Unit 6

• The nervous system means that humans can react to their surroundings and coordinate their behaviour
• Responding to their environment helps organisms survive

  • Animals increase their chances of survival by responding to changes in their external environment
  • They also respond to changes in their internal environment to make sure that the conditions are always right for their metabolism
  • Plants also increase their chances of survival by responding to changes in their environment
• Stimulus


  Any change in the internal or external environment
• Receptors and effectors


  • Receptors detect stimuli
  • Effectors produce a response
• How receptors and effectors communicate

  1. Via the nervous system

  2. Via the hormonal system

  3. Sometimes both
• Central nervous system (CNS)


  • Consists of the brain and spinal cord
  • Coordinates the response to a stimulus
• How the CNS coordinates a response

  1. Receptors in sense organs detect a stimulus

  2. Sensory neurones send electrical impulses to the CNS

  3. CNS sends electrical impulses to an effector along a motor neurone

  4. Effector responds accordingly
• Coordinated responses always need a stimulus, a receptor and an effector
• Neurones transmit information using high speed electrical impulses, allowing the nervous system to bring about very rapid responses
• Synapse


  The connection between two neurones
• How information is transferred at a synapse

  1. Neurotransmitters are released and diffuse across the gap

  2. They set off a new electrical signal in the next neurone
• Reflexes


  • Automatic responses to certain stimuli
  • Can reduce the chances of being injured
• The reflex arc


  1. Impulses travel along a sensory neurone to the CNS

  2. Relayed to a motor neurone

  3. Impulse travels to the effector (muscle) which contracts
• Reflexes bypass the conscious brain, allowing for a quicker response
• Parts of the eye

  • Conjunctiva
  • Sclera
  • Cornea
  • Iris
  • Pupil
  • Lens
  • Retina
  • Optic nerve
• Iris reflex


  • Bright light makes the pupil smaller
  • Dim light makes the pupil larger
• Accommodation


  1. Ciliary muscles relax to focus on distant objects

  2. Ciliary muscles contract to focus on near objects
• Short-sightedness


  Unable to focus on distant objects due to cornea/lens bending light too much or eyeball being too long
• Long-sightedness


  Unable to focus on near objects due to cornea/lens not bending light enough or eyeball being too short
• Hormones are chemical messengers sent in the blood to target cells
• Hormones and their roles

  • Adrenaline - readies body for fight or flight
  • Insulin - controls blood sugar level
  • Testosterone - main male sex hormone
  • Progesterone - supports pregnancy
  • Oestrogen - main female sex hormone
  • ADH - controls water content
  • FSH - causes egg maturation
  • LH - stimulates egg release
• Nerves


  Send very fast messages
• Hormones


  Send slower messages
• Homeostasis


  Balancing body functions to maintain a constant internal environment
• Nerves


  One way information can be sent around the body
• There are two ways information can be sent round the body - via the nervous system and the hormonal system
• Nerves


  • Very FAST message
  • Act for a very SHORT TIME
  • Act on a very PRECISE AREA
• Homeostasis


  Balancing body functions to maintain a "constant internal environment"
• Homeostasis involves balancing inputs (stuff going into your body) with outputs (stuff leaving)
• Ways water is lost from the body

  • Through the skin as sweat
  • Via the lungs in breath
  • Via the kidneys as urine
  • In faeces
• On a hot day, or when you're exercising

  You sweat a lot, produce less urine which is more concentrated
• Body temperature


  • Kept at about 37°C
  • Enzymes work best at this temperature
  • Brain acts as a thermostat to maintain this temperature
• When you're too hot

  1. Sweat is produced

  2. Blood vessels near skin widen

  3. Hairs lie flat
• Plants, like animals, increase their chances of survival by responding to changes in their environment
• Examples of plant responses

  • Sensing direction of light and growing towards it
  • Sensing gravity and growing roots downwards and shoots upwards
  • Climbing plants sensing touch to find things to climb
• Plants are more likely to survive if they respond to the presence of predators to avoid being eaten
• Plants are more likely to survive if they respond to abiotic stress - anything harmful that's natural but non-living, like a drought
• Smaller organisms


  • Have bigger surface area to volume ratios
  • Can gain or lose heat faster
• Larger organisms


  • Have smaller surface area to volume ratios
  • Gain or lose heat more slowly
• Auxins


  Plant growth hormones that control growth at the tips of shoots and roots