Communication and Homeostasis

• Cell signalling: the way in which cells communicate with each other
• All living things need to maintain conditions inside the cell for enzymes to work efficiently:

  • suitable temperature
  • suitable pH
  • aqueous environment that keeps substrates and products n solution
  • freedom from toxins and excess inhibitors
• Cells in multicellular organisms must communicate to coordinate activities as they are specialised and rely on each other.
• Changes in external environment places stress on the living organism e.g cooler environment causes greater heat loss
• Environmental change is a stimulus and the way that organism changes its behaviour or physiology to reduce the stress is its response
• Gradual changes in environment as seasons pass elicit a gradual response e.g arctic fox has thicker white coat in winter and thinner grey/brown coat in summer; provides greater insulation and camouflage in winter so animal can survive but does not overheat in summer.
• Appearance of predator or moving from burrow into sunlight are rapid changes in the environment and a stimulus must be monitored for the organism to respond to the change.
• Most multicellular organisms have range of tissues and organs, many of which are protected by epithelial tissues and organs such as skin or bark so aren't exposed to the external environment. Environment of cells is the internal bathing of cells and tissues in tissue fluid.
• Use of substrates and production of new products as cells undergo metabolic pathways means some may be toxic or unwanted so have to move out of cells into tissue fluid. Hence, activities of cells alter their own environment.
• Build up of waste products in tissue fluid can also act directly on cells which reduce their activity in response to produce less waste. But this response may not be good for the entire organism.
• Build up of waste product carbon dioxide in tissue fluid outside cells alters pH of tissue fluid and disrupts action of enzymes and other proteins. Accumulation of toxins or excess waste must act as a stimulus to cause removal of products so cell can survive. In example of carbon dioxide, reduced pH of blood stimulates a greater breathing activity which expels carbon dioxide.
• Blood maintains compostition of tissue fluid.
• Blood flows throughout body and transports substances to and from cells. Any waste products or toxins in tissue fluid likely to enter the blood and be carried away and so in order to prevent their accumulation inside the blood itself they are removed from the body by excretion.
• Concentrations of waste products and other substances in blood monitored closely to ensure body does not excrete too much any useful substance but removes enough waste products to keep in good health. Also ensures cells are provided with substrates they need.
• A good communication system will:

  • enable specific communication
  • enable rapid communication
  • enable both short and long term responses
  • enable cells to communicate with each other
  • cover the whole body
• 2 major systems of communication working by cell signalling:

  • neuronal
  • hormonal
• Neuronal communication:

  • interconnected network of neurones signalling to each other across a synapse junction
  • can conduct signal very quickly
  • enable rapid responses to stimuli that could be changing quickly
• Hormonal system:

  • uses blood to transport signals
  • Cells in endocrine organ release signal (hormone) directly into blood where it is transported throughout the body but only recognised by specific target cells
  • enables longer-term responses to be coordinated
• Effector: a cell, tissue or organ that brings about a response
• Homeostasis: maintaining a constant internal environment despite changes in external and internal factors. Aspects maintained by homeostasis include:

  • body temperature
  • blood glucose concentration
  • blood salt concentration
  • water potential of blood
  • carbon dioxide concentration
  • blood pressure
• Negative feedback: mechanism that reverses a change and brings system back to optimum
• Positive feedback: mechanism that increases a change and takes system further away from the optimum
• Sensory receptors: cells/sensory nerve endings responding to a stimulus in the internal or external environment of organism and can create action potentials
• Standard response pathway:

  Stimulus -> receptor-> communication pathway (cell signalling) -> effector -> response
• What are sensory receptors specialized structures for?

  They are specialized structures that detect changes in the environment.
• Where are temperature receptors located?

  Temperature receptors are located on the surface of the body, such as in the skin.
• What do sensory receptors monitor?

  They monitor changes in the external environment.
• What is an example of an internal receptor?

  Temperature receptors in the brain.
• What happens when a receptor detects a change?

  A message is sent to an effector when a receptor detects a change and is stimulated.
• What are the two communication systems mentioned?

  The neuronal system and the hormonal system.
• How do the neuronal and hormonal systems function?

  They act by signaling between cells to transmit messages from receptor cells to effectors.
• What are messages from receptors to the coordination center called?

  Input.
• What are messages sent to effectors known as?

  Output.
• What role do effector cells play?

  Effector cells bring about a response.
• Give two examples of effector cells.

  Liver cells and muscle cells.
• What is the overall process of sensory reception and response?

  1. Sensory receptors detect changes in the environment.
  2. Messages are sent to the coordination center (input).
  3. Coordination center processes the information.
  4. Messages are sent to effectors (output).
  5. Effectors bring about a response.
• Feedback: changes in input when effectors respond to output from coordination center and bring about response that changes internal conditions which are then detected by receptors which in turn has an effect on response pathway
• Negative feedback is required to maintain a constant internal environment so any changes away from optimum must be revered to bring them back towards the optimum.
• Receptors detect stimulus and send input to coordination center which sends an output to effectors and brings about a change reversing the initial change in conditions. Thus, the system moves closer to the optimum and the stimulus is reduced. Receptors detect this and reduce input to the coordination center which means output from coordination center is also reduced so effectors reduce their activity too. As system reaches optimum, response is reduced.
• For a negative feedback to work:

  1. Change to internal environment must be detected
  2. Change must be signalled to other cells
  3. Must be an effective response that reverses changes in conditions