Cell Structure & Function — Part 6: 40 MCQs with Explanations

Vitamin A (retinol) is required for synthesis of rhodopsin (visual pigment in rod cells); deficiency impairs dark adaptation and causes night blindness.

Secretin released by duodenal S cells in response to acidic chyme stimulates pancreas to secrete bicarbonate-rich fluid that neutralizes acidity.

Water moves through root cortex via apoplast (cell walls) and symplast (through plasmodesmata) pathways driven by water potential gradients.

Heterozygote advantage maintains genetic variation; e.g., sickle cell trait (HbA/HbS) provides resistance to malaria while homozygous normal (HbA/HbA) is susceptible and homozygous sickle (HbS/HbS) causes disease.

B cells (B lymphocytes) differentiate into plasma cells that secrete specific antibodies against antigens; T cells mediate cell-mediated immunity.

Guard cells change shape due to K⁺ influx/efflux altering turgor pressure: turgid guard cells create open stoma; flaccid guard cells close stoma.

Sickle cell anemia results from homozygous recessive genotype (HbS/HbS) for mutant β-globin gene on chromosome 11.

Oxytocin from posterior pituitary causes contraction of myoepithelial cells surrounding mammary alveoli, ejecting milk into ducts.

Chloroplasts in plant cells and algae contain chlorophyll and other pigments that capture light energy for photosynthesis.

Anaphase begins when centromeres divide; spindle fibers shorten pulling chromatids (now chromosomes) toward opposite poles.

Glucagon from pancreatic alpha cells acts primarily on liver to break down glycogen to glucose and synthesize glucose from non-carbohydrate precursors.

Lactic acid fermentation: pyruvate + NADH → lactate + NAD⁺ regenerates NAD⁺ for continued glycolysis when oxygen is limited.

Meiosis I separates homologous chromosomes (reduction division); meiosis II separates sister chromatids (equational division).

Water moves into root hairs by osmosis because soil water has higher water potential (less negative) than root hair cell sap.

Hexokinase (or glucokinase in liver) phosphorylates glucose using ATP, trapping it inside the cell for further metabolism.

Prokaryotes have circular DNA in nucleoid, 70S ribosomes, cell wall containing peptidoglycan (in bacteria), and lack membrane-bound organelles.

Auxin accumulates on shaded side of shoot causing greater cell elongation there, resulting in bending toward light source (positive phototropism).

During prophase, chromatin condenses into discrete chromosomes, nucleolus disappears, and mitotic spindle begins to form.

Osmosis is passive movement of water from region of higher water potential (lower solute concentration) to lower water potential (higher solute concentration).

Translation occurs on ribosomes where tRNA molecules bring amino acids corresponding to mRNA codons, forming polypeptide chains.

Insulin facilitates glucose uptake by cells, promotes glycogen synthesis in liver/muscles, and inhibits gluconeogenesis.

Calcitonin from thyroid C cells inhibits osteoclast activity, reducing calcium release from bones; important in calcium homeostasis especially in children.

With few exceptions (mitochondria, some protists), the same codons specify the same amino acids across bacteria, archaea, and eukaryotes.

Erythrocytes (RBCs) contain hemoglobin with iron-containing heme groups that bind O₂ reversibly in lungs and release it in tissues.

Phloem consists of sieve tube elements (with companion cells) that transport photosynthates via pressure flow mechanism from sources to sinks.

Transduction: bacteriophage carries bacterial DNA from donor to recipient cell during viral infection cycle.

Passive artificial immunity provides immediate but temporary protection (weeks to months) because recipient does not produce own antibodies or memory cells.

Plant cells have rigid cell wall composed primarily of cellulose outside plasma membrane; animal cells lack cell walls.

Granulosa cells of developing follicles secrete estrogen, which thickens endometrium and triggers LH surge at mid-cycle causing ovulation.

Glycogen is branched polymer of glucose stored in liver and muscle cells; broken down to glucose when blood sugar drops.

Ribosomal RNA (rRNA) constitutes ~60% of ribosome mass and catalyzes peptide bond formation (peptidyl transferase activity).

Cytokinins (e.g., zeatin) promote cell division in plant roots/shoots, delay leaf senescence, and work with auxin to control apical dominance.

Neutrophils are most abundant WBCs (50-70%); they phagocytose bacteria and release antimicrobial substances within hours of infection.

Actin-myosin contractile ring pinches plasma membrane inward forming cleavage furrow that deepens until daughter cells separate.

HIV binds to CD4 receptors on helper T cells (and some macrophages/dendritic cells), replicates inside them, and destroys them, leading to AIDS.

Oxidative phosphorylation: electron transport chain complexes embedded in inner membrane create proton gradient driving ATP synthesis via ATP synthase.

Cancer cells show genomic instability (high mutation rate), uncontrolled proliferation, loss of contact inhibition, immortality, angiogenesis, and metastasis.

Crossing over exchanges genetic material between non-sister chromatids of homologous chromosomes, producing recombinant chromosomes with new allele combinations.

Viruses lack cellular organization (no cytoplasm, organelles, or metabolism); they are obligate intracellular parasites requiring host machinery for replication.

Follicle-stimulating hormone (FSH) acts on Sertoli cells to support spermatogenesis; LH stimulates Leydig cells to produce testosterone.