Cellular Organelles – Quick Review

Nucleus

• Double‐membraned nuclear envelope with pores; continuous with ER
• Contains chromosomes (DNA) and nucleolus
• Nucleolus produces rRNA ➜ ribosome subunits
• Produces $rRNA$, $mRNA$, $tRNA$
• Regulates traffic in/out via nuclear pores

Ribosome

• Two subunits (large + small) of rRNA + protein
• Free in cytosol (cytosolic proteins) or bound to RER (secretory/endomembrane proteins)

Endoplasmic Reticulum (ER)

• Continuous membrane cisternae; lumen separate from cytosol
• Smooth ER: lipid synthesis, carbohydrate metabolism, $Ca^{2+}$ storage, drug detox
• Rough ER: ribosome-studded; synthesizes secretory proteins, forms glycoproteins, adds membrane

Golgi Apparatus

• Stacked cisternae with polarity (cis ➜ trans)
• Modifies proteins/lipids, makes polysaccharides
• Sorts & packages into vesicles; cisternal maturation model

Endomembrane System – Protein Secretion Pathway

1. DNA (nucleus) codes protein ➜ $mRNA$
2. $mRNA$ to ribosome on RER ➜ polypeptide enters RER lumen
3. Transport vesicle ➜ cis-Golgi ➜ modification ➜ trans-Golgi
4. Secretory vesicle ➜ plasma membrane ➜ exocytosis (e.g., milk proteins)

Lysosome

• Golgi-derived acidic vesicle with hydrolytic enzymes
• Digest food (phagocytosis) & recycle organelles (autophagy)

Vacuole (plants)

• Large, membrane-bound central compartment (cell sap)
• Storage, waste, digestion, turgor pressure, pigments

Peroxisome

• Single membrane; oxidative enzymes transfer H to $O<em>2$ ➜ $H</em>2O_2$ ➜ water
• Lipid breakdown & detox

Plastids (plants)

• Chloroplast: double membrane, stroma, thylakoids/grana; photosynthesis
• Chromoplast: yellow–red pigments
• Leucoplast: colorless; stores starch/oil

Mitochondrion

• Double membrane; inner folds (cristae), matrix with DNA & ribosomes
• Site of cellular respiration; ATP production

Plasma Membrane

• Phospholipid bilayer + proteins
• Selective barrier for $O_2$, nutrients, wastes

Cytoskeleton

• Microtubules (tubulin): shape, organelle positioning, centrioles, cilia/flagella
• Microfilaments (actin): muscle contraction, cleavage furrow, shape
• Intermediate filaments (keratin): tension resistance, organelle anchoring, cell junctions

Maintenance & Energy Mnemonics

• Cell maintenance: L-P-V = Lysosome, Peroxisome, Vacuole
• Energy: Mitochondria & Chloroplasts (own DNA, double membrane)