Cellular Organelles: Structure and Function

Membranous Organelles

Membranous organelles are specialized structures within eukaryotic cells, each surrounded by a lipid bilayer membrane. These organelles perform essential cellular functions, including synthesis, packaging, and degradation of cellular materials.

• Golgi Apparatus (Golgi Body, Golgi Complex, Dictyosome):

  • Composed of flattened, membrane-bound stacks called cisternae (singular: cisterna).

  • Functions in packaging materials for lysosomes, peroxisomes, secretory vesicles, and membrane segments used to replenish the cell membrane.

  • Receives transport vesicles from the rough endoplasmic reticulum (RER) at the cis-Golgi (convex end) and processes them to the trans-Golgi (concave end).

  • Secretory products are discharged from the cell via exocytosis, a process where cytoplasmic materials are ejected by fusion of a membranous vesicle with the cell membrane.

  • Example: The Golgi apparatus packages digestive enzymes into lysosomes and hormones for secretion.

• Lysosomes:

  • Spherical, membrane-walled sacs containing digestive enzymes known as acid hydrolases.

  • Function in digesting unwanted substances, such as excess or worn-out organelles, food particles, and engulfed viruses or bacteria.

  • Example: Lysosomes break down bacteria engulfed by white blood cells.

• Peroxisomes:

  • Smaller, membrane-walled sacs containing enzymes, especially oxidases and catalases.

  • Function in removing toxic peroxides from the body by breaking down hydrogen peroxide (H2O2).

  • Peroxide: A compound containing an oxygen-oxygen single bond (e.g., H2O2).

  • Example: Peroxisomes in liver cells detoxify alcohol and other harmful substances.

Non-Membranous Organelles

Non-membranous organelles are not enclosed by lipid membranes and are always exposed to the cytosol. They play critical roles in protein synthesis, cell structure, and intracellular transport.

• Ribosomes:

  • Complexes of two subunits (ribosomal RNA and protein).

  • Function as the site of protein synthesis.

  • Two types:

    • Free ribosomes: Located within the cytosol; synthesize proteins for use within the cell.

    • Fixed ribosomes: Bound to the endoplasmic reticulum (ER); synthesize proteins for export or for use in membranes.

  • Example: Ribosomes on the rough ER synthesize digestive enzymes for secretion.

• Cytoskeleton:

  • An elaborate, internal network of protein rods spanning throughout the cytosol.

  • Confers strength and flexibility to the cytoplasm, provides support and shape to the cell, and enables intracellular movement.

  • Components:

    • Microtubules: Composed of the protein tubulin; function as the main support of the cell, allow the cell to change shape, enable organelle movement, and play a role in cell division by moving and separating DNA strands.

    • Microfilaments: Mainly composed of thin strands of the actin protein; involved in cell movement and shape changes.

    • Intermediate Filaments: Provide strength, stabilize organelles, and assist in the transport of materials within the cytoplasm.

  • Example: Microtubules form the mitotic spindle during cell division.

Summary Table: Membranous vs. Non-Membranous Organelles

Key Terms and Definitions

• Exocytosis: The process by which cells expel materials in vesicles by fusing them with the plasma membrane.

• Acid Hydrolases: Enzymes within lysosomes that break down biomolecules under acidic conditions.

• Oxidases and Catalases: Enzymes in peroxisomes that neutralize toxic substances, especially hydrogen peroxide.

• Microtubules: Hollow tubes of tubulin protein that provide structural support and facilitate intracellular transport.

• Microfilaments: Thin filaments of actin protein involved in cell movement and shape.

• Intermediate Filaments: Protein fibers that provide mechanical strength to cells.

Additional info:

• The Golgi apparatus is especially prominent in cells specialized for secretion, such as glandular cells.

• Lysosomal storage diseases result from defects in lysosomal enzymes, leading to accumulation of undigested materials.

• Peroxisomes are abundant in liver and kidney cells due to their role in detoxification.