Question 4 of 6: Metabolic Pathways and Cell Structure

Question 4: Metabolic Pathways and Cell Structure (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.

(A) Anaerobic vs. aerobic glucose catabolism

Both pathways begin identically: glycolysis, occurring in the cytoplasm, splits one glucose molecule into two pyruvate molecules via ten enzymatic steps, with a net yield of 2 ATP (by substrate-level phosphorylation) and 2 NADH per glucose. This first stage requires no oxygen and is common to both routes. What happens to the pyruvate and the NADH afterward is where the two pathways diverge.

Anaerobic (fermentation) pathway. In the absence of an external electron acceptor, pyruvate itself is reduced using the NADH generated in glycolysis — e.g. to lactate (lactic-acid fermentation, many bacteria and muscle cells) or to ethanol + CO₂ (alcoholic fermentation, yeast). The sole biochemical purpose of this step is to regenerate NAD⁺ so glycolysis can keep running; no further ATP is produced. Net yield: 2 ATP per glucose, and the carbon remains largely in the reduced fermentation product (energy is not fully extracted from the glucose).

Aerobic (respiration) pathway. With O₂ available, pyruvate crosses into the mitochondrion (eukaryotes) or is processed at the plasma membrane (aerobic bacteria) and is oxidatively decarboxylated to acetyl-CoA, releasing CO₂ and more NADH. Acetyl-CoA enters the TCA (Krebs) cycle, which fully oxidises the two remaining carbons to CO₂, generating further NADH, FADH₂, and GTP/ATP. The NADH and FADH₂ then feed the electron transport chain, where O₂ serves as the terminal electron acceptor (reduced to H₂O), driving chemiosmotic oxidative phosphorylation. Net yield: roughly 30–32 ATP per glucose — an order of magnitude more energy extracted from the same starting molecule, because the carbon skeleton is fully oxidised rather than left as a reduced fermentation product.

(B) Prokaryotic vs. eukaryotic cell structure

Prokaryotic cell (e.g. a bacterium such as E. coli): DNA is a single circular chromosome located in an unbound region of cytoplasm called the nucleoid — there is no nuclear envelope. Small (70S) ribosomes float free in the cytoplasm and carry out all protein synthesis; there are no membrane-bound organelles (no mitochondria, ER, or Golgi), so functions like respiration occur across the plasma membrane itself. A rigid cell wall (peptidoglycan in bacteria) outside the plasma membrane maintains shape and resists osmotic lysis; some cells add a protective capsule, and motility/attachment structures such as flagella or pili. Typical size 1–10 µm.

Eukaryotic cell (e.g. the yeast Saccharomyces cerevisiae used in fermentation industries): a true membrane-bound nucleus houses linear chromosomes packaged with histones, with a nucleolus inside for ribosome subunit assembly. Mitochondria (double membrane, own DNA) are the site of the TCA cycle and oxidative phosphorylation — the organelle that makes the aerobic ATP yield in part (A) possible. The endoplasmic reticulum (rough ER, studded with ribosomes, for membrane/secreted protein synthesis; smooth ER for lipid synthesis and detoxification) and the Golgi apparatus (protein modification, sorting and packaging into vesicles) form an internal membrane-trafficking system prokaryotes lack. Larger (80S) ribosomes occur both free and ER-bound. A cytoskeleton (microtubules, microfilaments) gives shape, enables transport, and drives cell division. Many eukaryotic cells also have a cell wall (chitin in fungi, cellulose in plants) or none (animal cells) and, where present, a vacuole for storage/turgor. Typical size 10–100 µm — roughly 10× larger in linear dimension than a typical prokaryote.