All Hormones and Their Functions - Class 11 & 12 Biology - NEET 2023

Table of Contents:

• Hormones
• Auxins
• Gibberellins
• Cytokinins
• Ethylene
• Abscisic Acid
• Endocrine System
• Hypothalamus
• Pituitary
• Pineal
• Thyroid
• Parathyroid Gland
• Thymus
• Adrenal Glands
• Pancreas
• Testis
• Ovary
• Hormones of Heart
• Hormones of Kidney
• Hormones of Gastrointestinal Tract

Hormones

Hormones are non-nutrient chemicals that act as intercellular messengers and are usually produced in trace amounts. There are two types of hormones discussed in this article:

1. Phytohormones or Plant Hormones
2. Animal Hormones (Only Humans)

Phytohormones

These are organic compounds that give signals to regulate and control the growth of plants. They are also known as plant growth regulators, plant growth substances or plant hormones. They are derivatives of gases (ethylene), terpenes (gibberellins), indole (auxins), carotenoids (abscisic acid) and adenine (cytokinins). These hormones can be broadly classified as growth promoting and growth inhibiting hormones.

Phytohormones promote growth by means of cell elongation, cell division, organ development, formation of reproductive structures, tropic growth, seed formation, fruiting and flowering. Likewise, they inhibit growth by means of abscission and dormancy.

Animal Hormones

In Animals (Humans), this chemical messenger (hormone) is synthesised by specialised glands called the endocrine glands. These glands are distributed throughout the body and are vital in maintaining homeostasis. Oestrogen, Progesterone, Cortisol and Melatonin are some important human hormones.

In this article, let’s learn more about the endocrine glands and the hormones secreted by them, along with a brief note on phytohormones.

Auxins

Auxin comes from the Greek word ‘Auxein’ which means ‘to grow’. Naturally occurring auxins include Indole-3-acetic acid (IAA). The other endogenous auxins are indole-3-butyric acid (IBA), indole-3-propionic acid (IPA), phenylacetic acid and 4 chloroindole-3-acetic acid (4-Cl-IAA).

Auxin was first isolated from human urine. These hormones are generally found in the growing apices of stems and roots. Auxins can be classified into 2 types –

• Natural Auxins – IAA, IBA, 4-Cl-IAA, etc.
• Synthetic Auxins – 2,4-dichlorophenoxyacetic acid, Naphthalene acetic acid.

Functions

The two major functions of auxins are –

• Tropic movements – Phototropism and gravitropism
• Developmental effects – Apical dominance, feminising effect, root initiation, flower initiation, delay of abscission, parthenocarpy, xylem differentiation and cell division.

Apical Dominance – It is an adaptive feature seen in plants where the auxin produced in the apical buds prevents the growth of the lateral buds. It helps in keeping the central stem dominant and there is a loss of lateral buds.

Applications of Auxin

• It is used in tea plantations and hedge making.
• Used in plant propagation and initiation of rooting in stem cuttings.
• Used in producing seedless fruits. Example – Seedless tomatoes.
• It is also used as a herbicide. Example – 2,4-D.

Gibberellins

Gibberellins (GAs) are one of the longest known phytohormones. GAs are tetracyclic diterpene acids. There are around 136 GAs that have been identified from fungi, plants and bacteria. The functions of gibberellins are as follows –

• Stimulates stem growth – It promotes bolting, i.e., internode elongation just prior to flowering in beetroot and cabbages.
• It promotes seed germination.
• GAs break bud dormancy.
• It also delays senescence.

Applications

• It is used to increase the length of the stem and thereby increase the yield.
• GAs are used to speed up the malting process of barley in breweries.
• It is used to induce flowering in many conifers.
• GAs are also used to improve fruit shape and size.

Cytokinins

Cytokinins were discovered as kinetin (a modified form of adenine, a purine). This kinetin does not occur naturally in plants. Cytokines, along with auxins, promote cell divisions. As per their name, they play a vital role in the cytokinesis process. Naturally, cytokinin can be found as zeatin in corn kernel and coconut milk. The natural cytokinins occur in regions of rapid cell division like the developing shoot buds, root apices and young fruits.

Functions

• It regulates cell division. The auxin:cytokinin ratio plays an important role in regulating morphogenesis.
• It is used to overcome apical dominance and promote lateral growth.
• It promotes chloroplast development. It also helps to produce new leaves and adventitious shoots.
• It enhances the mobilisation of nutrients.
• It delays senescence.

Ethylene

Ethylene is a gas that naturally acts as a plant hormone. It is a simple alkene gas with the following functions:

Effects on seeds and seedlings

• Breaks seed dormancy
• Initiates germination
• Horizontal growth of seedlings
• Swelling of embryonic axis
• Apical root formation in dicot seedlings

Effects on the shoot

• Breaks bud dormancy
• Sprouting of potato tubers
• Internode or petiole elongation in deep-water rice plants

Effects on the root

• Root hair formation
• Root growth

Effects on fruits and flowers

• Induces flowering in mangoes, pineapples
• Promotes female flowers in cucumbers and hence increases yield
• Synchronises fruit-set in pineapples
• Ripening of fruits and enhances respiration rate during ripening

Effects on various organs

• Senescence
• Abscission

Applications

• It is used in agriculture for thinning of fruits like apple, cherry, walnut and cotton
• It is commercially used to ripen fruits after picking

Abscisic Acid

Abscisic acid (ABA) was discovered for its role in regulating dormancy and abscission. In the mid 1960s, 3 different types of inhibitors were identified and characterised. They are – inhibitor ꞵ, dormin and abscisin Ⅱ

Effects

• They are general plant growth inhibitors and are considered as inhibitors of plant metabolism.
• Increases tolerance of plants to various stresses.
• It stimulates the closure of stomata.
• It is an antagonist to GAs.
• It inhibits seed germination.
• It is also responsible for seed development, maturation and dormancy.

Endocrine System

It is a system of endocrine glands and hormone-producing diffused tissues or cells present in different parts of our body. These endocrine glands that make up the endocrine system are ductless glands. They release their secretions (hormones) directly into the bloodstream.

The major glands of the endocrine system includes – hypothalamus, pituitary, pineal gland, thyroid, parathyroid, thymus, adrenal glands, pancreas and gonads.

Apart from these major glands, there are also other parts in our body that have secondary endocrine functions. This includes heart, liver, kidney, bones and gastrointestinal tract.

Hypothalamus

Hypothalamus is a part of the brain that acts as a connecting link between the nervous system and the endocrine system. It synthesises and secretes inhibiting hormones and releasing hormones whose main role is to control the release of other hormones by either inhibiting or stimulating their release.

Releasing hormones – It stimulates the secretion of pituitary hormones. Example – Gonadotropin releasing hormone (GnRH).

Inhibiting hormones – Inhibits the secretion of pituitary hormones. Example – Somatostatin.

Pituitary Gland

Pituitary gland can be anatomically divided into adenohypophysis (anterior pituitary lobe) and neurohypophysis (posterior pituitary lobe). The anterior lobe can further be classified into pars tuberalis, pars distalis and pars intermedia. Par tuberalis is just a sheath-like structure that extends from the pars distalis and joins with the pituitary stalk. Its function is not fully understood. The pars intermedia lies between the pars tuberalis and pars distalis.

Pituitary Hormones

• Pars intermedia secretes MSH (melanocyte stimulating hormone) that acts on the melanin containing cells (melanocytes) and regulates pigmentation of the skin.
• Pars distalis produces GH (growth hormone) which results in gigantism in terms of over secretion and dwarfism in terms of low secretion. PR or prolactin regulates the growth of mammary glands and formation of milk. Likewise, TSH (thyroid-stimulating hormone) stimulates the synthesis and secretion of thyroid hormones from the thyroid gland. The ACTH (adrenocorticotropic hormone) stimulates the synthesis and secretion of steroid hormones called glucocorticoids from the adrenal cortex.
• The pars distalis also produces LH (luteinizing hormone) and FSH (follicle-stimulating hormone). The LH in males stimulates the synthesis and secretion of hormones called androgen from testis. The LH in females induces ovulation of fully mature graafian follicles. It also maintains corpus luteum formed from the remnants of the graafian follicles. Likewise, the FSH and androgen in males regulate spermatogenesis. The FSH in females stimulates the growth and development of the ovarian follicles.
• Majority portion of the posterior pituitary or neurohypophysis is constituted by the pars nervosa or the neural lobe. It stores and releases oxytocin and vasopressin. These are actually synthesised by the hypothalamus and are transported axonally to the neurohypophysis.
• Oxytocin acts on the smooth muscles and stimulates their contraction. In females, it stimulates vigorous contraction of the uterus at the time of child birth. It also stimulates milk ejection from the mammary gland.
• Vasopressin acts at the kidney and stimulates reabsorption of water and electrolytes by the distal tubules. It reduces the loss of water through diuresis (urine). Hence, it is also called antidiuretic hormone(ADH).

Pineal Gland

The pineal gland is also located in the brain. It secretes a hormone called melatonin that regulates the 24-hour (diurnal) rhythm of the body. The pineal gland regulates the following –

• Diurnal rhythm
• Body temperature
• Metabolism
• Menstrual cycle
• Body defence

Thyroid

Thyroid gland is composed of follicles and stromal tissues. The thyroid follicles are composed of follicular cells that synthesise two hormones – T₄(thyroxine or tetraiodothyronine) and T₃ (triiodothyronine).

Thyroid Disorders

• Deficiency of iodine results in thyroid related problems like hypothyroidism. It causes enlargement of the thyroid gland, commonly called the goitre. During pregnancy it can cause cretinism (stunted growth of growing baby), mental retardation, low intelligence quotient, deaf-mutism, abnormal skin, etc. In adult women hypothyroidism may cause the menstrual cycle to become irregular.
• Thyroid cancer or development of nodules of thyroid glands increases the rate of synthesis and secretion of thyroid hormones. An abnormally high level secretion of thyroid hormone results in hyperthyroidism.

Other Roles of Thyroid Hormone

• It regulates the basal metabolic rate.
• It supports the process of red blood cell formation.
• It controls the metabolism of proteins, carbohydrates and fats.
• It maintains the water and electrolyte balance.
• Thyroid gland also secretes a protein hormone called TCT (thyrocalcitonin) which regulates the blood calcium levels.

Parathyroid Gland

Parathyroid gland secretes a peptide hormone called parathyroid hormone (PTH) which regulates the circulating levels of calcium ions. This PTH increases the calcium levels in the blood. It acts on bones and stimulates the process of bone resorption (demineralisation/dissolution). It stimulates the reabsorption of calcium by the renal tubules and increases the calcium absorption from the digested food.

Thymus

Thymus secretes peptide hormones called the thymosins. Thymosins play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity. It also promotes the production of antibodies to provide humoral immunity.

In old individuals, the thymus is degenerated resulting in a decreased production of thymosins.

Adrenal Glands

Adrenal glands are a pair of glands located on top of the kidneys. They are composed of two types of tissues – adrenal medulla (centrally located tissue) and adrenal cortex (outside tissue).

• The adrenal medulla secretes 2 hormones – adrenaline or epinephrine and noradrenaline or norepinephrine.
• Adrenaline and noradrenaline are rapidly secreted during emergency situations and are called emergency hormones or hormones of fight or flight.
• The chromaffin cells of the medulla secretes catecholamines that stimulate the breakdown of glycogen resulting in an increased concentration of glucose in blood.
• The adrenal cortex secretes many hormones called corticoids – glucocorticoids and mineralocorticoids.
• Cortisol is the main glucocorticoid that is involved in maintaining the function of the cardiovascular system and kidneys. It produces anti-inflammatory reactions and suppresses the immune response. It also stimulates RBC production.
• Mineralocorticoids regulate the balance of water and electrolytes in the body. Aldosterone is the main mineralocorticoid in humans. It acts mainly at renal tubules by stimulating the reabsorption of water and sodium and the excretion of potassium and phosphate ions. It helps in the maintenance of body fluid volume, electrolytes, osmotic pressure and blood pressure.
• The adrenal cortex also secretes small amounts of androgenic steroids that play a vital role in the growth of axial hair, pubic hair and facial hair during puberty.

Pancreas

Pancreas is a composite gland that acts as both exocrine and endocrine gland. The endocrine pancreas consist of Islets of Langerhans. The two main types of cells in the islet of Langerhans are –

1. ɑ-cells that secretes glucagon
2. β-cells that secretes insulin

Glucagon is a peptide hormone that maintains normal blood glucose levels. It acts mainly on the liver cells (hepatocytes) and stimulates glycogenolysis resulting in an increased blood sugar (hyperglycemia). It also stimulates the process of gluconeogenesis which contributes to hyperglycemia. Glucagon reduces the cellular glucose uptake and utilisation. It is a hyperglycemic hormone.

Insulin is also a peptide hormone and it regulates glucose homeostasis. It acts mainly on hepatocytes and adipocytes and enhances cellular glucose uptake and utilisation. Rapid movement of glucose from blood to hepatocytes and adipocytes causes hypoglycemia. Insulin also stimulates conversion of glucose to glycogen (glycogenesis) in the target cells.

Testis

In males, a pair of testes are present in the scrotal sac. It performs dual functions as a primary sex organ as well as an endocrine gland. It is composed of seminiferous tubules and stromal or interstitial tissue. The interstitial cells or the leydig cells produce a group of hormones called androgens, mainly testosterone. Androgens regulate the development, maturation and functions of male accessory sex organs like epididymis, vas deferens, seminal vesicles, urethra, prostate gland, etc.

They have a major stimulatory role in the process of spermatogenesis. Androgens act on the central neural system and influence the male sexual behaviour (libido). These hormones produce anabolic (synthetic) effects on protein and carbohydrate metabolism.

Ovary

Ovary in females also produces two groups of steroid hormones – oestrogen and progesterone. Ovary is composed of ovarian follicles and stromal tissues.

• Oestrogen is synthesised and secreted mainly by the growing ovarian follicles. After ovulation, the ruptured follicle is converted into corpus luteum, which mainly secretes progesterone. Estrogen has a wide-range of actions like stimulation of growth and activities of female secondary sex organs, development of growing ovarian follicles, appearance of female secondary sex characters and mammary gland development.
• Progesterone mainly supports pregnancy. It also acts on the mammary glands and stimulates the formation of alveoli (sac-like structures that store milk) and milk secretion.

Hormones of Heart

The atrial wall of our heart secretes a peptide hormone called atrial natriuretic factor (ANF). It decreases blood pressure. When blood pressure is increased, ANF is secreted which causes dilation of the blood vessels. Thus, it reduces blood pressure.

Hormones of Kidney

Juxtaglomerular cells of the kidney produce a peptide hormone called erythropoietin. It stimulates the formation of RBC (erythropoiesis).

Hormones of Gastrointestinal Tract

The endocrine cells in the gastrointestinal tract secretes four major peptide hormones –

• Gastrin – It acts on gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen.
• Secretin – It acts on exocrine pancreas and stimulates secretion of water and bicarbonate ions.
• Cholecystokinin (CCK) – It acts on both pancreas and gallbladder and stimulates secretion of pancreatic enzymes and bile juice respectively.
• Gastric inhibitory peptide (GIP) – Inhibits gastric secretion and motility.

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