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Background information Conditions within the body must be maintained at a constant level in order to achieve optimal metabolic efficiency; enzymes, responsible for controlling all metabolic activities within the cell, will only operate within a limited range of temperature and acidity. Solute concentration must in turn be kept constant in order for these reactions to occur properly. Homeostasis is the process by which organisms maintain a relatively stable environment. Homeostasis consists of two stages: detecting changes from the stable state - this is made possible by the presence of receptors in living organisms. Receptors in animals consist of nerve cells that detect stimuli (a stimulus is any information that causes a response). Receptors in plants usually include the shoot and root tips working together with plant hormones. counteracting changes from the stable state - this is brought about in living organisms by effectors. In mammals, effectors are usually muscles or glands and in plants hormones such as auxins and cytokinins act as effectors. Changes in an organism's surroundings are detected by receptors, processed by a control centre (usually the brain), and counteracted by effectors. 1) Match the stimulus in column A with the receptors that detect it in column B below. Stimulus Changes in the concentration of blood chemicals such as glucose, amino acids, CO, oxygen and dissolved ions Temperature changes Light _ Pressure, gravity, sound 2) Receptor Thermoreceptors Mechanoreceptors Chemorece tors Photoreceptors Match the effector in column A with the response it produces in column B below. Effector Response Muscle Gland Plant hormones Secretion of hormones or other chemicals; examples of their action include control of solute levels in the blood, diverting blood to the muscles, secondary sexual characteristics. Responses include control of flowering and fruiting, growth of buds, stem elongation. Examples include contraction to produce movement, blood vessel constriction and shivering. 3) In a 'feedback loop' the receptors detect the response and send messages back to the control centre to stop further adjustment. This is often referred to as 'negative feedback' because it results in a negative response by the effector; for example the accumulation of a hormone in the blood automatically cuts down its production. Insert the words listed into the feedback loop shown below. effector , stimulus , conductor (usually nerve fibres), receptor, response, control centre A Feedback Loop 4) Give a specific example of a negative feedback system which operates in mammals. Use a diagram in your answer. 5) List three things that must be maintained at a constant level in order to maintain a stable internal environment in living organisms. 6) Describe what a receptor is. 7) Define the term 'stimulus'. 8) State the two main stages in homeostasis. THE ROLE OF THE NERVOUS SYSTEM 1N DETECTING AND RESPONDING TO ENVIRONMENTAL CHANGE Read the following passage and answer the questions. In endotherms a constant internal temperature is controlled by an inbuilt 'thermostat'; the hypothalamus, which is located in the brain. The hypothalamus is a region of neurones and secretory cells and it performs a variety of roles involving nervous and hormonal messages. In its role as a temperature regulator, it acts by first detecting and then responding to changes in the surrounding temperature. Receptor cells within the hypothalamus itself detect temperature changes from either nervous impulses sent from 'thermoreceptor' cells in the skin, or from changes in the internal blood temperature of the organism. The hypothalamus responds to higher temperature signals by sending nervous messages to the peripheral nervous system, which in turn stimulate cooling processes such as 'vasodilation' (blood vessels dilate and move closer to the skin's surface, allowing heat to escape), sweating and panting. The resulting effect is a lowering of the overall blood temperature. When the hypothalamus detects lower temperatures it directs the peripheral nervous system to stimulate heating mechanisms such as increased muscle activity (shivering), 'vasoconstriction' (blood vessels constrict, so reducing heat loss to the surroundings) and the contraction of hair erector muscles to allow hair, fur or feathers to stand on end. The result of these processes is a warmer blood temperature. The involvement of the hypothalamus and peripheral nervous system in dealing with environmental change is an example of a 'feedback loop', in which the response alters the stimulus, as shown in the diagram below. The Role of the Nervous System in Responding to Environmental Change 1 1) Copy and complete the following passage, using the words listed below. peripheral, receptors, blood, shivering, vasoconstriction, hypothalamus, thermoreceptors, vasodilation, sweating, panting The …………………… is an area of the brain involved in both nervous and hormonal control. It contains sensory ……………….. that receive stimuli from …………….. in the skin and from temperature changes in the ………………. . The hypothalamus then sends information via the ……………….. nervous system to produce heating responses such as …………….. and …………………, and cooling responses such as …………………. , ……………… and ……………….. . 3) Draw and label a diagram to describe how both endotherms and ectotherms would use a simple reflex action pathway to respond to extreme environmental stimuli such as heat or acidity. Include the following terms in your labelling: effector, receptor, motor neurone, sensory neurone, connecting neurone. 4) Match column A with column B. A Hypothalamus Thermoreceptors Endotherm Vasoconstriction Peripheral Nervous System Ectotherm B Stimulates sensory receptors in hypothalamus to respond to heat and cold. An organism that cannot regulate its body temperature internally. A mechanism in which blood vessels dilate to allow heat loss to occur. Mammal that utilises vasoconstriction to reduce heat loss. Area of brain that regulates internal temperatures. Mammal that utilises vasodilation to allow heat loss in its desert environment. Mitchell's Hopping Mouse An organism capable of regulating its internal environment. Arctic Fox A mechanism in which blood vessels constrict to prevent heat loss. Blood Temperature Vasodilation The neurones outside the central nervous system. Specialised heat sensitive neurones located in the skin. LIFE'S NARROW TEMPERATURE LIMITS Read the passage below and answer the questions. The temperature ranges in which life on earth can occur are dictated by the heat tolerance of enzymes and other proteins within cells and by the need for a liquid medium in which molecules can react together. At temperatures of only a few degrees below 0°C the cytoplasmic fluid of cells freezes. As a result, molecules are either left at a concentration that is too high for satisfactory metabolic activity or immobilised completely. Most proteins begin to denature at temperatures around 50°C - their three-dimensional structure is altered and enzymes can no longer function. The occurrence of life is limited to external environments within the narrow range of just below freezing to 45-50°C. Exceptions include some forms of cyanobacteria found in hot springs that have adapted so that metabolism can occur at temperatures of about 100°C. Certain species of lichen and moss have been known to withstand temperatures below -200°C and the seeds of some plants are capable of surviving temperatures of over 100°C during bushfires. Organisms living in large bodies of water such as oceans experience very little temperature variation (usually no more than 10°C annually) because of the large volume to surface area ratio of their surroundings. As a result, marine organisms do not need to posses as many adaptations for coping with temperature changes. Instead, their body temperatures usually remain the same as the ambient water temperature. Nevertheless, oceans in different areas of the world range from 0°C to around 40°C and aquatic life forms can be found at all these temperatures. Terrestrial organisms, on the other hand, must be able to adapt to wide temperature fluctuations, sometimes within a few hours. In some places annual terrestrial temperatures may vary by as much as 70°C. Globally, terrestrial temperatures range from approximately -60°C to 60°C. Temperatures in areas near the tropics are usually warm and relatively constant whereas regions closer to the poles experience more seasonal variation and larger temperature extremes. 1) Explain why most forms of life cannot exist at temperatures a few degrees below 0°C. 2) Outline the reasons why most forms of life cannot exist at temperatures above 50°C. 3) Name some organisms which are exceptions in that they can survive outside the range of 0°C -50oC. 4) Why don't aquatic organisms need to possess as many adaptations to temperature variations as terrestrial organisms? 5) Which areas of the earth experience greater seasonal variation and temperature extremes? PLANT RESPONSES TO TEMPERATURE CHANGE 1) Match Column A with Column B. A Waxy, shiny, hairy or light coloured leaf surfaces Vernalisation B Transpiration Surface area One of the major ways temperature affects plants Water loss and subsequent evaporation from plants which helps to cool the plant Dormancy Rhizomes, tubers and bulbs This is reduced in the leaves of some desert plants to reduce heat absorption Help to reflect light and reduce the temperature of the plant in hot conditions Process in which some plants lose leaves to avoid damage in cold conditions Process in which plants must be subjected to cold temperatures before the will flower Underground reproductive structures which survive when the plant above the round dies Eucalypt leaves hang this way to reduce the surface area exposed to the sun Some Australian plants need this for their seed pods to open or for their seeds to germinate 2) Growth rate The heat of a bushfire Vertically The table below shows the effect of temperature changes on growth in barley plants. Temperature °C 20 22 24 26 28 30 Growth in mm/hr 4 6 6.8 7.2 8 9.1 Convert these figures into a line graph. Comment on the effect temperature has on the growth rate of barley. 3) Explain why heat loss through the evaporative cooling of transpiration poses a problem for desert plants.