HORMONES AND ENDOCRINE GLANDS
Hormones are chemical substances produced or secreted by endocrine glands (ductless glands) in response to various stimuli. Hormones are secreted into the bloodstream and circulated to exert their effect on their target organs. This effect could be to speed up or slow down biological reactions.
A few hormones like thyroxine and other growth hormones exert their effects on all body cells. A hormone can therefore be defined as a chemical messenger that is produced in one part of an organism and brings about a specific effect in a target organ some distance away. Hormones (usually needed in small amount) are active in homeostasis, growth and development. After their actions, hormones are inactivated in the liver and excreted in urine.
EVALUATION
- What is hormone?
- State four characteristics of hormones
ANIMAL HORMONES
There are basically five groups of animal hormones which are secreted by different glands
PITUITARY HORMONE
The pituitary gland can be found below the hypothalamus. It consists of anterior and posterior parts. Both parts of the pituitary gland release hormones. The anterior pituitary gland secretes growth hormones and several tropic hormones that regulate the activities of other endocrine glands through homeostatic mechanism. Hence, the pituitary is called “master gland”. These hormones include
- Gonadotropins i. e. follicle stimulating hormone (which cause the ovary to produce mature eggs and the testis to produce sperms) and lutenizing hormone (which causes ovulation in female and production of testosterone in by the testis)
- Adrenocorticotropic hormone which stimulates the adrenal cortex to secrete corticosteroids
- Thyroid stimulating hormones
- Prolactin which stimulates the secretion of milk by the mammary gland
- Somatotropin which stimulates the growth of long bones and increases metabolic rate in the cells. Growth hormones are produced during childhood and adolescence.
Over secretion of growth hormones in children causes gigantism while under secretion causes dwarfism.
The posterior pituitary gland produces
- Anti-diuretic hormone (ADH)
- Oxytocin which stimulates the contraction of the uterine wall and dilation of the cervix during parturition and milk letdown from the nipples.
EVALUATION
- List four hormones secreted by the pituitary gland and their functions.
- Describe how the pituitary act as the master gland
THYROID HORMONES
The thyroid gland is located in the region close to the larynx (the neck region). It produces three main hormones, the most important of which is thyroxine (others are triiodothyronine and calcitonin). Thyroxine
- regulates the growth and development of all body cells.
- increases the rate at which glucose is oxidized in the body cells and increases heat production as well.
Under secretion of thyroxine causes cretinism that is observed in physically underdeveloped and mentally retarded children, sluggishness and goiter in adults. Over secretion of thyroxine causes hyper activeness and restlessness.
PARATHYROID HORMONE
Parathyroid glands are attached to the thyroid gland and are observed as four tiny glands. The hormone, called parathormone raises blood calcium level by:
- Releasing calcium from bone
- Increasing calcium absorption in the gut.
- Reducing calcium excretion by the kidney.
Calcitonin (a thyroid hormone) lowers the blood calcium level.
Over secretion of parathormone causes the bones to become fragile, soft and prone to fracture while under secretion causes muscle spasms.
EVALUATION
- What is the effect of under secretion and over secretion of thyroxine?
- List two parathyroid hormones and their functions.
PANCREATIC HORMONES
The pancreas is a digestive as well as an endocrine gland. Hence, it has both exocrine and endocrine functions. Most cells in the pancreas produce digestive enzymes. Among these cells are hormone – producing cells called Islets of Langerhans which produce two hormones, insulin and glucagon.
Insulin lowers the blood glucose level by stimulating the liver cells to convert excess glucose to glycogen for storage in the liver and muscles. Deficiency of insulin causes diabetes mellitus i. e. excretion of glucose in urine, less appetite and great thirst. Over secretion of insulin causes fall in blood sugar level and incessant hunger. Glucagon raises the blood glucose level by stimulating the liver to convert stored glycogen to glucose.
ADRENAL HORMONES
The adrenal glands are located above the kidney and they produce two distinct groups of hormones. The adrenal cortex produces corticoids while adrenal medulla produces adrenaline and noradrenaline (the emergency hormones). Corticoids include; glucocorticoids (which raise blood glucose level during stress e.g. cortisol) and mineralocorticoids which regulate the levels of sodium and potassium ions in body fluids e.g. aldosterone increases sodium ions absorption.
Adrenalin as an emergency hormone prepares the body for immediate action in time of fright, danger and anger by
- increasing muscular tone
- increasing heart beat rate and respiration
- aiding pupils dilation
- increases the rate of conversion of glycogen to glucose by the liver
Under secretion of adrenalin results in slow response to emergency, low blood pressure and heart beat while over secretion results in over anxiety and excitement.
EVALUATION
- Describe how adrenaline act as an emergency hormone
- What are the functions of pancreatic hormones?
REPRODUCTIVE HORMONES
Certain cells in the reproductive organs produce hormones. The testes produce testosterone, the male sex hormone and the ovaries produce oestrogen and progesterone (female sex hormones). Sex hormone production begins at puberty (a period in late childhood, between 10 and 14 year of age), then the production of sex hormone is greatly increased, causing the body to grow rapidly and change into a sexually mature form.
The production of sex hormones is stimulated by gonadrotropic hormones secreted by the pituitary gland
Male sex hormone, testosterone stimulates:
- the growth and maturation of the penis, testes and accessory sex
- the development of male sex characteristics such as muscular body, growth of hair in the pubic region, armpit, chest and the face and deepening of the voice.
Female sex hormone oestrogen which performs the following functions.
- brings about the development of secondary sexual characteristics such as the enlargement of the breasts, growth of hair in the pubic region and armpit, widening of hip and fats distribution.
- regulates the reproductive or menstrual cycle.
Progesterone
- inhibits egg production (ovulation) during pregnancy
- prepares and maintain the lining of the uterus
- aids implantation and development of the embryo in the uterus
Over-secretion of reproductive hormones causes excessive development of sexual organs and abnormal urge for sex while under-secretion results in poor development of secondary sexual characters, sexual organs and low urge for sex.
EVALUATION
- List three reproductive hormones and their functions
- Describe how female sex hormones controls the reproductive cycle
PLANT HORMONES
Co-ordination in plants is simpler than in animals. It is ensured by chemicals known as plant hormones. These are similar to animal hormones in that they are:
- only needed in small amount to bring about their effects.
- produced in one part of the body and transported to another part where they exert their effects.
Animal hormones are produced in specific glands and bring about response in specific target organs. In plants, hormones are not produced in tissues specialized for their production. Their effects are also more general varying with concentration and types of organs. Plant hormones stimulate or inhibit growth in the target tissues in response to external stimuli such as light, temperature, gravity and touch. These substances are known as plants growth hormones, but they affect other activities such as fruits formation, roots development and leaf fall. Combination of plant hormones brings about responses that are different from what each hormone will produce alone. This type of interaction is also common in animals. The responses of plant hormones are usually much slower than most of animals. This is because responses in plants are mainly brought about by growth.
TYPES OF PLANTS HORMONE
One of the most important groups of plants hormones are auxins. Other groups include gibberellins, cytokinins, abscisic acid and ethene (ethylene).
AUXINS
The most important naturally occurring auxin is indoleacetic acid (IAA). It is produced at the apices of a shoot and it is transported in one direction away from the tip. It moves across short distance by diffusion and longer distance through the phloem. Auxin influences cell division, elongation and differentiation. It
- influence the growth of stem towards light (positive phototropism) and root away from it (negative phototropism). Similarly, it causes the growth of roots towards the force of gravity (geotropism).
- stimulates the development of lateral and adventitious roots for increased water and mineral absorption.
- causes apical dominance by inhibiting the growth of lateral buds
- promotes the development of fruits.
- breaks dormancy in seeds ensuring early germination
- delay leaf fall
GIBBERELLINS
These plant hormones are found in root and stem apices. They promote growth by stimulating both cell elongation and cell division. They also stimulate growth in dwarf varieties. Other effects of gibberellins include
- Inducing dormant seeds to germinate
- Causing dormant auxiliary buds to grow
- Increasing fruit size.
CYTOKININS
Cytokinins are produced in roots. They are also growth promoting hormones like auxins and gibberellins. Together with auxins, they stimulate cell division so that stems and roots grow normally. Unlike auxins, they stimulate the lateral buds to grow into branches. They delay ageing in plants.
ABSCISIC ACID
This hormone is produced in mature green leaves, fruits and root caps. It is a growth inhibitor whose effects generally oppose both auxins and gibberellins. It:
- suppresses the growth of bud.
- induces dormancy
- brings about ageing in leaves.
- controls the opening and closing of stomata.
The effects of abscisic acid enable the plants to withstand severe environmental condition.
ETHENE
This is a simple hydrocarbon which is produced in leaves, stems and young fruits. It retards lateral bud development and hastens the ripening of fruits.
EVALUATION
- List four plants hormones and their functions
- Describe how auxins act as growth hormone in plant
APPLICATION OF HORMONES TO AGRICULTURE
Natural plant hormones and synthetic ones are used in horticulture and agriculture. The uses are:
- Artificial vegetative propagation: Auxins are used in rooting powders which are applied at the end of cutting to induce root formation. Synthetic auxins are used to knit together parts of plants after grafting by inducing wound tissue formation.
- Weed control: The synthetic auxin is used as a selective weed killer (herbicide) e. g. 2, 4 – D
- Harvesting: Auxins are used to ensure a longer stay of fruits on the plants.
- Parthenocarpy: Auxin and gibberellins causes flowers to develop into fruits without fertilization
- Preservation: Cytokinins used in storing vegetables prevent yellowing. Growth inhibitors (abscisic acid) are used to prevent onions and potatoes in stores from sprouting.
EVALUATION
Outline five applications of plant hormones to agriculture.
GENERAL EVALUATION
- What are hormones?
- What are ductless glands
- Outline five general functions of hormones
- Discuss the exocrine and endocrine functions of the pancreas
- Describe how the female sex hormones controls the reproductive cycle
- In two ways, show how plant and animal hormones are similar.
- State four differences between hormonal coordination in plants and animals
- How relevant are plant hormones to agriculture
WEEKEND ASSIGNMENT
- Which of the following statement is correct? Hormones are (a) secreted into the blood through ducts (b) secreted directly into the blood stream (c) inactive chemical substances in the blood stream (d) not important
- The pituitary is called “master glands” because (a) it is located in the brain (b) its secretions are more numerous than any other glands. (c) it secretion controls other glands (d) it is used in mastering learning.
- The condition known as cretinism is caused by the deficiency of (a) thyroxine (b) adrenaline (c) vitamin A (d) oxytocin.
- Auxins are produced in the (a) apical region of roots and shoot (b) edges of leaves (c) epidermis of roots (d) epidermis of shoots.
- All these are plant hormones except (a) thyroxine (b) auxins (c) abscisic acid (d) ethylene
THEORY
- State six effects of adrenaline as an emergency hormones on body organs
- In a tabular form state the gland, location, functions, effects of undersecretion and over secretion of (a) thyroxine (b) insulin (c) somatotropin
See also