Question 8 of 8: Cytoplasm, Nucleus, Ribosome/Chloroplast/Mitochondria, Bacterial Shape Classes

Question 8: Cytoplasm, Nucleus, Ribosome/Chloroplast/Mitochondria, Bacterial Shape Classes (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) Cytoplasm. The cytoplasm is the semi-fluid, gel-like contents of a cell enclosed by the plasma membrane (excluding the nucleus in eukaryotes), comprising the cytosol (water, ions, dissolved metabolites, enzymes) plus, in eukaryotes, the suspended organelles and cytoskeletal filaments. It is the medium in which most metabolic reactions (glycolysis, protein synthesis at free ribosomes, etc.) physically occur.

(b) Nucleus vs. nuclear zone (nucleoid). The nucleus (eukaryotes) is a true membrane-bound organelle: a double-membrane nuclear envelope (perforated by nuclear pores for regulated macromolecular transport) encloses the chromosomal DNA, organised with histone proteins into chromatin, plus a nucleolus for ribosomal RNA synthesis/ribosome assembly. The nuclear zone (nucleoid) of prokaryotes is not membrane-bound at all — it is simply the region of the cytoplasm where the single circular chromosome (with little to no histone packaging, condensed instead by supercoiling and nucleoid-associated proteins) is concentrated, in direct physical contact with ribosomes and the rest of the cytoplasm. The key contrast is the presence/absence of a bounding envelope, which is what allows eukaryotes (but not prokaryotes) to physically separate transcription from translation.

(c) Ribosome, chloroplast, mitochondria. Ribosomes are ribonucleoprotein particles (rRNA + protein, no membrane) that translate mRNA into protein; prokaryotic ribosomes are the smaller 70S type (30S+50S subunits), eukaryotic cytoplasmic ribosomes are the larger 80S type (40S+60S) — both function identically in principle (codon-by-codon polypeptide synthesis) but differ in size/composition, which is exploited by ribosome-targeting antibiotics that inhibit 70S but spare 80S ribosomes. Mitochondria are double-membrane organelles (present in nearly all eukaryotes) that carry out oxidative phosphorylation/the electron transport chain to generate ATP aerobically; the inner membrane is folded into cristae to maximise surface area for this process. Chloroplasts are double-membrane organelles (present only in photosynthetic eukaryotes — plants, algae) containing thylakoid membranes (stacked into grana) bearing chlorophyll, where light-dependent photosynthetic reactions occur, plus a stroma where CO₂ fixation (Calvin cycle) proceeds. The two organelles share an endosymbiotic signature: mitochondria and chloroplasts contain their own circular DNA and 70S ribosomes (resembling their free-living bacterial ancestors), distinguishing them from the surrounding eukaryotic cell's own 80S ribosomal machinery.

(d) Spirilla, cocci, bacilli. These are the three principal bacterial morphological (shape) classes. Cocci are spherical/ovoid cells, which may occur singly, in pairs (diplococci), chains (streptococci), or clusters (staphylococci) depending on the plane(s) and completeness of division. Bacilli are rod-shaped cells, occurring singly, in pairs, or in chains, generally longer than they are wide. Spirilla are rigid, helically curved/spiral-shaped cells (distinct from the flexible spirochetes), typically motile via polar flagella. Cell shape in bacteria is genetically determined (governed largely by cytoskeletal proteins such as MreB/FtsZ homologues and the pattern of peptidoglycan synthesis) and is a stable, taxonomically useful characteristic rather than an environmentally induced variation.