Practice question

Eukaryotic cytoplasm contains membrane-bound and non-membrane organelles distinguished by presence of phospholipid bilayer enclosing aqueous compartment that allows compositional specialization. Ribosomes are 80S megaDalton protein-RNA assemblies consisting of small and large subunits that translate mRNA in cytosol or on rough endoplasmic reticulum, lacking membrane and exchanging subunits dynamically. Nucleolus is membraneless nuclear subdomain formed by liquid-liquid phase separation around rDNA repeats for ribosome biogenesis. Centrosome comprises two centrioles surrounded by pericentriolar material nucleating microtubules, also non-membrane. Membrane-bound organelles physically segregate metabolic reactions and maintain distinct pH, calcium, redox and enzyme contents via active transport and impermeable bilayer. Peroxisome is archetypal small spherical organelle 0.1 to 1 micrometer diameter bounded by single membrane, equipped with specific import machinery for PTS1 and PTS2 signal proteins and containing oxidative enzymes and catalase. Its lumen is topologically isolated, enabling hydrogen peroxide generation without cytosolic damage. Other membrane-bound examples include endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids, while non-membranous structures rely on phase separation or protein scaffolds. This compartmentalization underpins eukaryotic metabolic complexity and regulation of incompatible reactions.