Cell Structure — Set 6

Correct Answer: C. Golgi Apparatus

• **Golgi Apparatus** = A stack of flattened membrane sacs (cisternae) that receives proteins from the rough ER, modifies them (glycosylation, phosphorylation), sorts them, and dispatches them in vesicles to their destinations — acting as the cell's post office and packaging centre • Proteins arrive at the cis face (entry) from the ER and leave from the trans face (exit) as secretory vesicles, lysosomes, or vesicles destined for the plasma membrane • Lysosomes are formed when Golgi vesicles containing hydrolytic enzymes bud off and mature — making the Golgi the factory for lysosome production • 💡 Option A (Nucleus) stores DNA and directs gene expression but does not form lysosomes; Option B (Mitochondria) produces ATP and is entirely unrelated to lysosome biogenesis; Option D (Smooth ER) synthesises lipids and detoxifies drugs but does not produce lysosomes

Correct Answer: C. Electron Microscope

• **Electron Microscope** = A microscope that uses beams of electrons (wavelength ~0.004 nm) instead of visible light to produce images at magnifications up to 500,000× or more, allowing visualization of the internal ultrastructure of organelles • The resolving power of a light microscope is limited to ~200 nm; an electron microscope achieves resolution of ~0.1–0.2 nm, enabling detailed imaging of ribosomes, membranes, and even large protein complexes • Two main types: Transmission Electron Microscope (TEM) for internal cross-sections and Scanning Electron Microscope (SEM) for 3D surface views • 💡 Option A (Light Microscope) can only resolve structures larger than ~200 nm — sufficient for whole cells but not organelle ultrastructure; Option B (Simple Microscope) is a single-lens magnifying glass with very low magnification (~10×); Option D (Binocular Microscope) is a compound light microscope for low-magnification viewing and cannot reveal fine internal organelle structure

Correct Answer: A. Cell Wall

• **Cell Wall** = A rigid outer layer made of cellulose (plants), chitin (fungi), or peptidoglycan (bacteria) that provides structural support and protection — it is absent in all animal cells • Animal cells lack a cell wall because they rely on the cytoskeleton and extracellular matrix (collagen, fibronectin) for shape and structural support; a rigid cell wall would prevent animal cells from changing shape for movement and phagocytosis • The absence of a cell wall means animal cells are more flexible but also more susceptible to osmotic lysis if placed in hypotonic solutions • 💡 Option B (Golgi body) is present in animal cells — it processes and packages proteins; Option C (Ribosomes) are found in all living cells including animal cells; Option D (Mitochondria) are present in animal cells as the ATP-producing powerhouse

Correct Answer: D. Theodor Schwann

• **Theodor Schwann** = German zoologist who in 1839 extended the cell theory to animals, proposing that all animals are made of cells — complementing Matthias Schleiden's 1838 proposal for plants • Together, Schleiden and Schwann formulated the Cell Theory; Rudolf Virchow (1855) later added the third tenet: 'Omnis cellula e cellula' (every cell arises from a pre-existing cell) • Schwann also discovered the Schwann cells that form the myelin sheath around peripheral nerve fibres, named in his honour • 💡 Option A (Robert Brown) discovered the cell nucleus in 1831; Option B (Matthias Schleiden) proposed that all plants are made of cells in 1838 — he worked on plants, not animals; Option C (Rudolf Virchow) stated that cells only arise from pre-existing cells in 1855, a different contribution

Correct Answer: A. Cristae

• **Cristae** = Shelf-like infoldings of the inner mitochondrial membrane that dramatically increase the membrane's surface area, providing more sites for the electron transport chain (ETC) and ATP synthase enzymes • Without cristae, the inner membrane would have far less surface area — the greater the cristae density, the higher the ATP output; cells with the highest energy demands (like heart muscle) have the most densely packed cristae • The space enclosed by the inner membrane is the matrix, and the space between inner and outer membranes is the intermembrane space — the proton gradient across the inner membrane drives ATP synthesis • 💡 Option B (Matrix) is the fluid-filled interior of the mitochondria enclosed by the inner membrane, not a membrane structure; Option C (Grana) are stacks of thylakoids found in chloroplasts, not mitochondria; Option D (Stroma) is the fluid inside a chloroplast, entirely unrelated to mitochondria

Correct Answer: C. Golgi body

• **Golgi Body** = A stack of flattened membrane cisternae that receives proteins and lipids from the ER, modifies them (adds sugar chains, cleaves signal peptides), then sorts and dispatches them in vesicles — acting as the cell's packaging and courier centre • The Golgi also produces secretory vesicles (for exocytosis), lysosomes, and plant cell wall components; it has a distinct polarity — cis face (receives from ER) and trans face (dispatches to destinations) • Albert Claude, Christian de Duve, and George Palade shared the 1974 Nobel Prize for discoveries related to cell organelles including the Golgi • 💡 Option A (Endoplasmic reticulum) synthesises proteins and lipids but is the source material, not the packaging centre; Option B (Lysosome) is a product of the Golgi, not the packaging centre itself; Option D (Vacuole) stores materials in plant cells but does not process or dispatch proteins

Correct Answer: B. Nucleoid

• **Nucleoid** = An irregularly shaped, non-membrane-bound region in prokaryotic cells where the single circular chromosome (and plasmids) are concentrated — it is the functional equivalent of a nucleus but has no enclosing membrane • Since prokaryotes lack a membrane-bound nucleus, their DNA floats in the cytoplasm in this condensed region; it is compacted by nucleoid-associated proteins (NAPs) like HU and H-NS rather than histones • The absence of a nuclear membrane means transcription and translation can occur simultaneously in prokaryotes — mRNA is translated even as it is being made • 💡 Option A (Nucleus) is a membrane-bound organelle found only in eukaryotes, not prokaryotes; Option C (Nucleoplasm) is the fluid inside a eukaryotic nucleus — prokaryotes have no such enclosed compartment; Option D (Nucleolus) is a sub-nuclear region in eukaryotes for rRNA synthesis, absent in prokaryotes

Correct Answer: A. Smooth ER

• **Smooth ER (Smooth Endoplasmic Reticulum)** = A network of membranous tubules lacking ribosomes that is the primary site for synthesis of phospholipids, cholesterol, and fatty acids that make up cell membranes • Smooth ER is abundant in liver cells (lipid metabolism and drug detoxification), steroid-producing cells (adrenal cortex, gonads), and muscle cells (stores Ca²⁺ as sarcoplasmic reticulum) • Unlike the rough ER (which has ribosomes and makes proteins), the smooth ER specialises in lipid synthesis and detoxification of drugs and toxins • 💡 Option B (Golgi body) modifies and packages lipids after they arrive from the ER — it is the next stop, not the synthesis site; Option C (Rough ER) is studded with ribosomes and synthesises proteins destined for secretion, not lipids; Option D (Ribosome) translates mRNA into protein, it has no role in lipid synthesis

Correct Answer: B. Waterproofing

• **Waterproofing** = The primary function of the Casparian Strip — a band of suberin (a waxy, hydrophobic substance) deposited in the radial and transverse walls of endodermal cells in plant roots that blocks the apoplastic (cell-wall) pathway of water and mineral transport • The Casparian Strip forces all water and minerals to pass through the endodermal cell membranes (symplastic pathway), giving the plant selective control over what enters the vascular tissue (xylem) • This structure is named after botanist Robert Caspary (1865); it is found in the root endodermis and is a critical checkpoint for mineral ion selection • 💡 Option A (Sugar transport) is handled by phloem sieve tubes, not the Casparian Strip; Option C (Energy production) occurs in mitochondria via cellular respiration, not in root cell walls; Option D (Food storage) occurs in vacuoles and plastids (amyloplasts), not in the endodermis

Correct Answer: B. Ribosome

• **Ribosome** = The molecular machine responsible for translation — it reads the mRNA sequence codon by codon and catalyses the formation of peptide bonds between amino acids to build a protein chain • Ribosomes are the sole site of translation in gene expression; they are composed of rRNA and proteins and exist as two subunits (large + small); the ribosome itself is not membrane-bound and can be free in the cytoplasm or attached to the rough ER • Each codon (3 nucleotides) on mRNA is recognised by a complementary anticodon on tRNA carrying a specific amino acid — this decoding is translation • 💡 Option A (Mitochondria) carries out cellular respiration but plays no role in the translation of mRNA; Option C (Nucleus) is the site of transcription (DNA → mRNA), not translation; Option D (Lysosome) digests waste materials using hydrolytic enzymes and is completely unrelated to translation