Survival and Response

• What is a stimulus ?

  A change in the external or internal environment of an organism.
• What is a receptor ?

  a structure which detects the stimulus
• What is an effector ?

  a structure e.g. muscle that produces a response
• Definition of taxis.

  A movement of the entire organism in response to a directional stimulus.
• What are the two types of taxes ?

  Positive- move towards the greatest intensity of the stimulus

  Negative- move away from the greatest intensity
• What is the taxis for light called ? Give an example for +ive and -ive ?

  Phototaxis

  +euglena

  -earthworms, cockroaches, woodlice
• What is the taxis for chemical called ? Give an example for +ive and -ive ?


  Chemotaxis

  +ive: moss

  -ive: mosquitoes
• What is the taxis for air called ? Give an example for +ive and -ive ?

  Aerotaxis

  +motile aerobic bacteria

  -
• What is the taxis for resistance called ? Give an example for +ive and -ive ?

  Rheotaxis

  + moths and butterflies

  -
• A motile, photosynthetic protoctist swims towards optimum light intensity. State the taxis and describe the adaptive value of this response.

  Phototaxis

  By moving to an area of high light intensity it can photosynthesis at greater rates and produce more glucose which will allow it to grow and reproduce.
• A water flea swims directly towards low-intensity light and towards oxygen-rich environments; What taxis is displayed and describe the adaptive value of this response ?

  + positive aerotaxis

  • negative phototaxis

  Low light intensities ensure lower temperatures and less visibility to

  predators are more likely to survive and reproduce. The oxygen is used for a higher rate of aerobic respiration to make ATP.
• Definition of kenisis ?

  A non-directional movement response in which the rate of movement is related to the intensity of the stimulus and not the direction of the stimulus.
• Describe the kinesis of an organism in an unfavourable environment.

  Moves faster and turns less to get away from the starting position.
• Describe the kinesis of an organism in a favourable environment

  Moves slower, turns more to stay in the same area.
• What is the stimulus for geotaxis ?

  gravity
• Why might an organism have negative geotaxis ?

  To fly so they aren't in competition
• Definition of tropism.

  The response of a plant to a directional stimulus (a stimulus coming from a particular direction). Plants respond to stimuli by regulating their growth.
• Give three examples of tropisms.

  Phototropism

  Geotropism

  Hydrotropism
• What is phototropism ?

  The growth of a plant in response to light.
• What is geotropism ?

  The growth of a plant in response to gravity.
• State the tropics of shoots and roots of plants.

  Shoots = positive phototropism and negative geotropism

  Roots = negative phototropism and positive geotropism
• Where does IAA move on shoots and roots ?

  the shaded areas
• What are the three types of plant growth ?

  Cell division

  Cell elongation

  Cell differentiation
• Where are auxins made ?

  shoot apex and young leaves (apical meristems)
• What is the process by which auxins move ?

  diffusion
• Explain why shoots grow towards light.


  1.Cells in the tip of the shoot produce IAA.

  2. The IAA diffuses down the shoot.

  3. IAA moves from the light side to the shaded

  side of the shoot (by active transport).

  4. IAA concentration is greater on the shaded side

  of the shoot than on the light side.

  5. IAA causes elongation of cells so the cells on

  the shaded side elongate more.

  6. The shaded side of the shoot grows faster,

  causing the shoot to bend towards the light.