Aqa Alevel Biology

• Tropism

  A directional growth response in plants
• Types of tropism

  • Phototropism
  • Geotropism/Gravitropism
• Phototropism

  Shoots and roots grow in response to light
• Phototropism in shoots

  1. IAA (auxin) produced in tip

  2. IAA diffuses to shaded side

  3. Promotes growth and cell elongation on shaded side

  4. Shoot bends towards light
• Phototropism in roots

  • IAA diffuses to shaded side
  • Inhibits growth and cell elongation
  • Root bends away from light (negative phototropism)
• Geotropism/Gravitropism

  Shoots and roots grow in response to gravity
• Geotropism in shoots

  1. IAA diffuses to bottom side

  2. Promotes growth and cell elongation on bottom side

  3. Shoot bends upwards against gravity (negative geotropism)
• Geotropism in roots

  1. IAA diffuses to bottom side

  2. Inhibits growth and cell elongation

  3. Root bends downwards towards gravity (positive geotropism)
• Survival and response in mobile organisms

  • Taxes
  • Kinesis
  • Reflexes
• Taxes

  Directional responses in movement due to a stimulus
• Types of taxes

  • Positive taxes (move towards stimulus)
  • Negative taxes (move away from stimulus)
  • Chemotaxis
  • Phototaxis
  • Geotaxis
  • Rheotaxis
• Kinesis

  Non-directional responses in movement due to a stimulus
• Kinesis

  • Humans moving more when cold to keep warm
  • Woodlice making more turns when searching for favourable environment
• Reflexes

  Extremely rapid responses that do not require thought, creating a protective effect
• Reflex arc

  1. Stimulus detected by receptors

  2. Sensory neuron transmits signal to spinal cord

  3. Relay neuron transmits signal to motor neuron

  4. Motor neuron causes effector (muscle) to contract
• Similarities between taxes and tropisms: both involve directional responses to a stimulus
• Differences between taxes and tropisms: in taxes the whole organism moves, in tropisms it is a growth response</b>
• Taxes occur in mobile organisms, tropisms occur in plants
• Roots grow towards gravity

  But away from salt
• Salt has a greater effect on root growth than gravity
• The roots grow downwards in the agar gel containing no salt, but away from the salt in the agar gel containing salt
• Roots

  • Grow towards gravity
  • Grow away from salt
• Salt has more of an effect than gravity

  On the growth direction of roots
• IAA (indole acetic acid)

  • Transported out of cells by carrier proteins in root tips
  • High concentrations inhibit cell elongation
• Keeping seedlings in the dark

  Necessary to only measure the effects of gravity, not light (phototropism)
• Root tip removal

  Indicates cells in the root tip respond to gravity