3.2 Cell Structure: Prokaryotic vs Eukaryotic

Two Cell Plans, One Common Ancestry

Every cell on Earth shares four traits: a plasma membrane, cytoplasm, ribosomes, and DNA. Beyond that, life splits into two structural plans. Prokaryotic cells (domains Bacteria and Archaea) are small, simple, and lack a true nucleus. Eukaryotic cells (animals, plants, fungi, protists) are larger, compartmentalized, and contain a true membrane-bound nucleus plus an entourage of organelles.

Prokaryotes

Prokaryotic cells are typically 1–10 micrometers in diameter — about one tenth the size of a typical eukaryotic cell. Their genome is a single circular chromosome held in an irregular region called the nucleoid; many also carry small accessory loops called plasmids. Ribosomes are 70S (a 50S large subunit + a 30S small subunit), which is why many antibiotics (tetracyclines, aminoglycosides) selectively target bacteria without damaging human 80S ribosomes.

Prokaryotic cell walls are a Praxis favorite for telling Bacteria and Archaea apart:

Many bacteria add a capsule (sticky polysaccharide layer), pili (attachment fibers, including the sex pilus used for conjugation), and flagella (rotary motors driven by a proton gradient).

Eukaryotes

Eukaryotic cells partition labor into membrane-bound organelles. Memorize the following inventory; the Praxis routinely pairs an organelle name with a structural or functional clue.

• Nucleus — double membrane (the nuclear envelope) pierced by nuclear pores; houses linear chromatin (DNA + histone proteins). The nucleolus inside makes ribosomal RNA.
• Rough endoplasmic reticulum (rough ER) — flat sacs studded with ribosomes; synthesizes membrane and secreted proteins and threads them into the lumen for folding.
• Smooth endoplasmic reticulum (smooth ER) — tubular, no ribosomes; synthesizes lipids and steroids, detoxifies drugs (especially in liver hepatocytes), and stores Ca2+ in muscle cells (sarcoplasmic reticulum).
• Golgi apparatus — stack of flattened cisternae that modifies, sorts, and packages proteins arriving from the rough ER; the cis face receives, the trans face ships.
• Mitochondria — double-membraned power plants; the inner membrane folds into cristae that host the electron transport chain. Contain their own 70S ribosomes and circular DNA.
• Chloroplasts (plants and algae only) — double-membraned with internal thylakoid sacs stacked into grana; convert light energy into chemical energy via photosynthesis. Also have 70S ribosomes and circular DNA.
• Lysosomes — single-membrane vesicles full of acidic hydrolases (pH ~5); digest worn organelles by autophagy and engulfed material by phagocytosis.
• Peroxisomes — single-membrane vesicles that break down very-long-chain fatty acids and detoxify hydrogen peroxide (H2O2) using catalase.
• Cytoskeleton — three filament systems that provide shape, transport, and motility:
  • Microfilaments (actin) — thin; muscle contraction, cell crawling, cytokinesis.
  • Intermediate filaments (e.g., keratin) — mechanical strength.
  • Microtubules (tubulin) — thick; chromosome movement, cilia, flagella, organelle trafficking.
• Centrioles (animal cells) — paired microtubule cylinders that organize the mitotic spindle.
• Cell wall — in plants (cellulose), fungi (chitin), and many protists; absent in animal cells.
• Central vacuole (plants) — large fluid-filled sac that supports the cell with turgor pressure.

The Plasma Membrane and the Fluid Mosaic Model

The plasma membrane is a phospholipid bilayer with embedded proteins, cholesterol, and surface carbohydrates. Singer and Nicolson's fluid mosaic model captures the essential image: the lipids are a two-dimensional fluid in which proteins drift like icebergs in a sea.

• Integral (transmembrane) proteins span the bilayer and form channels, pumps, and receptors.
• Peripheral proteins sit on either surface, often anchored to the cytoskeleton or to lipid heads.
• Cholesterol wedges between phospholipids and buffers fluidity — keeping membranes from becoming too fluid at high temperature and too rigid at low temperature.
• Glycoproteins and glycolipids on the outer face act as cellular ID badges (e.g., ABO blood groups).

Endosymbiotic Theory

Mitochondria and chloroplasts both look — and behave — suspiciously like miniature prokaryotes. Lynn Margulis's endosymbiotic theory proposes that they descend from free-living bacteria engulfed by an ancestral eukaryote roughly 1.5–2 billion years ago. Evidence the Praxis expects you to cite:

1. Both organelles have double membranes (the outer one from the engulfing host, the inner from the original bacterium).
2. Both contain their own circular DNA, separate from the nuclear genome.
3. Both have 70S ribosomes, identical in size to bacterial ribosomes.
4. Both reproduce by binary fission, independent of the rest of the cell.

Quick Comparison Table