Organisms Respond to Changes in their Internal and External Environments

Question 1

define stimulus

Answer 1

any change in the internal or external environment

Question 2

define receptors

Answer 2

these are cells or proteins on cell membranes which detect stimuli.

Question 3

define effectors

Answer 3

these are cells that bring about a response to a stimulus, to produce an effect. effectors include muscle cells and cells found in glands.

Question 4

what are the three main types of neurons?

Answer 4

sensory neurons
motor neurons
relay neurons.

Question 5

what is the function of sensory neurons?

Answer 5

sensory neurons transmit electrical impulses from receptors to the central nervous system; the brain and spinal cord.

Question 6

what is the function of motor neurons?

Answer 6

motor neurons transmit electrical impulses from the CNS to effectors.

Question 7

what is the function of relay neurons?

Answer 7

relay neurons transmit electrical impulses between sensory neurons and motor neurons.

Question 8

how does the nervous system send information as electrical impulses?

Answer 8

a stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neuron.
when an electrical impulse reaches the end of a neuron, chemicals called neurotransmitters take the information across to the next neuron, which then sends an electrical impulse.
the central nervous system processes the information and sends impulses along the motor neurons to an effector.

Question 9

define reflex.

Answer 9

a reflex is where the body responds to a stimulus without making a conscious decision to respond.

Question 10

how do simple reflexes help organisms?

Answer 10

they help to protect the body because they’re rapid.

Question 11

define reflex arc

Answer 11

the pathway of neurons linking receptors to effectors in a reflex.

Question 12

explain the hand-withdrawal response to heat as an example of a simple reflex arc.

Answer 12

thermoreceptors in the skin detect the heat stimulus.
the sensory neuron carries impulses to the relay neuron.
the relay neuron connects to the motor neuron.
the motor neuron sends impulses to the effector (your biceps muscle).
your muscle contracts to withdraw your hand and stop it from being damaged.

Question 13

what happens if there is a relay neuron involved in the simple reflex?

Answer 13

its possible to override the reflex, e.g. like in the hand-withdrawal example where a relay neuron was involved, your brain could tell your hand to withstand the heat.

Question 14

how is the nervous system communication localised, short-lived and rapid?

Answer 14

when an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto target cells; so the nervous response is localised.
neurotransmitters are quickly removed once they’ve done their job, so the response is short-lived.
electrical impulses are really fast, so the response is rapid; this allows animals to react quickly to stimuli.

Question 15

how are flowering plants able to increase their chances of survival by responding to stimuli?

Answer 15

-they can sense the direction of light and grow towards it to maximise light absorption for photosynthesis.
-they can sense gravity, so their roots and shoots grow in the right direction.
-climbing plants have a sense of touch, so they can find things to climb up and reach the sunlight.

Question 16

define tropism

Answer 16

the response of a plant to a directional stimulus (a stimulus coming from a particular direction).

Question 17

how do plants respond to stimuli?

Answer 17

by regulating their growth.

Question 18

define positive tropism

Answer 18

growth towards the stimulus

Question 19

define negative tropism

Answer 19

growth away from the stimulus.

Question 20

define phototropism

Answer 20

this is the growth of a plant in response to light

Question 21

how do shoots respond to phototropism?

Answer 21

shoots are positively phototropic and grow towards light.

Question 22

how do roots respond to phototropism?

Answer 22

roots are negatively phototropic and grow away from light.

Question 23

define gravitropism

Answer 23

this is the growth of a plant in response to gravity.

Question 24

how do shoots respond to gravitropism?

Answer 24

shoots are negatively gravitropic and grow upwards.

Question 25

how do roots respond to gravitropism?

Answer 25

roots are positively gravitropic and grow downwards.

Question 26

how are responses in plants brought about?

Answer 26

they are brought about using specific growth factors.

Question 27

what are growth factors?

Answer 27

these are hormone-like chemicals that speed up or slow down plant growth.
they are produced in the growing regions of the plant (shoot tips, leaves) and they move to where they’re needed in the other parts of the plant.

Question 28

what are auxins?

Answer 28

these are a growth factor which stimulates the growth of shoots by cell elongation; this is where cell walls become loose and stretchy, so the cells get longer.
high conc. of auxins inhibit growth in roots.

Question 29

what is Indoleacetic Acid (IAA)?

Answer 29

it is an important auxin that’s produced in the tips of shoots in flowering plants.
IAA is moved around the plant to control tropisms; it moves by diffusion and active transport over short distances, and via the phloem over long distances.
this results in different parts of the plant having different concentrations of IAA. uneven distribution of IAA means there’s uneven growth.

Question 30

how does IAA act with phototropism in plants?

Answer 30

the IAA moves to the more shaded parts of the shoots and roots, so there’s uneven growth.

Question 31

how does IAA act in shoots in phototropism?

Answer 31

IAA concentration increase on the shaded side; cells elongate and the shoot bends towards the light.

Question 32

how does IAA act in roots in phototropism?

Answer 32

IAA concentration increases on the shaded side; growth is inhibited so the root bends away from the light.

Question 33

how does IAA act with gravitropism in plants?

Answer 33

IAA moves to the underside of shoots and roots, so there’s uneven growth.

Question 34

how does IAA act in shoots in gravitropism?

Answer 34

IAA concentration increases on the lower side; cells elongate so the shoot grows upwards.

Question 35

how does IAA act in roots in gravitropism?

Answer 35

IAA concentration increases on the lower side; growth is inhibited so the root grows downwards.

Question 36

what are the two simple responses that keep simple organisms in a favourable environment?

Answer 36

tactic responses (taxes)
kinetic responses (kineses)

Question 37

define taxes

Answer 37

the organisms move towards or away from a directional stimulus e.g light.

Question 38

explain an example of taxes using woodlice

Answer 38

woodlice show a tactic response to light (phototaxis). they move away from a light source. this helps them survive as it keeps them concealed under stones during the day (safe from predators) and keeps them in damp conditions (which reduces water loss).

Question 39

define kineses

Answer 39

the organisms’ movement is affected by a non-directional stimulus, e.g humidity.

Question 40

explain an example of kineses using woodlice

Answer 40

woodlice show a kinetic response to humidity. in high humidity they move slowly and turn less often, so that they stay where they are. as the air gets drier, they move faster and turn more often, so that they move into a new area.
(may need editing with another example).

Question 41

why are receptors described as specific?

Answer 41

they only detect one particular stimulus.

Question 42

what are the different types of receptors?

Answer 42

cells and proteins on cell surface membranes

Question 43

how do receptor cells that communicate information via the nervous system work?

Question 44

what are pacinian corpuscles?

Answer 44

they are mechanoreceptors; they detect mechanical stimuli, e.g pressure and vibrations.
they are found in your skin.

Question 45

what do pacinian corpuscles contain?

Answer 45

they contain the end of a sensory neurone, imaginatively called a sensory nerve eneding. the sensory nerve ending is wrapped in lots of layers of connective tissue called lamallae.

Question 46

what happens when a pacinian corpuscle is stimulated?

Answer 46

the lamellae are deformed and this presses on the sensory nerve ending.
this causes the sensory neurone’s membrane to stretch, deforming the stretch-mediated sodium ion channels. the channels open and sodium ions diffuse into the cell, creating a generator potential.
if the generator potential reaches the threshold, it triggers an action potential.