Key Points: Eukaryotic Cell Organelles

Nucleus: The control center of the eukaryotic cell. Bounded by a double-membraned nuclear envelope with nuclear pore complexes (~3,000–4,000 per nucleus) that regulate transport. Contains chromatin (DNA + histone proteins) that condenses into chromosomes during division. The nucleolus within the nucleus synthesizes rRNA and assembles ribosome subunits. The outer nuclear membrane is continuous with the rough ER.

Endoplasmic Reticulum: An extensive network of interconnected membranes throughout the cytoplasm. Rough ER has ribosomes attached → site of protein synthesis and initial processing (signal cleavage, N-glycosylation). Smooth ER lacks ribosomes → lipid and steroid synthesis, drug detoxification (cytochrome P450 in liver), $Ca^{2+}$ storage.

Golgi Apparatus: Receives proteins from ER at its cis (forming) face. Processes them through stacked cisternae (glycosylation, phosphorylation, sulfation, proteolytic cleavage). Releases mature vesicles from trans (maturing) face toward: lysosomes, plasma membrane (secretion), or vacuoles. Also produces cell plate material during plant cell division.

Lysosomes: Membrane-bound vesicles derived from Golgi. Contain over 50 acid hydrolases (proteases, lipases, nucleases) active at pH ~5. Functions: heterophagy (digestion of externally ingested material) and autophagy (digestion of cell's own damaged organelles). Called "suicide bags" — rupture causes autolysis.

Mitochondria: Double-membraned organelles (outer smooth + inner folded into cristae). Matrix contains circular DNA, 70S ribosomes, and Krebs cycle enzymes. Cristae house ETC and ATP synthase. Semi-autonomous: can divide by binary fission. Evidence for endosymbiotic origin.

Chloroplasts: Found only in plant/algal cells. Double-membraned envelope; inside: thylakoids stacked into grana (light reactions: photosystems I and II, ETC, ATP synthase); stroma (Calvin cycle: $CO_{2}$ fixation). Contain circular DNA and 70S ribosomes. Semi-autonomous.

Ribosomes: Non-membranous, made of rRNA + proteins. Cytoplasmic ribosomes: 80S (60S + 40S). Organellar ribosomes (mito/chloro): 70S. Both types translate mRNA into proteins but are structurally distinct — enabling selective antibiotic activity.

Vacuoles: Single-membrane (tonoplast) vesicles. Large central vacuole in plants: osmotic regulation, turgor pressure, storage (anthocyanins, waste), and digestive functions. Animal cell vacuoles are small and temporary (food vacuoles, contractile vacuoles in protists).

Peroxisomes: Single-membrane organelles containing oxidative enzymes and catalase. Break down long-chain fatty acids and detoxify hydrogen peroxide ($H_{2}O_{2}$ → $H_{2}O$ + $O_{2}$). Abundant in liver and kidney cells.