2.2 Eukaryotic Organelles: Nucleus, Mitochondria, Ribosomes, ER, Golgi, and Lysosomes

2.2 Eukaryotic Organelles: Nucleus, Mitochondria, Ribosomes, ER, Golgi, and Lysosomes

Quick Summary: Eukaryotic cells utilize membrane-bound organelles to create distinct microenvironments for metabolic processes. The nucleus protects genetic instructions and directs protein synthesis. Ribosomes translate mRNA into polypeptides, working with the endomembrane system—the rough ER, smooth ER, and Golgi apparatus—to synthesize, fold, and package proteins and lipids. Mitochondria produce ATP through aerobic respiration and provide evidence of endosymbiotic origins. Lysosomes and peroxisomes safely degrade cellular wastes and toxic byproducts.

The Compartmentalization Advantage

Eukaryotic cells are larger in volume than prokaryotes. As volume increases, the surface-area-to-volume ratio drops, rendering unassisted diffusion inefficient. Eukaryotes resolve this through membrane-bound compartmentalization:

1. Catalytic Concentration: Reactants and enzymes are concentrated in localized compartments, accelerating reaction rates.
2. Segregation of Incompatible Pathways: Opposing reactions proceed simultaneously (e.g., fatty acid synthesis in cytosol while breakdown occurs in mitochondria).
3. Containment of Hydrolytic Enzymes: Destructive digestive enzymes are isolated in lysosomes, preventing damage to cytoplasm.
4. Specialized Microenvironments: Specific conditions, such as the acidic pH 4.5–5.0 of lysosomes, are maintained without disrupting the neutral pH 7.2 of the cytosol.
5. Amplified Membrane Area: Folds like mitochondrial cristae maximize surface area for transport proteins.

The Nucleus and Ribosomes

The Nucleus: Genetic Control Center

The nucleus houses genomic DNA and directs cell activity:

• Nuclear Envelope: A double lipid bilayer with nuclear pore complexes that selectively import proteins (polymerases, histones) and export mRNA, tRNA, and ribosomal subunits.
• Chromatin: DNA complexed with histone proteins, organized as loose, active euchromatin and dense, inactive heterochromatin. It condenses into distinct chromosomes during division.
• Nucleolus: A dense subnuclear region where ribosomal RNA (rRNA) is transcribed and assembled with proteins into ribosomal subunits.

Ribosomes: Protein Translation

Ribosomes are non-membranous complexes of rRNA and proteins:

• Structure: Eukaryotic 80S ribosomes consist of a small 40S subunit (mRNA binding) and a large 60S subunit (peptide bond catalysis).
• Free Ribosomes: Suspended in the cytosol; synthesize soluble proteins functioning in the cytoplasm and nucleus (e.g., glycolytic enzymes, actin).
• Bound Ribosomes: Docked on the rough ER; translate proteins destined for membranes, lysosomes, or secretion (e.g., insulin, antibodies).

The Endomembrane System: ER and Golgi

The endomembrane system coordinates protein and lipid maturation:

1. Rough Endoplasmic Reticulum (RER)

• Structure: Membranous sacs (cisternae) studded with ribosomes on the cytosolic face.
• Function: Polypeptides enter the RER lumen, where chaperones assist folding and carbohydrate chains are added (glycoproteins). Correctly folded proteins are packaged into coated transport vesicles directed to the Golgi.
• Abundance: Prominent in secretory cells, such as pancreatic beta cells and plasma B cells.

2. Smooth Endoplasmic Reticulum (SER)

• Structure: Tubular membrane network lacking ribosomes.
• Function: Synthesizes lipids and steroid hormones (abundant in testes, ovaries, and adrenal glands). In liver hepatocytes, SER enzymes detoxify drugs and toxins. In muscle cells, specialized SER (sarcoplasmic reticulum) stores and releases calcium ions ($Ca^{2+}$) to regulate contraction.

3. Golgi Apparatus: Sorting and Packaging

The Golgi apparatus consists of stacked, polarized cisternae:

• Cis Face: Receives transport vesicles from the rough ER.
• Medial Cisternae: Enzymatically modify carbohydrate groups, phosphorylate residues, and tag molecules.
• Trans Face: Packages sorted products into targeted vesicles: secretory vesicles (exocytosis to extracellular space), lysosomes (retaining hydrolytic enzymes), and membrane delivery vesicles (replenishing plasma membrane lipids and receptors).

Mitochondria and Endosymbiotic Theory

Mitochondrial Energetics

Mitochondria produce roughly 90% of cellular ATP via aerobic respiration:

• Outer Membrane: Smooth and permeable to small molecules through porins.
• Intermembrane Space: Accumulates protons ($H^+$) pumped during electron transport.
• Inner Membrane & Cristae: Convoluted folds (cristae) maximize surface area for electron transport complexes and ATP synthase.
• Matrix: Fluid interior containing citric acid (Krebs) cycle enzymes, circular mtDNA, and ribosomes.

Endosymbiotic Theory

Proposed by Lynn Margulis, the Endosymbiotic Theory states that mitochondria evolved from an aerobic bacterium engulfed by an ancestral eukaryote. Evidence includes: autonomous circular mtDNA resembling bacterial genomes, internal 70S ribosomes sensitive to bacterial antibiotics, independent replication via binary fission, and a double membrane with bacterial cardiolipin in the inner layer.

Lysosomes and Peroxisomes

Lysosomes: Digestive Vesicles

Membrane-bound sacs containing hydrolytic enzymes (acid hydrolases):

• Acidic Lumen: Active proton ($H^+$) pumps maintain an internal pH of 4.5–5.0, activating enzymes while protecting the cell if breakage occurs.
• Autophagy & Phagocytosis: Recycles damaged organelles (autophagy) and digests engulfed pathogens (phagocytosis).
• Clinical Defect: In Tay-Sachs disease, an absent lysosomal enzyme causes toxic lipid buildup in neural tissue.

Peroxisomes: Metabolic Neutralization

Single-membrane organelles that oxidize fatty acids via beta-oxidation, producing toxic hydrogen peroxide ($H_2O_2$). The enzyme catalase decomposes this byproduct into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$).