Eukaryotic Cells

Core Concepts

The nucleus: the control center

The nucleus is the defining organelle of eukaryotes. A double membrane, the nuclear envelope, surrounds it, perforated by nuclear pores that control the traffic of molecules such as mRNA and proteins. Inside, DNA is packaged as chromatin — DNA wound around histone proteins — and condenses into chromosomes during cell division. The nucleolus, a dense region within the nucleus, is where ribosomal RNA is made and ribosome subunits begin assembly. The nucleus directs cell activities by controlling which genes are transcribed — the "front office" holding the blueprints.

Organelles of the endomembrane system

Several organelles work as a coordinated traffic network (the focus of Topic 4): the endoplasmic reticulum (ER) — rough ER studded with ribosomes for protein synthesis, and smooth ER for lipid synthesis and detoxification; the Golgi apparatus, which modifies, sorts, and packages proteins; vesicles, the membrane sacs that carry materials between compartments; and lysosomes, digestive sacs of enzymes that break down worn-out parts and engulfed material.

Energy organelles: mitochondria and chloroplasts

Mitochondria are the power plants of nearly all eukaryotic cells: their double membrane encloses a folded inner membrane (cristae) where the reactions of cellular respiration produce most of a cell's ATP. Chloroplasts, found in plant and algal cells, carry out photosynthesis: chlorophyll in internal thylakoid membranes captures light energy and builds sugars. Both organelles resemble prokaryotes — they have their own circular DNA, their own 70S ribosomes, and a double membrane — which is the central evidence for the endosymbiotic theory: mitochondria and chloroplasts descend from ancient prokaryotes engulfed by a larger host cell, settling into a permanent partnership.

Plant cells versus animal cells

The two share the nucleus, ER, Golgi, mitochondria, and 80S ribosomes, but each adds distinctive structures:

• Plant-only: a rigid cell wall of cellulose outside the plasma membrane (plants have no skeleton, so the wall provides support); a large central vacuole that stores water and solutes and maintains turgor pressure; chloroplasts; and plasmodesmata, channels through the wall that connect neighboring cells' cytoplasm.
• Animal-only (in most animals): lysosomes for intracellular digestion; centrosomes with centrioles that organize the mitotic spindle; and, in some cells, cilia and flagella for movement or sensing.

This is a classic comparison: both cell types have mitochondria, but only plants have chloroplasts, a cell wall, and a central vacuole; lysosomes and centrioles are typical of animal cells.

Cytoplasm, cytosol, and internal organization

The material inside the plasma membrane is the cytoplasm: the fluid cytosol plus all organelles and inclusions. The cytosol is where many metabolic pathways run (for example, glycolysis and protein synthesis on free ribosomes). A protein cytoskeleton (Topic 5) crisscrosses the cell, giving shape, anchoring organelles, and providing tracks for movement. Together, cytosol, organelles, and cytoskeleton make the eukaryotic cell a crowded, organized, constantly moving city rather than a bag of enzymes.

The endosymbiotic theory: evidence and implications

Why believe mitochondria and chloroplasts were once free-living prokaryotes? The commonly taught evidence: (1) two membranes, consistent with engulfment; (2) their own circular DNA resembling bacterial chromosomes; (3) 70S ribosomes like prokaryotes, not the 80S ribosomes of the eukaryotic cytoplasm; and (4) division independent of the cell, resembling binary fission. A practical consequence: mitochondrial genes are inherited through the cytoplasm, typically from the mother's egg — a pattern used in ancestry and forensics.

Common Confusions