Tissues — Set 2

Correct Answer: B. Cardiac muscle

• **Cardiac muscle** = involuntary, striated muscle tissue found exclusively in the walls of the heart (myocardium) • **Key fact** — cardiac cells are branched and joined by intercalated discs containing gap junctions, allowing electrical impulses to spread instantly across the heart for synchronised pumping • The heart beats about 72 times per minute throughout life without fatigue because cardiac cells are densely packed with mitochondria supplying constant ATP • 💡 Option A (Smooth muscle) is wrong because smooth muscle is non-striated, involuntary, and found in blood vessels and viscera, not the heart wall; Option C (Striated muscle) is wrong because in this context it refers to skeletal/voluntary muscle attached to bones, not heart muscle; Option D (Skeletal muscle) is wrong because skeletal muscle is under conscious control and attached to the skeleton, not to the heart

Correct Answer: B. Neurons

• **Neurons** = structural and functional units of nervous tissue with a cell body (soma), dendrites for receiving signals, and an axon for transmitting them • **Key fact** — nerve impulses travel along axons at 1-120 m/s depending on myelination; communication between neurons occurs at synapses via neurotransmitters like acetylcholine and dopamine • The human brain contains about 86 billion neurons; mature neurons are post-mitotic and do not divide, which is why brain injuries often cause permanent deficits • 💡 Option A (Nephrons) is wrong because nephrons are the microscopic filtration units of the kidney, not nerve cells; Option C (Chondrocytes) is wrong because chondrocytes are cartilage cells embedded in matrix responsible for cartilage maintenance, not impulse conduction; Option D (Platelets) is wrong because platelets are small cell fragments in blood involved in clot formation, not electrical signal transmission

Correct Answer: D. Parenchyma

• **Aerenchyma** = a specialised modification of parenchyma where large interconnected air cavities (lacunae) replace some cells, dramatically reducing tissue density • **Key fact** — air spaces provide buoyancy to aquatic plants like lotus, Hydrilla, and water hyacinth, and supply oxygen to submerged roots and rhizomes • Aerenchyma also forms in roots of terrestrial plants like rice under waterlogged conditions as an adaptive response to low soil oxygen • 💡 Option A (Collenchyma) is wrong because collenchyma provides mechanical support through pectin-thickened corners, with no air-space adaptation for buoyancy; Option B (Xylem) is wrong because xylem is vascular conducting tissue that has no lacunae modification for floatation; Option C (Sclerenchyma) is wrong because sclerenchyma is dead, densely lignified, and rigid, the structural opposite of light buoyant aerenchyma

Correct Answer: A. Epidermis

• **Epidermis** = outermost single-cell-thick protective layer covering all plant organs, secreting a waxy cuticle on aerial surfaces to limit water loss • **Key fact** — the cuticle of cutin and wax reduces transpiration water loss by up to 90%; epidermal cells lack chloroplasts except guard cells, which have them to power stomatal opening • Root epidermis produces unicellular root hairs that massively increase water and mineral absorption surface area • 💡 Option B (Cortex) is wrong because cortex is multilayered internal ground tissue below the epidermis used for storage and gas exchange; Option C (Endodermis) is wrong because endodermis is an internal layer surrounding the vascular bundle that controls mineral entry via the Casparian strip; Option D (Pith) is wrong because pith is the central ground tissue of stems used for storage, not the outermost plant covering

Correct Answer: A. Cartilage

• **Cartilage** = flexible connective tissue of chondrocytes embedded in an avascular matrix of collagen and proteoglycans (chondroitin sulphate) • **Key fact** — elastic cartilage, containing elastin fibres, forms the ear pinna and epiglottis, giving them springy flexibility that returns to shape after bending • Cartilage is avascular (no blood vessels) and aneural (no nerves), so it heals very slowly; nutrients reach chondrocytes by diffusion through the matrix • 💡 Option B (Ligament) is wrong because ligaments are dense fibrous bands joining bone to bone at joints, not forming flexible frameworks for ear or nose; Option C (Bone) is wrong because bone is rigid and mineralised with calcium phosphate, unsuitable for the springy ear pinna or nose tip; Option D (Areolar) is wrong because areolar tissue is soft loose connective tissue filling spaces between organs, lacking the structural rigidity to maintain the shape of ear or nose

Correct Answer: B. Phloem

• **Phloem** = complex vascular tissue that transports photosynthates (sugars and organic compounds) from leaves to all parts of the plant • **Key fact** — sieve tube elements (perforated end walls) and companion cells are the functional units of phloem; companion cells provide ATP via symplastic loading to drive active phloem transport • Unlike xylem, phloem transport is bidirectional (source-to-sink) and requires metabolic energy — it is an active, pressure-flow process • 💡 Option A (Xylem) is wrong because xylem transports water and minerals upward, not food; its conducting cells (tracheids/vessels) are dead, not sieve tubes; Option C (Cambium) is wrong because cambium is a meristematic layer that produces xylem and phloem but does not conduct substances; Option D (Cortex) is wrong because cortex is ground tissue used for storage, lacking sieve tubes or companion cells

Correct Answer: C. Areolar tissue

• **Areolar tissue** = loose connective tissue with scattered fibroblasts, collagen fibres, elastin fibres, mast cells, and macrophages in a semi-fluid matrix • **Key fact** — areolar tissue is the most widely distributed connective tissue in the body; it fills spaces between organs, anchors skin to underlying muscles, and supports blood vessels and nerves • It also acts as a reservoir for tissue fluid and plays a role in immune defence through its macrophages and mast cells • 💡 Option A (Adipose tissue) is wrong because adipose tissue is specialised for fat storage with closely packed adipocytes, not for filling gaps; Option B (Dense tissue) is wrong because dense connective tissue (tendons, ligaments) has tightly packed fibres leaving little ground substance; Option D (Nervous tissue) is wrong because nervous tissue consists of neurons and glia transmitting signals, not filling structural spaces

Correct Answer: B. Lateral meristem

• **Lateral meristem** = meristematic tissue running along the length of stems and roots responsible for secondary growth that increases girth • **Key fact** — vascular cambium (one type of lateral meristem) produces secondary xylem inward and secondary phloem outward, forming growth rings visible in tree cross-sections • Cork cambium (phellogen), another lateral meristem, produces cork cells outward replacing the epidermis in woody plants • 💡 Option A (Intercalary meristem) is wrong because it is located at internodes and leaf bases, enabling grass stems to regrow from the base, not increasing width; Option C (Apical meristem) is wrong because apical meristem drives primary elongation growth at tips, not secondary girth increase; Option D (Primary meristem) is wrong because primary meristems differentiate from apical meristems and produce primary tissues, not secondary thickening

Correct Answer: D. Voluntary muscles

• **Voluntary muscles** = skeletal muscles attached to bones via tendons that contract under conscious control of the nervous system • **Key fact** — voluntary (skeletal) muscle fibres are multinucleated, cylindrical, and show cross-striations due to alternating actin and myosin filament bands (A and I bands) • They fatigue quickly compared to cardiac muscle because they rely partly on anaerobic respiration during intense activity, producing lactate • 💡 Option A (Cardiac muscles) is wrong because cardiac muscles contract involuntarily and are found only in the heart, not attached to the skeleton; Option B (Smooth muscles) is wrong because smooth muscles are involuntary, non-striated, and found in visceral organs like the stomach and uterus; Option C (Involuntary muscles) is wrong because involuntary muscles work without conscious control, the exact opposite of muscles moved at will

Correct Answer: D. Sclerenchyma

• **Sclerenchyma** = simple permanent tissue of dead, elongated cells with uniformly thick walls impregnated with lignin, providing maximum mechanical support • **Key fact** — sclerenchyma fibres are dead at maturity because lignin deposition fills the cell lumen; they are the stiffest plant cells and give strength to vascular bundles and stem edges • Commercial fibres like jute, hemp, and flax are sclerenchyma fibres extracted from the phloem of their respective plants • 💡 Option A (Xylem parenchyma) is wrong because xylem parenchyma cells are living, thin-walled, and used for storage of starch and oils, not mechanical support; Option B (Parenchyma) is wrong because parenchyma consists of living cells with thin walls serving storage and metabolic functions; Option C (Collenchyma) is wrong because collenchyma is living, flexible, and provides support only in young growing organs