Cell Structure — Set 4

Correct Answer: D. Stroma

• **Stroma** = The fluid-filled matrix inside a chloroplast that surrounds the thylakoid membranes, distinct from the thylakoid interior • Contains enzymes of the Calvin cycle (light-independent reactions) including RuBisCO, plus chloroplast DNA and ribosomes • Stroma is to chloroplast what matrix is to mitochondria — the soluble interior compartment where carbon fixation occurs • 💡 Option A (Matrix) is the fluid inside mitochondria, not chloroplasts; Option B (Lumen) is the interior space inside thylakoid sacs; Option C (Cytosol) is the liquid component of cytoplasm outside organelles

Correct Answer: B. Microfilaments

• **Microfilaments** = Thin protein filaments (~7 nm diameter) made of polymerized actin that form a contractile ring at the cell's equator during cytokinesis • The actin-myosin contractile ring constricts like a purse-string, pinching the cytoplasm into two daughter cells — a process unique to animal cells • Plant cells use a cell plate instead of a cleavage furrow because the rigid cell wall prevents pinching • 💡 Option A (Microtubules) form the mitotic spindle that separates chromosomes, not the cleavage furrow; Option C (Centrioles) organize spindle fiber assembly; Option D (Intermediate filaments) provide structural stability to the cell shape

Correct Answer: C. Vacuole

• **Vacuole** = A membrane-bound sac (tonoplast membrane) that stores water, dissolved minerals, waste products, and pigments in a single large central compartment in plant cells • In a fully mature plant cell the central vacuole can occupy up to 90% of cell volume, pushing the cytoplasm and organelles to a thin peripheral layer • Vacuoles also maintain turgor pressure that keeps plant cells firm, and store toxic compounds as a defense against herbivores • 💡 Option A (Golgi body) processes and packages proteins for secretion; Option B (Endoplasmic reticulum) synthesizes proteins and lipids; Option D (Peroxisome) oxidizes fatty acids and detoxifies hydrogen peroxide

Correct Answer: D. Mitochondria

• **Mitochondria** = ATP-producing organelles densely packed in the midpiece (middle segment) of sperm cells, directly behind the flagellum • The flagellum requires continuous ATP to power its dynein motor proteins; hundreds of mitochondria spiral around the axoneme in the midpiece to meet this high energy demand • Cells with high energy needs (cardiac muscle, liver cells, kidney tubule cells) similarly contain large numbers of mitochondria • 💡 Option A (Lysosomes) contain hydrolytic digestive enzymes used in waste breakdown; Option B (Vacuoles) are storage compartments mostly prominent in plant cells; Option C (Ribosomes) translate mRNA into protein but do not produce ATP

Correct Answer: D. Tubulin

• **Tubulin** = A globular protein that exists as α-tubulin and β-tubulin heterodimers, which polymerize end-to-end to form the hollow cylindrical walls of microtubules (~25 nm diameter) • Microtubules form the mitotic spindle, cilia, flagella, and serve as tracks for motor proteins like kinesin and dynein to move organelles within the cell • Drugs like colchicine (used in gout treatment) and taxol (cancer drug) work by disrupting tubulin polymerization, halting cell division • 💡 Option A (Keratin) is a structural protein that forms hair, nails, and skin; Option B (Actin) polymerizes into microfilaments (~7 nm), not microtubules; Option C (Myosin) is a motor protein that interacts with actin in muscle contraction

Correct Answer: B. Chloroplast

• **Chloroplast** = The site where RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) performs its oxygenase reaction, binding O₂ instead of CO₂ to produce 2-phosphoglycolate — the first step of photorespiration • Photorespiration is a wasteful process occurring in C3 plants under high O₂/low CO₂ conditions, reducing photosynthetic efficiency by up to 25%; C4 plants like maize have evolved mechanisms to suppress it • The full photorespiration pathway spans three organelles: chloroplast (initiation), peroxisome (glycolate oxidation), and mitochondria (decarboxylation) • 💡 Option A (Mitochondria) is the site of aerobic cellular respiration producing ATP; Option C (Peroxisome) processes glycolate produced during photorespiration but is not the initiating site; Option D (Golgi apparatus) modifies and packages proteins for secretion

Correct Answer: A. Osmosis

• **Osmosis** = The net movement of water molecules across a selectively permeable membrane from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential) • Osmosis is a passive process — no energy is required; it is driven entirely by the water potential gradient and stops when equilibrium is reached or turgor pressure equals osmotic pressure • Osmosis explains phenomena like wilting of plants (water loss from cells), swelling of raisins in water, and the preservation of food by salt/sugar • 💡 Option B (Diffusion) is the movement of any solute particles from high to low concentration, not specifically water; Option C (Active Transport) moves substances against their concentration gradient using ATP energy; Option D (Facilitated Diffusion) uses protein channels/carriers but moves solutes, not specifically water

Correct Answer: A. Nuclear Pore

• **Nuclear Pore** = A large protein complex (~120 nm diameter, ~120 MDa) embedded in the nuclear envelope that acts as a selective gate for molecular traffic between nucleus and cytoplasm • Each nuclear pore complex consists of ~30 different nucleoporin proteins; small molecules (<40 kDa) pass freely, while large proteins and RNA molecules require specific signal sequences for active transport • A single nucleus typically contains 3,000–4,000 nuclear pores, collectively allowing the export of thousands of mRNA and ribosomal subunits per minute • 💡 Option B (Nucleoplasm) is the fluid-like substance filling the nucleus interior; Option C (Nuclear Envelope) is the double-membrane boundary of the nucleus but does not itself regulate exchange; Option D (Nucleolus) is the dense region within the nucleus where ribosomal RNA is synthesized and assembled

Correct Answer: C. Cilia

• **Cilia** = Short, hair-like projections (~5–10 µm long) on the surface of respiratory epithelial cells that beat in a coordinated wave-like rhythm to sweep mucus and trapped particles toward the throat • Cilia contain a '9+2' arrangement of microtubule doublets powered by dynein ATPase motors; this 'mucociliary escalator' is a critical first-line defense against pathogens and dust • Kartagener syndrome (immotile cilia syndrome) results from defective dynein, causing repeated respiratory infections because mucus cannot be cleared • 💡 Option A (Flagella) are longer whip-like projections (~50–200 µm) used for cell locomotion, as seen in sperm, not for mucus transport; Option B (Microvilli) are finger-like projections that increase absorption surface area in the intestine, not movement; Option D (Pseudopodia) are temporary cytoplasmic extensions used by amoeba and white blood cells for engulfing particles

Correct Answer: B. Thylakoid

• **Thylakoid** = Flattened, membrane-bound sacs inside the chloroplast whose membranes are embedded with photosystems I and II, electron carriers, and ATP synthase — the complete machinery for the light reactions of photosynthesis • Stacked groups of thylakoids are called grana (singular: granum); the light reactions capture solar energy to produce ATP and NADPH, which are then used in the stroma for the Calvin cycle • Chlorophyll a, chlorophyll b, and carotenoids are all located in the thylakoid membranes within antenna complexes that funnel light energy to the reaction centres • 💡 Option A (Stroma) is the fluid surrounding thylakoids where the light-independent Calvin cycle reactions occur, not the light reactions; Option C (Inner membrane) refers to the mitochondrial inner membrane, not a chloroplast structure; Option D (Outer membrane) is a boundary layer of the chloroplast that plays no role in photochemistry