option h.1 – hormonal control
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Option H.1 from the IB Biology syllabus. Notes created by IB Screwed http://ibscrewed4biology.blogspot.com/TRANSCRIPT
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Option H.1 – Hormonal Control
H.1.1 – State that hormones are chemical messengers secreted by endocrine glands into
the blood and transported to specific target cells
Hormones are used in the form of long-distance communication of the endocrine system.
The endocrine glands, such as the pancreas, produce hormones and secrete them into the
bloodstream. The blood carries the hormones through the body. When the hormone
reaches the cells that have the right receptor, it will bind to the target cell and produce a
response.
H.1.2 – State that hormones can be steroids, proteins and tyrosine derivatives, with one
example of each
Steroid Hormones
Steroids are a type of molecule made up of four organic rings.
An example of a steroid hormone is cortisol, a stress hormone,
shown right. The sex hormones œstradiol and testosterone are
also steroids.
Protein Hormones
These hormones are made up of polypeptides.
An example of a protein hormone is insulin, which is used in the control
of blood glucose concentration.
Tyrosine Derivative Hormones
Tyrosine is one of the amino acids. Some hormones are derived from it.
An example is the thyroid hormone, thyroxine (T4), which helps to
regulate metabolism.
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H.1.3 – Distinguish between the mode of action of steroid hormones and protein
hormones
Steroid Hormones
Since steroid hormones are non-polar molecules, they are
able to move through the lipid bilayer and into the cell. It
then bonds to an intracellular receptor protein to form a
hormone-receptor complex, which enters the nucleus.
It affects the DNA by causing expression of genes and
consequently protein synthesis.
This process is used by steroid hormones, such as the sex
hormones, to regulate growth and development.
Protein Hormones
These hormones tend to be larger and are polar molecules. Hence, they cannot cross the
lipid bilayer. Instead, they attach to receptor proteins on the outer cell membrane. The
binding triggers the action of a second messenger in the cytoplasm. This activates the
cellular response.
H.1.4 – Outline the relationship between the hypothalamus and the pituitary gland
Within the human body, there are many endocrine glands. The main ones that control
homeostasis are the hypothalamus and the pituitary gland.
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Hypothalamus
The hypothalamus integrates the nervous and endocrine systems. It contains
neurosecretory cells that perform the endocrine functions including synthesising some
hormones such as oxytocin and ADH. It also produces releasing hormones, which cause the
secretion of other hormones.
Underneath the hypothalamus is the pituitary gland, which is made up of two lobes.
Posterior Pituitary
This is an extension of the hypothalamus, connected by the neurosecretory cells. The
hormones that are produced in the hypothalamus travel along the axons of the
neurosecretory cells. When the nerve impulses move along the axons, it stimulates the
release of the hormones into the bloodstream at the posterior pituitary to be stored for
release.
Anterior Pituitary
This is regulated by hormones that are released by the hypothalamus into portal blood
vessels, or the portal vein. It synthesises and releases TSH and PRL (prolactin).
H.1.5 – Explain the control of ADH (vasopressin) secretion by negative feedback
Negative feedback means that the response reduces the initial stimulus. This is used to
prevent excessive pathway activity.
Vasopressin, or Anti-Diuretic Hormone, is produced in the hypothalamus and released from
the posterior pituitary. It increases levels of urination. The neurosecretory cells in the
hypothalamus synthesis and transport ADH along the axons of nerves for storage in the
synaptic ends.
When the body loses water, the osmolarity of the blood increases. Osmoreceptors of the
hypothalamus detect the change in the water content of blood as it passes through the
hypothalamus. The osmoreceptors send an action potential through the neurosecretory
cells to the posterior pituitary is the water contant is low, stimulating the release of
vasopressin into the bloodstream.
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The target cells for ADH are mainly in the distal tubules and collecting duct of the kidney. It
causes the epithelium to become more permeable to
water, increasing renal reabsorption of water and
reducing urine volume. Consequently, the urine is more
concentrated. This is achieved by acting upon the
aquaporins.
Once the osmolarity is returned to the homeostatic set
point, a negative feedback loop is used to signal to the
hypothalamus to reduce vasopressin production.
Conversely, if there is excessive water consumption, then
ADH secretion is reduced so that a greater volume of
urine is produced.