Plant and Animal Hormones: Functions and Mechanisms
Hormones: Functions and Regulation
Hormones are substances that regulate chemical reactions. Hormones are produced in small quantities, synthesized where they are to act, and transported through the blood, saline, or air.
Phytohormones: Plant Growth Regulators
Phytohormones are organic compounds responsible for plant growth and development. They are synthesized in one part of the plant and typically act elsewhere, producing a physiological response at very low concentrations. Major stimuli affecting plant development include humidity, temperature, gravity, daylight hours, and contact with objects.
Plant Receptors: Sensing the Environment
Plants use pigments like phytochrome to sense red and far-red light, enabling them to detect gravity. Touch-sensitive cells capture tactile stimuli, triggering hormonal responses and rapid movements.
Plant Hormones: Types and Functions
- Auxins: Synthesized in apical buds. They regulate stem growth and root development. Light activates their mechanism of action.
- Cytokinins: Produced in roots. They promote lateral branch growth and prevent leaf aging. Red light and high temperatures activate them.
- Ethylene: Produced in fruits. It promotes fruit ripening. Auxin, water deficit, and wounds activate it, while abscisic acid inhibits it.
- Abscisic Acid: Found throughout tissues like leaves and stems. It controls stomatal opening and closing, inhibits plant growth and seed development. Drought, waterlogging, cold, and red light increase its concentration.
- Gibberellins: Found in stems and young leaves. They promote seed germination. Long photoperiods actively stimulate their effects.
Mechanism of Action of Auxin
Auxin, produced at the stem’s tip, causes its elongation through a process called acid growth. This effect allows the stem to curve towards light. Auxins migrate from the stem to the roots, promoting growth at low concentrations or inhibiting it at high concentrations.
The Feedback Mechanism: Hormonal Control
The feedback mechanism, controlled by the hypothalamus, maintains adequate hormone levels in the blood. Upon receiving stimuli, the hypothalamus releases neurohormones that target the pituitary gland. The pituitary then releases tropic hormones into the blood, which act on target glands. These glands synthesize specific hormones that reach and act on target organs.
The Hypothalamus: A Central Regulator
The hypothalamus, composed of nervous tissue, receives information from the cerebral cortex about external and internal stimuli. It synthesizes neurohormones that have stimulatory or inhibitory functions on the anterior pituitary gland.
Key Hormones and Their Functions
- Growth Hormone: Increases protein synthesis and stimulates bone growth.
- Prolactin: Stimulates milk synthesis in mammary gland cells after birth.
- Thyroid-Stimulating Hormone: Stimulates thyroid gland cells.
- Adrenocorticotropic Hormone: Stimulates cortisol production in the adrenal cortex.
- Follicle-Stimulating Hormone: Activates sperm production and stimulates oocyte maturation.
- Luteinizing Hormone: Stimulates testosterone production and triggers oocyte maturation and ovulation.
- Oxytocin: Stimulates uterine muscle contractions during childbirth and triggers milk release from mammary gland cells.
- Antidiuretic Hormone: Stimulates water reabsorption in kidney nephrons.