Cell Membrane Structure and Function

Cell Membrane

• Outermost membrane surrounding the cell.
• Thickness: 7.5-10 nm, visible only with electron microscopy (E.M.).

Cell Membrane Structure

• Trilaminar appearance: Two electron-dense lines (black) separated by an electron-lucent line (white) under E.M.
• Consists of Lipid molecules, Protein molecules & Carbohydrate molecules.

Lipid Molecules

• Phospholipids:
  • Head: Hydrophilic (water-attracting).
  • Tail: Hydrophobic (water-repelling).
  • Arranged in a double layer (bilayer), hydrophobic tails are directed towards the center.
• Cholesterol:
  • Found between phospholipid molecules.
  • Firmly embedded in the lipid bilayer.

Protein Molecules

• Integral Proteins:
  • Firmly embedded in the lipid bilayer.
  • Transmembrane proteins - large molecules that act as channels.
• Peripheral Proteins:
  • Loosely attached to the membrane.

Carbohydrate Molecules

• Present as Glycolipids and Glycoproteins.
• Project from the external surface of the membrane, forming the cell coat.

Functions of Cell Membrane

• Endocytosis:
  • Phagocytosis: For solid particles; cells can engulf bacteria (e.g., macrophages).
  • Pinocytosis: For fluids; cells can engulf fluids.
  • Receptor-mediated endocytosis: For large molecules like hormones and drugs; needs receptors (integral proteins).
• Exocytosis: The opposite of endocytosis; expels waste products from the cell.

Hematoxylin & Eosin stain (H&E)

• Routine stain used for light microscopy (L.M.).
• Hematoxylin (H): Basic (alkaline), stains acidic structures blue; stains DNA (in nucleus) and RNA (in ribosomes and RER).
• Eosin (E): Acidic, stains basic structures red; stains cytoplasm, which is rich in mitochondria.

Mitochondria

• Membranous organelle containing enzymes necessary for energy production (ATP) - the "powerhouse of the cell".
• Variable in size and shape: Elongated, rod-shaped, or spherical.
• Number varies depending on cell activity:
  • Liver cells (active) contain numerous mitochondria.
  • Lymphocytes (less active) contain few mitochondria.
• Located at sites of maximum energy requirement (e.g., between myofibrils in cardiac muscle cells).

Mitochondria Structure (E.M.)

• Double-membranous organelle:
  • Outer membrane: Smooth, no folds.
  • Inner membrane: Forms complex folds called cristae (number increases in active cells).
  • Mitochondrial matrix: Contains mitochondrial DNA and enzymes.

Functions of Mitochondria

• Provide the cell with ATP through aerobic respiration, which occurs within the matrix and on the inner membrane.

Ribosomes

• Non-membranous organelles involved in protein synthesis.
• Small in size (20-30 nm in diameter).
• Under L.M., aggregation of ribosomes causes cytoplasmic basophilia (due to rRNA).
• Under E.M., composed of small and large subunits, both made of rRNA and protein molecules.

Types of Ribosomes

• Free ribosomes (polyribosomes or polysomes): Multiple ribosomes bound to a single mRNA molecule.
• Attached ribosomes: Ribosomes attached to the surface of the endoplasmic reticulum (forming RER).

Functions of Ribosomes

• Free ribosomes: Synthesize proteins for use within the cell (e.g., cytoskeletal proteins).
• Attached ribosomes: Synthesize proteins that:
  • Will be secreted outside the cell.
  • Remain in the cytoplasm as primary lysosomes.

Rough Endoplasmic Reticulum (RER)

• Membranous organelle involved in protein synthesis, primarily for secretion outside the cell.
• Found in cells specialized for protein synthesis and secretion (e.g. fibroblasts, plasma cells).
• RER in L.M. is basophilic due to its content of ribosomes.
• Under E.M., appears as parallel cisternae (long and flattened) with attached ribosomes and polyribosomes.

Functions of RER

• Segregation of proteins synthesized by the attached ribosomes for transport to the Golgi apparatus.

Smooth Endoplasmic Reticulum (SER)

• Endoplasmic reticulum cisternae with no attached ribosomes (not basophilic).
• Under E.M., appears as tubules and vesicles.
• Sites and Functions of SER:
  • Liver cells: Glycogen metabolism, detoxification of drugs and toxins.
  • Adrenal gland: Lipid biosynthesis (e.g. cortisone hormone).
  • Muscle cells: Calcium metabolism.

Golgi Apparatus

• Membranous organelle involved in protein secretion, synthesized by RER.
• Under L.M., appears as an unstained area near the nucleus (negative Golgi image) with silver staining.
• Under E.M., appears as a stack of 4-10 saccules:
  • Immature face (convex): Receives proteins from RER.
  • Mature face (concave): Releases secretory vesicles.
  • Secretory vesicles: Large, arise from the mature face and either are secreted outside the cell (exocytosis) or remain in the cytoplasm as primary lysosomes.

Functions of Golgi Apparatus

• Concentration and secretion of proteins.
• Forms primary lysosomes.

Lysosomes

• Membranous organelles containing approximately 40 hydrolytic enzymes, also known as "suicide bags".
• Found in all cells but more abundant in cells with phagocytic activity (e.g. macrophages).

Lysosome Structure (E.M.)

• Primary lysosomes: Freshly synthesized, spherical and homogenous core, involved in intracytoplasmic digestion.
• Secondary lysosomes: Start cytoplasmic digestion, irregular in shape, larger, heterogeneous core.

Functions of Lysosomes

• Digestion and lysis of phagocytosed particles into simple molecules.
• Digestion and lysis of dead cell organelles.

The Cytoskeleton

• Non-membranous structures important for:
  • Maintaining cell shape.
  • Movement of the entire cell.
  • Movement of cell organelles.
• Includes:
  • Microfilaments (Actin filaments): Diameter 5-9 nm, composed of actin protein.
  • Intermediate filaments: Diameter 10 nm.
  • Microtubules: Diameter 25 nm.

Microfilaments (Actin Filaments)

• Diameter 5-9 nm.
• Composed of actin protein synthesized by free ribosomes.
• Found in:
  • Muscle cells.
  • Microvilli.
  • Contractile rings formed during cell division.
• Functions of Actin:
  • Muscle contraction.
  • Preservation of microvilli.
  • Cell division.

Microtubules

• Hollow tubules.
• Diameter 25 nm, variable in length.
• Formed of tubulin protein synthesized by free ribosomes.

Functions of Microtubules

• Supporting cell shape.
• Major role in cell division (formation of mitotic spindle).
• Forms:
  • Centrioles.
  • Cilia.
  • Flagella.

Centrioles

• Non-membranous structures important for cell division.
• Two centrioles are perpendicular to each other.
• Each centriole is a short cylinder, the wall of which is composed of 27 microtubules arranged in 9 triplets.

Functions of Centrioles

• Cell division.
• Formation of basal bodies of cilia.

Cilia

• Hair-like processes projecting from the free surface of certain epithelial cells.
• Found in the respiratory system and female genital organs.

Cilia Structure (E.M.)

• Shaft: Contains 9 peripheral doublets of microtubules and 2 central singlets of microtubules.
• Basal body: Similar to a centriole with 9 triplets of microtubules.
• Rootlets: Formed of fibers.

Functions of Cilia

• Movement of fluids or foreign bodies on the surface of cells.

Cell Inclusions

• Stored metabolites or other substances inside the cytoplasm.

Types of Cell Inclusions

• Glycogen granules: Stained by Best's carmine, found in the liver and muscle fibers.
• Lipid droplets: Stained by Osmic acid and Sudan black.
• Pigments:
  • Exogenous: Dust particles, carotene.
  • Endogenous: Hemoglobin, melanin.

Nucleus

• Found in all cells except red blood cells and platelets.
• Usually mononucleated, but some cells can be binucleated or multinucleated.

Nucleus Size and Shape

• Largest structure in the cell, ranging from 3-14 μm in diameter.
• Rounded, oval, kidney-shaped, or multilobed.

Nucleus Staining and Appearance (L.M.)

• Vesicular (open-face) nucleus: Lightly stained, details are visible (e.g. nucleolus), found in active cells (liver cells).
• Condensed nucleus: Deeply basophilic, no apparent details, found in less active cells (lymphocytes).

Nucleus Structure (E.M.)

• Contains nuclear envelope, nucleoplasm, nucleolus, and chromatin.

Explore the intricate details of cell membrane structure in this quiz. Learn about the various components like lipid molecules, protein molecules, and carbohydrates that play crucial roles in membrane functionality. Test your knowledge on the trilaminar appearance and the unique properties of membrane constituents.