polysaccharides that form glycoproteins and glycolipids, contributing to the cell coat.
• The cell coat (glycocalyx) is a layer of glycoproteins and glycolipids on the external surface of the cell membrane.
• Functions of the cell coat:
  • Immunological specificity: Contains blood group antigens and acts as a protective mechanical barrier.
  • Cell-cell interactions
  • Formation of basement membranes
• Functions of the cell membrane
  • Protection of cellular integrity
  • Regulation of cell-cell interactions
  • Recognition of antigens and foreign cells
  • Phagocytosis, pinocytosis, and exocytosis
  • Controlling movement of substances in and out of the cell (selective permeability)

Transport of Materials through Cell Membrane

• Passive transport: Simple diffusion and facilitated diffusion
• Active transport: Requires energy (e.g., Na+/K+ pump)
• Bulk transport:
  • Endocytosis: Movement of substances into a cell
    • Phagocytosis: Engulfment of solid particles.
    • Pinocytosis: Engulfment of liquids.
    • Receptor-mediated endocytosis: Highly selective process that requires specific receptors.
    • Endocytic vesicles become associated with clathrin proteins.
  • Exocytosis: Movement of substances out of a cell
    • Extrusion of residual bodies

Mitochondria

• The mitochondria are responsible for energy (ATP) production.
• They are enclosed by two membranes: an outer smooth membrane and an inner membrane that folds into cristae.
• The inner membrane contains respiratory chain enzymes and ATP synthase.
• The mitochondrial matrix is rich in oxidative enzymes and dense granules that store calcium.
• Mitochondria can move and divide independently, as they contain their own DNA, ribosomes, and systems for protein synthesis.
• Mitochondria are more abundant in cells with high metabolic activity, such as liver cells.

Medical Application of Mitochondria

• Abnormal mitochondrial DNA can lead to failure of oxidative metabolism, impairing mitochondrial function.
• Most common patterns of mitochondrial disorders:
  • Muscle weakness (mitochondrial myopathy)
  • Degenerative central nervous system disease (e.g., loss of optic nerve fibers, cerebellar tissue, white matter)
  • Metabolic disturbances (e.g., lactic acidosis)
• Mitochondrial diseases are maternally inherited and can manifest at any age.
• Diagnosis can be assisted by muscle biopsy and analysis of mitochondrial DNA.

Subtypes of Mitochondrial Myopathies

• MELAS: Mitochondrial encephalomyopathy, lactic acidosis, recurring stroke-like episodes
• Other findings include muscle weakness, seizures.
• MERRF: Myoclonic epilepsy with ragged red fibers; destruction of proteins involved in oxidative phosphorylation
• Other findings include generalized seizures.

Golgi Apparatus

• Found between the nucleus and the cell periphery.
• The secretory apparatus of the cell.
• Structure by light microscopy (LM):
  • Negative Golgi images: Intensely basophilic cells (e.g., plasma cells)
  • Special techniques (e.g., silver stain) reveal darkly staining granules and fibrils
• Structure by EM:
  • Flattened vesicles: Elongated, parallel sacs piled upon the other, forming stacks.
    • Cis (forming, immature) face: Faces the cell base
    • Trans (mature) face: Faces the cell surface.
  • Transfer vesicles (microvesicles): Carry proteins from rough endoplasmic reticulum (rER) to the cis face of the Golgi apparatus.
  • Macrovesicles (condensing vacuoles): Large, distended vesicles that separate from the trans face of the Golgi apparatus and mature into secretory vesicles or secretory granules.

Functions of the Golgi Apparatus

• Packaging and storage of proteins.
• Modification of proteins: Adding carbohydrates, glycogen, or phosphate groups.
• Formation of secretory proteins.
• Renewal of the cell membrane.
• Formation of lysosomes.

Case Scenario

• A 32-year-old woman presents with recurrent generalized seizures, diffuse muscular weakness, and episodes of transient left-sided paresis.
• This clinical presentation suggests a mitochondrial disorder, possibly MELAS or MERRF.
• Genetic consultation is essential to confirm the diagnosis and provide guidance for management.