The Eukaryotic Cell

Cutaway of an animal cell — organelles, cytoskeleton and the secretory pathway

A generalised human (animal) cell cut open to show its compartments, drawn roughly to the proportions of a 20–30 µm secretory epithelial cell. The apical surface (top) carries microvilli and a single cilium; the rest of the plasma membrane is underlain by a cortex of actin microfilaments. The nucleus, wrapped in its double nuclear envelope with pores, holds chromatin (dark heterochromatin along the envelope, paler euchromatin inside) and a dense nucleolus. Sheets of rough ER studded with ribosomes surround the nucleus and continue into tubular smooth ER; the Golgi apparatus faces the ER with its cis face and the cell surface with its trans face, next to the centrosome from which microtubules radiate.

Animations show the main traffic of the cell: mRNA leaving the nucleus through nuclear pores; secretory vesicles budding from the trans-Golgi network, carried outward along microtubules by kinesin and fusing with the plasma membrane to release their cargo (exocytosis); a clathrin-coated vesicle pinching off from a coated pit (endocytosis) and travelling to an early endosome; mitochondria (with cristae and mtDNA) and slowly drifting free ribosomes; and a beating cilium. Hover or tap any organelle for details, or play the steps to follow a protein from gene to secretion.

Step-by-step

1 · The gene in the nucleus
The protein's gene lies in chromatin. Active genes are in loosely packed euchromatin, where RNA polymerase II transcribes them into pre-mRNA, which is capped, spliced and polyadenylated in the nucleus.
2 · mRNA exits through nuclear pores
Mature mRNA is exported through the nuclear pore complexes (≈2000–4000 per nucleus) of the double nuclear envelope. The nucleolus has meanwhile assembled the ribosomal subunits that will translate it.
3 · Translation on the rough ER
Translation starts on a free ribosome; when the N-terminal signal sequence emerges, the signal recognition particle docks the ribosome onto the rough ER and the growing chain is threaded into the ER lumen, where it is folded, disulfide-bonded and N-glycosylated.
4 · Processing in the Golgi apparatus
COPII vesicles carry the folded protein to the cis face of the Golgi. Passing through the cisternae it is further glycosylated and sorted; lysosomal enzymes are tagged with mannose-6-phosphate, secretory proteins are packaged at the trans-Golgi network.
5 · Transport along microtubules
Secretory vesicles bud from the trans-Golgi network and are carried by kinesin motors toward the plus ends of microtubules, which radiate from the centrosome to the cell periphery.
6 · Exocytosis
At the plasma membrane, SNARE proteins zipper the vesicle and plasma membranes together; they fuse and the cargo is released outside the cell, while the vesicle membrane becomes part of the cell surface. In regulated secretion this step waits for a rise in cytosolic Ca²⁺.
Energy · Mitochondria
Mitochondria oxidise pyruvate and fatty acids and use the electron transport chain and ATP synthase on their folded inner membrane (cristae) to make most of the cell's ATP — ≈30–32 ATP per glucose.
Lipids and detoxification
The tubular smooth ER makes membrane lipids and steroids, stores Ca²⁺ and detoxifies drugs; peroxisomes oxidise very-long-chain fatty acids and destroy H₂O₂; glycogen and lipid droplets are stored as inclusions.
Recycling · Endosomes, lysosomes and autophagy
Material taken in by endocytosis passes through endosomes to lysosomes (pH ≈4.5–5), whose acid hydrolases digest it. Worn-out organelles are enclosed in double-membraned autophagosomes that fuse with lysosomes, and short-lived or misfolded proteins are destroyed by proteasomes.
Surface specialisations
Microvilli (actin cores, ≈1 µm) multiply the absorptive surface; a cilium (9 + 2 microtubule axoneme, dynein-driven) beats to move fluid. Both are anchored in the cytoskeleton beneath the apical membrane.

The Eukaryotic Cell

Step-by-step

Labelled parts