Cell Structure — Set 3

Correct Answer: D. Jan Evangelista Purkyně

• **Jan Evangelista Purkinje** = Czech anatomist who coined the term 'protoplasm' in 1839 to describe the living, jelly-like substance that fills all cells — distinguishing it from the non-living cell wall and other structural components. • **Key fact** — The word 'protoplasm' comes from Greek: 'protos' (first) + 'plasma' (formed thing); Hugo von Mohl later popularised the term in botany in 1846 specifically for the living content of plant cells. • 💡 Option A (Robert Hooke) is wrong because Hooke coined the term 'cell' in 1665; Option B (Rudolf Virchow) is wrong because Virchow formulated the principle 'Omnis cellula e cellula' (all cells from pre-existing cells) in 1855; Option C (Robert Brown) is wrong because Brown discovered and named the cell nucleus in 1831.

Correct Answer: D. Rough ER

• **Rough Endoplasmic Reticulum (RER)** = ER studded with ribosomes on its cytoplasmic face, making it the primary site for synthesis of secretory proteins, membrane proteins, and proteins destined for organelles — the newly made proteins thread into the ER lumen for folding and initial glycosylation. • **Key fact** — Cells that are highly active in protein secretion — such as pancreatic acinar cells (digestive enzymes), plasma B-cells (antibodies), and goblet cells (mucus) — have an enormously expanded rough ER network. • 💡 Option A (Lysosome) is wrong because lysosomes contain digestive hydrolase enzymes and break down cellular waste, not synthesise proteins for export; Option B (Peroxisome) is wrong because peroxisomes oxidise fatty acids and detoxify harmful chemicals like alcohol; Option C (Smooth ER) is wrong because smooth ER lacks ribosomes and instead specialises in lipid synthesis, steroid hormone production, and drug detoxification.

Correct Answer: B. Attachment and conjugation

• **Pili (fimbriae)** = Short, hair-like protein appendages on the surface of certain bacterial cells that enable attachment to host tissues or surfaces (fimbriae) and, in the special case of sex pili, facilitate conjugation — the transfer of plasmid DNA from a donor to a recipient bacterium. • **Key fact** — Sex pili are hollow tubes that form a conjugation bridge between two bacteria; many antibiotic-resistance genes are transferred between bacteria via plasmids through this pili-mediated conjugation process. • 💡 Option A (Energy production) is wrong because energy is produced at the cell membrane via the electron transport chain in prokaryotes — pili have no role in ATP synthesis; Option C (Movement) is wrong because bacterial movement is driven by rotating flagella, which are structurally and functionally distinct from pili; Option D (Protection from heat) is wrong because heat resistance in bacteria is due to heat-shock proteins and endospore formation, not pili.

Correct Answer: A. Cell Wall

• **Cell Wall** = A rigid, protective layer outside the plasma membrane in bacteria (composed of peptidoglycan) that maintains the characteristic shape of the cell and acts as a pressure vessel, preventing the cell from lysing (bursting) when internal osmotic pressure exceeds that of the surrounding medium. • **Key fact** — Bacteria can be broadly classified by their cell wall structure using the Gram stain: Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet dye, while Gram-negative bacteria have a thin layer sandwiched between two membranes. • 💡 Option B (Ribosome) is wrong because ribosomes synthesise proteins and have no structural role in maintaining cell shape; Option C (Capsule) is wrong because the capsule is a loose, outer polysaccharide layer that protects against phagocytosis and desiccation — it is too soft to maintain cell shape; Option D (Plasma Membrane) is wrong because the plasma membrane is a flexible, semi-fluid lipid bilayer that cannot resist turgor pressure by itself.

Correct Answer: A. Peroxisome

• **Peroxisome** = A single-membrane-bound organelle that generates hydrogen peroxide (H₂O₂) as a toxic byproduct of oxidative reactions — particularly beta-oxidation of very long-chain fatty acids — and immediately neutralises it using the enzyme catalase, converting it to water and oxygen. • **Key fact** — Peroxisomes were discovered by Christian de Duve in 1966 (the same scientist who found lysosomes); liver and kidney cells are richest in peroxisomes because these organs carry the heaviest detoxification burden. • 💡 Option B (Ribosome) is wrong because ribosomes translate mRNA into protein and produce no metabolic byproducts like H₂O₂; Option C (Lysosome) is wrong because lysosomes produce acid hydrolases for cellular digestion, not oxidative reactions that generate H₂O₂; Option D (Vacuole) is wrong because vacuoles store water, pigments, and waste products — they have no oxidative metabolic activity.

Correct Answer: A. Chromatin

• **Chromatin** = The loose, thread-like complex of DNA wrapped around histone proteins that makes up the genetic material in the nucleus of a non-dividing (interphase) cell — accessible to transcription and replication machinery in this decondensed form. • **Key fact** — Chromatin exists in two forms: euchromatin (loosely packed, transcriptionally active regions) and heterochromatin (tightly packed, transcriptionally silent regions); during mitosis, chromatin condenses into the compact, visible structures we recognise as chromosomes. • 💡 Option B (Chromoplast) is wrong because chromoplasts are pigment-containing organelles in plant cells that store coloured carotenoids, unrelated to nuclear genetic material; Option C (Centromere) is wrong because a centromere is the constricted region of a condensed chromosome where spindle fibres attach during cell division; Option D (Chromosome) is wrong because chromosomes are the highly condensed, visible form of chromatin that appears specifically during cell division.

Correct Answer: B. Mitochondria

• **Mitochondria** = Generates the vast majority of a cell's ATP through oxidative phosphorylation: electrons from NADH and FADH₂ flow down the electron transport chain embedded in the inner mitochondrial membrane, driving ATP synthase (complex V) to produce ATP from ADP and phosphate. • **Key fact** — A single glucose molecule yields approximately 30–32 ATP molecules through the complete aerobic pathway; roughly 34 of those come from oxidative phosphorylation in mitochondria, versus only 2 ATP from glycolysis in the cytoplasm. • 💡 Option A (Chloroplast) is wrong because chloroplasts use light energy to synthesise glucose from CO₂ via photosynthesis — the reverse energy direction from mitochondria; Option C (Endoplasmic Reticulum) is wrong because ER synthesises proteins and lipids but has no direct role in ATP production; Option D (Ribosome) is wrong because ribosomes synthesise proteins from amino acids, with no involvement in ATP generation.

Correct Answer: D. mRNA

• **mRNA (messenger RNA)** = A single-stranded RNA molecule transcribed from a DNA template in the nucleus that carries the genetic code (as codons of three nucleotides) from the gene to cytoplasmic ribosomes, where it is translated into a specific protein sequence. • **Key fact** — In eukaryotes, pre-mRNA is processed in the nucleus before export: a 5' cap and poly-A tail are added, and introns are spliced out; this processed, mature mRNA is then exported through nuclear pores to the ribosomes. • 💡 Option A (tRNA) is wrong because transfer RNA carries specific amino acids to the ribosome and matches them to codons on the mRNA — it is the adaptor, not the information carrier; Option B (rRNA) is wrong because ribosomal RNA is a structural and catalytic component of the ribosome itself, not the genetic message being translated; Option C (snRNA) is wrong because small nuclear RNA forms spliceosomes that remove introns from pre-mRNA during processing, not carry genetic information.

Correct Answer: A. Cells arise only from pre-existing cells

• **'Cells arise only from pre-existing cells'** = The third and most critical postulate of cell theory, added by Rudolf Virchow in 1855 (Omnis cellula e cellula), establishing that new cells can only be produced by the division of existing cells — ruling out spontaneous generation. • **Key fact** — The original two-part cell theory was proposed in 1838–39 by Matthias Schleiden (all plants are made of cells) and Theodor Schwann (all animals are made of cells); Virchow's 1855 addition completed the three-part theory we use today. • 💡 Option B (Viruses are the smallest living cells) is wrong because viruses are not cells at all — they are acellular entities that lack metabolism and cannot reproduce independently; Option C (All cells contain a membrane-bound nucleus) is wrong because prokaryotic cells have no nuclear membrane, yet they are still true cells; Option D (All cells have a cell wall) is wrong because animal cells have no cell wall — only plant, fungal, and bacterial cells do.

Correct Answer: C. Golgi Apparatus

• **Golgi Apparatus** = During plant cell division (cytokinesis), the Golgi apparatus produces vesicles packed with cell wall precursors (polysaccharides and glycoproteins) that migrate to the equatorial plane of the dividing cell and fuse to form the cell plate — the precursor of the new middle lamella and primary cell wall. • **Key fact** — This Golgi-directed cell plate formation is unique to plant cells; animal cells divide by a completely different mechanism — a contractile actin-myosin ring that pinches the cell membrane inward (cleavage furrow). • 💡 Option A (Mitochondria) is wrong because mitochondria produce ATP to power cell division but do not contribute structural materials for the cell plate; Option B (Lysosome) is wrong because lysosomes contain digestive enzymes for waste breakdown and have no role in building the new cell wall; Option D (Centriole) is wrong because centrioles organise the mitotic spindle that separates chromosomes — they do not produce the cell plate materials.