Tissues — Set 4

Correct Answer: D. Parenchyma

• **Parenchyma** = The most basic and widely distributed permanent tissue made of thin-walled, loosely packed living cells, found in leaves, stems, and roots — its simplicity and abundance make it the "default" plant tissue. • **Key fact** — Parenchyma cells retain the ability to divide and can redifferentiate into other cell types, a property neither Collenchyma nor Sclerenchyma shares. • These cells serve multiple roles: photosynthesis (chlorenchyma variant), storage of starch and water, and wound healing. • 💡 Option A (Collenchyma) is wrong because it has unevenly thickened walls and provides flexible support, making it more specialised; Option B (Sclerenchyma) is wrong because it has heavily lignified dead cells meant for rigid mechanical support; Option C (Xylem) is wrong because it is a complex vascular tissue, not a simple permanent tissue.

Correct Answer: A. Movement of mucus

• **Ciliated Epithelium** = A type of epithelial tissue whose cells bear hair-like projections (cilia) that beat rhythmically, sweeping mucus and trapped dust or pathogens upward out of the airway — this is the correct answer because mucus movement is its defining function. • **Key fact** — The coordinated wave-like beating of cilia is called metachronal rhythm; failure of this mechanism (as in Kartagener syndrome) leads to chronic respiratory infections. • Goblet cells interspersed in this epithelium secrete the mucus that the cilia then propel, making the two cell types a functional team. • 💡 Option B (Absorption of nutrients) is wrong because absorption is the job of microvilli-bearing columnar epithelium in the intestine; Option C (Protection from acids) is wrong because stratified squamous epithelium in the oesophagus handles acid resistance; Option D (Sensation of smell) is wrong because olfactory epithelium with specialised receptor neurons serves that purpose.

Correct Answer: B. Dendrite

• **Dendrite** = Short, branched extensions of the neuron that act as antenna-like receivers, collecting electrochemical signals (via neurotransmitters) from the axon terminals of adjacent neurons and converting them into local graded potentials. • **Key fact** — A single neuron can have thousands of dendrites, vastly increasing its ability to integrate signals from multiple sources simultaneously — a capacity the single axon does not have. • The dendritic spines (tiny protrusions on dendrites) are the physical sites of most excitatory synapses and play a central role in learning and memory. • 💡 Option A (Cell body/Soma) is wrong because it integrates signals but does not directly receive them from other neurons; Option C (Nucleus) is wrong because it houses DNA and regulates gene expression, not signal reception; Option D (Axon) is wrong because it transmits signals away from the cell body to the next neuron or effector.

Correct Answer: B. Sieve tubes

• **Sieve tubes** = The principal conducting elements of phloem, not xylem — they transport dissolved sugars and organic molecules from leaves to the rest of the plant, making them structurally and functionally distinct from xylem components. • **Key fact** — Sieve tubes are associated with companion cells that provide metabolic support; xylem's water-conducting tracheids and vessels are dead at maturity and have no such companion cells. • Xylem is a complex tissue comprising tracheids, vessels (both dead, for water transport), xylem fibres (support), and xylem parenchyma (storage and lateral transport) — none of which are sieve tubes. • 💡 Option A (Xylem parenchyma) is wrong because it is the living storage component of xylem; Option C (Tracheids) is wrong because they are elongated dead cells with pitted walls that conduct water in xylem; Option D (Vessels) is wrong because they are wide, dead, tubular elements in xylem that carry water upward.

Correct Answer: C. Stratified squamous

• **Stratified squamous epithelium** = A multilayered tissue in which the outermost cells are flat (squamous) and are continually shed and replaced, acting as a durable mechanical barrier that protects underlying tissues from friction, abrasion, and microbial invasion. • **Key fact** — In the skin the surface cells are also keratinised (filled with the tough protein keratin), adding a waterproof quality that no single-layered epithelium provides. • The deepest layer (stratum basale) constantly divides to replenish the worn-out outer layers, meaning the epithelium regenerates itself in a conveyor-belt fashion. • 💡 Option A (Columnar) is wrong because tall columnar cells line secretory and absorptive surfaces like the stomach and intestine, not wear-resistant skin; Option B (Simple cuboidal) is wrong because its single cube-shaped layer suits secretion in glands and kidney tubules but cannot withstand friction; Option D (Simple squamous) is wrong because its single thin layer allows diffusion but tears easily under mechanical stress.

Correct Answer: C. Base of leaves or nodes

• **Intercalary meristem** = A region of actively dividing cells located at the base of leaves or internodes (nodes), positioned between regions of permanent tissue — this location between mature zones is what makes it "intercalary" (inserted between). • **Key fact** — This meristem is especially prominent in grasses and monocots, enabling rapid re-growth of the leaf blade after grazing or mowing, since the growing zone sits safely at the base rather than at the vulnerable tip. • Intercalary meristem is considered a detached portion of apical meristem retained at the leaf base during the plant's development. • 💡 Option A (Root tips) is wrong because root tips contain apical meristem, not intercalary; Option B (Shoot apex) is wrong because the shoot tip houses apical meristem responsible for primary elongation; Option D (Cambium) is wrong because vascular cambium is a lateral meristem that increases girth, unrelated to intercalary activity.

Correct Answer: A. Hemoglobin

• **Hemoglobin** = An iron-containing protein found in red blood cells (RBCs); its iron atom (in the haem group) binds oxygen and reflects red wavelengths of light, giving oxygenated blood its characteristic bright red colour. • **Key fact** — Deoxygenated hemoglobin (venous blood) is dark red or maroon because the absence of bound oxygen changes the haem group's light absorption, yet it is still hemoglobin — not a different pigment — responsible for the colour. • Each RBC contains about 270 million hemoglobin molecules, and the sheer abundance of RBCs (4–6 million per µL) makes hemoglobin the dominant colour-imparting molecule in blood. • 💡 Option B (Platelets) is wrong because they are colourless cell fragments involved in clotting, not colour; Option C (WBCs) is wrong because white blood cells are colourless and form less than 1% of blood cells; Option D (Plasma) is wrong because plasma is a pale yellow liquid due to bilirubin and proteins, not red.

Correct Answer: D. Chlorenchyma

• **Chlorenchyma** = A specialised form of parenchyma tissue whose cells are packed with chloroplasts, making it the primary photosynthetic tissue of plants — it is named directly after chlorophyll, clarifying why it is the correct answer. • **Key fact** — The mesophyll of leaves (palisade and spongy layers) is composed entirely of chlorenchyma, explaining why leaves are the main photosynthetic organs; no other simple tissue type carries chloroplasts in this density. • Chlorenchyma can also occur in the cortex of young green stems, allowing minor photosynthesis in the stem under bright light conditions. • 💡 Option A (Sclerenchyma) is wrong because it consists of dead, lignified cells with no chloroplasts, functioning solely for mechanical support; Option B (Aerenchyma) is wrong because it is parenchyma with large air spaces for buoyancy and gas exchange in aquatic plants, not photosynthesis-dedicated; Option C (Collenchyma) is wrong because its thickened-corner cells provide flexible support in young growing parts and contain little to no chlorophyll.

Correct Answer: C. Smooth muscle

• **Smooth muscle** = Involuntary muscle tissue whose cells are long, tapering (spindle-shaped) at both ends and lack the sarcomeric cross-striations seen in other muscle types — the absence of regular myosin-actin banding is precisely why they are called "unstriated." • **Key fact** — Smooth muscle is under autonomic (involuntary) nervous control and is found in the walls of hollow organs like the intestine, uterus, and blood vessels, where slow, sustained contractions regulate internal processes without conscious effort. • Unlike skeletal and cardiac muscle, smooth muscle cells are uninucleate (single nucleus) and can regenerate by mitosis, making repair after injury possible. • 💡 Option A (Skeletal muscle) is wrong because it is striated, voluntary, and cylindrical with multiple nuclei; Option B (Striated muscle) is wrong because "striated" itself means having cross-striations, the opposite of the question's "unstriated" condition; Option D (Cardiac muscle) is wrong because it is striated with intercalated discs and is involuntary but never spindle-shaped.

Correct Answer: C. Immune defense

• **White blood cells (WBCs/Leucocytes)** = Nucleated cells of the immune system that identify and destroy pathogens, foreign particles, and abnormal cells — immune defence is their primary and defining function, which is why they are also called immunocytes. • **Key fact** — WBCs include five types: neutrophils (phagocytose bacteria), lymphocytes (produce antibodies and mediate cellular immunity), monocytes (mature into macrophages), eosinophils (fight parasites), and basophils (release histamine) — every type is oriented toward defence, not transport or clotting. • Unlike RBCs, WBCs can exit blood vessels (diapedesis) and enter tissues where infections occur, a mobility that is essential for immune surveillance. • 💡 Option A (Waste removal) is wrong because excretion of metabolic waste is handled by kidneys and the liver, not blood cells; Option B (Oxygen transport) is wrong because that is the exclusive role of haemoglobin-rich red blood cells (RBCs); Option D (Blood clotting) is wrong because platelets (thrombocytes) and clotting factors in plasma are responsible for coagulation, not WBCs.