Cell Structure — Set 2

Correct Answer: C. Smooth ER

• **Smooth Endoplasmic Reticulum (SER)** = Ribosome-free membrane networks that are the primary cellular site for synthesis of phospholipids, cholesterol, and steroid hormones such as testosterone and oestrogen in gonadal and adrenal cortex cells. • **Key fact** — In muscle cells, the SER is specialised as the sarcoplasmic reticulum, which stores Ca²⁺ ions and releases them to trigger muscle contraction — a function entirely separate from hormone synthesis. • 💡 Option A (Ribosome) is wrong because ribosomes synthesise proteins via translation, not lipids; Option B (Lysosome) is wrong because it contains digestive hydrolases for breaking down intracellular waste, not for synthesising biomolecules; Option D (Rough ER) is wrong because its surface ribosomes make it specialised for protein synthesis, not lipid or steroid production.

Correct Answer: A. Vacuole

• **Tonoplast** = The selectively permeable membrane that surrounds the large central vacuole in plant cells, regulating passage of water, ions, and solutes between the vacuolar sap and the cytoplasm through aquaporin and ion-channel proteins. • **Key fact** — The vacuolar sap maintained by the tonoplast can reach very acidic pH (as low as 3–4) and stores pigments such as anthocyanins, metabolic wastes, and toxic compounds to protect the plant. • 💡 Option B (Mitochondrion) is wrong because mitochondria are enclosed by their own distinct inner and outer membranes with cristae, not a tonoplast; Option C (Nucleus) is wrong because the nucleus is surrounded by a double-layered nuclear envelope studded with pores; Option D (Chloroplast) is wrong because chloroplasts are enclosed by an outer and inner envelope membrane with an internal thylakoid membrane system — no tonoplast.

Correct Answer: C. Chlorophyll

• **Chlorophyll** = A magnesium-containing porphyrin pigment embedded in the thylakoid membranes of chloroplasts that absorbs red light (~680 nm) and blue-violet light (~430 nm) most strongly, reflecting green wavelengths — giving plants their characteristic colour. • **Key fact** — There are two main forms: chlorophyll a (primary, directly participates in light reactions) and chlorophyll b (accessory, broadens light absorption); Richard Willstätter received the Nobel Prize in Chemistry in 1915 for first crystallising and characterising chlorophyll. • 💡 Option A (Anthocyanin) is wrong because anthocyanins are water-soluble vacuolar pigments that produce red, purple, and blue colours in petals and autumn leaves; Option B (Xanthophyll) is wrong because xanthophylls are yellow oxygen-containing carotenoid accessory pigments; Option D (Carotene) is wrong because carotenes are orange-red hydrocarbon carotenoid accessory pigments, not the primary green pigment.

Correct Answer: C. Golgi Apparatus

• **Golgi Apparatus** = A polarised stack of cisternae with a convex cis face (entry side, facing the ER) that receives protein-laden vesicles, and a concave trans face (exit side, facing the plasma membrane) that dispatches modified proteins in secretory or storage vesicles. • **Key fact** — Between the cis and trans faces lie medial cisternae where glycosylation enzymes sequentially add sugar chains to proteins; this structural polarity ensures proteins are progressively and correctly modified as they move through the stack. • 💡 Option A (Nucleus) is wrong because the nucleus communicates with the cytoplasm via nuclear pores, which are protein-channel complexes — not cis or trans faces; Option B (Endoplasmic Reticulum) is wrong because the ER is a continuous, interconnected membrane network with no cis-trans polarity designation; Option D (Mitochondria) is wrong because mitochondria have an outer membrane and an inner membrane folded into cristae — not cis-trans faces.

Correct Answer: C. Robert Brown

• **Robert Brown** = Scottish botanist who, in 1831, systematically examined cells of over 200 plant species and identified the nucleus as a consistent, opaque, oval body present in every living plant cell — naming it the 'nucleus' from the Latin word for kernel. • **Key fact** — Brown is also famous for discovering Brownian motion in 1827 — the random jiggling of pollen grains in water — which later became key experimental evidence for the existence of atoms and molecules. • 💡 Option A (Robert Hooke) is wrong because Hooke named 'cells' in 1665 while looking at dead cork tissue, not the nucleus in living cells; Option B (Matthias Schleiden) is wrong because Schleiden co-developed cell theory in 1838, building on Brown's earlier nucleus discovery; Option D (Rudolf Virchow) is wrong because Virchow added 'Omnis cellula e cellula' — all cells arise from pre-existing cells — to cell theory in 1855.

Correct Answer: A. Chitin

• **Chitin** = A long-chain polymer of N-acetylglucosamine (a nitrogen-containing amino sugar) that forms the rigid cell walls of fungi, giving them structural strength comparable to cellulose in plants. • **Key fact** — Chitin is the second most abundant natural polysaccharide on Earth after cellulose; it is also found in the exoskeleton of insects and crustaceans, making it one of the most versatile biological structural materials. • 💡 Option B (Peptidoglycan) is wrong because peptidoglycan is the mesh-like polymer found exclusively in the cell walls of bacteria, not fungi; Option C (Lignin) is wrong because lignin is a complex polymer that strengthens the secondary cell walls of vascular plants and wood; Option D (Cellulose) is wrong because cellulose is the beta-glucose polysaccharide that makes up plant cell walls, not fungal ones.

Correct Answer: D. Chloroplast

• **Chloroplast** = The organelle nicknamed the 'kitchen of the cell' because it manufactures glucose — the cell's primary food molecule — by using sunlight, CO₂, and water through the process of photosynthesis. • **Key fact** — Chloroplasts contain two distinct compartments for photosynthesis: the thylakoid membranes (where light-dependent reactions capture solar energy) and the stroma (where the Calvin cycle fixes CO₂ into glucose). • 💡 Option A (Vacuole) is wrong because it is a storage compartment for water, salts, and waste products, not a site of food production; Option B (Mitochondria) is wrong because it is called the 'powerhouse' — it releases energy from glucose via respiration, it does not produce food; Option C (Ribosome) is wrong because ribosomes are the 'protein factories,' synthesising polypeptide chains from mRNA.

Correct Answer: C. Lysosome

• **Lysosome** = A membrane-bound vesicle containing about 50 different hydrolytic enzymes (lipases, proteases, nucleases) that function optimally at the acidic pH (~4.5–5) maintained inside by proton pumps — breaking down worn-out organelles, foreign particles, and cellular debris. • **Key fact** — Lysosomes were discovered by Christian de Duve in 1955; their accidental rupture releases digestive enzymes into the cytoplasm and causes 'autolysis' (self-digestion) — a process exploited during apoptosis (programmed cell death). • 💡 Option A (Golgi Body) is wrong because it modifies and packages proteins into vesicles, not breaking them down; Option B (Centrosome) is wrong because it is the microtubule-organising centre containing centrioles that form the spindle apparatus during cell division; Option D (Peroxisome) is wrong because peroxisomes degrade fatty acids by beta-oxidation and neutralise H₂O₂ — they do not contain the broad array of hydrolytic enzymes lysosomes do.

Correct Answer: B. Cytoplasm

• **Cytoplasm** = Glycolysis, the first stage of cellular respiration, occurs in the cytosol (the soluble portion of the cytoplasm), where one glucose molecule is split into two pyruvate molecules, yielding a net gain of 2 ATP and 2 NADH — without requiring oxygen. • **Key fact** — Because glycolysis requires no oxygen and no mitochondria, it is the only energy-generating pathway available to both prokaryotes and eukaryotes, and it is thought to be one of the most ancient metabolic processes in life's history. • 💡 Option A (Inner membrane) is wrong because the inner mitochondrial membrane is the site of the electron transport chain and oxidative phosphorylation; Option C (Mitochondrial matrix) is wrong because the matrix is the site of the Krebs (citric acid) cycle, which processes pyruvate after glycolysis; Option D (Nucleoplasm) is wrong because nucleoplasm is the fluid inside the nucleus that bathes chromosomes and contains transcription machinery.

Correct Answer: C. Plasmodesmata

• **Plasmodesmata** = Microscopic channels (~40–50 nm diameter) lined with plasma membrane that pass through the cell walls of adjacent plant cells, allowing direct cytoplasmic continuity and the exchange of water, nutrients, signalling molecules, and even viruses between cells. • **Key fact** — The connected network of cytoplasm linked via plasmodesmata is called the symplast, as opposed to the apoplast (cell wall space); the diameter of plasmodesmata can be regulated by the cell to control molecular traffic. • 💡 Option A (Tight junctions) is wrong because tight junctions seal neighbouring animal epithelial cells to prevent leakage between them — a feature of animal, not plant, tissue; Option B (Desmosomes) is wrong because desmosomes are anchoring junctions found in animal tissues, especially skin and heart muscle; Option D (Gap junctions) is wrong because gap junctions are protein-channel connections between adjacent animal cells that allow ions and small molecules to pass — the animal functional equivalent of plasmodesmata.