key points: #1) Hormones: are certain chemicals used as signals, secreted by cells inside the body (into the blood), for the purpose of stimulating or inhibiting certain other cells, often all over the body. Note that adrenaline is used both as a neurotransmitter (locally) AND as a hormone (all over the body) Glands that secrete hormones are called "endocrine" glands "endocrinology" is the name of the science that studies them. #2) Most hormones belong to 3 categories of chemicals: steroids: of which examples include male and female sex hormones; insect molting hormone; cortisone proteins/peptides: insulin, secretin, all the pituitary hormones and amino acid derivatives: adrenaline, thyroxine & there are also some others, such as prostaglandins; and NO which is a gas. #3) Hormones are secreted (most often) by specific glands; such as the adrenal gland the thyroid gland, gonads (in males, the gonads are testes (singular is "testis") #4) Hormones produce effects (often large effects) at very low concentrations; because hormone molecules bind to (fit exactly into binding site) on receptor proteins. Proteins can't diffuse through plasma membranes into cells, so receptors for protein hormones are always on the outer surface of the plasma membrane; and when they bind, this causes some internal change. But steroids can diffuse through plasma membrane; so usually steroid receptors are inside cells(& inside nuclei) and in many cases steroid receptors are transcription factors; bind to DNA at certain base sequences, control whether RNA copies of those genes are or are not transcribed. #5) Many hormones produce effect by second messengers; of which the classic example is cyclic AMP. This is synthesized from ATP by adenyl cyclase enzyme. The effects of those hormones can therefore be mimicked or amplified by (for example) dibutyryl cyclic AMP Cyclic GMP is also used as a second messenger, and so are inositol phosphates and diacylglycerol; but it is enough for you to learn cyclic AMP #6) Stabilization of hormone concentrations and many variables by negative feed-back cycles: an example of "Homeostasis" an example: FSH and LH stimulate gonads to secrete more steroid sex hormones But secretion of LH and FSH by the pituitary become less in response to more steroid sex hormones. This is analogous to a thermostat regulating temperature, notice that heating a thermostat will cause the furnace to be turned down. In such a cycle, an odd number of the effects need to be inhibitory #7) By what methods have hormones been discovered, and their chemical nature identified. a) Surgical removal of glands: produces what effect? Which chemical extracts will compensate for this removal? Bioassays: comparing effects of different fractions of extracts. The example of serotonin. b) Diseases in which too much or too little of a given hormone. #8) In contrast, knowledge about hormone receptors comes more and more from molecular genetic methods (DNA sequences, from which amino-acid sequences can then be deduced) #9) Pheromones: Outside the body influence other individual animals, almost always of the same species; and usually of the opposite sex; often for attraction for mating; These are what your dog is sniffing so vigorously when you go on walks. Are there human pheromones? of which we are not consciously aware? Observations on synchronization of menstrual cycles in women in dorms. Read about this in the textbook Perfumes are made partly from extracts of glands from civet cat glands, which make pheromones. ----- Questions that might be on an exam: a) Are there any hormones that are also neurotransmitters? b) What is endocrinology? c) Are all steroids also hormones? Are any steroids hormones? What are two specific examples of hormones that are steroids? d) What is an example of a hormone that can diffuse right through the plasma membranes of cells, into their cytoplasm? e) Most hormones belong to which general categories of molecules? (name 2 such categories) f) Some hormones also belong to what third category of molecules, besides steroids and chains of amino acids? g) Endocrine glands secrete what? h) What are 3 or 4 specific examples of endocrine glands? i) Do gonads secrete certain hormones, in addition to producing sperm and egg cells? [hint: yes; but what effects do these hormones produce?] j) What do hormones bind to, in cells? k) How is this binding related to the specificity of hormones, and their ability to produce large effects, often even at very, very low concentrations? *l) The time needed to reverse a hormone's effect tends to be longer for those hormones whose receptors are either especially specific (much less affected by other chemicals with nearly the same structure as the hormone), or which act at unusually low concentrations: figure out the logical connection between these 3 properties: high specificity; high sensitivity; and slow reversibility. [Hint: it has to do with the tightness of the bonding between hormones and their receptor proteins.] m) Which kinds of hormones can diffuse (somewhat) freely into cells, diffusing right through the plasma membrane and the nuclear membrane? n) The receptors for which class of hormones are often transcription factors, which become able to bind specifically to DNA having certain base sequences, but only when also bound to their hormones? o) Receptors for which kinds of hormones have to be located on the outer surface of the plasma membrane? p) What is an example of a "second messenger" (for hormones)? q) What is the raw material from which cells synthesize molecules of this second messenger? What sort of stimulus causes cells to synthesize molecules of a second messenger? *r) By what simple chemical trick can molecules of this second messenger artificially be enabled to diffuse through plasma membranes, so that they can diffuse into cells from the outside? s) "Homeostatic" regulation of hormone concentrations or other variables is the result of what kind of feedback cycles? Hint: What are the differences between negative feedback cycles versus positive feedback cycles? Further hint: the propagation of nerve action potentials results from a positive feedback cycle, in which a certain change stimulates more change in the same direction? *t) Before puberty (the onset of sexual maturity in humans and other animals, the pituitary gland is more sensitive to inhibition by steroid sex hormones; then puberty results indirectly from the pituitary becoming LESS sensitive to this inhibition. This results in secretion of more of two kinds of hormones. Figure out what sense this makes. How can less sensitivity to a hormone cause increases in hormone effects? *u) The active ingredients in birth control pills are certain steroid hormones. How can these produce an inhibition of egg cells production? Wouldn't it make more sense of more of a hormone resulted in more egg cells being produced? v) Explain how a bioassay is used to identify the chemical nature of a certain hormone, or other biologically important molecule. w) If a molecule were analogous to a hormone, but were secreted to the outside of a given kind of animal, and produced its effects on other individual animals of the same species, then what would be a 10 letter word starting with P that is the name for any such molecule? *x) Suppose such a chemical produced strong but unconscious psychological effects, then what nefarious uses might it have? *y) Invent (in general terms) a bioassay by which the chemical nature of such an external signalling molecule might be discovered or confirmed? **z) Would sensitivity to such a chemical count as "extra-sensory perception" (ESP) in your opinion? **!) Could action potentials in certain nerves in one animal be sensed by another animal? How? Could the second animal deduce the meaning of those signals? Hint: if all kinds of nerves use the same kind of action potential, then why does that make ESP less likely? Conversely, invent how a nervous system might need to work in order to make ESP more possible. (So that the significance of nerve signals could be decoded at a distance? Or is that perhaps not the essence of ESP?)

 

back to syllabus