coordination and response

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• Edexcel IGCSE Biology: Double Science
• Contents


  • 2.80 Response to Changes in the Environment
  • 2.81 Homeostasis
  • 2.82 Co-ordinating Response
  • 2.83 Response to Stimuli: Plants
  • 2.84 Geotropism & Phototropism
  • 2.85 Role of Auxin in Phototropism
  • 2.86 Nervous & Hormonal Control
  • 2.87 Human Nervous System: Structure
  • 2.88 Human Nervous System: Function
  • 2.89 Role of Neurotransmitters at Synapses
  • 2.90 Simple Re}ex Arc
  • 2.91 The Human Eye: Structure
  • 2.92 The Human Eye: Function
  • 2.93 The Role of Skin in Temperature Regulation
  • 2.94 The Role of Hormones: Basic
• Your notes
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• Response to Change


  Animals and plants need to be able to respond to changes in their internal and external environment
• Homeostasis


  Physiological control systems that maintain the internal environment within restricted limits
• Examples of physiological factors controlled by homeostasis in mammals


  • Core body temperature
  • Metabolic waste (eg. carbon dioxide and urea)
  • Blood pH
  • Concentration of glucose in the blood
  • Water potential of the blood
  • Concentration of respiratory gases (carbon dioxide and oxygen) in the blood
• Communication systems in mammals


  • The nervous system
  • The endocrine system
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• Homeostasis


  The control or regulation of the internal conditions of a cell or organism
• Examples of internal conditions maintained by homeostasis


  • Water content (of an individual cell or of the body }uids of an organism)
  • Temperature
  • pH
  • Blood pressure
  • Blood glucose concentration
• Importance of homeostasis


  It is important for an organism to keep these internal conditions within set limits to ensure they stay healthy and to maintain optimum conditions to allow the organism to function in response to internal and external changes
• Consequence of exceeding homeostatic limits

  If these limits are exceeded, the organism may die
• Homeostasis maintains optimal conditions for enzyme action and all cell functions
• Examples of homeostasis in humans


  • Control of body temperature
  • Control of body water content
• Control of body temperature in humans


  • The core body temperature of humans is kept close to 37 °C
  • This is very tightly controlled as a change in core body temperature of more than 2 °C can be fatal
• Reason for tight control of body temperature

  Such a temperature change would stop essential enzymes from functioning optimally
• Thermoregulatory centre


  It monitors and controls body temperature
• Skin temperature receptors


  They send nervous impulses to the thermoregulatory centre
• Control of body water content in humans


  Water loss via the lungs (during breathing) or skin (during sweating) cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys
• Nephrons


  They contain structures called tubules, through which |ltrate passes on its way to the bladder
• Water reabsorption in collecting ducts

  Water can be reabsorbed from this |ltrate as it passes along these tubules
• If the water content of the blood is too high

  Less water is reabsorbed
• If the water content of the blood is too low

  More water is reabsorbed
• Blood water content receptors

  They are detected by receptors in the base of the brain as blood passes through it
• Antidiuretic hormone (ADH)


  It is constantly released by the pituitary gland in the brain
• Role of ADH


  It affects the permeability of the collecting ducts to water
• If the water content of the blood falls below a certain level


  • The blood is too concentrated
  • Receptors detect this and stimulate the pituitary gland to release more ADH
  • This causes the collecting ducts of the nephrons to become more permeable to water
  • This leads to more water being reabsorbed from the collecting ducts
  • The kidneys produce a smaller volume of urine that is more concentrated (contains less water)
• If the water content of the blood rises above a certain level


  • The blood is too dilute
  • Receptors detect this and stimulate the pituitary gland to release less ADH
  • This causes the collecting ducts of the nephrons to become less permeable to water
  • This leads to less water being reabsorbed from the collecting ducts
  • The kidneys produce a larger volume of urine that is less concentrated (contains more water)
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• Homeostasis


  It is under involuntary (automatic) control
• Brain stem and spinal cord


  They are involved in maintaining homeostasis
• Components of a control system that carries out co-ordinated responses


  • A stimulus (a change in the environment)
  • A receptor (receptor cells that detect stimuli)
  • A coordination centre (such as the brain, spinal cord and pancreas), which receives and processes information from receptors
  • An eyector (a muscle or gland), which brings about responses to restore optimum levels
• A co-ordinated response (such as that required when there is not enough water in the blood) requires a stimulus, a receptor and an eyector
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• Response to Stimuli: Plants

  Plants need to be able to grow in response to certain stimuli
• Tropisms


  The directional growth responses made by plants in response to light and gravity