Biology · Lesson 14

Hormones, plant hormones & tropisms

JAMBWAECNECO

What the syllabus asks

JAMB: co-ordination and control: hormones in animals, plant hormones and growth responses

WAEC: co-ordination: hormonal (endocrine) system: endocrine glands, their hormones, functions and effects of deficiency or excess; comparison of nervous and hormonal co-ordination; plant hormones (auxins, gibberellins, cytokinins, abscisic acid, ethylene) and their uses; responses in plants: tropisms (photo-, geo-, hydro-, thigmo-, chemotropism), nastic movements and taxes; experiments on tropisms…

NECO: co-ordination: hormonal (endocrine) system: endocrine glands, their hormones, functions and effects of deficiency or excess; comparison of nervous and hormonal co-ordination; plant hormones (auxins, gibberellins, cytokinins, abscisic acid, ethylene) and their uses; responses in plants: tropisms (photo-, geo-, hydro-, thigmo-, chemotropism), nastic movements and taxes; experiments on tropisms…

What you will learn

Define a hormone and distinguish endocrine from exocrine glands.
Name the main endocrine glands, their hormones and functions.
Describe the effects of deficiency and excess of key hormones.
Explain the control of blood glucose by insulin and glucagon, and negative feedback.
Compare nervous and hormonal co-ordination.
State the effects and agricultural uses of plant hormones.
Describe and explain tropisms, nastic movements and taxes, and interpret classic experiments.

Key points

Hormones: chemical messengers from ductless glands, carried in blood to target organs.
Pituitary (master gland): GH, TSH, ACTH, FSH, LH, prolactin, ADH, oxytocin; controlled by the hypothalamus.
Growth hormone: too little → dwarfism; too much → gigantism (child) or acromegaly (adult).
Thyroxine (needs iodine) controls metabolic rate; deficiency → cretinism, myxoedema, goitre; excess → hyperthyroidism.
Adrenaline prepares the body for fight or flight; ADH reduces urine; lack of ADH → diabetes insipidus.
Insulin lowers blood glucose (glucose → glycogen); glucagon raises it; diabetes mellitus when insulin fails.
Nervous: fast, short, localised; hormonal: slow, long-lasting, widespread.
Auxins: elongation, apical dominance, rooting, weedkillers; gibberellins: stem growth, germination; cytokinins: cell division; ABA: dormancy; ethylene: ripening.
Shoots: +phototropic, −geotropic; roots: +geotropic, +hydrotropic; auxin collects on shaded/lower side.
Tropisms (directional growth), nastic movements (non-directional, e.g. Mimosa), taxes (whole organism).

Start here: Chemical co-ordination and hormones

Your body has two systems that co-ordinate its activities. The nervous system sends rapid electrical impulses along nerves. The endocrine (hormonal) system sends chemical messengers through the blood.

Hormone: an organic chemical substance produced in small quantities by an endocrine (ductless) gland, released directly into the blood, and carried to a target organ where it produces a specific effect.

Hormones are often proteins or steroids. They are needed in tiny amounts, and once they have acted they are broken down (mainly in the liver) and removed by the kidneys.

Endocrine gland	Exocrine gland
ductless	has a duct
secretes into the blood	secretes onto a surface or into a cavity
e.g. thyroid, adrenal, pituitary	e.g. salivary, sweat, tear glands

The pancreas is both: its exocrine part sends pancreatic juice (digestive enzymes) along the pancreatic duct to the duodenum, while small clusters of cells, the islets of Langerhans, release insulin and glucagon into the blood. Ovaries and testes are also both: they produce gametes and secrete hormones.

Worked example 14.1

Why is the pancreas described as both an exocrine and an endocrine gland?

Exocrine: secretes pancreatic juice through a duct into the duodenum.
Endocrine: islets of Langerhans secrete insulin and glucagon directly into the blood.

It has a duct for juice and ductless islets for hormones

⚠️ Common mistake: saying hormones travel along nerves. They travel in the blood plasma.

Try it yourself. Give two characteristics of hormones.

Show solution

Any two: produced by ductless glands, carried in blood, act on target organs, needed in small amounts, slow but long-lasting effects.

Continue this lesson in the free app →

Practice questions

1. Insulin is produced by the

pancreas
thyroid
pituitary
adrenal gland

Show answer

A: pancreas
Islets of Langerhans in the pancreas secrete insulin to lower blood sugar.

2. The "fight or flight" hormone is

thyroxine
oestrogen
insulin
adrenaline

Show answer

D: adrenaline
Adrenaline (from the adrenal glands) prepares the body for emergencies.

3. The plant hormone responsible for phototropism is

abscisic acid
thyroxine
auxin
ethylene

Show answer

C: auxin
Auxin gathers on the shaded side causing more growth there, so the shoot bends toward light.

4. Cells of the islets of Langerhans in the pancreas secrete

bile
insulin and glucagon
adrenaline
thyroxine

Show answer

B: insulin and glucagon
They control blood glucose.

5. Insulin

lowers blood glucose
raises blood glucose
controls growth
controls water balance

Show answer

A: lowers blood glucose
It promotes glucose uptake and storage as glycogen.

Key words

Hormone: A chemical messenger secreted in small amounts by an endocrine gland and carried in blood to a target organ.
Endocrine gland: A ductless gland that releases hormones directly into the blood.
Exocrine gland: A gland that releases secretions through a duct, e.g. salivary gland.
Target organ: The organ on which a hormone acts.
Negative feedback: A control system in which a change triggers a response that reverses the change.
Diabetes mellitus: A disorder in which blood glucose stays too high because of lack of insulin or resistance to it.
Goitre: Swelling of the thyroid gland in the neck, often due to iodine deficiency.
Auxin: A plant hormone (e.g. IAA) that promotes cell elongation in shoots.
Tropism: A growth movement of part of a plant in response to a directional stimulus.
Phototropism: Growth response to light from one direction.
Geotropism (gravitropism): Growth response to gravity.
Nastic movement: A non-directional response to a stimulus, e.g. folding of Mimosa leaves when touched.

Learn Hormones, plant hormones & tropisms step by step, free.
The Cara's Tips app teaches every topic from the very basics, with quick checks, worked examples and exam practice. It works offline.
Open this lesson in the free app Download for Android