Unit 6 - Response to Stimuli

Question 1

what changes in their environment do organisms respond to & what is the effect?

Answer 1

organisms detect & respond to internal & external stimuli

–> increases survival chances & increases chances of reproduction so passes on beneficial alleles

Question 2

why is there always a strong selection pressure?

Answer 2

to avoid danger/predation
to detect prey
to avoid toxic build up e.g. CO2
to ensure effective O2 delivery by altering heart rate

Question 3

what is the purpose of taxis & kinesis?

Answer 3

they are simple movements that can maintain a mobile organism in a favourable environment

Question 4

describe kinesis

Answer 4

simple, non-directional movement of mobile organism
in response to unfavourable stimulus
changes the speed at which the organism moves & the rate at which it changes direction depending on conditions
in response to non-directional stimulus e.g. temperature

Question 5

in kinesis, what happens if an organism is in favourable conditions (or has just moved from favourable to unfavourable conditions)?

Answer 5

rate of changing direction increases to increase chances of returning to favourable conditions quickly

Question 6

in kinesis, what happens if an organism is in unfavourable conditions?

Answer 6

rate of changing direction decreases so organism moves in straighter line to increase chances of finding a location with favourable conditions (surrounded by +ve stimuli)

Question 7

describe taxis

Answer 7

more advanced than kinesis
directional movement of mobile organism towards favourable conditions & away from unfavourable conditions
+ve taxis = towards stimulus
-ve taxis = away from stimulus
in response to directional stimulus e.g. light, chemicals, gravity etc.

Question 8

describe tropism & example

Answer 8

plant growth response (or part of a plant)
in response to directional stimulus
enable favourable conditions for max. growth

e.g. shoots show +ve phototropism & -ve gravitropism
roots show -ve phototropism & +ve gravitropism & +ve hydrotropism

Question 9

what causes tropism?

Answer 9

uneven distribution of IAA auxin, which causes uneven cell elongation & growth

Question 10

what do plants produce to control growth & responses to light & gravity?

Answer 10

hormones

Question 11

what is the benefit of phototropism?

Answer 11

to aid photosynthesis

Question 12

what is the benefit of gravitropism?

Answer 12

to obtain water, mineral ions & better anchorage

Question 13

what does IAA stand for?

Answer 13

indolacetic acid

Question 14

describe the response of shoots to light from directly above? (phototropism)

Answer 14

IAA diffuses evenly to both sides of the shoot
so even cell elongation & growth on both sides
so shoot grows straight up

Question 15

describe the response of shoots to light from one direction? (phototropism)

Answer 15

IAA diffuses to shaded side of the shoot
so cells on shaded side elongate more & grow faster than cells on sunny side
so shoot grows towards light

Question 16

what is the effect of the force of gravity on IAA?

Answer 16

the force of gravity causes IAA to accumulate on the underside of roots & shoots

Question 17

describe gravitropism (response to gravity) in roots

Answer 17

1. cells in root tip produce IAA
2. IAA diffuses & accumulates on underside of root due to the force of gravity
3. IAA inhibits cell growth & elongation on underside of root
4. so cells on upperside grow faster & elongate more than underside cells
   –> so roots grow downwards in the direction of gravity
   +vely gravitropic

Question 18

describe gravitropism (response to gravity) in shoots

Answer 18

1. cells in shoot tip produce IAA
2. IAA diffuses & accumulates on underside of shoot due to force of gravity
3. IAA stimulates cell growth & elongation on underside of shoot
4. so underside cells grow faster & elongate more than upperside cells
   –> shoot grows upwards against gravity
   -vely gravitropic

Question 19

describe the organisation of the nervous system

Answer 19

CNS: brain, spinal cord

peripheral nervous system (PNS):
sensory pathways (S neurones from receptor to CNS)

motor pathways:
somatic/voluntary NS - conscious control e.g. movement
autonomic/involuntary NS - subconscious control e.g. heart rate: sympathetic - stimulate effectors & speed up
parasympathetic - inhibits effectors & slows down

Question 20

what is a reflex & e.g.?

Answer 20

a rapid, short-lived, localised & involuntary response to a dangerous/harmful stimulus

e.g. removing hand from hot object

Question 21

what makes a reflex rapid?

Answer 21

very few synapses (chemical message is slower than electrical impulse)
short neurone pathway
does not go to conscious part of brain

Question 22

why are reflexes important? classic exam Q

Answer 22

to decrease or avoid damage - give e.g. related to Q
to escape from predators
to maintain balance/posture
role in homeostasis

Question 23

describe the reflex arc (in exam, relate to Q)

Answer 23

1. stimulus e.g. sharp pin
2. receptor - pressure/mechanoreceptors in skin detect stimulus & generate potential in sensory neurone
3. sensory neurone transmits action potential to spinal cord in CNS
4. relay/intermediate neurone links sensory neurone to motor neurone
5. motor neurone transmits action potential from spinal cord (CNS) to effector = muscle or gland e.g. muscles on finger/arm
6. effector - muscle contracts/gland secretes e.g. finger/arm muscle contracts
7. response e.g. pull finger/hand away from sharp object

Question 24

from Seneca: function of dendrites, axon & cell body

Answer 24

dendrites carry impulse towards cell body
axon - away
cell body - where nucleus is located