Hormones – Plant hormones and growth regulation.Animal hormones and their functions.


Hormones are chemical messengers that are secreted directly into the blood, which carries them to organs and tissues of the body to exert their functions. There are many types of hormones that act on different aspects of bodily functions and processes. Some of these include:

  • Development and growth
  • Metabolism of food items
  • Sexual function and reproductive growth and health
  • Cognitive function
  • Mood Maintenance of body temperature and thirst

Animal hormones

Endocrine system is the system formed by ductless glands which secrete chemical substances called as hormones. Endocrine glands release hormones directly into the blood.

Endocrine glands Different types of endocrine glands present in our body are the pituitary gland, the pineal gland, the hypothalamus, the thyroid, the parathyroid, the thymus, the adrenal gland, the pancreas, the testes and the ovary.

The pituitary gland     

  • It is a pea-shaped gland located at the base of the brain.
  • It is considered to be master gland as it secretes many hormones to regulate the organs as well as the other glands.
  • Different hormones secreted by this gland include Growth hormone, TSH, FSH, LH, ACTH, MSH, Vasopressin and Oxytocin.

The hypothalamus  

  • It is a neuro-endocrine part of the brain.
  • It links the nervous system and the endocrine system through the pituitary gland.
  • Different hormones secreted by this gland include TRH, GnRH, GHRH, CRH, Stomatostatin, Dopamine.


The thyroid gland

  • It is located in the neck, ventral to the larynx.
  • It is the one of the largest endocrine glands.
  • The principal hormones produced by this gland are triiodothyronine and thyroxine.

Thyroxine is a hormone that regulates the metabolism of carbohydrates, proteins and fats in the body.  Hyposecretion of thyroxine leads to cretinin in children, and myxoedema in adults.  Hypersecretion of thyroxine leads to exopthalmic goitre in adults.  Goitre is caused due to deficiency of iodine in food.  Iodine is essential for the synthesis of thyroxine.

Parathyroid glands      

  • These are two pairs of small, oval-shaped glands embedded on the dorsal surface of the thyroid gland present in the neck.
  • They secrete parathormone.
  • Parathormone helps in regulation of calcium and phosphate ions in the bones and blood.
  • Hyposecretion leads to parathyroid tetany and hypersecretion causes osteoporosis.

The adrenal glands  

  • These are located above the kidneys and hence are called as suprarenal glands.
  • Two regions of the adrenal gland are adrenal cortex and adrenal medulla.
  • Adrenal cortex secretes the hormones like cortisol, aldosterone and androgens.
  • Adrenal medulla secretes the hormones like adrenaline and noradrenaline.
  • Adrenaline is also called the “hormone of fight or flight,” or the emergency hormone.
  • It prepares the body to face an emergency condition of physical stress, like danger, anger and excitement.

The pancreas

  • It is located just below the stomach within the curve of the duodenum.
  • It is both exocrine and endocrine in function.
  • It secretes hormones such as insulin, glucagon, somatostatin and pancreatic polypeptide.
  • Insulin regulates the sugar level in our blood. Insulin  secreted in small amounts increases the sugar level in our blood which in turn causes a disease called diabetes mellitus.



  • Two types of gonads present in human beings are female gonads and male gonads. Female gonads
  • A pair of ovaries forms the gonads in female.
  • Ovaries are the female sex organs that lie one on either side of the abdominal cavity. Ovaries produce two hormones, namely, oestrogen and progesterone.
  • Oestrogen controls the changes that occur during puberty, like feminine voice, soft skin and development in mammary glands.
  • Progesterone controls the uterine changes in the menstrual cycle, and helps in the maintenance of pregnancy.

Male gonads

  • A pair of testes forms the gonads in males.
  • A pair of testes is the male sex organ located in the scrotum, which is outside the abdomen.
  • Testes produce the hormone testosterone.
  • Testosterone controls the changes, which occur during puberty, like deeper voice, development of penis, facial and body hair.

The pineal gland       

  • It is located near the centre of the brain, dorsal to the diencephalon.
  • It produces the hormone melatonin.
  • Melatonin affects reproductive development, modulation of wake and sleep patterns, and seasonal functions.

The thymus gland

  • It is located in front of the heart, in the upper part of the sternum.
  • It produces the hormone thymosine.
  • It helps in the maturation of T-lymphocytes.


Plant hormones

Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, moved to other locations, in other functional parts of the plant. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit.

There are five major types of plant hormones: auxin, gibberellin, cytokinin, ethylene, and abscisic acid.



Auxins are a powerful growth hormone produced naturally by plants. They are found in shoot and root tips and promote cell division, stem and root growth. They can also drastically affect plant orientation by promoting cell division to one side of the plant in response to sunlight and gravity.

Auxins have four key effects on plant growth:

  • Stimulating shoot elongation: Auxins positively influence gibberlins that promote cell elongation. This increases plant length. Essentially, gibberlins and thereby auxins, increase the distance between nodes, spacing the branch points further apart.
  • Controlling seedling orientation: It was the infamous Charles Darwin and his son Francis who first noticed that seedlings bend toward the light. However, whether a new shoot grows into the soil or towards light, depends on where auxins are located and how they influence cells within the plant. Auxins will move downward due to gravity and laterally, away from light. Cells grow more in areas of the plant where auxins are highly concentrated.
  • Stimulating root branching : When an auxin is applied to a cut stem, the stem will initiate roots at the cut.
  • Promoting fruit development: Auxins in the flower promote maturation of the ovary wall and promote steps in the full development of the fruit.

Auxins can be produced naturally (by the plant) or synthetically (in a lab). When produced synthetically, they can be used in high concentrations as a pesticide, causing drastic growth. The herbicide, 2-4-D, is an example of an auxin-based pesticide, specifically engineered to cause dicots (plants like dandelions) to grow quickly and uncontrollably, ultimately killing the plant.

Gibberellin: Gibberellin causes some similar effects in plants as auxin, but it is a very different hormone. Gibberellins were discovered originally in Japan. A fungus called Gibberella fujikuroi infected rice plants and caused them to grow too tall and fall over. The infectious fungus produced a chemical that stimulated the growth in rice plants. The chemical was isolated and named Gibberellin after the fungus. It was later found that plants naturally produce variations of these chemicals!  Gibberellins play an important role in several developmental stages in plants, but their claim to fame is making stems longer. Gibberellins promote stem elongation between nodes on the stem. A node is a place on a stem where a leaf attaches, so gibberellins elongate the internodes. It is easiest to see the absence of gibberellin in dwarf plants and rosette plants – there is very little space between nodes on a stem and the leaves are clustered toward the base of the plant.  What’s the big deal about knowing how to control stem elongation in plants? Well, when would it be beneficial to know how to make a plant stem shorter or longer? Biologists can prevent plants in a greenhouse from making gibberellins to keep them a manageable size. That’s handy. Or what if you’re a farmer and your business is something that comes from the stem of a plant? Longer stems would mean more profit for you, right? Gibberellins sprayed on sugar cane in Hawaii elongate the stem between the nodes. Longer stems mean more stored sugar. More sugar to sell means more coin! Knowing about plant hormones just makes cents.

Cytokinins: Cytokinins are a group of hormones that promote cell division in plant roots and shoots and the growth of buds. These hormones have been found in all complex plants as well as mosses, fungi, and bacteria. There are about 200 different natural and synthetic  cytokinins known to botanists today.  Most cytokinins are produced in the meristem of the roots. Meristem is the name for a region of tissue within the plant that actively promotes cell division. In other words, the meristem is any place that’s still growing (like the tip of the roots or the top of the stem).  Once the cytokinin has been produced in the roots, it travels up the xylem, or vascular tissue, to other parts of the plant where continued growth takes place (such as young leaves, developing fruits, and seeds).

Cytokinins increase cell division by stimulating the production of proteins needed for mitosis. Mitosis is non-sexual cell division that occurs in all living things producing additional cells for body growth.  In your body, mitosis is occurring every day, replacing dead and damaged cells and allowing for growth. If you skin your knee, it’s mitosis that grows back the cells you lost.  In plants, mitosis creates additional cells that make the plants grow. If you have ever played with building blocks that snap together, you can think of them like plant cells. Every time the process of mitosis occurs, a new cell is formed and moved to the end of the plant making it longer or taller (just like adding a building block to your structure).

Ethylene: Ethylene serves as a hormone in plants. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves. Commercial ripening rooms use “catalytic generators” to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening (20 °C; 68 °F) has been seen to produce CO2 levels of 10% in 24 hours.

Ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seeds. Ethylene production is regulated by a variety of developmental and environmental factors. During the life of the plant, ethylene production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers. Ethylene production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators. The pathway for ethylene biosynthesis is named the Yang cycle after the scientist Shang Fa Yang who made key contributions to elucidating this pathway.

Abscisic acid: Abscisic acid, is involved in many developmental plant processes, including leaf abscission, responding to environmental stress, and inhibiting fruit ripening. Abscisic acid is produced in the roots of the plant as well as the terminal buds at the top of the plant.

Functions of Abscisic Acid: The following are some of the physiological responses known to be associated with abscisic acid:

  • Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis).
  • Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. Induces seeds to synthesize storage proteins.
  • Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase.
  • It Has some effect on induction and maintanance of dormancy.
  • It Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.
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