Hormones | Animal Hormones & Plant Hormones

What are Hormones?

  1. Hormones are chemical substances produced or secreted by endocrine glands, which are ductless glands in the body.
  2. They are released into the bloodstream and travel to target organs to exert their effects.
  3. Hormones can speed up or slow down biological reactions in the body.
  4. They act as chemical messengers, produced in one part of the body and affecting specific target organs.
  5. Hormones play a role in homeostasis, growth, and development.
  6. After their actions, hormones are inactivated in the liver and excreted in urine.

Animal Hormones

There are five groups of animal hormones secreted by different glands.


The pituitary gland is located below the hypothalamus and consists of anterior and posterior parts. The anterior pituitary gland releases growth hormones and tropic hormones that regulate other endocrine glands. It is often referred to as the “master gland.” The hormones secreted by the anterior pituitary include:

  1. Gonadotropins (follicle-stimulating hormone and luteinizing hormone) which regulate reproductive functions.
  2. Adrenocorticotropic hormone, which stimulates the adrenal cortex to secrete corticosteroids.
  3. Thyroid-stimulating hormone, which regulates the thyroid gland.
  4. Prolactin, which stimulates milk production in mammary glands.
  5. Somatotropin (growth hormone), which promotes bone growth and metabolic rate.

The posterior pituitary gland secretes:

  1. Anti-diuretic hormone (ADH), which regulates water balance in the body.
  2. Oxytocin, which stimulates uterine contractions during childbirth and milk ejection.


The thyroid gland, located in the neck region close to the larynx, produces three main hormones: thyroxine, triiodothyronine, and calcitonin.

The primary hormone is thyroxine.


  1. Regulates the growth and development of all body cells.
  2. Increases the rate of glucose oxidation in body cells and heat production.
  3. Under secretion of thyroxine causes cretinism in children and sluggishness/goiter in adults.
  4. Over secretion of thyroxine causes hyperactivity and restlessness.


The parathyroid glands, which are small glands attached to the thyroid gland, secrete a hormone called parathyroid hormone (PTH). PTH plays a crucial role in regulating blood calcium levels. It achieves this by:

  1. Releasing calcium from the bones
  2. Increasing calcium absorption in the gut
  3. Reducing calcium excretion by the kidneys

The hormone calcitonin, produced by the thyroid gland, has the opposite effect and lowers blood calcium levels. Over secretion of parathyroid hormone leads to fragile bones prone to fractures, while under secretion causes muscle spasms.


The pancreas serves both as a digestive gland and an endocrine gland. It contains specialized cells called Islets of Langerhans, which produce two important hormones: insulin and glucagon.

Insulin lowers blood glucose levels by stimulating liver cells to convert excess glucose into glycogen for storage in the liver and muscles. Deficiency of insulin leads to diabetes mellitus, characterized by glucose excretion in urine, reduced appetite, and increased thirst. Excessive secretion of insulin causes a drop in blood sugar levels and constant hunger.

Glucagon, on the other hand, raises blood glucose levels by stimulating the liver to convert stored glycogen into glucose.


The adrenal glands, located above the kidneys, produce two distinct groups of hormones. The adrenal cortex produces corticosteroids, while the adrenal medulla produces adrenaline and noradrenaline, known as emergency hormones.

Corticosteroids include glucocorticoids, such as cortisol, which raise blood glucose levels during times of stress, and mineralocorticoids, such as aldosterone, which regulate sodium and potassium ion levels in body fluids.

Adrenaline, as an emergency hormone, prepares the body for immediate action in response to fear, danger, or anger. It achieves this by:

  1. Increasing muscular tone
  2. Accelerating heart rate and respiration
  3. Dilating pupils
  4. Enhancing the conversion of glycogen to glucose by the liver

Under-secretion of adrenaline results in a slow response to emergencies, low blood pressure, and heart rate, while over-secretion leads to excessive anxiety and excitement.


Reproductive hormones are produced by specific cells in the reproductive organs. In males, the testes produce testosterone, the male sex hormone, while in females, the ovaries produce estrogen and progesterone, the female sex hormones.

Testosterone stimulates:

  1. Growth and maturation of the penis, testes, and accessory sex structures
  2. Development of male sex characteristics such as muscle growth, pubic, armpit, chest, and facial hair, and deepening of the voice

Estrogen performs the following functions in females:

  1. Development of secondary sexual characteristics, including breast enlargement, pubic and armpit hair growth, widening of the hips, and fat distribution.
  2. Regulation of the reproductive or menstrual cycle


  1. Inhibits egg production (ovulation) during pregnancy
  2. Prepares and maintains the lining of the uterus
  3. Aids in implantation and development of the embryo in the uterus

Excessive secretion of reproductive hormones leads to excessive development of sexual organs and abnormal sexual urges, while under secretion results in poor development of secondary sexual characteristics, sexual organs, and decreased sexual drive.

Plant Hormones

Plant coordination relies on chemicals known as plant hormones. These hormones share similarities with animal hormones in that they:

  1. Are required in small amounts to exert their effects
  2. Are produced in one part of the plant and transported to other parts to elicit their effects

However, plant hormones differ in that they are not produced exclusively in specialized glands but have more generalized effects based on concentration and organ type. Plant hormones regulate growth, fruit formation, root development, leaf fall, and other activities. Combinations of plant hormones can result in unique responses beyond what each hormone can accomplish alone. Although slower than animal hormones, plant hormone responses mainly occur through growth processes.


The major groups of plant hormones include auxins, gibberellins, cytokinins, abscisic acid, and ethylene.


The primary naturally occurring auxin is indoleacetic acid (IAA), produced at the shoot apex. It influences cell division, elongation, and differentiation. Auxin:

  1. Influences stem growth towards light (positive phototropism) and root growth away from light (negative phototropism)
  2. Stimulates the development of lateral and adventitious roots for increased water and mineral absorption
  3. Inhibits the growth of lateral buds, causing apical dominance
  4. Promotes fruit development
  5. Breaks seed dormancy, promoting early germination
  6. Delays leaf fall


Gibberellins are found in root and stem apices. They promote growth by stimulating both cell elongation and division. They are also effective in promoting growth in dwarf plant varieties. Other effects of gibberellins include inducing seed germination and increasing fruit size.


Cytokinins are produced in roots and promote growth, similar to auxins and gibberellins. They stimulate cell division, ensuring normal stem and root growth. Unlike auxins, cytokinins stimulate lateral buds to grow into branches and delay aging in plants. 


Produced in mature green leaves, fruits, and root caps, abscisic acid acts as a growth inhibitor that generally opposes the effects of auxins and gibberellins. It:

  1. Suppresses bud growth
  2. Induces dormancy in seeds and buds
  3. Promotes leaf aging
  4. Regulates the opening and closing of stomata, the tiny openings on leaves

The effects of abscisic acid help plants withstand adverse environmental conditions.


Ethylene, a simple hydrocarbon, is produced in leaves, stems, and young fruits. It inhibits lateral bud development and accelerates fruit ripening.


Both natural and synthetic plant hormones find applications in horticulture and agriculture. Some common uses include:

  1. Artificial vegetative propagation: Auxins are used in rooting powders to promote root formation in cuttings. Synthetic auxins aid in grafting by inducing wound tissue formation.
  2. Weed control: Synthetic auxins are used as selective herbicides to control weeds (e.g., 2,4-D).
  3. Harvesting: Auxins are applied to extend the shelf life of fruits on the plants.
  4. Parthenocarpy: Auxins and gibberellins induce fruit development without fertilization.
  5. Preservation: Cytokinins are used to prevent yellowing in stored vegetables, while growth inhibitors (e.g., abscisic acid) prevent sprouting in onions and potatoes.

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