Cell Structure — Set 1

Correct Answer: B. Smooth Endoplasmic Reticulum

• **Smooth Endoplasmic Reticulum (SER)** = A network of tubular membranes without ribosomes that hosts cytochrome P450 enzymes, which oxidise and conjugate drugs and toxins to make them water-soluble for excretion. • **Key fact** — SER is dramatically expanded in liver (hepatocyte) cells and adrenal cortex cells precisely because of their heavy detoxification and steroid-synthesis workload. • 💡 Option A (Golgi Apparatus) is wrong because it modifies and packages already-synthesised proteins, not toxins; Option C (Lysosome) is wrong because it digests cellular debris with acid hydrolases, not chemical detoxification; Option D (Rough ER) is wrong because its ribosomes specialise it for protein synthesis, not detox reactions.

Correct Answer: B. Ribosome synthesis

• **Nucleolus** = A dense, non-membrane-bound sub-nuclear body that transcribes ribosomal RNA (rRNA) genes and assembles the large and small ribosomal subunits before they are exported to the cytoplasm. • **Key fact** — A single human cell nucleus typically contains 1–4 nucleoli; cells with high protein-secretion demand have larger, more active nucleoli to meet ribosome supply needs. • 💡 Option A (Lipid production) is wrong because lipid synthesis occurs in the smooth ER; Option C (Protein folding) is wrong because chaperone proteins in the ER lumen and cytosol handle folding; Option D (DNA replication) is wrong because replication happens throughout the nucleus at replication forks, not specifically inside the nucleolus.

Correct Answer: C. Plasma Membrane

• **Plasma Membrane** = A fluid mosaic of a phospholipid bilayer—hydrophilic heads facing outward, hydrophobic tails inward—embedded with integral and peripheral proteins that perform transport, signalling, and cell-recognition functions. • **Key fact** — The fluid-mosaic model was proposed by Singer and Nicolson in 1972; the bilayer is approximately 7–10 nm thick and lateral diffusion of lipids gives it fluidity at body temperature. • 💡 Option A (Nuclear Pore) is wrong because it is a protein-channel complex spanning the double nuclear envelope, not a standalone phospholipid bilayer; Option B (Cytoskeleton) is wrong because it is an internal protein-filament network with no lipid bilayer component; Option D (Cell Wall) is wrong because it is a rigid, polysaccharide-based layer outside the membrane, entirely absent in animal cells.

Correct Answer: D. Robert Hooke

• **Robert Hooke** = English scientist who, in 1665, used his compound microscope to examine thin slices of cork and described the tiny box-like compartments as 'cells,' comparing them to monks' rooms (cellulae) in a monastery. • **Key fact** — Hooke published these observations in 'Micrographia' (1665), one of the most influential science books of the 17th century; he was observing dead cork cells, not living ones. • 💡 Option A (Robert Brown) is wrong because he discovered the nucleus in living plant cells in 1831, not the cell itself; Option B (Theodor Schwann) is wrong because he co-founded cell theory in 1839 alongside Matthias Schleiden; Option C (Anton van Leeuwenhoek) is wrong because he first observed living microorganisms — protists and bacteria — after Hooke's cork observation.

Correct Answer: C. Absence of nuclear membrane

• **Absence of nuclear membrane** = The defining feature of prokaryotes: their DNA is not enclosed by any lipid-bilayer envelope but instead lies exposed in a region called the nucleoid, freely accessible to cytoplasmic ribosomes for direct translation. • **Key fact** — Prokaryotes also possess 70S ribosomes (30S + 50S subunits) rather than the 80S ribosomes of eukaryotes; this difference is exploited by antibiotics like streptomycin and erythromycin, which selectively target 70S ribosomes. • 💡 Option A (Presence of mitochondria) is wrong because mitochondria are exclusively eukaryotic organelles with their own DNA; Option B (Membrane-bound nucleus) is wrong because a nucleus enclosed by a nuclear envelope is the definition of a eukaryote, the opposite of a prokaryote; Option D (Large 80S ribosomes) is wrong because prokaryotes have smaller 70S ribosomes.

Correct Answer: B. Golgi Apparatus

• **Golgi Apparatus** = A stack of flattened, membrane-bound cisternae that receives newly synthesised proteins from the rough ER, chemically modifies them (glycosylation, phosphorylation, sulphation), and packages them into vesicles for secretion or delivery to organelles. • **Key fact** — Discovered by Camillo Golgi in 1898; the organelle is polarised — the cis face (entry) faces the ER and the trans face (exit) faces the plasma membrane, ensuring one-way protein traffic. • 💡 Option A (Ribosome) is wrong because ribosomes only synthesise polypeptide chains; they cannot modify sugar chains or sort proteins into destinations; Option C (Peroxisome) is wrong because it oxidises fatty acids and neutralises hydrogen peroxide, not protein processing; Option D (Vacuole) is wrong because it is a storage compartment, not a protein-modification station.

Correct Answer: C. Cementing adjacent cells

• **Middle Lamella** = The outermost layer of the plant cell wall, composed mainly of calcium and magnesium pectates, that cements adjacent plant cells together; it is the first layer deposited during the formation of the cell plate after mitosis. • **Key fact** — The enzyme pectinase dissolves the pectin in the middle lamella; this is why over-ripe fruit becomes soft — the cells lose their adhesive layer and begin to separate. • 💡 Option A (Photosynthesis) is wrong because photosynthesis occurs in the thylakoid membranes and stroma of chloroplasts; Option B (DNA storage) is wrong because DNA is housed in the nucleus; Option D (Water storage) is wrong because water is stored in the large central vacuole, which is bounded by the tonoplast membrane.

Correct Answer: B. Centrioles

• **Centrioles** = Pairs of cylindrical structures in animal cells, each built from nine triplets of microtubules in a 9+0 arrangement; they form the centrosome and organise the mitotic spindle fibres that pull chromosomes apart during cell division. • **Key fact** — Higher plant cells generally lack centrioles yet divide successfully using other microtubule-organising centres; centrioles also serve as basal bodies that anchor cilia and flagella in animal and lower plant cells. • 💡 Option A (Chloroplasts) is wrong because chloroplasts are plant organelles for photosynthesis — they are absent in animal cells, not the other way around; Option C (Cell Wall) is wrong because cell walls are a plant and fungal feature absent in animal cells; Option D (Large central vacuole) is wrong because this is a prominent plant-cell feature, often occupying up to 90% of cell volume.

Correct Answer: C. Mitochondria

• **Mitochondria** = Double-membrane organelles whose inner membrane is folded into cristae; they contain their own circular DNA (mtDNA), 70S ribosomes, and divide by binary fission — all features inherited from an ancient alpha-proteobacterial ancestor, supporting the endosymbiotic theory. • **Key fact** — Lynn Margulis formally proposed the endosymbiotic theory in 1967; human mtDNA is 16,569 base pairs long, encodes 37 genes, and is inherited exclusively through the maternal line. • 💡 Option A (Ribosome) is wrong because ribosomes are non-membrane-bound particles with no DNA; Option B (Golgi Body) is wrong because it is a processing organelle derived from the ER, with no DNA and no bacterial origin; Option D (Lysosome) is wrong because it is a single-membrane vesicle produced by the Golgi apparatus and contains no genetic material.

Correct Answer: C. Cytoskeleton

• **Cytoskeleton** = A dynamic, interconnected network of microtubules (tubulin), microfilaments (actin), and intermediate filaments that spans the cytoplasm, maintaining cell shape and enabling the directed movement of organelles via motor proteins. • **Key fact** — Motor proteins kinesin (moves cargo toward the cell periphery) and dynein (moves cargo toward the nucleus) walk along microtubule tracks; anti-cancer drugs colchicine and taxol work by disrupting microtubule dynamics. • 💡 Option A (Cytoplasm) is wrong because the cytoplasm is the gel-like matrix that fills the cell interior — it is the medium, not the structural framework; Option B (Cell Membrane) is wrong because it is the outer boundary of the cell, not an internal scaffold; Option D (Nucleoplasm) is wrong because it is the fluid within the nucleus, completely separate from the cytoskeletal network.