response to stimuli

• Stimulus

  A detectable change in the internal or external environment of an organism that will lead to a response in the organism
• Stimulus

  • Increases the chance of survival
  • Examples: predators, extreme conditions, moving towards food
• Organisms with greater chance of survival

  Have a greater chance of passing their alleles to the next generation
• There is selection pressure favouring organisms that have better response to their environment
• Receptors

  Specific to one type of stimulus
• Stimulus response

  1. Stimulus detected by receptors

  2. Coordinator formulates suitable response

  3. Response produced by effector
• Coordination

  May be at a molecular level or involve a large organ (such as the brain)
• Response

  May be at molecular level or involve the behaviour of a whole organism
• Hormonal communication

  Relatively slow process, found in both plants and animals
• Nervous system communication

  Rapid, has many different receptors and control effectors, each receptor linked to central coordinator
• Stimulus response sequence

  Stimulus → receptor → coordinator → effector → response
• Taxes

  • Taxis is a response determined by the direction of the stimulus.
  • As a result, a motile organism responds directly to environmental changes either by moving its whole body towards a favourable stimulus or moving away from an unfavourable one.
  • Taxes classified by movement towards (positive taxis) or away (negative taxis)
  • Some examples:
  • Single-celled algae: move towards light (positive phototaxis).
  • earthworms: move away from the light (negative phototaxis)
  • Some species of bacteria: move towards regions of more highly concentrated glucose (positive chemotaxis)
• Kinesis

  Where the organism will change the speed at which it moves and the rate at which it changes direction
• If an organism crosses the line dividing its favourable condition and unfavourable condition

  Its rate of turning increases
• If moved into an unfavourable condition

  1. It will move in a long straight line

  2. Then sharply turn

  3. This type of response often brings the organism to a new region with favourable conditions
• Kinesis

  • Important when stimulus is less directional
  • E.g. humidity, temperature, do not always produce a clear gradient from one extreme to another
• Kinesis

  • Woodlice: lose water so move to damp area quickly and slow down once in one then move rapidly when not damp
• Plant responses to external stimuli

  • Light
  • Gravity
  • Water
• Positively phototropic


  Shoots grow towards the light
• Positively gravitropic


  Roots grow in the direction of gravity's pull
• Positively hydrotropic

  Roots grow towards water
• Plants need light for photosynthesis
• Plants need to be firmly anchored in soil
• Plants need water for photosynthesis and other metabolic processes, as well as support
• Plant growth factors

  Hormone-like substances that affect plant growth, made by cells throughout the plant rather than in particular organs, produced in small quantities
• Auxins


  A group of plant growth hormones that control cell elongation, including Indoleacetic acid (IAA)
• Unlike animal hormones, some plant growth factors affect the tissues that release them rather than acting on a distant target organ
• Control of tropisms by IAA

  • Tropism is the directional growth of a plant in response to a directional stimulus.
  • Such as light, can observe in a young shoot it will grow towards the light that it is directed at it from one side (unilateral light), known as positive phototropism.
• Response of shoots of flowering plants to unilateral light

  1. Cells in the top of the shoot produce IAA

  2. IAA is initially transported evenly throughout all regions as it begins to move down the shoot

  3. Light causes the movement of IAA from the light side to shaded side of the shoot

  4. Greater concentrations of IAA build up on shaded side of shoot than light side

  5. As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, cells on this side elongate more

  6. The shaded side of the shoot elongates faster than the light side, causes the shoot tip to bend toward the light
• IAA also controls the bending of roots in response to light
• • High concentrations of IAA increase cell elongation in shoots, it inhibits cell elongation in roots
• Response of horizontally-growing root to gravity

  1. Cells in tip of root produce IAA

  2. IAA is transported along root

  3. Gravity influences movement of IAA from upper side to lower side of root

  4. Greater concentration of IAA builds up on the lower side of the root than upper side

  5. IAA inhibits elongation of root cells, so there is greater concentration of IAA on the lower side, cells on this side elongates less than those on the upper side

  6. Relatively greater elongation of cells on the upper side compared to the lower side causes root to bend downwards towards the force of gravity
• In shoots, greater concentrations of IAA on the lower side

  Increases cell elongation and causes this side to elongate more than upper side, so shoot grows upwards away from the force of gravity
• some plants cells can detect gravity:

  • Columellar cells near the root tip possess heavy organelles called amyloplasts.
  • Amyloplasts are densely packed with starch, so they sink to the bottom of the cell
  • When a root is moved from the vertical plane to the horizontal plane these organelles fall to where the bottom of the cell is
• IAA (Indole-3-acetic acid)


  A plant hormone that plays a key role in elongation growth
• Transport of IAA

  • One direction (mainly away from the tip of shoots and roots where it is produced)
• Effects of IAA on plant cells

  • Increases plasticity (ability to stretch) in cell walls, especially young cell walls where cells can elongate
  • As cell walls mature, they develop rigidity- which means the older part of the shoot, root is not able to respond
• How IAA increases plasticity

  1. Acid growth hypothesis

  2. Active transport of hydrogen ions from the cytoplasm into spaces in the cell wall

  3. Causes the cell wall to be more plastic

  4. Allows the cell to elongate by expansion
• Elongation of cells on one side only of the stem or root


  Can lead to bending
• Plants respond relatively quickly to environmental stimuli e.g., light and gravity