STIMULI AND RESPONSE

• Define stimulus


  a stimulus is a detectable change in the environment
• what are receptors?


  they are cells that are specific to one type of stimulus which they detect
• Organisms increase their chance of survival by responding to stimuli via?

  response mechanisms
• a response is produced by

  an effector
• The response in flowering plants is called?

  Tropism
• What is Tropism?


  When plants respond via growth to a stimuli

  They can be positive or negative (growing towards or growing away from stimulus)
• What do plants respond to?

  Light and Gravity
• An example of a specific growth factor that controls tropisms?

  indoleacetic acid (IAA)
• What is IAA?


  A type of auxin

  can control cell elongation in shoots

  inhibit growth of cells in roots
• Where is IAA made?

  In the tip or the roots and shoots

  BUT can diffuse to other cells
• What is phototropism?


  When plant is responding to LIGHT
• Explain phototropism in shoots?

  Light is needed in shoots for the LDR in photosynthesis

  so plants grow and bend towards the light (positive phototropism)

  1. Shoot tip cells produce IAA - causing cell elongation (towards light source)
  2. IAA diffuses to other cells (shaded side)
  3. If there is a unilateral light - IAA will diffuse towards the shaded side of the shoot and resulting in a higher conc of IAA there
  4. The cells on the shaded side elongate more + result in plant bending towards the light source
• Explain phototropism in the roots

  Roots do not photosynthesis so do not require light

  SO must anchor plant deep in soil - which is good for survival as will reach more water sources

  IN ROOTS - a high conc of IAA inhibits cell elongation

  causes root cells to elongate more on lighter side so roots bend away from the light

  • NEGATIVE PHOTOTROPISM in roots
• What is gravitropism?


  when plant responds to GRAVITY
• Gravitropism in shoots?


  IAA will diffuse from upper side to lower side of a shoot -> diffusing down towards gravity

  If a plant is vertical, this causes a plant cell to elongate and the plant grows upwards

  If a plant is on its side, it will cause the shoot to bend upwards (away from gravity)

  • NEGATIVE GRAVITROPISM IN SHOOTS
• Gravitropism in roots


  IAA moves to the lower side of roots - so upper side elongates and root bends down towards gravity + anchors plant in

  Plant moves down towards gravity

  • POSITIVE GRAVITROPISM IN ROOTS
• Define Reflex


  a rapid, automatic response to protect you from danger
• What are the three neurones a reflex arc is made up of?

  Sensory, Relay and Motor - only 2 synapses hence reflex is so rapid
• What are the 2 simple responses?

  Taxes and Kinesis - they are simple as keep organisms within the favourable conditions of their environment (light, moisture and chemicals)
• Explain Taxes?


  An organism will move its ENTIRE BODY TOWARDS a favourable stimulus or AWAY from an unfavourable stimulus
• What is positive taxis?


  when an organism moves towards a stimulus
• What is negative taxis?

  when an organism moves away
• Kinesis 


  An organism changes SPEED of movement and RATE it changes direction
• What do receptors do

  DETECT stimuli
• What happens when a receptor responds to its specific stimuli?

  stimulation of receptor leads to the establishment of a generator/ action potential which can cause a response
• What are the three receptors?

  Pacinian Corpuscle

  Rods

  Cones
• Two examples of PHOTO/LIGHT receptors?

  Rods and cones
• What changes do Pacinian corpuscle detect?

  Pressure
• Where are Pacinian corpuscle located?

  Deep in Skin, mainly in fingers and feet
• Describe Pacinian Corpuscle structure?

  Sensory neurone - wrapped in layers of plasma membrane (which have special channel proteins)

  In between each layer there is connective tissue with gel
• Describe the 'special channel proteins' in membranes of Pacinian corpuscle?

  They are stretch-mediated sodium channels

  These open and allow Na+ to enter the sensory neurone when they are stretched and deformed
• Explain how Pacinian corpuscle membrane channels become stretched and deformed.

  When pressure is applied it deforms

  The neurone plasma membrane stretches and widens the Na+ channels so Na+ diffuses in

  which leads to establishment of generator potential

  when Na+ has reached threshold value
• Where are rods and cones found?


  Human Retina
• What does the generator potential create?

  An action potential that passes to CNS (Central Nervous System)
• Draw and label a Pacinian Corpuscle?

  :

  
• When pressure is applied where do the sodium ions release into?

  nerve ending - to create a generator potential
• Rod cells only process images in?

  BLACK AND WHITE
• Why can rod cells on see images in black and white?

  As they cannot differentiate different wavelengths
• To create a generator potential in rod cells what needs to happen?

  Rhodopsin (pigment of rod cells) must be broken down by light energy
• At what intensity can rod cells detect light and why?

  very low intensity, as many rod cells connect to one sensory neurone (retinal convergence)