12.4 Cell Structures, Membranous Organelles & Cell Theory
Key Takeaways
- Cell theory: all living things consist of one or more cells, the cell is the basic unit of life, and all cells arise from pre-existing cells
- Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both, and contain membrane-bound organelles such as the nucleus, ER, Golgi, mitochondria, chloroplasts, lysosomes, and peroxisomes
- Mature mammalian red blood cells (erythrocytes) lack nuclei and organelles—this is the PA-CAT sample item: RBCs are devoid of genetic material
- Non-membranous organelles include ribosomes, the cytoskeleton (microtubules, microfilaments, intermediate filaments), and centrioles
- The endomembrane system (nuclear envelope, ER, Golgi, lysosomes, vesicles, plasma membrane) is functionally connected by vesicular traffic
12.4 Cell Structures, Membranous Organelles & Cell Theory
Quick Answer: Cell theory states that all living things are made of cells, the cell is life's basic unit, and cells come only from pre-existing cells. Eukaryotic cells contain membrane-bound organelles (nucleus, ER, Golgi, mitochondria, chloroplasts, lysosomes, peroxisomes); prokaryotic cells do not. Mature mammalian red blood cells are a striking exception among eukaryotes: they lose their nucleus and organelles during maturation, which is exactly the PA-CAT sample item—RBCs are devoid of genetic material.
Cell Theory
The classical cell theory has three tenets:
- All organisms are composed of one or more cells (Schleiden and Schwann, 1838–1839).
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells by division (Virchow, 1855).
A modern extension adds: cells carry hereditary information that passes to daughter cells, and metabolic chemistry occurs within cells. Note that viruses are not cells and are not considered living outside a host.
Prokaryote vs Eukaryote
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | Absent (nucleoid region) | Present, membrane-bound |
| Membrane-bound organelles | Absent | Present |
| DNA | Single circular chromosome + plasmids | Multiple linear chromosomes in nucleus |
| Ribosomes | 70S (30S + 50S) | 80S (40S + 60S); mitochondria/chloroplasts have 70S |
| Cell wall | Usually present (peptidoglycan in bacteria) | Plants (cellulose), fungi (chitin); animals none |
| Reproduction | Binary fission | Mitosis and meiosis |
| Size | Typically 0.1–5 µm | Typically 10–100 µm |
The eukaryotic organelles mitochondria and chloroplasts likely arose by endosymbiosis of a prokaryote within a host cell—an idea supported by their circular DNA, 70S ribosomes, and double membranes.
Membranous Organelles
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Double membrane (nuclear envelope) with pores; contains chromatin and nucleolus | Houses DNA; site of transcription; nucleolus makes rRNA |
| Endoplasmic reticulum (rough) | Continuous with nuclear envelope; studded with ribosomes | Co-translational folding and modification of secreted/membrane proteins |
| Endoplasmic reticulum (smooth) | Tubular, no ribosomes | Lipid synthesis, detoxification (cytochrome P450), Ca²⁺ storage |
| Golgi apparatus | Stacked cisternae | Modifies, sorts, packages proteins into vesicles; forms lysosomes |
| Mitochondria | Double membrane; inner folds = cristae; matrix contains enzymes | ATP synthesis by oxidative phosphorylation; has own DNA |
| Chloroplast (plant/algae) | Double membrane + thylakoid membranes | Photosynthesis; has own DNA |
| Lysosomes | Single-membrane vesicle with hydrolytic enzymes | Digestion of macromolecules and worn organelles; apoptosis |
| Peroxisomes | Single-membrane vesicle | Oxidative reactions; β-oxidation of very-long-chain fatty acids; catalase breaks down H₂O₂ |
Non-Membranous Organelles
Ribosomes (80S in cytosol, 70S in mitochondria/chloroplasts) translate mRNA into protein. The cytoskeleton has three filament systems:
- Microtubules (tubulin, 25 nm) — mitotic spindle, cilia/flagella structural core, tracks for vesicle transport.
- Microfilaments (actin, 7 nm) — muscle contraction, cytokinesis cleavage furrow, cell shape.
- Intermediate filaments (keratin, lamins, ~10 nm) — mechanical strength, nuclear lamina.
Centrioles (9+0 microtubule triplets) organize the mitotic spindle in animal cells and lie within the centrosome. Plant cells lack centrioles but still assemble a spindle from microtubule-organizing centers at the nuclear envelope.
The Endomembrane System
The endomembrane system is a functionally continuous network: nuclear envelope → ER → Golgi → vesicles → lysosomes → plasma membrane. A secreted protein is synthesized into the rough ER, packaged into COP-coated vesicles, modified in the Golgi, sorted into secretory vesicles, and released by exocytosis. Lysosomes bud from the trans-Golgi. The plasma membrane itself is part of this system, receiving and dispatching vesicles.
The RBC Exception (PA-CAT Sample Item)
Mammalian erythrocytes are enucleated during maturation: the nucleus, mitochondria, ribosomes, and most other organelles are extruded so the cell can be packed with hemoglobin. This is the rationale for the PA-CAT Bulletin (rev. 20240815) sample item stating that red blood cells are devoid of genetic material—they have neither a nucleus nor mitochondria, so neither nuclear nor mitochondrial DNA is present in mature RBCs. Birds and reptiles retain nucleated RBCs, so this is a mammalian-specific adaptation. Practical consequence: RBCs cannot divide, cannot synthesize new protein, and rely entirely on anaerobic glycolysis for ATP (no mitochondria = no oxidative phosphorylation), giving them a lifespan of ~120 days in humans.
Endomembrane System: A Worked Secretion Pathway
To solidify how the endomembrane system operates as a connected conveyor, trace a secreted protein such as insulin from gene to extracellular release. (1) The insulin gene is transcribed in the nucleus; the mRNA exits through nuclear envelope pores. (2) Ribosomes translate the mRNA at the rough ER, where the nascent polypeptide is threaded into the ER lumen, folded, and given core N-linked glycosylation. (3) COPII-coated vesicles bud from ER exit sites and travel to the cis face of the Golgi. (4) As the protein passes through the medial and trans cisternae, glycosylation is trimmed and refined, and the protein is sorted by signal patches. (5) Clathrin-coated secretory vesicles bud from the trans-Golgi and mature, often concentrating insulin into dense-core granules in pancreatic beta-cells. (6) Upon a Ca2+ trigger, vesicles dock at the plasma membrane and fuse via SNARE proteins, releasing insulin by exocytosis. Each handoff is vesicular: the protein never touches cytosol. Lysosomal hydrolases follow the same route but are tagged with mannose-6-phosphate in the cis-Golgi, diverting them to lysosomes rather than secretion. This single narrative lets you answer PA-CAT items asking which organelle modifies, sorts, or packages a secreted protein, and which marker routes cargo to the lysosome.
Mature mammalian red blood cells cannot carry out oxidative phosphorylation because they:
Which organelle is the site of co-translational protein folding and is studded with ribosomes?
Which pair correctly matches organelle to function?