Hormones

Hormones are chemical substances produced by glands and carried in the blood to specific target organs, where they alter activity. Some, like thyroxine and growth hormones, affect all body cells. Hormones act as chemical messengers, regulating homeostasis, growth, and development. After their function, hormones are inactivated in the liver and excreted in urine.

Functions of Hormones

1. Homeostasis: Hormones regulate the internal environment's composition.
2. Coordination: They coordinate organ functions to meet specific conditions, such as adrenaline preparing the body for danger.
3. Growth: Hormones stimulate and control growth and other unconscious body activities.
4. Sexual Maturity: They regulate processes like growth and the development of sexual maturity characteristics.
5. Cellular Changes: Hormones enable processes like nutrient absorption and waste preparation for excretion.

Endocrine Glands

Endocrine glands in vertebrates include:

1. Pituitary Gland: Located below the hypothalamus, this "master gland" controls other glands and secretes hormones like:
   • Gonadotropins: Follicle-stimulating hormone (stimulates egg/sperm production) and luteinizing hormone (triggers ovulation and testosterone production).
2. Thyroid Gland: Found in the neck, it produces thyroxine, which regulates growth and development by increasing the basal metabolic rate, and calcitonin, which lowers blood calcium levels.
3. Parathyroid Glands: Four small glands attached to the thyroid produce parathormone, which raises blood calcium by:
   • Releasing calcium from bones.
   • Reducing calcium excretion by the kidneys.
   • Increasing calcium absorption in the gut.
4. Pancreas: This gland produces insulin and glucagon:
   • Insulin: Lowers blood glucose by stimulating liver cells to store glucose as glycogen.
   • Glucagon: Raises blood glucose by converting glycogen to glucose.
5. Adrenal Glands: Located above the kidneys, they produce:
   • Corticoids: Regulate glucose and sodium levels.
   • Adrenaline: Prepares the body for action by increasing heart rate, respiration, and glucose availability.

Sex Hormones

Reproductive organs produce hormones like testosterone, oestrogen, and progesterone, influenced by pituitary gonadotropins.

Testosterone

Testosterone stimulates:

1. Growth and development of male reproductive organs.
2. Male characteristics like muscle growth, facial hair, and a deeper voice.

Oestrogen

Oestrogen functions include:

1. Development of female secondary sexual characteristics.
2. Regulation of the menstrual cycle.

Progesterone

Progesterone functions include:

1. Inhibiting ovulation during pregnancy.
2. Maintaining the uterine lining for embryo development.

Plant Hormones

Plants use hormones to coordinate growth and respond to environmental stimuli like light, gravity, and touch. Unlike animals, plants do not have specialized hormone-producing tissues.

Types of Plant Hormones

Auxins

Auxins, like indole acetic acid (IAA), are produced at shoot tips and influence cell division, elongation, and differentiation.

Functions:

• Promote stem growth toward light and root growth toward gravity.
• Stimulate lateral root development.
• Encourage fruit development and seed germination.

Gibberellins

Found in roots and stems, gibberellins promote growth and stimulate seed germination, bud growth, and fruit enlargement.

Cytokinins

Produced in roots, cytokinins work with auxins to stimulate cell division and delay plant aging.

Abscisic Acid

Produced in leaves, fruits, and roots, abscisic acid inhibits growth and induces dormancy during unfavorable conditions.

Ethene (Ethylene)

This gas promotes fruit ripening and inhibits lateral bud growth.

Uses of Plant Hormones in Agriculture

• Vegetative Propagation: Auxins in rooting powders encourage root formation in cuttings.
• Weed Control: Synthetic auxins act as selective herbicides.
• Preservation: Cytokinins prevent yellowing in stored vegetables.
• Parthenocarpy: Auxins and gibberellins induce seedless fruit production.