Aerobic vs. Anaerobic Metabolism; Organelles of Eukaryotic Cells: 23-Chem-B4 Biochemical Engineering, May 2016, Q3

Question 3: Aerobic vs. Anaerobic Metabolism; Organelles of Eukaryotic Cells (20 marks)

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(i) Aerobic vs. anaerobic metabolism

Aerobic metabolism uses molecular oxygen as the terminal electron acceptor of the electron transport chain (ETC). Glucose is fully oxidized via glycolysis → pyruvate oxidation → the TCA cycle → the ETC/oxidative phosphorylation, ultimately to CO₂ and H₂O, netting ∼30–32 ATP per glucose. Because O₂ has a very favourable (highly positive) reduction potential, the ETC can extract a large amount of free energy per electron pair, which is why aerobic metabolism supports such a high ATP yield.

Anaerobic metabolism covers two distinct strategies that avoid using O₂: fermentation (strict sense) regenerates NAD⁺ by passing electrons from glycolytic NADH to an INTERNAL organic metabolite (e.g. pyruvate→lactate, or pyruvate→acetaldehyde→ethanol), with no ETC involved at all and a net yield of only 2 ATP/glucose (substrate-level phosphorylation only); and anaerobic respiration, which still uses an ETC and oxidative phosphorylation but substitutes an EXTERNAL inorganic terminal acceptor other than O₂ (e.g. NO₃⁻, SO₄²⁻, CO₂), yielding more ATP than fermentation but less than aerobic respiration because these acceptors have less favourable reduction potentials than O₂.

Feature	Aerobic respiration	Anaerobic (fermentation / anaerobic respiration)
Terminal electron acceptor	O₂	none (internal metabolite) / external inorganic (NO₃^-, SO₄^2-, …)
ETC used?	Yes	No (fermentation) / Yes (anaerobic respiration)
ATP yield / glucose	∼30–32	2 (fermentation) / intermediate (anaerobic respiration)
End products	CO₂ + H₂O	lactate, ethanol+CO₂, etc. / reduced acceptor (e.g. N₂, H₂S)
Typical environment	Oxic	Anoxic

(ii) Organelles of eukaryotic cells

Eukaryotic cells are defined by extensive internal membrane-bound compartmentalization, each organelle specializing in a subset of cellular functions:

Fig. 4 — generalized eukaryotic cell: nucleus, mitochondrion, rough ER, Golgi apparatus, and lysosome, each performing a distinct function.

Nucleus — houses the linear chromosomal DNA inside a double membrane (nuclear envelope) perforated by nuclear pores; the nucleolus within it assembles ribosomal subunits. Controls gene expression and is the site of DNA replication and transcription.
Mitochondria — double-membraned, with the inner membrane folded into cristae to maximize surface area for the electron transport chain and ATP synthase; the matrix houses the TCA cycle enzymes. The site of aerobic respiration's energy payoff (oxidative phosphorylation).
Endoplasmic reticulum (ER) — rough ER (ribosome-studded) synthesizes and folds membrane/secretory proteins; smooth ER synthesizes lipids and, in some cells, detoxifies compounds.
Golgi apparatus — a stack of flattened membrane cisternae that modifies (e.g. glycosylates), sorts, and packages proteins arriving from the ER into vesicles for secretion or delivery to other organelles.
Lysosomes — membrane-bound vesicles containing acid hydrolases that degrade macromolecules, worn-out organelles (autophagy), and material taken in by endocytosis.
Ribosomes — (free or ER-bound) the site of protein synthesis (translation), present in both prokaryotes and eukaryotes but larger (80S) in the eukaryotic cytosol.
Cytoskeleton — microtubules, microfilaments, and intermediate filaments that give the cell shape, enable intracellular transport, and drive cell division.

Topic: Terminal-electron-acceptor chemistry distinguishing aerobic and anaerobic metabolic modes; membrane-bound organelle compartmentalization in eukaryotic cells.

Key relations: Aerobic ATP yield ≈30–32/glucose vs. fermentation's fixed 2/glucose (substrate-level phosphorylation only); organelle compartmentalization separates otherwise incompatible biochemical environments (e.g. the mitochondrion's proton-motive-force-generating membrane vs. the lysosome's acidic hydrolytic interior).

Why this works: The ATP-yield gap between aerobic and anaerobic metabolism is explained entirely by which molecule accepts the final electrons: O₂'s very favourable reduction potential lets the ETC pump far more protons (and hence make far more ATP) per NADH than any anaerobic acceptor can. Compartmentalization by membrane-bound organelles similarly lets a eukaryotic cell run chemically incompatible processes (e.g. oxidative ATP synthesis and hydrolytic digestion) simultaneously without one destroying the other.

Common pitfall: Treating "fermentation" (strict, no-ETC sense) and "anaerobic respiration" (ETC present, non-O₂ acceptor) as synonyms — they have very different ATP yields; and forgetting that ribosomes, though not membrane-bound, are still counted among a eukaryotic cell's key organelle-level machinery.

Source: Madigan et al., Brock Biology of Microorganisms, 13th ed., Ch. 3 (cell structure) and Ch. 13 (respiration, fermentation); Shuler & Kargi, Bioprocess Engineering, 2nd ed., Ch. 3 (microbial metabolism).

Question 3 of 5: Aerobic vs. Anaerobic Metabolism; Organelles of Eukaryotic Cells

Question 3: Aerobic vs. Anaerobic Metabolism; Organelles of Eukaryotic Cells (20 marks)

(i) Aerobic vs. anaerobic metabolism

(ii) Organelles of eukaryotic cells