SURVIVAL AND RESPONSE

• • A stimulus is a change in the internal or external environment.
  • The internal environment is inside an organism.
  • The external environment is outside an organism.
• A receptor detects a stimulus.
• • A coordinator formulates a suitable response to a stimulus.
• • An effector produces a response.
• • Receptors are specific to one type of stimulus.
• • Nerve cells pass electrical impulses along their length. A nerve impulse is specific to a target cell only because it releases a chemical messenger directly onto it, producing a response that is usually rapid, short-lived and localised.
• • Mammalian hormones stimulate their target cells via the blood system. They are specific to the tertiary structure of receptors on their target cells and produce responses that are usually slow, long-lasting and widespread.
• • Plants control their response using hormone-like growth substances.
• Stimuli, both internal (inside an organism) and external (outside an organism), are detected and lead to a response
• • Taxes and kineses are simple responses that can maintain a mobile organism in a favourable environment.
• • Taxis is a directional response where a whole organism moves either towards (positive taxis) or away from (negative taxis) the stimulus.
• • Kinesis involves random movements by the whole organism.
  • If the organism is in unfavourable conditions, it moves fast and turns little.
  • If the organism is in favourable conditions, it moves slowly and turns a lot.
• Tropism

  • This is a directional response that involves an organism either growing towards (positive tropism) or away from (negative tropism) a stimulus.
• • In flowering plants, specific chemical growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli.

  Plants use tropisms to ensure the leaves get enough light for photosynthesis and roots get access to enough water and inorganic nutrients for growth.
• IAA

  • IAA is a growth factor made in tips of roots and shoots.
  • IAA diffuses around plant.

  There are light and gravity receptors in the root tips and shoot tips that can cause IAA to move and redistribute within the tip, which in turn affects its diffusion pathway from the tip.
• How roots and shoots grow

  • New cells are produced by mitosis at the tip.
  • Growth continues due to cell elongation behind the tip.
  • Regulating cell elongation is important in photo- and gravitropism.
• The nervous system is useful for controlling responses that are:

  • rapid
  • short-lived
  • localised.
• Neurones communicate with each other chemically via neurotransmitter release at a junction called a synapse. Chemical transmission is slower than electrical transmission.
• Key features of a reflex response

  Have basic survival functions, such as:

  • Reducing damage to tissues e.g. from burning;
  • Allowing escape from predators;
  • Homeostasis.

  Rapid, because they are:

  • involuntary (don’t require conscious thought)
  • controlled by a reflex arc with a small number of synapses (chemical transmission slower than electrical impulses)

  Do not have to be learned.
• Reflex responses are also known as:

  • Reflex actions
  • Reflexes
• REFLEX ARC

  A) Stimulus (e.g. heat)

  B) receptor nerve ending in skin sensitive to heat

  C) sensory neurone passes nerve impulses to spinal cord

  D) intermediate neurone pass impulses across the spinal c

  E) effector contracts

  F) motor neurone passes impulse to the muscle

  G) response - hand is moved away quickly

  H) TS spinal cord (mag x5)

  
• The Pacinian corpuscle should be used as an example of a receptor to illustrate that:

  1. receptors respond only to specific stimuli
  2. stimulation of a receptor leads to the establishment of a generator potential.
• Transducers

  • Pacinian corpuscles are pressure receptors, mainly in the skin.
  • They are thought to be particularly important in detecting vibrations, which may play a role in how our skin can detect different textures.
  • We say that a Pacinian corpuscle “transduces” mechanical energy into electrical energy.
  • A transducer can change energy from one form to another.
  • Receptors act as transducers.
• How a Pacinian corpuscle works

  • Increased pressure stretches the neurone membrane;
  • This deforms the stretch-mediated sodium ion channels embedded in the membrane of the sensory neurone;
  • These channels therefore open and sodium ions now diffuse into the neurone;
  • This depolarises the membrane leading to a generator potential;
  • The greater the pressure, the more stretch-mediated sodium ion channels open and the larger the generator potential;
  • If the generator potential is large enough, a critical voltage called threshold is reached and nerve impulses are sent along the neurone.