B3

• Central nervous system (CNS)


  Brain and spinal cord
• How the nervous system coordinates responses

  1. Receptor cells convert stimulus into electrical impulse

  2. Electrical impulse travels along sensory neurons to CNS

  3. Information processed in CNS, appropriate response coordinated

  4. Electrical impulse sent along motor neurons to effectors

  5. Effectors carry out response
• Reflex arc


  1. Stimulus detected by receptors

  2. Impulses sent along sensory neuron

  3. Impulse passes to relay neuron in CNS

  4. Impulses sent along motor neuron

  5. Impulse reaches effector, resulting in response
• Eye


  • Cornea refracts light to reach retina
  • Iris controls amount of light entering eye
  • Lens focuses light onto retina
  • Retina contains light receptors (rods and cones)
  • Optic nerve carries impulses between eye and brain
• Brain


  • Cerebrum: intelligence, vocabulary, personality, conscious thought
  • Cerebellum: coordinates voluntary movements, helps with balance
  • Medulla: controls heart rate, blood pressure, breathing rate
  • Hypothalamus: controls temperature, water balance
  • Pituitary gland: releases hormones
• Limitations in investigating and treating brain damage
• Endocrine system


  Hormones are chemical messengers secreted by endocrine glands into bloodstream, transported to target organ receptors
• Hormones


  • Adrenaline
  • Thyroxine
• Negative feedback


  If thyroxine levels too low, hypothalamus produces TRH, pituitary releases TSH, thyroid produces more thyroxine. When levels normal, inhibition stops further increases.
• Puberty


  • Testosterone responsible for male secondary sexual characteristics

  Oestrogen responsible for female secondary sexual characteristics
• Menstrual cycle


  Oestrogen causes uterus lining thickening

  FSH causes egg maturation, LH causes ovulation

  Progesterone maintains uterus lining, inhibits FSH and LH
• Fertility treatments


  Fertility drugs stimulate egg maturation and release

  IVF: eggs collected, fertilised, embryos implanted
• Plant hormones


  • Auxin: stimulates growth, causes phototropism and gravitropism
• Phototropism


  Light on one side, auxin moves to shaded side, stimulates more growth, shoot bends towards light
• Gravitropism


  Auxin moves to lower side of horizontal shoot, stimulates more growth, shoot bends away from ground

  Roots show positive gravitropism, growing towards gravity
• Phototropism


  Response to light
• Gravitropism

  Response to gravity
• Geotropism


  Response to gravity
• Hormones move from the place they are made to where they are needed in order to produce the appropriate response
• Auxins


  A growth hormone
• Positive phototropism


  1. Plant is exposed to light on one side

  2. Auxin moves to the shaded side of the shoot

  3. Auxin stimulates cells to grow more here

  4. Shoot bends towards the light

  5. Plant receives more light, meaning photosynthesis can occur at a faster rate
• Negative gravitropism (shoots)


  1. Auxin moves to the lower side

  2. Cells of the shoot grow more on the side with most auxin

  3. Shoot bends and grows away from the ground

  4. Beneficial as light levels are likely to be higher further away from the ground
• Positive gravitropism (roots)


  1. Auxin moves to the lower side

  2. Cells of the root grow more on the side with less auxin

  3. Root bends and grows downwards

  4. Beneficial as there are more likely to be increased levels of water and nutrients lower down, and it provides stability for the plant
• When the auxin distribution becomes equal on both sides it grows straight in that direction
• Investigating effect of light or gravity on newly germinated seedlings

  1. Placing in cardboard box and shining light from one side

  2. Attaching a petri dish containing the seedlings to a wall (effects of gravity)
• Weed killers


  • Contain auxin and only affect broad-leaved plants
  • Increased amount of auxin causes the cells to grow too rapidly, resulting in the weed dying
• Rooting powders


  Contain auxin and are applied to plant cuttings to promote root growth
• Tissue culture


  Cells from the plant are placed in a growth medium containing lots of nutrients and hormones such as auxins to form roots and shoots
• Gibberellins


  Another plant hormone important to stimulate seed germination, flowering and shedding of leaves
• Uses of gibberellins


  • Ending seed dormancy
  • Promoting flowering
  • Increasing fruit size
• Ethene


  Involved in cell division and the ripening of fruits
• Using ethene to ripen fruit

  1. Fruit is picked when it is not ripe

  2. It is exposed to ethene and warmer temperatures

  3. Ethene is involved in controlling cell division and stimulates enzymes that result in fruit ripening

  4. This reduces wastage as more fruit is suitable to be sold and it does not ripen too early
• Homeostasis


  The maintenance of a constant internal environment
• Thermoregulatory centre


  Monitors and controls body temperature, found in the brain
• Maintaining body temperature when too high

  1. Sweat is produced from sweat glands

  2. Vasodilation means more blood flows closer to the surface of the skin, resulting in increased energy transfer from the body
• Maintaining body temperature when too low

  1. Sweating stops

  2. Skeletal muscles contract rapidly (shivering) to generate heat from respiration

  3. Hairs stand on end to create an insulating layer, trapping warm air

  4. Vasoconstriction means blood does not flow so close to the surface, resulting in less heat lost
• Insulin


  A hormone produced by the pancreas that lowers blood glucose levels
• How insulin works


  1. Eating foods that contain carbohydrates increases the glucose levels in the blood

  2. If the glucose levels are too high, the pancreas produces insulin

  3. Insulin binds to cells in target organs (muscles and liver) causing glucose to move from the blood into muscle cells for respiration and excess glucose to be converted into glycogen which is stored in the liver

  4. The blood glucose concentration is reduced
• Glucagon


  A hormone produced by the pancreas that raises blood glucose levels
• How glucagon works


  1. If glucose levels decrease, the pancreas produces glucagon

  2. Glucagon binds to liver cells causing glycogen to be broken down into glucose

  3. Glucose is released into the blood, increasing the blood glucose concentration