Organelles and compartmentalization explain how eukaryotic cells divide work among specialized structures, controlled internal spaces, and directed vesicle transport routes.
An organelle is a distinct cellular subunit adapted for a particular function. Membranes create local conditions, concentrate enzymes and substrates, and isolate incompatible reactions.
boundary → controlled composition → efficient reaction conditions; vesicles and transport proteins then connect the separate spaces into one coordinated cell
The nuclear envelope keeps DNA and transcription in the nucleus while translation occurs on cytoplasmic ribosomes. This separation permits pre-mRNA processing before mature mRNA reaches a ribosome.
DNA transcription → 5′ cap, intron removal and poly-A tail → mature mRNA export through a nuclear pore → several ribosomes can translate it as a polysome
The matrix contains enzymes for the link reaction and Krebs cycle. The inner membrane carries electron-transfer chains and ATP synthase; cristae enlarge its area, while the narrow intermembrane space permits rapid H⁺ accumulation.
reduced carriers donate electrons → inner-membrane complexes pump H⁺ → electrochemical gradient forms → H⁺ returns through ATP synthase → ATP is produced
Thylakoid membranes contain chlorophyll, electron carriers and ATP synthase. Their lumen accumulates H⁺; the surrounding stroma contains enzymes for the Calvin cycle. Grana increase thylakoid membrane area.
light excites electrons → H⁺ accumulates in thylakoid lumen → ATP and reduced NADP form at the membrane → both enter the stroma and support carbon fixation
The nucleus has inner and outer membranes separated by a perinuclear space. The outer membrane is continuous with rough ER; nuclear pore complexes selectively control traffic between nucleoplasm and cytoplasm.
out: processed mRNA, tRNA and assembled ribosomal subunits
in: histones, polymerases, transcription factors and other nuclear proteins
lamina: supports nuclear shape and organizes chromatin
All cytosolic ribosomes begin translation free. A signal peptide on the growing polypeptide can redirect the ribosome to rough ER, where translation continues into the ER lumen or membrane.
The Golgi is a polarized stack of flattened cisternae. The cis face receives ER vesicles; different cisternae modify proteins and lipids; the trans face sorts cargo into vesicles for distinct destinations.
rough ER → transport vesicle → cis Golgi → sequential modification through cisternae → trans Golgi sorting → lysosome, plasma membrane or secretion
A vesicle buds with selected membrane and soluble cargo, travels to a target, and fuses so that cargo enters the correct compartment without exposure to cytosol.
cargo selection → membrane curvature and budding → scission → uncoating → target recognition → fusion; clathrin helps form selected coated buds during receptor-mediated endocytosis