HOMEOSTASIS

1 EJC H2 Biology T2W2 Homeostasis Homeostasis 1. Introduction In multicellular organisms, different parts of the body perform different functions. Communication is of utmost importance to ensure that bodily functions can be coordinated and carried out efficiently. Communication ensures that there is coordination between organs so that the organisms can function as a whole in responses to changes. In particular, the composition of the internal environment must be maintained at a narrow, optimum range to support the proper functioning of the body cells. In this segment, we will be learning about the concept of homeostasis to appreciate how such optimum conditions of the internal environment may be maintained, so that cells can function in a state of maximum efficiency. In particular, we will focus on insulin and glucagon regulation of the blood glucose concentration in the body. 2. Learning Outcomes 3 (p) Outline how insulin and glucagon regulate the concentration of blood glucose through the respective tyrosine kinase receptor and G-protein linked receptor. (The outline should be limited to describing how the ligand induces a conformational change in a membrane-bound receptor to trigger downstream signaling pathways that elicit physiological changes in blood glucose concentration. Details of different second messengers and specific kinases activated in the pathway are not required.) Students should be able to use the knowledge gained in this section in new situations or to solve related problems. 3. References  Campbell, N.A. and Reece, J.B. (2008). Biology, 8th edition. Pearson.  Sherwood, Fundamental of Human Physiology, 4th edition  Brooker et al., Biology, McGraw-Hill  Ho YK (2003) A-level course in Biology – Core syllabus, Longman

2 EJC H2 Biology T2W2 Homeostasis 4. Organisation of Lecture Content 1. Introduction 1 2. Learning Outcomes 1 3. References 1 4. Organisation of Lecture Content 2 5. Overview of Homeostasis 3 A. Definition: What is Homeostasis 3 B. The Importance of Homeostasis 4 C. The Principles of Homeostasis 4 6. Communication Systems 6 A. Hormonal Control: The Endocrine System 6 B. Hormones 7 C. Mechanisms of Hormone Action 7 D. The Pancreas: Islets of Langerhans 8 E. Blood Glucose Regulation by Insulin and Glucagon 9 I. Role of Insulin – decrease blood glucose concentration 11 II. Role of Glucagon – Increase blood glucose concentration 12

3 EJC H2 Biology T2W2 Homeostasis 5. Overview of Homeostasis A Definition: What is Homeostasis? o Homeostasis is the maintenance of a constant internal cellular environment of an organism, providing it with a degree of independence from its external environment. o It involves self-regulating and negative feedback mechanisms to control the level of substances that are to be maintained. Note: The concept of ‘self-regulating and negative feedback’ will be detailed later.  The external environment refers to the environment in which the organism lives, whereby the conditions fluctuate beyond the control of the organism.  In complex multicellular organisms, most living cells of the body are not directly exposed to the external environment. They are bathed in a liquid internal environment known as interstitial fluid.  The internal environment refers to the immediate surrounding which the individual cells live (e.g. the interstitial fluid, shown in Fig. 1).  Components of the internal environment which are homeostatically regulated and kept constant: - a) Concentration of chemical constituents in blood, e.g. glucose, ions, nitrogenous waste products b) Concentration of respiratory gases e.g. oxygen and carbon dioxide c) Body temperature d) pH e) Osmotic pressure Notes to self Figure 1: Interstitial Fluid (The fluid between cells, also known as the internal environment of individual cells)

4 EJC H2 Biology T2W2 Homeostasis B The Importance of Homeostasis  Homeostasis provides optimal conditions for cellular activities. This is crucial to cells because cells can only function within a narrow range of conditions, so as to ensure maximum efficiency.  Homeostatic mechanisms buffer the body against fluctuations from optimal conditions.  This provides the organisms with a degree of independence from the external environment.  As a result, a stable internal environment with minimal disturbances is maintained.  What are the consequences if homeostasis is disrupted? o Altered conditions can be detrimental. It can lead to illness and death. o E.g. Increased temperature/ acidity may denature enzymes. C The Principles of Homeostasis  Organisms are open systems; they interact with the external environment and have a continuous exchange of matter with it.  To maintain stability in the internal environment and prevent it from coming to equilibrium with the external environment, control systems are required .  Control systems are homeostatic mechanisms that exhibit two principles: i. Self- Regulation o This involves corrective mechanisms which are triggered by the very entity/ parameter that it serves to regulate. E.g. In the control of blood glucose level , the secretion of hormones (glucagon and insulin) to regulate any fluctuation is triggered by changes in blood glucose level. ii. Negative Feedback o Negative feedback is a corrective/ regulatory mechanism in which a deviation from the set/ reference point serves as a stimulus to trigger a response by the control system. o The response is in the opposite direction to the stimulus, and it opposes the initial deviation/ reduces the initial stimulus. o In other words, a change sets off events that counteract the change. Notes to self

5 EJC H2 Biology T2W2 Homeostasis  Homeostasis operates primarily via a negative feedback control loop (Fig. 2):  The table below summarises the various components of the homeostatic control system:  The response/output (changed condition) serves as a negative feedback to the receptor for monitoring of any displacement from the set/reference point that is caused by the stimulus.  Negative feedback is associated with increasing stability of the system because the disturbance triggers a sequence of events to restore the system back to norm / set point.  Homeostasis is a dynamic process: it works by constantly making adjustments to compensate for fluctuations of output. Thus it is more accurate to describe such system as steady state or dynamic equilibrium. o E.g. when a parameter (e.g. glucose concentration in blood) increases above the set point, the corrective mechanism will act to cause that parameter to decrease and return to set point. 1. Reference point  The set level or optimal level at which the system operates. 2. Detector (or receptor)  Continuously monitors the parameter for deviation from the reference point.  Signals the extent of any deviation from the reference point.  Relays information to regulator or control centre. 3. Control Centre/ Regulator  Coordinates the information from various detectors.  Sends out instructions to correct any deviations by activating the appropriate effectors. 4. Effector  Brings about the necessary change needed to restore system to the reference point. Detector/ receptor Control centre/ regulator Effector Stimulus (input) Signal Signal Output/ response Negative feedback Figure 2: Basic components of a homeostatic control system/ Negative feedback loop

6 EJC H2 Biology T2W2 Homeostasis 6 Communication systems  There are two major internal communications systems, namely the endocrine and the nervous systems (Fig. 3).  These systems detect any changes known as stimuli in th