A comprehensive, detailed study guide of all the reproduction section content required for the IEB Life Science paper 2 final examination. it includes the human endocrine system, reproduction of flowering plants, diversity of reproductive strategies in some animals and the human reproductive system
HUMAN ENDOCRINE SYSTEM:
● The environment in and around the body is continuously changing
● Homeostasis - the maintenance of a constant internal equilibrium
● The nervous system and hormones enable animals to respond to external changes and to control conditions inside
their bodies
● The endocrine system consists of endocrine glands that secrete hormones which are organic compounds that act as
chemical messengers
● Hormones interact continuously by means of hormone secretions which maintain homeostasis and control and
coordinate all metabolic activities
● Chemical coordination - the type of control and coordination brought about by hormones in the blood
Endocrine glands: Exocrine glands:
● Ductless glands that release their secretions directly into ● Glands that release their secretions via ducts to a
the bloodstream body cavity or to the outside
● The blood transports the hormones to the target organs ● Eg: Saliva in the mouth
where they perform their function
● Eg: pituitary gland
Hormones:
● Organic chemical messengers
● Usually proteins, however some are steroids (lipid-like organic compounds)
● Secreted in small quantities directly into the bloodstream by endocrine glands
● Transported to target organs by the bloodstream
● Target organs - an organ that is a ected by a specific hormone and that has receptors for a specific hormone,
enabling it to respond to the hormone
● Hormones have a regulatory function that can be stimulating or inhibiting (suppressing)
● They may be related to positive/negative feedback mechanisms
Negative feedback:
Any deviation from the normal levels of a substance or temperature elicits a response that resources the normal levels
Example: hormone A and B
1. Hormone A stimulates the secretion of hormone B - Hormone B levels increase
2. Hormone B inhibits the secretion of Hormone A - Hormone A levels decrease
3. The lower levels of Hormone A reduces the stimulation for Hormone B - Hormone B levels decrease
4. The lower levels of Hormone B no longer inhibit the secretion of Hormone A - Hormone A levels return to normal
5. Thus both hormones are kept in balance through ongoing cycles
Nervous system vs endocrine system:
● Nervous system - rapid response to the external environment
● Endocrine system - delayed response with the use of chemical
coordination to the internal and external environments
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, ENDOCRINE GLANDS AND THEIR HORMONES:
1. HYPOPHYSIS/PITUITARY GLAND
○ Located at the base of the brain, attached to the hypothalamus by a short stalk
○ The hypothalamus is the part of the brain located just below the corpus callosum and
it controls the functions of the hypophysis. It is the link between the endocrine and
nervous systems
○ The hypophysis is known as the master gland in the body as it secretes many
hormones that a ect many of the other endocrine glands
○ Functions of the anterior lobe (adenohypophysis): thyroid, adrenal, reproductive
○ Functions of the posterior lobe (neurohypophysis): oxytocin and antidiuretic hormone
Relationship between the hypothalamus and the hypophysis:
● Connected by the blood vessels to the anterior lobe and by neurons to the posterior lobe
● The hypothalamus produces hormones that are transported via the neurons to the posterior lobe of the hypophysis by storage
● Thy hypothalamus controls the hypophysis by secreting released hormones via the neurons (to the posterior lobe) or via the
blood (to the anterior lobe). The releasing factors stimulate the hypothesis to produces and release hormones into the blood
Hormones of the anterior lobe:
TSH (thyroid stimulating hormone)
➢ TSH stimulates the the thyroid to secrete the hormone thyroxine
Growth hormone (STH/somatotropic hormone)
➢ Growth hormone promotes the growth of the skeleton and muscles by stimulating the synthesis of proteins
➢ The under-secretion of growth hormone in children causes dwarfism. The body is very short, with body parts in
proportion. Mental ability is normal, not the person does not reach sexual maturity
➢ The over-secretion of growth hormone in children causes gigantism. Usually the condition is the result of a tumor in the
hypophysis. The over-secretion of growth hormone in adults causes acromegaly. The long bones in the body have
completed growth and cannot lengthen anymore. The growth plates in the epiphyses of the long bones close during
puberty. The bones of the face, especially the jawbone, hands and feet, increase in size
FSH (follicle-stimulating hormone)
➢ FSH stimulates the development of primary follicles in the female ovary into mature Graafian follicles
➢ In males, it activates the germinal epithelium that produces sperm cells in the testes
LH (luteinising hormone)
➢ In females LH stimulates ovulation (release of the secondary oocyte) and the development of the corpus luteum in the
ovary
➢ In males it stimulates the cells of Leydig in the
testes, to produce the male hormone testosterone
Prolactin
➢ Prolactin stimulates the mammary glands to
produce milk after the birth of a baby
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,Hormones of the posterior lobe:
ADH (antidiuretic hormone)
➢ Controls the amount of water in the body and prevents dehydration
➢ ADH is produced in the hypothalamus
➢ ADH is transported to the posterior lobe of the hypophysis via neurons for storage
➢ When the volume of water in the bloodstream decreases, the osmoreceptors in the hypothalamus detect the change
and stimulate the neurons that connect the hypothalamus the the hypophysis
➢ The neurons stimulate the posterior lobe of the hypophysis to release stored ADH
➢ ADH controls the reabsorption of water by making the walls of the renal tubules (kidneys) and collecting ducts more
permeable to water. More water is reabsorbed from the filtrate and less water excreted with the urine
➢ High concentration of urine = high level of ADH & low concentration of urine = low ADH
Oxytocin
➢ Oxytocin is produced in the hypothalamus and released by the posterior lobe of the hypophysis
➢ It causes the contractions necessary for the birth process
➢ Contractions of the uterus stimulates the hypophysis to release more oxytocin
➢ Oxytocin stimulates contractions of the uterus which then stimulates the release of more oxytocin
➢ This is an example of positive feedback, where increasing levels of one hormone stimulates the increase of more of the
same hormone
➢ Positive feedback continues after the baby is born
2. THYROID:
● The thyroid consists of two lobes located on either side of the trachea, just below the larynx
● The thyroid produces and secretes the hormone thyroxin
● Simple goitre - the shortage of iodine in the body which leads to an enlarged thyroid
● Thyroxine - functions to cease the basal metabolic rate (the amount of energy the body needs when it is at rest)
● It increases the breathing rate and heart rate
● It is essential for normal growth and development of organs
● It is essential for normal functioning of the nervous system
Thyroid disorders:
● Hyperthyroidism - the over-secretion of thyroxin which causes an increase in metabolic rate, weight loss, increased
heart rate, increased respiration rate, increased blood pressure and increased irritability
● Hypothyroidism - the under-secretion of thyroxin which causes a very low metabolic rate. In children it leads to
Cretinism in which the child does not grow normally, is mentally disabled and does not reach sexual maturity. In adults
it leads to myxoedema characterised by mental decline, increase in body mass and thickening of skin.
Control of thyroxin levels in the blood:
● The amount of thyroxin in the blood is kept constant by a negative feedback mechanism between the hypophysis and
thyroid
● As soon as the thyroxin levels in the blood drop below the normal value, the hypophysis secrets more TSH
● TSH stimulates the thyroid to secrete more thyroxin and the thyroxine levels in the blood increase
● If the thyroxin levels rise above normal, the secretion of TSH is inhibited
● The lower levels of TSH levels results in the decreased thyroxin production
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, 3. PANCREAS
● The pancreas is made up of two types of cells: exocrine pancreatic cells and small groups of endocrine cells, the islets
of Langerhans, which are interspersed between the exocrine pancreatic cells
● The pancreas functions as both an exocrine and endocrine gland
● The exocrine pancreatic cells are so named due to their exocrine
function as they excrete pancreatic juice directly into the
pancreatic duct that leads to the duodenum (upper intestine).
Pancreatic juice contains various digestive enzymes that aid
chemical digestion
● The islets of Langerhans have an endocrine function as they secrete
hormones directly into the bloodstream
● Protease - protein
● Lipase - fats & lipids
● Amylase - carbohydrates
Hormones of the pancreas:
● The islets of Langerhans consist of two types of cells: alpha and beta cells
● Alpha cells produce glucagon
● Beta cells produce the hormone insulin
● Normal glucose level in humans - 3.5 - 5.5 mmol/litre
● Insulin is secreted when the blood glucose level rises above normal and it decreases the glucose level
● Glucagon is secreted when the blood glucose level drops below normal and it increases the glucose level
● Insulin and glucagon are antagonistic hormones with opposite e ects on the body
Flow diagram representing the control of glucose in the blood (negative feedback mechanism)
Doping in sport:
● Both Growth Hormone (GH) and insulin have been used as performance-enhancing drugs in the sports world. They regulate
the body’s metabolism and have an anabolic e ect on muscle development
● Athletes use these hormones to decrease body fat and increase muscle mass.
● It is di cult to distinguish between the changing levels of naturally occurring hormones and artificially injected drugs and
more advanced methods of drug tracing are being developed by scientists to ensure fair competition
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