IB DP Biology Topic 6: 6.6 Hormones, homeostasis and reproduction: Study Notes

In the unit of Hormones, Homeostasis and Reproduction we will learn how the internal environment of the body must remain constant despite changes that occur in the external environment. Hormones are used in feedback mechanism to help control this environment. We will look at how these hormones can control blood glucose, our metabolic rate, our body temperature, our appetite and sleep cycles. We will also look at how hormones are used in reproduction.

​This unit will last 4 school days.

​Essential idea:

• Hormones are used when signals need to be widely distributed. 6.6

​Nature of science:

• Developments in scientific research follow improvements in apparatus—William Harvey was hampered in his observational research into reproduction by lack of equipment. The microscope was invented 17 years after his death. (1.8)
  • Describe what Harvey was and was not able to observe in his reproduction research given the tools available at the time.

6.6.U1 Insulin and glucagon are secreted by β and α cells of the pancreas respectively to control blood glucose concentration.

• Explain the control of blood glucose concentration, including the roles of glucagon, insulin and the alpha and beta cells in the pancreatic islets.

The body needs glucose to make ATP (via cell respiration), however the amount required will fluctuate according to demand. High levels of glucose in the blood can damage cells (creates hypertonicity) and hence glucose levels must be regulated

• Blood glucose concentration is carefully monitored by negative feedback mechanisms.
• Cellular respiration is constantly lowering blood glucose levels.
• Receptors in the pancreas sense when the blood glucose level is too low.
• Alpha (α) cells in the islets of Langerhans in the pancreas secrete glucagon into the bloodstream.
• Glucagon stimulates the liver to breakdown stored glycogen into glucose which is released into the bloodstream.
• Blood glucose levels rise back to their normal limits.
• After a person eats, digestion breaks large carbohydrates into glucose molecules.
• Glucose levels rise in the blood.
• If the glucose levels get too high, receptors sense the increased glucose levels causing the pancreas to secrete insulin by the Beta cells (β) of the islets Langerhans.
• Insulin stimulates the absorption of glucose from the blood into skeletal muscles and fat tissue, and thus allowing the liver to convert glucose into glycogen (animal carbohydrate storage molecule).
• Glucose levels decrease back to the normal range. 

6.6.U2 Thyroxin is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature.​ 

• Describe the structure and function of thyroxin.
• Outline thyroxin’s role in body temperature regulation.
• List symptoms of thyroxin deficiency.

Thyroxin is a hormone secreted by the thyroid gland of the endocrine system

• Thyroxin contains iodine; therefore, prolonged deficiency to iodine in the diet prevents the production of thyroxin
• Thyroxin is important in the regulation of the body’s metabolic rate
• The body’s metabolic rate is the amount of energy a body uses at rest; combination of the catabolic and anabolic reactions
• Since thyroxin causes an increase in the body’s metabolic rate, there is an increase in oxygen consumption and the hydrolysis of ATP; thereby causing an increase in the body’s temperature
• Increase in thyroxin stimulates the breakdown of lipids and the oxidation of fatty acids
• Thyroxin also stimulates carbohydrate metabolism, including the uptake of glucose and the breakdown of glycogen into free glucose
• In a regular person, if the bodies temperature drops, a release in thyroxin will stimulate heat production causing the body’s temperature to rise
• If there is an excessive amount of thyroxin in the body, hyperthyroidism can occur
• If there is an insufficient amount of thyroxin in the body, hypothyroidism can occur
• Some of the symptoms of hypothyroidism are weight gain, loss of energy, feeling cold all the time, forgetfulness and depression

6.6.U3 Leptin is secreted by cells in adipose tissue and acts on the hypothalamus of the brain to inhibit appetite.

• State that leptin is a protein hormone.
• Outline the mechanism of action of leptin.
• Describe the role and discovery of the ob allele in obese mice

6.6.U4 Melatonin is secreted by the pineal gland to control circadian rhythms.​

• Define circadian rhythm.
• Describe the secretion and action of melatonin.
• Outline the mechanism that regulates melatonin secretion in response to the day-night cycle.

Mel​atonin is a hormone produced by the pineal gland within the brain in response to changes in light​

• The secretion of melatonin by the pineal gland is controlled by cells in the hypothalamus
• Light exposure to the retina is relayed to the suprachiasmatic nucleus (SCN) of the hypothalamus. These fibers from the hypothalamus relay a message to the nerve ganglia of the spinal cord which is relayed back to the pineal gland to release melatonin.

• Melatonin helps control your sleep and wake cycles (circadian rhythms).
• Very small amounts of melatonin are found in foods such as meats, grains, fruits, and vegetables.
• An internal 24- hour clock controls your natural cycle of sleeping and waking hours.
• Melatonin levels generally begin to rise in the mid to late evening, remaining high for most of the night, and then drop in the early morning hours.
• Light from the sun can also affect how much melatonin your body produces. During the shorter days of the winter months, your body may produce melatonin either earlier or later in the day than usual. This change can lead to symptoms of seasonal affective disorder (SAD), or winter depression.
• Natural melatonin levels slowly drop with age. Some older adults make very small amounts of it or none at all.

6.6.U5 A gene on the Y chromosome causes embryonic gonads to develop as testes and secrete testosterone.​ 

• Describe the mechanism by which the SRY gene  regulates embryonic gonad development.

Humans have 46 chromosomes in all diploid somatic cells – 22 pairs are autosomes and the 23rd pair are the sex chromosomes.

•  Females possess two copies of the X chromosome (XX), while males possess one X and a shorter Y chromosome (XY)
• The Y chromosome (small one below) has a gene called the SRY gene that causes the embryonic gonads to become testes and begin secreting testosterone
• SRY codes for a protein called TDF (testis-determining factor) that stimulates the expression of other genes located on the Y chromosome that  cause testis development
• If there are two X chromosomes, the gonads develop as ovaries

6.6.U6 Testosterone causes pre-natal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.​ 

• Outline role of testosterone in prenatal development of male genitalia.
• State testosterone’s role in stimulating the primary sexual characteristic of males.
• List secondary sexual characteristics triggered by testosterone at puberty.

​The main male reproductive hormone is testosterone, which is secreted by the testes and serves a number of roles:

• Secreted in the testes of males or the early stage testosterone-secreting cells that will become testes.
• Aid in the development and maturation of the male genitalia as a fetus at about the 8th to 9th week.
• During puberty, testosterone aids in the development of male secondary sexual characteristics such as pubic and facial hair, enlarged penis, broad shoulders, muscle mass, deepening of voice and bone density.
• Stimulates production of sperm and promotes the male libido (sex drive).

6.6.U7 Estrogen and progesterone cause pre-natal development of female reproductive organs and female secondary sexual characteristics during puberty.​ 

• State the sources of estrogen and progesterone used in embryonic development.
• Describe prenatal development of female reproductive organs.
• List secondary sexual characteristics triggered by estrogen and progesterone at puberty.

The main female reproductive hormones (secreted by the ovaries) are estrogen and progesterone, which serve several roles:

• If the SRY gene on the Y chromosome is not present in the embryo, the gonads develop into ovaries
• Estrogen and progesterone which are secreted by the mother’s ovaries and then by the placenta, will cause the female reproductive organs to develop in the absence of testosterone
• During puberty, estrogen and progesterone cause the development of secondary sexual characteristics in females, including breast development, menstrual cycle and pubic and armpit hair

6.6.U8 The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones​  [The roles of FSH, LH, estrogen and progesterone in the menstrual cycle are expected.]​ ​

• Outline events occurring during the follicular and luteal phases of the menstrual cycle.
• State the source and location of action of hormones in the menstrual cycle, including FSH (follicle stimulating hormone), LH (luteinising hormone), estrogen and progesterone.
• Outline the role of hormones in the menstrual cycle, including FSH (follicle stimulating hormone), LH (luteinising hormone), estrogen and progesterone.
• Describe the negative feedback loops that regulates secretion of FSH.
• Describe the positive feedback loop that regulates secretion of estrogen.
• Annotate a graph showing hormone levels in the menstrual cycle, illustrating the relationship between changes in hormone levels and follicular development, ovulation, changes to the corpus luteum, menstruation and the thickening of the endometrium.

The menstrual cycle describes recurring changes that occur within the female reproductive system to make pregnancy possible

FSH (Follicle stimulating hormone)

• Produced and secreted by the anterior pituitary gland.
• Stimulates the growth of the follicles in the ovaries to create a mature Graafian follicle.
• Promotes the thickening of the follicle wall.
• Stimulates the secretion of the hormone estrogen.

LH (luteinizing hormone)

• Produced and secreted by the anterior pituitary gland.
• Triggers the release of the egg (ovulation).
• Stimulates the growth of the corpus luteum (secretes estrogen and progesterone).
• Stimulates the secretion of hormone estrogen and progesterone.

Estrogen

• Produced by the developing follicles in the ovaries and the corpus luteum.
• Promotes the thickening of the uterine wall (endometrium) and the growth of blood vessels, in preparation of egg implantation.
• Inhibits FSH and LH when the estrogen levels are high (around same time as ovulation). This would prevent the development and release of another egg.

Progesterone

• Produced by the ovaries and the corpus luteum.
• Helps maintain the thickening of the uterine wall for egg implantation.
• Inhibit the production of FSH and LH.

6.6.A1 Causes and treatment of Type I and Type II diabetes.​ 

• Distinguish between causes of type I and type II diabetes.
• Distinguish between treatment of type I and type II diabetes.​

6.6.A2 Testing of leptin on patients with clinical obesity and reasons for the failure to control the disease. 

• Explain the double blind study that tested the effect of leptin treatment on human obesity.
• Outline role of leptin resistance in human obesity.

6.6.A3 ​Causes of jet lag and use of melatonin to alleviate it.

• State symptoms of jet lag.
• Outline the biological cause of jet lag.
• Describe use of melatonin in treatment for jet lag.​

The SCN of the hypothalamus and the pineal gland continual set the circadian rhythm of the place the person is departing from. Therefore, when a person lands in a country that is many time zones different than the origin, they feel sleepy in the day and awake at night

​Jet lag will only last a few days, as the body adjusts to the new times when the light is detected by the cells in the retina during a different time period

6.6.A4 ​The use in IVF of drugs to suspend the normal secretion of hormones, followed by the use of artificial doses of hormones to induce superovulation and establish a pregnancy. 

• Define in vitro fertilization.
• Outline the process of in vitro fertilisation including down-regulation, superovulation, harvesting, fertilization and implantation.

​ IVF is the process of fertilization by extracting eggs, retrieving a sperm sample, and then manually combining an egg and sperm in a laboratory dish. The embryo(s) is then transferred to the uterus​

• Generally, IVF treatment begins by taking drugs to halt the regular secretion of the hormones FSH and LH. This in turn stops the secretion of progesterone and estrogen and effectively allows the doctor to take control of the timing and egg production of the woman’s ovaries
• The woman is then injected with large amounts of FSH to induce the production of many Graafian follicles.
•  LH is also injected to promote the release of many ovules (eggs)
• This is called superovulation, which can produce between 10 and 20 eggs
• The eggs are then stimulated to mature by an injections of HCG (Human Chorionic Gonadotrophin), a hormone usually secreted by the developing embryo
• The eggs are surgically removed from the ovary of the woman.
• Sperm is collected from the male individual.
• Many sperm (50,000-100,000) are mixed with the eggs in a petri dish.
• The sperm and eggs in the petri dish are incubated at 37ºC (body temperature).
• The eggs are analyzed for successful fertilization (two nuclei inside the egg).
• Healthy embryos are selected and are transferred into the female uterus for implantation (up to 3 healthy embryos are transferred into the uterus to increase chance of implantation).

6.6.A5 ​William Harvey’s investigation of sexual reproduction in deer.  [William Harvey failed to solve the mystery of sexual reproduction because effective microscopes were not available when he was working, so fusion of gametes and subsequent embryo development remained undiscovered.]​

• Outline Harvey’s methods of studying reproduction.
• State Harvey’s discovery about reproduction.

William Harvey was best known for his discovery of the circulation of the blood, also was interested in sexual reproduction and how life is formed.

Aristotle’s theory was called the seed and soil theory, which stated that the male produces a seed (sperm) which then forms an egg when it mixes with menstrual blood of the mother. The egg then develops into a fetus inside the mother and eventually “voila” you have a baby

Harvey studied the uterus of the deer during mating season by killing them and then dissecting them expecting to find eggs; however, Harvey only found signs of fetal development 2 to 3 months after mating season. He concluded that Aristotle’s theory of seed and soil was incorrect, but then he was also incorrect in stating that the fetus doesn’t come from the mixture of the male and female seeds. He knew he had not come up with the correct method of sexual reproduction

6.6.S1  Annotate diagrams of the male and female reproductive system to show names of structures and their functions. 

• Label a diagram of the male reproductive system, including the bladder, sperm duct, penis (with foreskin and erectile tissue), urethra, testis, scrotum, epididymis, prostate gland and seminal vesicle.
• Outline the function of the following male reproductive structures:  testis, scrotum, epididymis, sperm duct, seminal vesicle, prostate gland, urethra and penis.
• Label a diagram of the female reproductive system, including the ovary, uterus, bladder, urethra, vulva, vagina, cervix and oviduct.
• Outline the function of the following female reproductive structures: ovary, oviduct, uterus, cervix, vagina, and vulva.​