General communication and homeostasis

• Homeostasis


  • control systems that keep the internal environment relatively constant
  • allows cells to function properly
  • stop damage
• Responding to the environment

  • to increase chances of survival
  • response to external and internal environment
  • ensures internal conditions are optimal for their metabolism
  • changes in environment is a stimulus
• Receptors detect stimuli


  receptors are specific, detect one type of stimulus
• Examples of receptors


  • Some receptors are cells
  • e.g. photoreceptor cells
  • some receptors are proteins
  • e.g. glucose receptors in cell membranes
• Effectors


  • cells that bring about a response to a stimulus, to produce an effect
  • e.g. muscle cells and cells found in glands
• Cell signalling between adjacent cells

  in the nervous system via chemicals (neurotransmitters)
• Cell signalling between distant cells

  in the hormonal system via chemicals (hormones) which travel in the blood
• Cell-surface receptors


  allow cells to recognise chemicals involved
• Negative feedback


  • receptors detect if level is too high or low
  • information is communicated
  • effectors act to counteract change
  • returns to the normal level
  • things stay close to normal
  • hard to counteract larger changes
• Positive feedback


  • not involved in homeostasis - does not keep levels constant
  • amplifies changes made to levels
  • effectors respond, increasing level away from normal
  • often rapidly activate things
  • e.g. hypothermia
• Positive feedback - blood clots after injury

  • platelets are activated, and release a chemical that signals for more platelets to be activated
  • platelets form a blood clot at injury site
  • process ends with negative feedback, when blood clot formation is detected