A2 Biology

INDEX PAGE COMMUNICATION SYSTEMS IN MULTICELLULAR ORGANISMS CELL SIGNALLING (NEURONAL AND HORMONAL) NEGATIVE/POSITIVE FEEDBACK HOMEOSTASIS PHYSIOLOGICAL AND BEHAVIOURAL RESPONSES IN ECTO AND ENDOTHERMS: CONSTANT CORE BODY TEMPERATURE PERIPHERAL TEMPERATURE RECEPTORS PERIPHERAL TEMPERATURE RECEPTORS THE HYPOTHALAMUS THE HYPOTHALAMUS SKIN AND MUSCLE EFFECTORS SKIN AND MUSCLE EFFECTORS

COMMUNICATION SYSTEMS HEINEMANN BIOLOGY 1.1.1, 1.1.2, START -- COMMUNICATION SYSTEMS

STIMULUS AND RESPONSE All living things must live within a limited range of conditions due to the specific range of conditions in which their enzymes will perform optimally. Factors that must be monitored include; pH, temperature, an aqueous medium in which substrate can be suspended and also toxin/inhibitor concentration. Constant monitoring is required so that an organism may change according to it’s environment. An environmental change (stimulus) may put pressure on an organism to change it’s behaviour or physiology; if it cannot do this then it will die. This change may be gradual such as with the changing of seasons, or it may be quick like the cycle of day and night. In a multi-cellular organism, not every cell will have direct contact with an external environment and so the internal environment needs to be monitored as well. Cells are suspended in the aqueous medium of tissue fluid, from which they can uptake substances and secrete waste products of metabolic activity (such as carbon dioxide or nitrogenous substances). These toxins or wastes will accumulate and cause harm to tissue and cells alike (carbon dioxide would alter pH and therefore enzyme activity). Therefore their presence must be monitored and high concentrations act as a stimulus for their removal. This stimulus could act directly on the cells and cause them to reduce their activity as to produce less waste products, however this is disadvantageous. The composition of tissue fluid is instead maintained by the blood. Substances may diffuse in and out of the tissue fluid down concentration gradients; in this way the blood facilitates the removal of waste products and toxins by taking them away from the cells to be excreted. The blood will also deliver useful substances and substrates to cells, whilst making sure that too many useful substances are not excreted from the body.

HOMEOSTATIS + NEGATIVE FEEDBACK Homeostasis is defined as the maintenance of a constant internal environment despite environmental changes. Various conditions in the body must be maintained, such as the; body temperature, blood glucose levels, water potential of the blood, blood carbon dioxide levels, blood pressure and blood ion/salt concentration. So that conditions can be maintained, the change must first be detected by cells which then transmit signals via a communication pathway to other cells that will then carry out the necessary change. The process of negative feedback is the response that returns conditions to their optimum/set levels. stimulus -> receptor -> communication pathway (signalling) -> effector -> response Sensory receptors (i.e. temperature or blood glucose) detect changes in the internal environment and send a message to other cells through cell signalling in the hormonal or neuronal systems. Effector cells such as the liver or muscle cells then carry out the necessary change to restore the conditions. Positive feedback next page. OPTIMUM/SET LEVELS DEVIATION FROM OPTIMUM LEVELS DETECTION BY RECEPTOR COMMUNIC- ATION VIA CELL SIGNALLING EFFECTOR ACTS TO REVERSE CHANGE OPTIMUM LEVEL RESTORED

HOMEOSTASIS + POSITIVE FEEDBACK Positive feedback is the opposite to what we have just covered; the response in which a change is exaggerated or magnified. It is much less common than negative feedback and is usually harmful. For example; a decrease in core body temperature causes levels of enzyme activity to drop, so exergonic reactions happen much slower and less heat is given out; causing the temperature to plummet further. It can be beneficial however; during the last stage of pregnancy the dilation of the cervix sends a message to the anterior pituitary gland, stimulating the release of oxytocin. This hormone causes more uterine contractions and further cervix dilation, which stimulates oxytocin release and so on…helping the baby to be born. When we talk about a constant level, we don’t actually mean a set value and no variation; this would be very hard and nigh impossible. We instead are referring to a very small range of values in which variation is tolerated.

MAINTAINING BODY TEMP IN ECTOTHERMS The maintenance of a near constant core body temperature is essential for all living things as proteins and in turn, enzymes, are affected dramatically by changes in temperature. We know that enzymes are globular proteins with high specificity regarding shape and function, meaning that deviations away from the optimum temperature cause changes to the shape of the protein (on a tertiary and quaternary level) and thus affect it’s ability to catalyse reactions. This can lead to denaturation and so irreversible damage to an individual’s metabolism through the changes caused by decreased enzyme activity. It is therefore of paramount importance that the body temperature is monitored and regulated as part of homeostasis. Ectotherms are organisms that do not use internal energy mechanisms to regulate core body temperature, instead adopting other physiological and behavioural strategies. For example; an ectotherm will not increase the respiration rate of liver cells to provide heat energy from exergonic reactions, but may instead bask in direct sunlight or on a hot surface to increase body temperature. In this way ectotherms are unable to regulate their body temperature, independently of their environment. They can however survive all but the most extreme conditions. Advantages of ectothermy include the lesser consumption of food as an energy source for exergonic reactions like respiration, so they can eat less food, of which more will be used for growth. Disadvantages include the fact that ectotherms will need to remain relatively stationary in hot conditions and also in cold conditions as they warm up in the Sun (for example), so they are open to predation at these times. Additionally, ectotherms may never become warm enough to be active and so will need energy stores to survive over winter without dying. Temperature regulation in ectotherms relies upon changing the exchange of heat with their environment.

MAINTAINING BODY TEMP IN ECTOTHERMS ADAPTATIONHELP IN TEMPERATURE REGULATIONEXAMPLE Expose body to the SunIncreases absorption of heat from environment and raises core body temperature. Snakes Orient body to the SunIncreases surface area exposed to the Sun and so maximises the ability for heat to be absorbed. Locusts Orient body away from the SunDecreases surface area exposed to the Sun and so minimises the absorption of heat, lowering body temperature. Locusts Hide in shade or a burrowReduces the exposure to the Sun and so the chance for heat to be absorbed. Lizards Alter body shapeExposes more or less surface area to the Sun for heat exchange.Horned lizards. Increase breathing movementsAllows body to cool faster as more water evaporates.Locusts

MAINTAINING BODY TEMP IN ENDOTHERMS Endotherms are able to regulate their body temperature independently of their environment as they use internal mechanisms to warm up and cool down. This is advantageous as endotherms may inhabit colder regions and live in extreme conditions, also; we don’t require a start up time to become active even in the cold. Mechanisms used range from the physiological to behavioural to biochemical. The maintenance of body temperature is regulated by the hypothalamus in the brain. It detects changes in blood temperature and signals accordingly to reverse the change in a negative feedback cycle. For example; a drop in blood temperature would cause the hypothalamus to stimulate liver cell respiration to produce heat, muscle contraction to produce heat and also prevent heat loss to the environment (vasoconstriction etc). Peripheral thermoregulators act as a pre-emptive defence that signals the hypothalamus to counteract temperature changes in the extremities. This is because changes in the temperature of extremities will eventually reach the core of the body and so peripheral receptors allow this to be prevented. RISE IN CORE TEMPERATURE THERMOREG CENTRE IN HYPOTHALAMAUS DETECTS CHANGE NERVOUS AND HORMONAL SYSTEMS CARRY SIGNALS TO SKIN, LIVER AND MUSCLES LESS HEAT GENERATED, MORE LOST TEMPERATURE FALLS DROP IN CORE TEMPERATURE TEMPERATURE RISES THERMOREG CENTRE IN HYPOTHALAMAUS DETECTS CHANGE NERVOUS AND HORMONAL SYSTEMS CARRY SIGNALS TO SKIN, LIVER AND MUSCLES MORE HEAT GENERATED, LESS LOST OPTIMUM TEMPERATURE - 37 °C

MAINTAINING BODY TEMP IN ENDOTHERMS COMPONENT INVOLVEDRESPONSE IF TOO HIGHTOO LOW Sweat glands in skinSecrete more sweat which latent body heat evaporates to cool individual down. No sweat produced so less loss of latent heat. Lungs, mouth and noseIndividual pants, evaporating water and cooling down as above. No panting, so less water evaporates. Hairs on skinHairs lie flat to not trap air and act as an insulating layer, allowing heat loss via convection and radiation. Hairs stand up to trap an insulating layer of air above the skin. Arterioles leading to skin capillaries Vasodilation; blood flows to skin where latent heat is exchanged with the environment. Vasoconstriction; blood doesn’t flow to the skin to lose heat. Skeletal musclesNo contraction.Spontaneous contractions. BEHAVIOUR IF TOO HIGHBEHAVIOUR IF TOO LOW Hide in burrow or shade to escape heat.Expose body to the Sun to absorb heat. Orient body from the Sun to increase exposed SA.Orient body from the Sun to decrease exposed SA. Remain inactive to limit heat produced from muscle contractions. Move around generate heat, except in extreme cold where this will increase heat exchange with environment and cool you down.