response and stimuli

• what is a stimulus?

  a detectable change in the internal or external environment of an organism that leads to a response in the organism.
• the ability to respond to stimuli increases chance of survival for an organism
• those who survive have a greater chance of raising offspring and of passing their alleles to the next generation. this means there is a selection pressure favouring organisms with more appropriate responses
• what are stimuli detected by?

  receptors - these are specific to one type of stimulus
• what does a coordinator do?

  formulates a suitable response to a stimulus
• what produces a response

  an effector
• what two means of bodily communication do animals have

  hormones and the nervous system
• what is a taxis

  a simple response whose direction is determined by the direction of the stimulus. a motile organism responds directly to environmental changes by moving its whole body towards a stimulus thats favourable or away from an unfavourable on.
• what is positive taxis

  towards the stimulus
• what is negative taxis

  away from the stimulus
• what taxes do single celled algae exhibit?

  positive phototaxis [they move towards light] which increases their chance of survival because they're photosynthetic and require light to manufacture food
• what taxes do earthworms exhibit

  negative phototaxis [they move away from light]. this increases their chance of survival because it takes them into the soil, where they are better able to conserve water, find food, and avoid some predators
• some species of bacteria will exhibit positive chemotaxis [where they move towards a region where glucose is more highly concentrated]. this increases their chance of survival because they use glucose as a food source
• what is a kinesis

  when the organism doesn't move towards or away from the stimulus and instead changes the speed at which it moves and the rate at which it changes direction.
• how do woodlice display kineses?

  they lose water from their bodies in dry conditions. when they move from a damp area into a dry one, they move more rapidly and change directions more often. this increases their chances of moving back into the damp area. once back in the damp area, they slow down and change direction less.
• what is a tropism

  the growth of part of a plant in response to a directional stimulus
• what type of tropism do plant shoots exhibit?

  positive phototropism - towards light

  negative gravitropism - away from gravity
• what type of tropism do plant roots exhibit?

  negative phototropism- away from light

  positive gravitropism- towards gravity
• what three factors do plants respond to?

  1. light - shoots grow towards light
  2. gravity- plants need to be firmly anchored in the soil
  3. water- almost all plant roots grow towards water [positively hydrotropic]
• what group indoleacetic acids [IAAs] belong to?

  Auxins
• what does IAA control

  plant cell elongation in shoots and inhibits plant cell elongation in roots
• outline the reponse of the shoots of flowering plants to unilateral light

  1. cells in the tip of the shoot produce IAA, which is then transported down the shoot
  2. the IAA is intitially transported evenly throughout all regions
  3. light causes the movement of IAA from the light side to the shaded side of the shoot
  4. a greater concentration of IAA builds up on the shaded side, so they elongate more
  5. shaded side elongates faster than light side so the shoot tip bends towards light
• outline the process of gravitropism in flowering plants

  1. cells in the tip of the root produce IAA, which is transported along the root
  2. gravity influences the movement of IAA and a greater concentration builds up on the lower side of the root
  3. the cells on this side elongate less than those on the upper side. cells on top elongate faster so the root bends down towards gravity
• what is the acid growth hypothesis?

  hydrogen ions are actively transported from the cytoplasm into spaces in the cell wall causing the cell wall to become more plastic which allows the cell to elongate by expansion.
• what are the two major divisions of the nervous system?

  • the central nervous system [CNS] - which consists of the brain and the spinal cord
  • the peripheral nervous system [PNS] - which is made up of pairs of nerves that originate from either the brain or spinal cord
• what two divisions come from the peripheral nervous system

  • sensory neurrones- these carry nerve impulsees from receptors towards the CNS
  • motor neurones- these carry nerve impulses away from the CNS to effectors
• what two divisions come from the motor nervous system?

  • the voluntary nervous system- this carrries nerve impulses to body muscles and is under voluntary control
  • the autoonomic nervous system- this carries nerve impulses to glands, smooth muscle, and cardiac muscle and is not under voluntary control
• what is a spinal cord?

  a column of nervous tissue that runs along the back and lies in the vertebral column for protection. at intervals along the cord there are pairs of nerves

  
• what is a reflex?

  an involuntary response to a sensory stimulus
• what is the pathway of neurons involved in a reflex

  a reflex arc
• describe the order of a reflex arc

  1. stimulus
  2. receptor
  3. sensory neurone
  4. coordinator [intermediate neurone]
  5. motor neurone
  6. effector
  7. response
• why are reflex arcs important?

  1. they're involuntary and therefore dont require the brains use
  2. they protect the body from harm and dont need to be learned
  3. they are fast, because the neurone pathway is short
  4. the absence of any decision making means the action is rapid
• what do pacinian corpurscles respond to

  changes in mechanical pressure
• what are the features of pacianian corpuscles

  1. its specific to a single type of stimulus
  2. it produces a generator potential by acting as a transducer
• where do pacinian corppuscles occur

  • deep in the skin [fingers, soles of feet, external genitalia]
  • joints
  • ligaments
  • tendons
• outline the process of the pacinian corpuscle creating a generator potential

  1. stretch mediated sodium channels are too narrow for sodium to enter at resting potential
  2. pressure deforms the pacinian corpuscle and the membrane around the neurone becomes stretched and widens stretch mediated sodium channels
  3. sodium ions diffuse into the neurone
  4. this depolarises the membrane, producing a generator potential which creates an action potential
• rod and cone cells in the eye act as transducers by conserving light energy into the electrical energy of a nerve impulse
• Rod Cells-

  1. rod shaped
  2. greater numbers than cone cells
  3. more of them at the periphery of the retina, absent at the fovea
  4. give poor visual acuity
  5. sensitive to low - intensity light
  6. one type only
• cone cells-

  1. cone shaped
  2. fewer numbers than rod cells
  3. fewer at the periphery of the retina, concentrated at the fovea
  4. give good visual acuity
  5. not sensitive to low - intensity light
  6. three types, each responding to different wavelengths of light
• many rod cells are connected to a single sensory neurone in the optic nerve.