3.6 Organisms Respond To Changes In Internal And External Environments

Question 1

What is a stimulus and why is it important to respond to them?

Answer 1

•A detectable change in the environment. These changes can be detected by cells that are called receptor.
•Increases chance of survival

Question 2

What is kinesis?

Answer 2

•if organism moves to an area with harmful stimuli= increase the rate it changes direction to return to the favourable conditions quickly
•if it is in an area with beneficial stimuli it decreases speed
•non-directional response to unfavourable conditions

Question 3

What is taxis?

Answer 3

A directional response, where they move towards or away from a stimulus

E.g -ve chemotaxis = moving away from beneficial chemical
/ +ve phototaxis = moving towards light

Question 4

Why do organisms respond to temperature and humidity via kinesis?

Answer 4

Less directional stimuli where there is often no clear gradient from one extreme to the other

Question 5

What is the effect of IAA in the shoot tip?

Answer 5

•in unilateral light, IAA will diffuse towards the shaded side
•it causes the cells to elongate more and this will bend the top to the light source
=positive phototropism

Question 6

What is the effect of IAA in the root tip?

Answer 6

•IAA will move to the lower side of the root
•this will inhibit cell elongation so that roots anchor into the soil
= positive gravitropism and negative phototropism

Question 7

What are the events involved in a response?

Answer 7

1.stimulus= change in internal/external environment
2.receptor= detects stimulus and respond by producing action potential in neurones
3.sensory neurone= carries impulses from receptor to CNS
4.coordinator=in CNS where info is interpreted
5.motor neurone= impulse from CNS to effector (gland/muscle)

Question 8

What are the features of the endocrine system?

Answer 8

•widespread action
•hormones transported in blood
•long lasting response
•slow effect

Question 9

What are the features of the nervous system?

Answer 9

•localised in a specific area
•neurotransmitters (chemical coordinator)
•short lived
•rapid effect
•impulses directly to target cells

Question 10

What is a reflex action?

Answer 10

•rapid, involuntary response to a stimulus
•innate (not learned)

Question 11

Why are reflexes important?

Answer 11

•increase survival =escape predators
•role of homeostasis
•leave brain free to carry out complex responses
•protect body from harm
•fast as neurone pathway is short

Question 12

What are the structures of a neurone and their functions?

Answer 12

•cell body
•dentrites: receive info and carry it towards cell body
•axon: transmit impulses away from cell body
•myelin sheath: fatty material which insulates the axon so no loss of impulses or crossing over occurs
•Schwann cells wrap around axon to form myelin sheath, gap between called nodes of ranvier
•axonites

Question 13

How is a resting potential established?

Answer 13

•3Na+ out and 2K+ into the axon by active transport via the Na-K pump
•electrochemical gradient produced = K+ diffuse out and Na+ in by facilitated diffusion
•membrane is more permeable to K+ as more K+ channels so more K+ moves out = -70 mv inside the axon

Question 14

What are the steps involved in an action potential?

Answer 14

Depolarisation:
•neurone stimulated opens voltage gates Na+ channels
•+ve feedback = more channels open
•Na+ flood down conc. gradient into axon = +40 mv in axon

Repolarisation:
•+40 mv reached, Na+ voltage gated channels close
•K+ voltage gated channels open and K+ flood down conc. gradient out of axon so -70 mv in axon

Hyper polarisation:
•too many K+ move out of axon so temporarily more -ve than -70 mv

Question 15

What is the all-or nothing principle?

Answer 15

•if depolarisation < -55mv= no action potential
•all at -55 mv will trigger a depolarisation of same magnitude to +40 mv max
•bigger stimuli increases the frequency of action potentials

This is important as only large stimuli are responded to for increased survival

Question 16

What is the refractory period and why is it important?

Answer 16

(Action potential cannot be stimulated right away after one already)=time delay between AP
•action potentials are separate form one another (distinct)
•AP will travel in one direction
•limits frequency of AP so prevents over reaction to a stimulus so senses not overwhelmed

Question 17

Why does a myelinated axon conduct impulses faster than non-myelinated axon?

Answer 17

•myelin sheath is an electrical insulator
•in myelinated, action potential (depolarisation) can only occur at the node
•nerve impulse jumps from node to node
•action potential does not travel along the whole length of the axon
(Saltatory conduction)

Question 18

Give the structure of a synapse and of a neurotransmitter junction

Answer 18

•synaptic cleft
•neurotransmitter (acetylcholine)
•pre and postsynaptic neurone
•neuro receptor: membrane of the post synaptic neurone has chemical gated ion channels

Question 19

What happens at the synapse?

Answer 19

1. An action potential arrives at the synaptic knob. Depolarisation of synaptic knob leads to opening of Ca2+ channels and Ca2+ diffuses in
   2.vesicles with neurotransmitter move towards and fuse with the pre synaptic membrane and is released in the cleft
   3.neurotransmitter diffuses down a conc. gradient to post synaptic membrane (binds by complementarity of shape of receptors)
   4.Na+ ion channels on post synaptic membrane open and Na+ diffuses in (above threshold= depolarisation)
   5.neurotransmitter is degraded and released from receptor (back to pre synaptic neurone to be recycled), Na+ channels close and the post synaptic= resting potential

Question 20

What is different in a cholinergic synapse?

Answer 20

•neurotransmitter is acetylcholine and will bind to receptors on post synaptic membrane
•it is broken down to acetyl and choline, moving back to pre synaptic knob.
•ATP energy is used to recombine it and it is stored in vesicles
•more acetylcholine can be made by the smooth endoplasmic reticulum

Question 21

Compare cholinergic synapses and neuromuscular junction

Answer 21

NJ CS
•unidirectional as receptors only on post synaptic membrane
•excitatory |•excitatory or
|inhibitory
•motor neurone|•2 neurones
to muscles |
•end point for AP|•new AP
•acetylcholine |•acetylcholine binds to muscle |binds to post
fibres | synaptic
|membrane

Question 22

Why are transmissions involved in synapses unidirectional?

Answer 22

•vesicles only released in the pre synaptic neurone
•receptors only on post synaptic neurone so only bind to one side

Question 23

What is temporal summation?

Answer 23

One presynaptic neurone releases neurotransmitter repeatedly over a short period of time to add up enough Na+ diffusing to exceed threshold

Question 24

What is spatial summation?

Answer 24

Many different pre synaptic neurones collectively trigger a new action potential by combining the neurotransmitter they release to exceed the threshold

Question 25

What occurs during inhibition by inhibitory synapses?

Answer 25

•Cl- will move into and some K+ move out of the post synaptic neurone
•-ve inside and +ve outside = -80 mv (hyperpolarisation so action potential is unlikely)

Question 26

How does the diameter of the axon affect speed of conductance?

Answer 26

•wider diameter, faster speed of conductance
•larger the diameter, lower the resistance to the flow of ions so faster wave of depolarisation travels along the neurone
•there will be less leakage of ions so action potentials travel faster

Question 27

How does temperature affect speed of conductance along the axon?

Answer 27

1.ions diffuse faster as more KE energy
2.enzymes involved in respiration work faster = so more ATP for active transport in the Na/K+ pump

Question 28

How does the pacinnian corpuscle work and where?

Answer 28

1)mechanical stimulation (pressure) deforms stretch mediated Na+ channels on the membrane of the ending of the sensory neurone
2)stretch mediated Na+ channels widen and allow some Na+ to diffuse into the sensory neurone
3)generator potential is produced
4)leads to an action potential in the neurone if threshold is met

Located in joints, tendons, skin and ligaments

Question 29

Why is the pacinian corpuscle described as a transducer?

Answer 29

Transfers energy from mechanical to electrochemical energy (stimuli into a generator potential)

Question 30

What are transducers and e.g?

Answer 30

Convert light energy to electrical energy e.g rod and cone cells

Question 31

What are rod cells and function?

Answer 31

•rhodopsin pigment is broken down by low light levels to generate action potential
•sensitive to light as many rod synapses with one bipolar neurone (retinal convergence) so neurotransmitters add together to reach threshold = action potential
•low acuity as different stimuli may produce only one action potential

Question 32

What are cone cells and function?

Answer 32

•iodopsin pigment is broken down by high light levels to generate an action potential
•3 different types of cells each sensitive to different wavelengths of light
•low sensitivity as one cone synapse with bipolar cell so lots of light is needed to release enough neurotransmitter to reach threshold
•high acuity as each cone synapses with one bipolar cell so will produce one action potential for each stimulus (can distinguish between separate light sources close together)