6.1.1 Survival and response

• What is a stimulus?


  A change in an organism's internal or external environment.
• Why is it important that organisms can respond to stimuli?

  Organisms increase their chance of survival by responding to stimuli.
• What is a tropism?

  The growth of a plant in response to a directional stimulus.
• What are the types of tropism and their directions?

  • Positive tropism: growth towards a stimulus
  • Negative tropism: growth away from a stimulus
• What are growth factors in flowering plants?

  Specific growth factors are hormone-like growth substances.
• How do auxins (such as IAA) move in flowering plants?

  They move via phloem or diffusion from growing regions to other tissues.
• What is the role of growth factors in flowering plants?

  They regulate growth in response to directional stimuli (tropisms).
• How does indoleacetic acid (IAA) affect cells in shoots?

  High concentrations of IAA stimulate cell elongation in shoots.
• How does indoleacetic acid (IAA) affect cells in roots?

  High concentrations of IAA inhibit cell elongation in roots.
• What are the steps of gravitropism in flowering plants?

  1. Cells in the tip of the shoot/root produce IAA.
  2. IAA diffuses down the shoot/root (evenly initially).
  3. IAA moves to the lower side of the shoot/root (increasing concentration).
  4. In shoots, this stimulates cell elongation; in roots, it inhibits cell elongation.
  5. Shoots bend away from gravity; roots bend towards gravity.
• What must an organism find in order to survive?

  Favourable living conditions
• Why is finding favourable living conditions easier for complex organisms like monkeys and wolves?


  They possess eyes, ears, and nervous systems to observe and process environmental information
• How do simpler organisms like flatworms find ideal living conditions?

  They display behaviours known as taxes and kineses
• What are taxes and kineses?

  • Taxes: Directional responses to stimuli
  • Kineses: Non-directional responses to stimuli
• What is kinesis?


  Kinesis is a non-directional response to a stimulus
• How does the intensity of a stimulus affect kinesis?

  The rate of movement of an organism is affected by the intensity of the stimulus
• What type of simple eye-like structures do planarians possess?

  Light-sensitive cells
• What happens to planarians when they are removed from their usual dark environment?


  They display kinesis and begin to move in random directions
• Why do planarians move randomly when a stone is removed or turned over?

  They have no way of detecting the nearest shaded space
• When do planarians stop moving during kinesis?

  When they return to a dark environment
• What is the purpose of kinesis for planarians?

  To protect themselves from predators by ensuring they are in a favourable environment
• What is taxis?


  Taxis is a directional response to a stimulus
• How does an organism move in response to taxis?

  The organism moves directly away from or towards the stimulus
• What single-celled organism exhibits taxis?

  Euglena
• What is phototaxis?


  It is the movement towards light
• Why is phototaxis valuable for Euglena?

  It brings the organism towards light where it can photosynthesize
• How can taxes and kineses behaviour be studied in small animals?

  • Using special apparatus like choice chambers and mazes
  • Common organisms studied include woodlice and maggots
• What is a challenge in distinguishing taxis from kinesis in experiments?

  It can be difficult to observe changes in turning frequency or movement rate
• What happens to the turning frequency of an organism when it detects a stimulus during taxis?

  The turning frequency would decrease
• What was the purpose of the experiment conducted with maggots in a choice chamber?

  • To investigate whether maggots exhibited negative phototaxis
  • To see if they moved away from bright light
• How was the choice chamber set up for the maggot experiment?

  One half was covered in opaque material to prevent light from entering
• How many maggots were placed into the choice chamber during the experiment?

  30 maggots
• What was done after placing the maggots in the choice chamber?

  The number of maggots in each half was counted after 10 minutes
• What did the results of the maggot experiment show?

  There were always more maggots in the shaded half of the chamber
• What conclusion can be drawn from the maggot experiment regarding their response to light?

  Maggots can detect bright light and respond by moving to a more favourable environment
• How can the choice chamber be altered for experiments?

  • Create subsections with different abiotic conditions