Membranous cell organelles l

🖇 I. Endoplasmic Reticulum (ER)

📌 Definition

• A membrane network within cytoplasm.
  “Endo” = inside, “plasmic” = cytoplasm, “reticulum” = network.

📌 Classification & Structure

• Two types:
• 1. Smooth Endoplasmic Reticulum (SER)
     → smooth surface.
  2. Rough Endoplasmic Reticulum (RER)
     → rough surface due to ribosomes attached.
     → Continuous tubes/sacs connected together.
     → Continuous with nuclear membrane.

🟠 Nature

• Classified as a membranous organelle.
• Structure: delicate tubules / cisternae (histological term).

📌 Distribution & Appearance

Present in all cells.

1. RER

• Abundant in cells specialized in protein synthesis.
• (Exam Point) “Rough = Protein.”

Light Microscopy (LM):

• Not directly visible.
• If abundant → cytoplasm appears basophilic (blue) due to RNA in ribosomes binding basic dyes (H&E).

Electron Microscopy (EM):

• Appears as connected tubules/sacs with rough outer surface (ribosomes = dark electron-dense granules).

2. SER

• Abundant in cells with lipid/fat metabolism.
• LM: not visible.
• EM: network of smooth-surfaced tubules.

📌 Functions of ER

1. RER – Protein Synthesis & Modification

• Step 1: Translation occurs at ribosomes using mRNA from nucleus.
• Step 2: Ribosomes attach to RER; new proteins enter RER tubules.
• Step 3: Proteins undergo initial modification (defects → hereditary diseases).
• Step 4: Modified protein + RER membrane pinch off → vesicle.
• Step 5: Vesicle moves to Golgi apparatus for further processing.

(Note: Proteins form structural, enzymatic, and hormonal elements except steroid hormones.)

2. SER – Lipid & Detoxification Functions

• (Exam Point) Synthesis of lipids, phospholipids, and steroid hormones.
• (Exam Point) Detoxification of toxins & drugs (mainly in liver).
• (Exam Point) Calcium storage in muscle cells for contraction.

🖇 II. Mitochondria

📌 Definition

• Known as the Powerhouse of the Cell.

📌 Distribution & Size

• Present in all cells except:
• • Red Blood Cells (RBCs)
  • Terminal Keratinocytes
• (Exam Point) Size not required for memorization.

📌 Appearance (Light Microscopy)

• Not visible with routine H&E.
• Seen only with special stains (Exam Point):
• 1. Iron Hematoxylin
  2. Janus Green
  3. Acid Fuchsin
• Appear as rods or spheres.

📌 Appearance (Electron Microscopy)

• Surrounded by two membranes (double unit membranes).

1. Outer Mitochondrial Membrane (OMM):

• Smooth, permeable.

2. Inner Mitochondrial Membrane (IMM):

• Folded inward → Cristae (shelf-like structures).
• Selectively permeable.
• Contains Cardiolipin → responsible for selective permeability.

3. Intermembrane Space:

• Between OMM & IMM.
• Contains H⁺ ions for respiratory chain reactions.

4. Mitochondrial Matrix:

• Fluid inside IMM.
• IMM folds (cristae) → increase surface area. (Exam Point)

5. F1 Particles (Elementary Particles):

• On IMM surface facing matrix.
• Tennis racket appearance. (Exam Point)
• Contain enzymes for ATP synthesis.

📌 Mitochondrial DNA & Division

• Contains its own DNA → can divide independently (non-synchronous).
• Mitochondrial DNA:
• • Circular shape, in matrix.
  • 1% of total cell DNA.
• Nuclear DNA:
• • Double helix, in nucleus.
  • 99% of total DNA.

📌 Types (EM Classification)

1. Orthodox Type: clear cristae & matrix → resting cells.
2. Condensed Type: dense cristae, indistinct matrix → active cells.

🖇 III. Golgi Apparatus (GA)

📌 Definition

• Membranous organelle responsible for secretion. (Exam Point)

📌 Distribution & Location

• Present in all cells.
• Prominent in secretory cells.

📌 Appearance (Light Microscopy)

• Not seen with routine H&E.
• Negative Golgi Image (Exam Point):
• • Seen as clear unstained area near nucleus in highly basophilic cells (e.g. plasma cells).
• Special Stains: Silver → brown filaments (Fittaniya).

📌 Appearance (Electron Microscopy)

• Series of flattened sacs (cisternae/saccules) stacked like plates.
• Two faces:
• 1. Cis / Forming / Convex Face: faces RER & nucleus.
  2. Trans / Mature / Concave Face: faces cell membrane → site of secretion.

📌 Functions

1. Post-translational modification & packaging of proteins from RER.
2. Modification:
3. • Glycosylation → glycoproteins.
   • Trimming of protein chains.
3. Secretion:
4. • Final vesicles pinch off from Trans face.
4. Membrane recycling: forms membranes of RER & cell membrane.
5. Lysosome formation: produces lysosomal enzymes.

🖇 IV. Lysosomes

📌 Definition

• Digestive system of the cell.

📌 Contents & Origin

• Contain hydrolytic enzymes (digestive enzymes).
• Enzymes synthesized in RER, packaged by Golgi apparatus.

📌 Appearance

• LM: not visible normally.
• EM: membrane-bound vesicles with enzyme content.

📌 Classification (Functional State)

1. Primary Lysosome:
2. • Newly formed, inactive, from Golgi.
   • Has not yet fused with target.
2. Secondary Lysosome:
3. • Formed after fusion with a target vesicle.
   • Subtypes:
   • • Phagolysosome: fusion with Phagosome (solid material).
     • Multivesicular Body: fusion with Pinocytic vesicle (fluid).
     • Autophagic Vacuole: digestion of cell’s own organelles (self-eating).
3. Residual Body (Exam Point):
   Lipofuscin Pigment (Exam Point):
4. • Indigestible waste left after digestion.
   • Normally expelled.
4. • Accumulates in nerve cells & cardiac muscle cells (non-dividing).
   • Represents undigested residual bodies.

📌 Functions

• Digestion & elimination of harmful substances (internal/external).
• Cell metabolism: activates thyroid hormone.
• Post-mortem autolysis:
• • After death, lysosomal membranes rupture → enzymes digest cell → decomposition.

🚨 Clinical Note

• Lysosomal Storage Diseases: genetic defects in lysosomal enzymes → substrate accumulation.
• • Example: Gaucher’s Disease – deficiency of Glucocerebrosidase → buildup of Glucocerebroside in tissues (muscle, brain, joints).

🖇 V. Peroxisomes

📌 Definition

• Structurally resemble lysosomes but contain oxidative enzymes.

📌 Contents & Functions

• (Exam Point) Contain oxidative enzymes (vs. lysosomal hydrolytic enzymes).
• Functions:
• 1. Fatty acid oxidation.
  2. Alcohol detoxification.
  3. Use oxidative enzymes for metabolism.

📌 Origin

• (Exam Point) Not derived from Golgi apparatus.

📌 Structure (EM)

• Membrane-bound vesicles.
• May appear as coated vesicles (protection from digestion).