communication and homeostasis

• Cell metabolism relies on enzymes and enzymes need a specific set of conditions in which to work efficiently
• Conditions needed for cells to function

  • Suitable temperature
  • Suitable pH
  • Aqueous environment that keeps substrates and products in solution
  • Freedom from toxins and excess inhibitors
• Without these conditions the cells will become inactive and die
• In multicellular organisms, cells are specialised and rely upon one another; therefore they must be able to communicate in order to coordinate their activities
• Changing external environments


  All living organisms have an external environment that consists of the air, water or soil around them
• Environmental change


  Stimulus and the organism's change in behaviour or physiology is its response
• Environmental changes


  • Seasonal changes
  • Appearance of a predator
  • Moving from a burrow into sunlight
• The composition of the tissue fluid is maintained by the blood
• Blood flows throughout the body and transports substances to and from the cells
• Any wastes or toxins accumulating in the tissue fluid are likely to enter the blood and be carried away
• It is important that the concentrations of metabolic products and other substances in the blood are monitored closely
• A multicellular organism is more efficient than a single-celled organism, because its cells are differentiated
• Groups of cells specialised to perform a particular function form tissues and organs
• A good communication system is required to ensure that the different parts of the body work together effectively
• Cell signalling


  Cells communicate with each other by the process of cell signaling, where one cell will send a chemical that is detected by another cell
• Two major systems of communication by cell signalling

  • Neuronal system
  • Hormonal system
• Cell signalling involves molecules that have a specific shape which is complementary to that of the cell surface receptor
• Stimulus


  A change in the external or internal environment that is detected by sensory receptors
• Response


  The way in which an organism changes its behaviour or physiology to reduce the stress caused by an environmental change
• Homeostasis


  The maintenance of a constant internal environment in living organisms despite changes in external and internal factors
• Aspects maintained by homeostasis

  • Body temperature
  • Blood glucose concentration
  • Blood salt concentration
  • Water potential of the blood
  • Blood pressure
  • Carbon dioxide concentration
• Sensory receptors


  Receptors that monitor changes in the external or internal environment and send messages to the coordination centre
• Communication system


  The neuronal system or hormonal system that transmits messages from receptors to the coordination centre and from the coordination centre to effectors
• Effector cells


  Cells that bring about a response to changes in the environment
• Negative feedback


  The mechanism that brings conditions back towards the optimum by reducing the response as the stimulus is reduced
• Positive feedback


  The mechanism that increases the original change, destabilising the system
• Negative feedback can maintain a reasonably constant set of conditions, with some variation around the optimum
• Positive feedback can be beneficial in some situations, such as stimulating an increase in a change
• Input


  What goes into a system
• Output


  What comes out of a system
• Negative feedback


  Feedback that reduces or opposes the original change
• Positive feedback


  Feedback that amplifies or increases the original change
• Ectotherms


  • Rely on external sources of heat to keep warm
  • Do not use up energy to keep warm
• Ways ectotherms control body temperature

  • Move into sunny areas
  • Lie on warm surfaces
  • Expose larger surface area to sun
  • Move out of sun
  • Move underground
  • Reduce body surface exposed to sun
• Ectotherms are less active in cooler temperatures, putting them at risk from predators
• Ectotherms can survive for long periods without food
• Ectotherms need to find less food than endotherms
• More of the energy and nutrients gained from food can be converted to growth in ectotherms
• Endotherms use a significant part of their energy intake to maintain body temperature in the cold
• Endotherms need more food than ectotherms