5.2 Eukaryotic Compartments and Cell Types

A eukaryotic nucleus is enclosed by a double nuclear envelope continuous with the endoplasmic reticulum. Nuclear pores regulate movement of proteins, RNA and ribosomal subunits. DNA associates with histones to form chromatin. The nucleolus is a region where ribosomal RNA is transcribed and ribosomal subunits begin assembly; it is not a membrane-bound organelle inside the nucleus.

The nuclear envelope separates transcription from cytosolic translation. This permits extensive RNA processing and additional control before an mRNA reaches ribosomes. Compartmentalization therefore does more than store DNA: it changes the sequence and regulation of information flow.

Rough endoplasmic reticulum has ribosomes attached to its cytosolic surface. Proteins destined for secretion, membranes or certain organelles enter the ER during synthesis, fold within its lumen and leave in transport vesicles. Free ribosomes generally synthesize proteins that remain in the cytosol or are imported into organelles by other targeting systems.

Smooth ER lacks attached ribosomes and participates in lipid synthesis, detoxification and calcium storage, with emphasis varying by cell type. The Golgi apparatus receives vesicles at one face, modifies and sorts their contents, and buds new vesicles from the other. Directional trafficking preserves the identity of compartments while moving cargo among them.

Lysosomes contain acid hydrolases that break down macromolecules, engulfed material and damaged organelles. Their membrane confines enzymes and maintains conditions different from the cytosol. Compartmentalization protects the rest of the cell and allows incompatible reactions to occur simultaneously.

Mitochondria carry out aerobic stages of respiration. Their inner membrane folds into cristae, increasing area for electron transport and ATP synthase. Chloroplasts in photosynthetic eukaryotes contain thylakoid membranes for light-dependent reactions and stroma for carbon fixation. Both organelles have double envelopes, circular DNA and bacterial-like ribosomes.

The cytoskeleton includes microtubules, actin microfilaments and intermediate filaments. It maintains shape, positions organelles, forms tracks for motor proteins and reorganizes during division. Centrioles help organize microtubules in many animal cells, but are not universal features of every eukaryote.

Plant cells add a cellulose wall, a large central vacuole and, in photosynthetic tissues, chloroplasts. Water entering the vacuole produces turgor pressure against the wall, supporting non-woody tissues. Fungal walls contain chitin and glucans rather than cellulose. Animal cells lack walls and can change shape more readily.

Structure predicts function probabilistically. A cell rich in rough ER and Golgi is likely to export protein; one with many mitochondria has high aerobic ATP demand; one with abundant smooth ER may synthesize lipids or detoxify compounds. The inference is strongest when several features agree.