Which of the Following is a Plant Hormone? (a) Insulin (b) Thyroxin (c) Oestrogen (d) Cytokinin — NCERT Class 10 Science
NCERT Class 10 Science | Texcellency Book Series
Answer: (d) Cytokinin
Cytokinin is the only plant hormone in this list. Insulin, Thyroxin, and Oestrogen are all animal hormones — produced by glands in the human and animal body — and have absolutely no role in plants.
But stop here for a moment. This question is not just asking you to identify one correct answer — it is quietly testing whether you know what all four hormones actually do, and where they belong. A student who only memorises “Cytokinin = plant hormone” will get 1 mark. A student who understands all four hormones will ace every related question in the exam. Let us build that understanding right now.
The Factory Analogy — Plant Factory vs Animal Factory
Every living organism — whether a mango tree or a human being — runs like a factory. The factory needs managers who give instructions: “start production here,” “slow down there,” “time to grow,” “release more energy.” These managers are hormones.
The key rule: Plant factory managers work ONLY in plant factories. Animal factory managers work ONLY in animal factories. They do not cross over.
Plant factory (a mango tree, a sunflower, a banyan tree) — uses plant hormones: Auxin, Gibberellin, Cytokinin, ABA, Ethylene.
Animal factory (a human, a cow, a dog) — uses animal hormones: Insulin, Thyroxin, Oestrogen, Testosterone, Adrenaline, and many more.
Cytokinin is a manager in the plant factory. Insulin, Thyroxin, and Oestrogen are managers in the animal factory. That is why (d) is the only correct answer.
Cytokinin — The Correct Answer Explained Fully
The Smartphone Production Analogy
Your smartphone factory produces a new phone only when the production manager gives the green light. Cytokinin is that production manager inside the plant — it gives the green light to plant cells to divide and produce new cells. Without Cytokinin — cells stop dividing — the plant cannot grow new tissue, cannot repair damage, cannot develop new leaves and shoots.
What exactly does Cytokinin do?
Promotes cell division (cytokinesis) — this is literally where its name comes from. Cyto = cell, Kinin = movement/division. Cytokinin triggers cells to divide — essential for growth of new tissues, new leaves, new roots.
Delays ageing of leaves (senescence) — leaves naturally yellow and fall as they age. Cytokinin pumps the brakes on this ageing process — keeping leaves green and photosynthetically active for longer. This is why cut flowers placed in Cytokinin solution stay fresh much longer.
Promotes lateral bud growth — plants have a main stem and lateral (side) buds. Normally the main stem suppresses lateral bud growth (a phenomenon called apical dominance, controlled by Auxin). Cytokinin counters Auxin’s dominance and encourages the lateral buds to sprout — making the plant bushier and fuller.
Produced mainly in roots — Cytokinin is primarily produced in the root tips and travels upward to the shoot and leaves through the plant’s conducting tissues.
One-line memory hook: Cytokinin = Cell division + stay young + grow sideways
Why Insulin is NOT a Plant Hormone
Insulin is a hormone produced by the beta cells of the Islets of Langerhans in the pancreas — a gland found only in animals. Its job is to regulate blood glucose levels. When you eat food and blood sugar rises — insulin is released — it signals body cells to absorb glucose from the blood and either use it for energy or store it as glycogen in the liver and muscles. Blood sugar drops back to normal. Perfect regulation.
When insulin fails or is insufficient — blood glucose remains dangerously high — a condition called Diabetes Mellitus. One of the most widespread hormonal disorders in the world. Millions of people inject insulin daily because their pancreas does not produce enough.
Plants have no pancreas. Plants have no blood. Plants have no blood glucose regulation system. Insulin has absolutely no role in plants.
One-line memory hook: Insulin = I (the pancreas) control Sugar levels
Why Thyroxin is NOT a Plant Hormone
Thyroxin (also called Thyroxine or T4) is produced by the thyroid gland — a butterfly-shaped gland at the base of the neck in humans and other vertebrates. It is one of the most powerful metabolic regulators in the animal body.
What does Thyroxin do?
Regulates the body’s metabolic rate — how fast or slow all chemical reactions in the body occur. Too much Thyroxin = metabolism too fast — weight loss, rapid heartbeat, anxiety, excessive sweating (Hyperthyroidism). Too little Thyroxin = metabolism too slow — weight gain, fatigue, cold intolerance, sluggishness (Hypothyroidism).
Essential for normal growth and development — especially brain development in children. Thyroxin deficiency in infants causes Cretinism — stunted physical and mental development. This is why iodine (essential for Thyroxin production) is added to common salt — a simple public health intervention that prevents millions of cases of developmental disorders.
Requires iodine for its production — Thyroxin contains iodine atoms in its structure. Iodine deficiency → insufficient Thyroxin → thyroid gland enlarges trying to produce more → visible swelling in neck called Goitre.
Plants have no thyroid gland. Plants have no metabolic rate regulation system of this kind. Thyroxin has no role in plants.
One-line memory hook: Thyroxin = Thyroid controls the Throttle of metabolism
Why Oestrogen is NOT a Plant Hormone
Oestrogen (also spelled Estrogen) is a sex hormone produced primarily by the ovaries in female animals and humans. It is the principal female sex hormone — responsible for the development and regulation of the female reproductive system and female secondary sexual characteristics.
What does Oestrogen do?
Development of female secondary sexual characteristics during puberty — breast development, widening of hips, distribution of body fat, regulation of menstrual cycle.
Regulates the menstrual cycle — Oestrogen rises and falls in a precise monthly cycle, coordinating the growth of the uterine lining, ovulation, and preparation for pregnancy.
Bone health — Oestrogen helps maintain bone density. This is why post-menopausal women (when Oestrogen levels drop) are at higher risk of osteoporosis — weakening of bones.
Produced in small amounts by adrenal glands and fat tissue in both males and females — but primarily by ovaries in females.
Plants have no ovaries (in the animal hormonal sense), no reproductive endocrine system, no secondary sexual characteristics to develop. Oestrogen has absolutely no role in plants.
One-line memory hook: Oestrogen = O for Ovaries, O for female
All Four Hormones — Side by Side Comparison Table
| Hormone | Plant or Animal | Produced By | Main Function |
|---|---|---|---|
| Cytokinin |
Plant | Root tips | Cell division, delays leaf ageing, promotes lateral buds |
| Insulin |
Animal | Pancreas (beta cells) | Regulates blood glucose levels |
| Thyroxin |
Animal | Thyroid gland | Regulates metabolic rate, growth and development |
| Oestrogen |
Animal | Ovaries | Female sexual development, menstrual cycle regulation |
Bonus — All 5 Plant Hormones at a Glance
Since this question is about plant hormones — here is a complete revision of all five plant hormones so you never confuse them again:
Auxin — cell elongation, phototropism, geotropism. Produced at shoot tip. Moves away from light. Makes plant bend toward light.
Gibberellin — stem elongation, seed germination, flowering. The plant’s growth booster. Responsible for making some plants grow very tall.
Cytokinin — cell division, delays ageing of leaves, promotes lateral buds. Produced in roots, travels upward. (The answer to this question.)
Abscisic Acid (ABA) — stress hormone. Closes stomata during drought. Inhibits growth. Prepares plant for bad conditions. Causes seed dormancy.
Ethylene — fruit ripening, leaf fall. Only gaseous plant hormone. Why ripe fruits near other fruits makes them ripen faster.
Rhyme to Remember — Plant vs Animal Hormones
“Cytokinin lives in the plant’s domain, Divides the cells again and again, Insulin lives in the pancreas instead, Keeps your blood sugar from going to your head, Thyroxin runs the thyroid show, Controls how fast your metabolism goes, Oestrogen belongs to the female side, In ovaries — that is where she resides! Only Cytokinin is the plant’s own friend — That is the answer — right to the end!”
Alliterations
“Cytokinin Creates and Cultivates new Cells in plants” “Insulin Instructs cells to Intake glucose Immediately” “Thyroxin Throttles the body’s metabolic Tempo” “Oestrogen Oversees female development and Ovulation“
Mnemonic — Never Confuse Plant and Animal Hormones Again
Plant hormones — remember “AGCAE”: Auxin • Gibberellin • Cytokinin • Abscisic acid • Ethylene → “A Gardener Can Always Evolve”
Animal hormones in this question — remember “ITO”: Insulin • Thyroxin • Oestrogen → “I Take Orders” (from the glands — pancreas, thyroid, ovaries)
If you see any of I-T-O in a plant hormone question — eliminate immediately. The answer will always be from the AGCAE family.
Exam-Ready Answer
Answer: (d) Cytokinin
Cytokinin is a plant hormone produced mainly in the root tips. It promotes cell division, delays the ageing of leaves, and encourages the growth of lateral buds. It belongs to the group of phytohormones (plant hormones) that also includes Auxin, Gibberellin, Abscisic Acid, and Ethylene.
The other options — Insulin, Thyroxin, and Oestrogen — are all animal hormones:
- Insulin is produced by the pancreas and regulates blood glucose levels.
- Thyroxin is produced by the thyroid gland and regulates metabolic rate and growth.
- Oestrogen is produced by the ovaries and regulates female sexual development and the menstrual cycle.
None of these are produced by or found in plants.
Key Points Checklist
Cytokinin = plant hormone = correct answer Cytokinin produced in root tips = promotes cell division = delays leaf ageing = promotes lateral buds Insulin = animal hormone = pancreas = regulates blood glucose Thyroxin = animal hormone = thyroid gland = regulates metabolism = needs iodine Oestrogen = animal hormone = ovaries = female sexual development Plant hormones = AGCAE: Auxin • Gibberellin • Cytokinin • Abscisic Acid • Ethylene Animal hormones in this question = ITO: Insulin • Thyroxin • Oestrogen Trick: if you see Insulin / Thyroxin / Oestrogen / Testosterone / Adrenaline — always animal, never plant Cytokinin name comes from “cytokinesis” = cell division — the name itself tells you the function
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