Topic 6

• What constitutes a stimulus for an organism?

  A change in internal or external environment
• Why is an organism's ability to respond to stimuli vital?

  It enhances their chance of survival
• What is tropism in plants?

  Growth in response to a directional stimulus
• What is positive tropism?

  Growth towards a stimulus
• What is negative tropism?

  Growth away from a stimulus
• Summarize the function of growth factors in flowering plants.

  • Specific growth factors (hormone-like substances), e.g., Auxins (IAA), move from growing regions like shoot/root tips.
  • Transport occurs via phloem or diffusion.
  • They regulate growth in other tissues responding to directional stimuli (tropisms).

  
• How does high concentrations of IAA affect cells in shoots?

  It stimulates cell elongation

  
• What effect does high concentrations of IAA have on cells in roots?

  It inhibits cell elongation

  
• Explain the process 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 there.
  4. In shoots, this stimulates cell elongation; in roots, it inhibits cell elongation.
  5. Shoots bend away from gravity, while roots bend towards gravity.

  
• Explain the process of phototropism 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 shaded side of the shoot/root, increasing concentration there.
  4. In shoots, this stimulates cell elongation; in roots, this inhibits cell elongation.
  5. Shoots bend towards the light, while roots bend away from the light.

  
• Describe simple responses that maintain a mobile organism in a favorable environment.

  1. Taxes (Tactic Response):

  • Directional response.
  • Movement towards or away from a directional stimulus.

  1. Kinesis (Kinetic Response):

  • Non-directional response.
  • Speed of movement or rate of direction change changes in response to a non-directional stimulus.
  • Depending on the intensity of the stimulus.
• What type of response is taxis?

  Directional
• What type of response is kinesis?

  Non-directional
• What type of response do woodlice exhibit when moving away from light to avoid predators?

  Taxis
• What type of response do woodlice exhibit when moving faster in drier environments to find humid areas?

  Kinesis
• Explain the protective effect of a simple reflex arc.

  • Rapid: Involves only 3 neurons and few synapses, making synaptic transmission quick.
  • Autonomic: Doesn’t involve conscious regions of the brain, so it's not learned.
  • Protects from harmful stimuli: Enables escape from predators or prevents damage to body tissues.
• Why are reflexes rapid?

  Only 3 neurones and few synapses
• Why are reflexes autonomic?

  They don't involve conscious brain regions
• Describe how to investigate the effect of an environmental variable on woodlice movement using a choice chamber.

  1. Set up a choice chamber with different compartments to create varying environmental conditions (e.g., humidity using a drying agent or damp filter paper; light using light and black card).
  2. Control other environmental conditions (e.g., light intensity when investigating humidity).
  3. Place 12 woodlice on the center of a mesh platform and cover with a lid.
  4. After a set time (e.g., 10 minutes), record the number of woodlice in each section.
  5. Repeat, gently returning woodlice to the center.
• When investigating humidity, what could be used to create different humidity conditions in a choice chamber?

  A drying agent and damp filter paper
• Why should other environmental conditions, such as light intensity, be controlled when investigating humidity?

  To isolate the effect of humidity
• In a woodlice choice chamber experiment, why were the woodlice left for 15 minutes before movement recording?

  For humidity to establish and woodlice to acclimatise
• How could you ensure the safe and ethical handling of animals like woodlice in an experiment?

  Cover wounds, wash hands, return to habitat ASAP
• Why is a mesh platform used when investigating the effect of humidity on woodlice?

  To keep woodlice from drying agent
• Describe how to investigate the effect of an environmental variable on maggots using a maze.

  1. Change the environment at one end of a T-shaped maze (e.g., add a food source).
  2. Place a maggot in the stem of the T.
  3. Record whether the maggot turns towards or away from the food source.
  4. Repeat with a large number of maggots, wiping/cleaning the maze between trials.
  5. Repeat with food on the other side of the T.
• In a maze experiment with maggots, what can be used to change the environmental conditions?

  A food source
• Why shouldn't the same maggot be used more than once in a maze experiment?

  Reduces stress and prevents learned behaviours
• Why should a clean petri dish/maze be used for each trial in a maggot experiment?

  To prevent influence from previous chemicals/scents
• What statistical test is appropriate for analyzing results from a maggot maze experiment?

  Chi-squared
• Why is the Chi-squared test used for analyzing maggot maze data?

  Data is categorical and comparing frequencies
• In a Chi-squared test for a maggot maze, what is the expected frequency?

  Equal numbers each side
• Describe the basic structure of a Pacinian corpuscle.

  • It is a receptor that responds to mechanical pressure.
  • It is found in the skin.
  • It consists of layers of tissue (lamellae) surrounding a sensory neurone.
• Outline how a generator potential is established in a Pacinian corpuscle.

  1. Mechanical stimulus (e.g., pressure) deforms lamellae, stretching Na+ channels.
  2. Na+ channels open, and Na+ diffuses into the sensory neurone.
  3. Depolarisation occurs, leading to a generator potential.
  4. If the generator potential reaches the threshold, it triggers an action potential.

  
• What type of stimulus does a Pacinian corpuscle respond to?

  Mechanical pressure
• How does greater pressure affect the generation of a potential in a Pacinian corpuscle?

  More Na+ channels open and more Na+ enter
• What happens when the generator potential in a Pacinian corpuscle reaches the threshold?

  An action potential is triggered
• Explain what the Pacinian corpuscle illustrates about receptors.

  • Receptors respond only to specific stimuli.
  • Stimulation of a receptor leads to the establishment of a generator potential.
  • When threshold is reached, an action potential is sent (all-or-nothing principle).
• Explain the differences in light sensitivity between rods and cones in the retina.

  Rods are more sensitive to light:

  • Several rods connected to a single neurone.
  • Spatial summation enables reaching threshold and generating an action potential.

  
  

  Cones are less sensitive to light:

  • Each cone connected to a single neurone.
  • No spatial summation occurs.
• Why are rods more sensitive to light than cones?

  Several rods connect to a single neurone
• What process allows rods to reach threshold in low light conditions?

  Spatial summation