16) Homeostasis _ Cell Signaling Summary_9744_2019

Energy and Equilibrium (9744) Homeostasis & Cell Signalling 2019 Prepared by Mrs.S.Nair, Mrs.Wong SH & Mdm.S.Cross Raffles Institution 1 Homeostasis and Cell Signalling  Homeostasis refers to the maintenance of a stable internal environment independent of fluctuations in the external environment by self- regulating & negative feedback mechanisms so that the organism can function optimally. Self-regulation: where a corrective mechanism is triggered by the very entity which is to be regulated (e.g. control of blood glucose levels is triggered by changes in blood glucose levels) Negative feedback: a mechanism which brings about increasing stability of a system i.e. it removes any deviations from the set point i.e. a change in a variable triggers a response that counteracts the initial change. (e.g. when blood glucose level goes higher than set point, insulin is secreted to return glucose levels to set point).  Hormones:  secreted by endocrine glands (ductless glands) directly into the bloodstream  effective in small quantities (as signal amplification, that occurs during signal transduction, will lead to the production of a strong cellular response)  act on specific target cells which have specific cell surface receptors  each type elicits different cellular responses & after having served their function, are rapidly broken down  e.g. insulin & glucagon are hydrophilic peptide hormones that bind to the specific receptors on the cell membrane (e.g. RTK & GPCR)  Pancreas:  is an organ that is both an endocrine (islets of Langerhans) gland & an exocrine (acinar cells) gland  the islets of Langerhans contain alpha cells which secrete glucagon and beta cells which secrete insulin into the bloodstream (insulin and glucagon (which are protein in nature) are secreted constantly and work in an antagonistic fashion; it is their relative concentrations and not their actual levels that are critical to maintain normal blood glucose levels at the set point which is 90mg/dL)  Glucose:  key respiratory substrate  Glycogen:  stored in liver and muscles  Insulin triggers the conversion of glucose to glycogen  Glucagon triggers the conversion of glycogen to glucose (It is incorrect to say that insulin converts glucose to glycogen as insulin binds to the insulin receptor which triggers a si gnal transduction pathway that eventually leads to the conversion of glucose to glycogen in the cell. Likewise, it is incorrect to say that glucagon converts glycogen to glucose.)  A deviation from the set point i.e. stimulus (e.g. high blood glucose levels)  is detected by detectors (e.g. beta cells in islets of Langerhans) (N.B. Sometimes detectors are referred to as rec