Unit 2: Cell Organelles

1. Endoplasmic Reticulum (ER)

The ER is a vast network of interconnected membranes (cisternae and tubules) that is continuous with the nuclear envelope. It is a key part of the endomembrane system.

Rough Endoplasmic Reticulum (RER)

• Structure: Appears "rough" because its surface is studded with ribosomes.
• Functions:
  1. Protein Synthesis & Modification: Synthesizes proteins destined for secretion, insertion into membranes, or delivery to organelles (like lysosomes). Proteins are folded and modified (e.g., glycosylation) inside the RER lumen.

Smooth Endoplasmic Reticulum (SER)

• Structure: Lacks ribosomes, giving it a "smooth" appearance.
• Functions:
  1. Lipid Synthesis: Synthesizes lipids, phospholipids, and steroids.
  2. Detoxification: Detoxifies drugs and poisons (abundant in liver cells).
  3. Calcium Storage: Stores and releases Ca²⁺ ions, critical for muscle contraction.

2. Golgi apparatus

The Golgi apparatus (or Golgi complex) is the "post office" of the cell. It receives, modifies, sorts, and packages molecules from the ER.

• Structure: A stack of flattened, membrane-bound sacs called cisternae. It has two faces:
  • Cis face: The "receiving" side, oriented towards the ER.
  • Trans face: The "shipping" side, oriented towards the plasma membrane.
• Functions:
  1. Modification: Further modifies proteins and lipids received from the ER.
  2. Sorting and Packaging: Sorts materials into different transport vesicles for their final destination.
  3. Lysosome Formation: Forms primary lysosomes.

3. Lysosomes

Lysosomes are the "digestive system" of the cell, containing powerful hydrolytic enzymes.

• Structure: Small, spherical, single-membrane vesicles that bud off from the Golgi apparatus.
• Internal Environment: Maintain a very acidic pH (around 5.0) by pumping H⁺ ions inside. This acidic environment is optimal for their enzymes.
• Functions:
  • Phagocytosis: Digesting food particles or engulfed pathogens (like bacteria).
  • Autophagy: The process of "self-eating," where lysosomes break down and recycle the cell's own old or damaged organelles.

4. Mitochondria

Mitochondria are the "powerhouses" of the cell, responsible for generating most of the cell's ATP through cellular respiration.

Structure

• Double Membrane: Has a smooth outer membrane and a highly folded inner membrane.
• Cristae: The folds of the inner membrane, which drastically increase the surface area for ATP synthesis.
• Intermembrane Space: The space between the two membranes, where protons (H⁺) are pumped.
• Matrix: The innermost compartment, containing enzymes for the Krebs cycle, mitochondrial DNA (mtDNA), and 70S ribosomes.

Function

• Cellular Respiration: The primary function. The Krebs cycle occurs in the matrix, and oxidative phosphorylation (Electron Transport Chain and Chemiosmosis) occurs on the inner membrane (cristae) to produce large amounts of ATP.

Semi-autonomous Nature

Mitochondria are called "semi-autonomous" because they retain some features of an independent organism.

• Own DNA: They possess their own circular DNA (mtDNA), similar to a prokaryotic chromosome.
• Own Ribosomes: They have their own 70S ribosomes (prokaryotic type).
• Self-Replication: They can grow and reproduce by binary fission, independent of the cell's main division cycle.
• Note: They are only "semi" autonomous because they still depend on the cell's nucleus for most of the proteins they need.

5. Peroxisomes

Peroxisomes are small, single-membrane-bound metabolic organelles.

Structure

• Small, spherical vesicles containing oxidative enzymes.
• They are not part of the endomembrane system (they don't bud from the Golgi).

Function

1. Breakdown of Fatty Acids: Perform beta-oxidation of very-long-chain fatty acids.
2. Detoxification: Contain enzymes (oxidases) that neutralize toxins (like alcohol) by producing hydrogen peroxide (H₂O₂).
3. Neutralizing H₂O₂: H₂O₂ is toxic, so peroxisomes also contain a high concentration of the enzyme catalase, which immediately breaks it down into harmless water and oxygen (2H₂O₂ → 2H₂O + O₂).

6. Nucleus and Nucleolus

The nucleus is the "control center" of the eukaryotic cell. It contains the cell's genetic material (DNA) and controls all cellular activities by regulating gene expression.

Structure of the Nucleus

• Nuclear Envelope: A double membrane that encloses the nucleus. The outer membrane is continuous with the ER.
• Nuclear Pore Complex: Large protein complexes that perforate the envelope, acting as regulated gateways for molecules to enter and exit the nucleus (e.G., mRNA exits, proteins enter).
• Nucleoplasm: The gel-like substance inside the nucleus.
• Chromatin: The complex of DNA and proteins (histones) that forms the chromosomes. (See Unit 3).

Function of the Nucleus

• Genetic Storage: Houses and protects the cell's primary genetic library (DNA).
• Control of Cell Activities: Controls protein synthesis and other activities by transcribing specific genes into mRNA.
• Ribosome Synthesis: Contains the nucleolus, where ribosomes are made.

Nucleolus

• Structure: A dense, non-membranous region within the nucleoplasm.
• Function: The primary site of ribosome synthesis. It is where ribosomal RNA (rRNA) is transcribed and assembled with proteins (imported from the cytoplasm) to form the large and small ribosomal subunits.