3.6.1.1 Survival and response

• A stimulus is a detectable change in the internal or external environment.
• A receptor is an organ or specialised cell that can detect a change in the environment (stimulus).
• Organisms respond to stimuli to improve their chances of survival, such as avoiding predators or seeking food.

  • Responses can be categorised into tactic (taxis) and kinetic (kinesis) movements.
• Taxis: A directional movement in response to a stimulus.

  • A positive taxis is towards the stimulus and negative taxis is away from the stimulus.
• Kinesis: A non-directional or random movement in response to a stimulus, where the speed and frequency of movement depends on the stimulus strength.
• Receptors are specific to certain types of stimuli e.g. light, temperature, pressure etc.
• Effectors carry out responses and are typically muscles or glands.
• A tropism is a growth response controlled by a directional stimulus.

  • Phototropism is where the direction of growth is determined by the direction of light.
  • Gravitropism is where the direction of growth is determined by the direction of gravity.
• The shoots of plants are positively phototropic and grow towards light while roots are negatively phototropic and grow away from light.
• The roots have positive gravitropism as they grow toward the direction of gravity (downwards), whereas shoots have negative gravitropism as they grow away from the direction of gravity (upwards).
• Indoleacetic acid (IAA) is a growth hormone in plants (auxin). It is synthesised in shoot and root tips and can move to other parts of the plant via active transport or diffusion.
• Indoleacetic acid (IAA) regulates cell elongation, which is key for tropisms (directional growth responses). An uneven distribution of IAA causes uneven growth of the plant to occur.
• In shoots, IAA stimulates growth of cells (cell elongation) while in roots, IAA inhibits cell elongation.
• Shoot phototropism:

  • In shoots, IAA accumulates on the shaded side of the plant in response to light.
  • This causes cells on the shaded side to elongate more than those on the lighted side.
  • Result: The shoot bends toward the light (positive phototropism).
• Root phototropism:

  • In roots, high concentrations of IAA inhibit cell elongation.
  • IAA accumulates on the shaded side, leading to less growth there.
  • Result: Roots grow away from light (negative phototropism).
• Root gravitropism:

  • IAA accumulates on the lower side of the root due to gravity.
  • High concentrations of IAA inhibit cell elongation on the lower side.
  • Result: The upper side grows more, and the root bends downward (positive gravitropism).
• Shoot gravitropism:

  • In shoots, IAA again accumulates on the lower side.
  • High IAA concentration promotes cell elongation on the lower side.
  • Result: The shoot bends upward (negative gravitropism).
• IAA is used in horticulture to stimulate growth in cuttings.
• IAA is used in weed killers, where high concentrations of synthetic auxins act as herbicides, causing uncontrolled growth in weeds causing them to die.
• IAA is used in combination with other hormones to induce root or shoot formation in plant tissue cultures.
• Reflexes are rapid automatic responses that can protect an organism from harmful stimuli and therefore help them to survive and avoid danger.
• Reflexes bypass the brain whiich is why they are rapid and automatic. They are coordinated by the spinal cord.
• The reflex arc consists of:

  • The sensory neuron - detects the stimulus and carries the nerve impulse from the receptor to the spinal cord.
  • The intermediate (relay) neuron - relays the nerve impulse from the sensory neuron to the motor neuron.
  • Motor neuron - carries the nerve impulse from the spinal cord to the effector.
• Explain the importance of reflex actions (3)


  • automatic / involuntary
  • reducing / avoiding damage to tissues / prevents injury
  • role in homeostasis
  • posture / balance
  • finding / obtaining food / mate
  • escape from predators
• Tropisms help plants grow toward light (phototropism) or water and nutrients (hydrotropism), aiding survival.