Tissues — Set 3

Correct Answer: C. Phloem

• **Phloem** = complex vascular tissue that transports food (photosynthates such as sucrose) from source organs (leaves) to sink organs (roots, fruits, seeds) • **Key fact** — food movement through phloem is bidirectional and driven by a pressure gradient built up by active loading of sugars at the source end (Munch pressure-flow hypothesis) • Phloem transport is 2-3 times faster than diffusion and can move up to 1 metre per hour in some plants • 💡 Option A (Xylem) is wrong because xylem moves water and minerals only upward, not food; Option B (Parenchyma) is wrong because parenchyma cells store food but do not transport it over long distances; Option D (Sclerenchyma) is wrong because sclerenchyma is dead, lignified support tissue with no transport function

Correct Answer: B. Osteon

• **Osteon** (Haversian system) = the basic structural unit of compact bone, consisting of concentric lamellae of mineralised matrix surrounding a central Haversian canal • **Key fact** — osteocytes trapped in lacunae within the lamellae communicate via cytoplasmic extensions through tiny channels called canaliculi, forming a nutrient and signal network • The Haversian canal contains blood vessels and nerves; Volkmann's canals connect adjacent Haversian canals perpendicularly • 💡 Option A (Chondrocyte) is wrong because chondrocytes are the cells of cartilage, not bone; Option C (Neuron) is wrong because neurons are the functional units of nervous tissue, not bone; Option D (Nephron) is wrong because nephrons are the filtration units of the kidney, completely unrelated to bone structure

Correct Answer: B. Iris muscle of the eye

• **Iris muscle of the eye** = smooth (involuntary) muscle that automatically adjusts pupil size in response to light intensity without conscious effort • **Key fact** — the iris contains two types of smooth muscle: the sphincter pupillae (constricts pupil in bright light) and dilator pupillae (widens pupil in dim light), both controlled by the autonomic nervous system • Because it is smooth muscle under autonomic control, we cannot consciously control pupil size the way we flex a bicep • 💡 Option A (Neck muscle) is wrong because neck muscles are skeletal and voluntary, allowing us to turn the head at will; Option C (Leg muscle) is wrong because leg muscles are skeletal voluntary muscles used for walking and running; Option D (Bicep muscle) is wrong because the bicep is a skeletal voluntary muscle that we can consciously contract to flex the arm

Correct Answer: B. Connective tissue

• **Connective tissue** = the most abundant tissue by weight in the human body, comprising bone, cartilage, blood, adipose, and areolar tissue • **Key fact** — connective tissue makes up about 50-60% of total body mass when all its forms (including bone and blood) are combined; it is derived from embryonic mesenchyme • Its abundance reflects its diverse roles: structural support, binding organs, transporting nutrients (blood), and storing energy (adipose) • 💡 Option A (Nervous tissue) is wrong because nervous tissue represents only about 2% of body weight (the brain plus spinal cord and nerves); Option C (Muscular tissue) is wrong because muscle constitutes roughly 40% of body weight, less than all connective tissue combined; Option D (Epithelial tissue) is wrong because epithelial tissue forms thin sheets covering surfaces and is far less in mass than connective tissue

Correct Answer: A. Epithelial tissue

• **Epithelial tissue** = covering and lining tissue of tightly packed cells with very little extracellular matrix, sitting on a basement membrane • **Key fact** — epithelium covers the entire external body surface (skin) and lines all internal cavities including the digestive tract, lungs, and blood vessels • It serves as a selective barrier: protecting against physical, chemical, and microbial threats while regulating the passage of substances • 💡 Option B (Nervous tissue) is wrong because nervous tissue forms the brain, spinal cord, and nerves, not surface coverings; Option C (Muscular tissue) is wrong because muscular tissue enables movement by contraction, not surface coverage; Option D (Connective tissue) is wrong because connective tissue provides structural support and binding between organs, not external or internal lining

Correct Answer: D. Gas exchange

• **Gas exchange** = the primary function of stomata, which are pores in the leaf epidermis flanked by two guard cells that control pore size • **Key fact** — stomata allow CO2 to enter for photosynthesis and O2 to exit as a by-product; simultaneously, they regulate water vapour loss (transpiration) and are typically open during the day and closed at night • Guard cells swell (open stomata) when turgid due to K+ ion influx, and shrink (close stomata) when flaccid, in a mechanism driven by light and CO2 concentration • 💡 Option A (Photosynthesis) is wrong because photosynthesis occurs inside mesophyll cells in chloroplasts, not at stomata themselves; Option B (Food storage) is wrong because food storage occurs in parenchyma cells of roots and tubers, not through stomatal pores; Option C (Water absorption) is wrong because water absorption occurs through root hairs in the root epidermis, not through leaf stomata

Correct Answer: D. Platelets

• **Platelets** (thrombocytes) = tiny, anucleate cell fragments derived from megakaryocytes in bone marrow that initiate blood clotting • **Key fact** — at a wound site, platelets adhere to exposed collagen, aggregate to form a platelet plug, and release clotting factors that trigger the coagulation cascade converting fibrinogen to fibrin • Normal platelet count is 1.5-4 lakh per microlitre of blood; a count below 1 lakh is called thrombocytopenia (seen in dengue fever) • 💡 Option A (Plasma) is wrong because plasma is the liquid component of blood carrying proteins and nutrients, not the clotting initiator; Option B (Red blood cells) is wrong because RBCs carry oxygen via haemoglobin and play no direct role in clot formation; Option C (White blood cells) is wrong because WBCs form part of the immune defence against pathogens, not blood clotting

Correct Answer: B. Sclerenchyma

• **Sclerenchyma** = the plant tissue whose cells are dead at functional maturity because lignin deposition progressively fills the cell lumen during development • **Key fact** — once lignified walls are complete, the protoplasm breaks down; the dead cell wall provides rigid mechanical support without any metabolic cost • Both fibres (long cells) and sclereids (short stone cells) of sclerenchyma are dead at maturity; stone cells in hard seed coats are a familiar example • 💡 Option A (Epithelium) is wrong because epithelial cells are living, continuously dividing to replace worn-out surface cells; Option C (Collenchyma) is wrong because collenchyma cells are living, which allows them to keep elongating as the plant grows; Option D (Parenchyma) is wrong because parenchyma cells are living and carry out metabolic functions like photosynthesis and storage

Correct Answer: D. Cork

• **Cork** = dead, impermeable protective tissue produced by the cork cambium (phellogen), replacing the epidermis in older woody stems • **Key fact** — cork cells are impregnated with suberin, a waxy waterproof substance that makes them completely impermeable to water and gases; lenticels (loose cells in cork) allow gas exchange • Commercial cork used for wine bottle stoppers comes from the bark of the cork oak (Quercus suber), which can be harvested every 9-12 years without harming the tree • 💡 Option A (Xylem) is wrong because xylem is vascular tissue inside the stem conducting water upward, not an outer protective layer; Option B (Phloem) is wrong because phloem transports sugars and is located just below the bark, not the outermost protective covering; Option C (Pith) is wrong because pith is the central ground tissue of young stems used for storage, not protection of the outer surface

Correct Answer: D. Ligament

• **Ligament** = dense regular connective tissue of tightly packed collagen and elastin fibres that connects bone to bone at joints • **Key fact** — ligaments are slightly elastic (due to elastin), allowing normal joint movement while preventing excessive displacement; the anterior cruciate ligament (ACL) in the knee is commonly torn in sports injuries • Ligaments have poor blood supply, so they heal slowly; severe ligament tears often require surgical repair • 💡 Option A (Cartilage) is wrong because cartilage cushions joint surfaces and forms flexible frameworks (ear, nose), not bone-to-bone connections; Option B (Areolar) is wrong because areolar tissue is loose connective tissue filling spaces between organs, not a strong joint-stabilising structure; Option C (Tendon) is wrong because tendons connect muscle to bone, not bone to bone