5.1 Functions of the Cellular Organelles

The Compartmentalized Eukaryotic Cell

Quick Answer: Organelles are membrane-bound compartments that partition biochemical work. The nucleus holds DNA, ribosomes build proteins, mitochondria make ATP, the endoplasmic reticulum (ER) and Golgi fold and route proteins, lysosomes and peroxisomes degrade and detoxify. The PA-CAT Bulletin of Information, rev. 20240815, pairs this content with a sample item in which a poison that inhibits protein synthesis is asked to identify the targeted organelle — the correct answer is the ribosome.

Physiology at the PA prerequisite level treats the cell as a set of cooperating machines. The PA-CAT Physiology blueprint (Bulletin Table 4) lists "The Cell" as the first content group, and many later topics — membrane potential, muscle contraction, blood cell production — depend on knowing which organelle does what.

Organelle Functions at a Glance

Ribosomes and Protein Synthesis Inhibition

Ribosomes are the non-membranous machines of translation. Each ribosome has a large subunit (60S in eukaryotes) and a small subunit (40S). They may be free in the cytosol (making cytosolic proteins) or bound to the RER (making membrane, lysosomal, or secreted proteins). Because ribosomes are the literal site of polypeptide assembly, any toxin that blocks protein synthesis must act at the ribosome or its associated factors.

Several clinically relevant toxins illustrate this principle and appear in PA-CAT-style reasoning:

• Diphtheria toxin inhibits elongation factor 2 (EF-2) by ADP-ribosylation, freezing the ribosome after translocation.
• Ricin removes a single adenine from the 28S rRNA of the 60S subunit, preventing elongation factor binding.
• Chloramphenicol inhibits the 50S peptidyl transferase (bacterial ribosomes — the basis of its selective toxicity).
• Puromycin mimics aminoacyl-tRNA and causes premature chain termination.

A PA-CAT sample item asks which organelle a poison that inhibits protein synthesis would target. The reasoning chain is: protein synthesis = translation → translation occurs at ribosomes → the affected organelle is the ribosome (or the RER, since bound ribosomes are part of the RER's machinery).

The Secretory Pathway

Proteins destined for secretion follow a defined route: ribosome → RER lumen → transport vesicle → Golgi → secretory vesicle → plasma membrane (exocytosis). Signal recognition particle (SRP) pauses translation until the ribosome docks on the RER, then translation resumes into the ER lumen where folding and disulfide bond formation begin. The Golgi performs N-linked glycosylation trimming, O-linked glycosylation, and sorting by signal patches.

Mitochondria: ATP and Apoptosis

Mitochondria are double-membrane organelles with an outer membrane (porous) and an inner membrane (folded into cristae, impermeable to protons). The inner membrane hosts the electron transport chain (Complexes I–IV) and ATP synthase (Complex V). The matrix contains the enzymes of the tricarboxylic acid (TCA) cycle and pyruvate dehydrogenase. Beyond ATP, mitochondria gate apoptosis by releasing cytochrome c into the cytosol, activating caspases. Cells with high ATP demand (cardiomyocytes, renal tubular cells, hepatocytes) are rich in mitochondria.

Lysosomes and Peroxisomes: Degradation and Detoxification

Lysosomes maintain an interior pH of ~4.7 via a vacuolar H⁺-ATPase. Their acid hydrolases (proteases, nucleases, lipases, glycosidases) digest endocytosed material, phagocytosed pathogens, and worn-out organelles through autophagy. Peroxisomes oxidize very-long-chain fatty acids and break down hydrogen peroxide via catalase (2 H₂O₂ → 2 H₂O + O₂). Defects in peroxisomal biogenesis cause Zellweger syndrome; absent lysosomal enzymes cause storage diseases such as Tay-Sachs (hexosaminidase A deficiency) and Gaucher (glucocerebrosidase deficiency).

The Cytoskeleton

The cytoskeleton has three filament systems. Microtubules (tubulin, 25 nm) organize the mitotic spindle and serve as tracks for kinesin (anterograde) and dynein (retrograde) motor proteins; they form the core of cilia and flagella (the "9+2" arrangement). Microfilaments (actin, 7 nm) drive cytokinesis, microvilli, and muscle contraction (with myosin). Intermediate filaments (10 nm) provide mechanical strength — keratin in epithelia, lamin in the nuclear envelope, vimentin in mesenchymal cells.

Knowing these organelle functions is the foundation for later PA-CAT physiology: membrane transport (ER and Golgi insert channels), blood cell production (ribosomes make hemoglobin), and cardiac conduction (mitochondria power the ATP-dependent Na⁺/K⁺ pump that sets resting potential).