3.6 organisms respond to changes in their environment

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  • 3.6 organisms respond the changes in environment
    • stimuli and response
      • stimulus- detectable change in environment of an organism that leads to a response in the organism
        • stimuli are detected by receptors - specific to one type of stimulus
          • coordinator formulates a suitable response to a stimulus - a response is produced by an effector
      • taxes - a simple response where organism moves towards or away from stimulus (positive or negative taxis)
        • kineses - changes the speed the organism moves and the rate at which it changes direction
          • tropisms - either positive or negative response e.g. phototropism. IAA- cells in the tip of the shoot produce IAA then it is transported down the shoot - light causes IAA to move to shaded side elongating those cells bending the tip of shoot towards the light. IAA in root - same thing
      • reflex arc -involuntary, stimulus - receptor - sensory neurone - coordinator - motor neurone - effector - response
      • pacinian corpuscle - responds to changes in mechanical pressure, acts as a transducer - converts energy from stimulus into nerve impulse
        • at resting state, the stretch mediated sodium channels remain shut, when it is stimulated they open allowing Na+ ions into neurone, depolarisation
      • eye - rod cells and cone cells - both act as transducers by conserving light energy into the electrical energy of a nerve impulse
        • there are more rod cells- they cannot distinguish different wavelengths of light - used to detect light of very low intensity, a certain threshold value has to be exceeded before a generator potential is created
          • many rod cells lead to a single bipolar cell - due to summation there is more of a chance of the threshold value being exceeded - only a single impulse travels to the brain
        • cone cells - three different types, each responding to different wavelengths of light, have their own bipolar cell - no summation
          • can only respond to high light intensity- they give accurate vision
    • nervous coordination
      • the nervous system - rapid and specific, release chemicals called neurotransmitters - short lived
        • neurones- dendrons- extensions of cell body that carry nerve impulses towards cell body, axon - carries impulse away from cell body
          • schwann cells- electrical insulation, make up the myelin sheath, nodes of Ranvier, constrictions between schwann cells where there is no myelin sheath
      • hormonal system- chemicals transported in the blood, longer lasting and more widespread
      • nerve impulse
        • speed - action potential remains the same all the way along the neurone, myelin sheath (electrical insulator) - jumps from one node of ranvier to another
          • the greater the diameter of an axon, faster the speed of conductance - less leakage of ions
            • temperature - affects rate of diffusion but could also denature enzymes if too high
        • all or nothing - stimulus has to be over threshold or no action potential - all action potentials are the same size so the strength of the stimulus is detected by the number of impulses in a given time


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