Cell Organelles II Quiz

Cell Organelles II

• Single-bound membrane organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.
• The endomembrane system encompasses the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane.
• The endoplasmic reticulum (ER) is a network of interconnecting channels called cisternae, making up more than half of the total membranes in eukaryotic cells.
• The ER extends from the outer nuclear membrane throughout most of the cytoplasm to the plasma membrane.
• The ER's function is to move molecules through its cisternae (lumen/cavity) from one part of the cell to another.

Types of Endoplasmic Reticulum

• Rough ER (RER): RER has ribosomes on its surface and is prevalent in cells specialized for protein secretion, such as pancreatic acinar cells (digestive enzymes), fibroblasts (collagen), hepatocytes (albumins), and plasma cells (immunoglobulins).

• Smooth ER (SER): SER lacks ribosomes and is prominent in Leydig cells of the testis (testosterone secretion), hepatocytes (detoxification), and other cells.

Rough Endoplasmic Reticulum (RER) Function

• RER is where proteins are synthesized, packaged, and then secreted.

Smooth Endoplasmic Reticulum (SER) Function

• Lipid synthesis: SER synthesizes phospholipids (major components of cellular membranes) and steroids or sex hormones (e.g., testosterone from Leydig cells).
• Detoxification: SER enzymes (e.g., cytochrome P450 family) detoxify harmful molecules like alcohol, toxins, and drugs.
• Bile processing: In liver cells, SER enzymes process bile components. Jaundice results from the accumulation of bilirubin or other pigments, which are usually metabolized and excreted as bile by SER enzymes.
• Muscle contraction: SER in skeletal muscle cells (sarcoplasmic reticulum) releases and re-absorbs calcium ions (Ca2+) during muscle contraction/relaxation.

Golgi Apparatus

• The Golgi apparatus is a series of flattened membranous sacs (cisternae).
• It receives vesicles containing proteins from the RER through its cis face.
• Proteins undergo modification within the Golgi apparatus.
• The Golgi apparatus has three parts:
  • Cis face: Receives incoming transport vesicles from RER.
  • Cisternae: Site of protein modifications.
  • Trans face: Releases secretory vesicles (containing modified proteins/vesicles).

Protein Modifications within the Golgi Complex

• Glycosylation: Addition of carbohydrates.
• Sulfation: Addition of sulfur.
• Phosphorylation: Addition of phosphate.
• Proteolysis: Cleavage of peptide bonds.

Lysosomes

• Lysosomes are membranous sacs containing hydrolytic enzymes used by eukaryotic cells to digest macromolecules.
• Lysosomal enzymes function best in the acidic pH of lysosomes.
• Leakage of lysosomal contents into the cytosol (which has a near-neutral pH) inactivates lysosomal enzymes.
• Excessive leakage can also lead to cell death via self-digestion.

Types of Lysosomes

• Primary lysosomes: Newly secreted from the Golgi apparatus and do not fuse with other vesicles.
• Secondary (heterolysosomes): Form by the fusion of primary lysosomes with other vesicles, leading to the degradation of ingested material.
  • Phagolysosomes: Formed by the fusion of a phagocytic vacuole with a lysosome.
  • Multivesicular bodies: Created by fusing an endocytic vesicle (endosome) with a lysosome.
  • Autophagolysosomes: Formed by an autophagic vacuole (formed by autophagy, which targets worn-out organelles) fusing with a lysosome.
• Tertiary lysosomes: Contain only indigestible material – residual bodies.

Vacuoles

• Vacuoles are large vesicles derived from the endoplasmic reticulum and Golgi apparatus.

• Types of vacuoles:

  • Food vacuoles: Formed by phagocytosis (engulfment of food particles) in phagocytic cells (e.g., amoebas, macrophages).
  • Contractile vacuoles: Found in freshwater protists; they pump out excess water.
  • Plant vacuoles (plant cell):
    • Small vacuoles: Store organic compounds (e.g., proteins in seeds).
    • Plant defense vacuoles: Protect the plant from animals by housing poisonous/unpalatable compounds.
    • Pigment vacuoles: Contain pigments (e.g., red/blue pigments in petals to attract pollinators).
    • Central vacuole: Large central vacuole in mature plant cells, containing cell sap rich in minerals and other stored compounds.