Question 3 of 6: Prokaryotes vs. Eukaryotes; Aerobic Respiration

Question 3: Prokaryotes vs. Eukaryotes; Aerobic Respiration (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Prokaryotes vs. eukaryotes

Prokaryotes (bacteria and archaea) and eukaryotes (fungi, protists, plants, animals) are the two fundamental domains of cellular organisation, and biotechnology exploits both (e.g. bacterial fermentations vs. yeast/mammalian cell culture), so an engineer needs to know which structural differences drive process decisions.

Despite these differences, both cell types share the same basic biochemical machinery: a phospholipid bilayer plasma membrane, DNA as the genetic material read via the same universal codon table, ribosome-based translation, and central catabolic pathways (glycolysis is essentially universal). The organizing principle behind the prokaryote/eukaryote split is compartmentalisation: eukaryotes wall off specialised functions (energy conversion in mitochondria, protein processing in ER/Golgi, genetic material in the nucleus) into membrane-bound organelles, allowing larger cell size, more complex regulation, and specialised subcellular chemistry, at the cost of the greater metabolic overhead of building and maintaining those membranes — part of why prokaryotic fermentations (e.g. E. coli, many industrial bacteria) grow faster and cheaper than eukaryotic cell culture, but eukaryotic hosts (yeast, CHO cells) are often required when post-translational processing (glycosylation, complex folding) is needed.

(ii) Aerobic respiration in cells

Aerobic respiration is the staged, fully oxidative breakdown of glucose to CO₂ and H₂O, coupling the released free energy to ATP synthesis. It proceeds in four connected stages:

1. Glycolysis (cytoplasm, no O₂ needed): one glucose (C₆) is split via ten enzymatic steps into two pyruvate (C₃) molecules, with a net yield of 2 ATP (substrate-level phosphorylation) and 2 NADH. This stage is common to both aerobic and anaerobic metabolism.

2. Pyruvate oxidation (link reaction) (mitochondrial matrix in eukaryotes; cytoplasm/ membrane in aerobic bacteria): each pyruvate is oxidatively decarboxylated by the pyruvate dehydrogenase complex to acetyl-CoA, releasing CO₂ and generating one NADH per pyruvate (2 NADH total per glucose).

3. TCA (Krebs) cycle (mitochondrial matrix): each acetyl-CoA (2 carbons) condenses with oxaloacetate to form citrate, then runs through eight enzymatic steps that fully oxidise those two carbons to CO₂. Per glucose (two turns of the cycle) this yields 6 NADH, 2 FADH₂, and 2 GTP/ATP (substrate-level phosphorylation) — and regenerates oxaloacetate to keep the cycle turning.

4. Electron transport chain (ETC) and oxidative phosphorylation (inner mitochondrial membrane): the NADH and FADH₂ collected in stages 1–3 donate electrons to a series of membrane-bound carrier complexes (I–IV), which pump protons across the inner membrane to build an electrochemical (proton-motive) gradient. ATP synthase uses that gradient to phosphorylate ADP (chemiosmotic coupling). Molecular oxygen is the terminal electron acceptor, reduced to H₂O at Complex IV — without O₂ to keep accepting electrons, the whole chain backs up and stops, which is exactly why aerobic respiration requires oxygen even though O₂ itself never touches glycolysis or the TCA cycle directly.