5.1.1

• Conditions needed for living organisms


  • Suitable temperature
  • Suitable pH
  • Aqueous environment that keeps substrates and products in solution
  • Freedom from toxins and excess inhibitors
• Communication systems are needed to maintain the constant internal environment in a changing external environment
• As the external environment changes, it places stress on living organisms
• In order to survive these changes must be monitored and the organism must change its behaviour and physiological process to reduce the stress
• Hormonal system


  Allows for slower, longer-term responses to environmental changes
• Neuronal system


  Allows for quicker responses to rapidly changing environmental conditions
• Communication systems allow organisms to coordinate their activities in order to respond to a changing internal environment
• Metabolic processes of cells alter the internal environment, reducing the concentration of substrates, and increasing the concentration of products, which may be toxic
• Communication between cells is required to offset the changing internal environment
• As multicellular organisms, cells are more efficient than single-celled organisms as they are differentiated to perform their various functions
• A good communication system is needed to ensure that different organs working together to perform the same function, such as excretion of waste work together effectively even if they are not adjacent to each other
• Characteristics of a good communication system

  • Covers the whole body
  • Enables cells to communicate with each other
  • Enables specific communication
  • Enables rapid communication
  • Enables short and long term responses
• Neuronal system


  Network of interconnected neurons that signal to each other across synapse junctions, enabling rapid responses
• Hormonal system


  Uses the blood to transport chemical signals, enabling longer-term responses
• Communication between adjacent cells


  1. Gap junctions may facilitate the movement of small molecules from cell to cell

  2. Contact dependent communication may occur between signalling molecules in the plasma membrane of the signalling cells and receptor proteins present in the plasma membrane of the target cell

  3. Paracrine cells secrete signalling molecules which diffuse locally through the extracellular fluid
• Homeostasis


  Maintenance of a constant internal environment despite changes in external and internal factors
• Homeostatic mechanism


  1. Deviation from the optimum is detected by sensory receptors

  2. Signal transmitted via hormonal or neuronal system to a coordination centre, usually the brain

  3. Effectors bring about a response that reverses the initial change in conditions

  4. Stimulus is reduced when the system moves closer to the optimum
• Positive feedback


  A deviation increases the original deviation, used to effect larger changes over a short period
• Ectotherms are not able to control their body temperature using their metabolism; they rely on external sources of heat and their body temperature fluctuates with the external temperature
• Behavioural mechanisms used by ectotherms to control body temperature

  1. Basking in the sun to gain heat

  2. Changing body orientation to increase/decrease surface area exposed to sun

  3. Curling into a ball or increasing ribcage size to change body size

  4. Increasing breathing rate to allow evaporative cooling

  5. Using burrows or crevices with stable air temperature to keep cool

  6. Increasing metabolic rate through movement
• Some ectotherms have physiological adaptations to control body temperature, such as darker colours in colder climates and ability to control heart rate
• Endotherms


  • Control body temperature within a small range, largely independent of external temperatures
  • Use internal exothermic metabolic activities to keep warm
  • Use energy-requiring physiological responses to cool down
• Thermoregulatory mechanism in endotherms


  1. Thermoregulatory centre in hypothalamus monitors blood temperature

  2. Peripheral temperature receptors in skin and muscles provide additional information

  3. Autonomic motor neurons activate effectors in skin and muscles to regulate heat loss/gain
• Behavioural responses of endotherms to temperature changes

  1. If too cold: basking, orienting body, pressing against warm surfaces, exercising, rolling into ball

  2. If too hot: bathing, finding shade, orienting body, remaining inactive, spreading out, licking skin/fur
• Physiological responses of endotherms to temperature changes

  1. If too hot: increased sweating, hair/feathers lying flat, vasodilation

  2. If too cold: reduced sweating, hair/feathers standing erect, vasoconstriction, increased respiratory rate, shivering
• Endotherms can maintain a fairly constant body temperature and remain active even in low external temperatures, but use more energy to do so
• Ectotherms use less energy for thermoregulation, allowing more energy for growth, but cannot be active at low temperatures