Unit 6: organisms respond to changes in their internal and external environments

Question 1

stimulus

Answer 1

a change in the internal or external environment

Question 2

receptor

Answer 2

detects a stimulus
-specific to one type of stimulus

Question 3

coordinator

Answer 3

formulates a suitable response to a stimulus

Question 4

effector

Answer 4

produces a response towards the stimulus

Question 5

taxis

Answer 5

a simple directional response determined by the direction of the stimulus

Question 6

positive taxis

Answer 6

motile organism moves towards the favorable stimulus

Question 7

negative taxis

Answer 7

motile organism moves away from the unfavorable stimulus

Question 8

positive phototaxis

Answer 8

organism moving towards light
e.g. single celled algae

Question 9

kinesis

Answer 9

form of response in which the organism does not move towards or away from the stimulus. instead it changes at the speed it moves and the rate at which it changes direction

Question 10

kinesis in favourable conditions

Answer 10

movement will be slower and direct, so the organism spends longer here

Question 11

kinesis in unfavourable conditions

Answer 11

movement is faster and involve more turns, to find the favourable condition

Question 12

tropism

Answer 12

the growth of a plant in response to a directional stimulus

Question 13

positive phototropism and negative gravitropism

Answer 13

plant shoots grow towards light and away from gravity

Question 14

negative phototropism and positive gravitropism

Answer 14

plant shoots grow away from light and towards gravity

Question 15

what do plants respond do

Answer 15

-light, shoots grow towards light because it is needed for photosynthesis
-gravity, plants need to be firmly anchored into the soil. roots are sensitive to gravity and grow in the direction of its pull
-water, almost all plants grow towards water (e.g. positively hydroptropic) in order to absorb it for use in photosynthesis and other metabolic processes

Question 16

what is plant growth controlled by

Answer 16

indoleacetic acid (IAA) which is an important auxin produced in the tips and shoots of flowering plants. the distribution of IAA around the plant controls tropisms.

Question 17

IAA on the effect of phototropism

Answer 17

when the shoot is illuminated from all side, the auxins are evenly distributed and move down the shoot tip thus causing elongation of cells across the cells of elongation.
whereas, if the shoot is only illuminated from one side, the auxins move towards the shaded part of the shoot thus causing elongation of the shaded side only which results in the bending of the shoot towards the light

Question 18

IAA on the effect of gravitropism

Answer 18

gravitropism in the roots is the opposite. IAA will build up on the lower side of the root. In roots IAA inhibits growth, therefore causing the cells on the upper side to grow faster, causing the root to bend downwards

Question 19

explain how the movement of IAA in shoots helps plants to survive

Answer 19

more IAA moves towards the shaded side of shoots than the light side when light is unidirectional. in repose to this uneven distraction of IAA, the cells on the long acted side elongate faster than those on the light side and the shoot bends towards the light
-this ensures that the shoot and the leaves attached to it have a greater chance of being illuminated
-as light is essential for photosynthesis, the process by which organic material for respiration is manufactured, the plant has a greater chance of survival

Question 20

what are the nervous systems two major division

Answer 20

central nervous system- made up of brain and spinal cord
peripheral nervous system- made up of pairs of nerves that originate from either the brain or the spinal cord

Question 21

what is the peripheral nervous system divided into

Answer 21

sensory nervous system- carry nerve impulses from the receptors the cns
motor nervous system- carry nerve impulses from the cns to the effectors

Question 22

what is the motor nervous system divided into

Answer 22

voluntary nervous system- which carries nerve impulses to body muscles and is under voluntary control
autnomic nervous system- carries nerve impulses to glands, smooth muscle and cardiac muscle. under involuntary control

Question 23

what is a reflex

Answer 23

type of involuntary response to a sensory stimulus

Question 24

what is a reflex arc

Answer 24

the pathway of neurons involved in a reflex

Question 25

order of the reflex arc

Answer 25

stimulus, receptor, sensory neuron, coordinator, motor neuron, effector, response

Question 26

importance of reflex arcs

Answer 26

-they are involuntary and therefore do not require the decision making powers of the brain. allows the brain to be free to carry out more complex responses -they protect the body from harm, present from birth and do not have to be learnt -they are fast, because the neuron pathway is short with very few synapses where neurons communicate with each other

Question 27

features of a Pacinian corpuscle

Answer 27

-is specific to a single type of stimulus, it only responds to mechanical pressure -produces a generator potential by acting as a transducer. Pacinian corpuscle transduces the mechanical energy of the stimulus into a generator potential

Question 28

basic structure of a Pacinian corpuscle

Answer 28

-single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule -stretch mediated sodium channels on plasma membrane -capillary runs along base layer of tissue

Question 29

what stimulus does a Pacinian corpuscle respond to and how

Answer 29

-pressure deforms membrane, causing stretch mediated sodium ion channels to open -if influx of sodium raises membrane to threshold potential (and the membrane becomes depolarised), a generator potential is produced -action potential moves along the sensory neuron

Question 30

2 types of photoreceptor cells located in the retina

Answer 30

-cone cells -rod cells

Question 31

location of rod cells

Answer 31

distribution more at the periphery of the retina -rod cells are in abundance because light intensity is lowest

Question 32

explain the low intensity response of rod cells

Answer 32

rhodopsin can break down the pigment in rod cells to create a generator potential. there is enough energy from low intensity light to cause this breakdown. -the stimulation of multiple rod cells can be combined to exceed the threshold value and therefore create a generator potential

Question 33

explain the visual acuity of rod cells

Answer 33

in perception, the brain cannot distinguish between the separate sources of light due to multiple rod cells only linking to one bipolar cell therefore it is only able to send a single nerve impulse

Question 34

location of cone cells

Answer 34

concentrated at the fovea as this is what receives the highest intensity of light

Question 35

explain the high intensity response of cone cells

Answer 35

each cone cell has their own sperate bipolar cell, so the stimulation of multiple cone cells cannot be combined to exceed the threshold value which produces a generator potential. -iodopsin requires a higher intensity for its breakdown and creation of a generator potential

Question 36

explain the visual acuity of cone cells

Answer 36

high visual acuity because each cone cell is connected to its own bipolar cell, which means that if two adjacent cone cells are stimulated, the brain receives two separate impulses. the brain cannot distinguish between the two separate sources of light that stimulated the cone cells

Question 37

what is visual acuity

Answer 37

the clarity of vision, determined by the individuals ability to recognise small details with precision

Question 38

what are the colours seen by rod + cone cells

Answer 38

rod- cannot distinguish different wavelengths of light so black and white only cone- each cone cell is sensitive to a specific range of wavelengths so can see in full colour (three types of cone cells)

Question 39

define myogenic

Answer 39

the hearts ability to initiate its own contraction

Question 40

pacemaker of the heart

Answer 40

sinoatrial node located in the wall of the right atrium -this initiates a wave of electrical stimulation which causes the atria to contract roughly at the same time

Question 41

describe the control of heart rate

Answer 41

-the SAN initiates a heart beat and sends a wave of electrical impulses across the atria, causing them to contract -after a short delay to allow the atria to empty before the ventricles contract, the AVN sends a wave of electrical impulses down the bundle of His -bundle of His receives electrical activity from the AVN and conducts a wave of excitation to the apex of the heart -purkyne tissues branch off the bundle of His, the wave of excitation is released from the purkyne tissues causing the ventricles to contract quickly at the same time from the bottom upwards

Question 42

what region of the brain controls heart rate

Answer 42

medulla oblangata

Question 43

2 divisions of the autonomic NS that control heart rate

Answer 43

sympathetic- increases heart rate parasympathetic- decreases heart rate

Question 44

control by chemoreceptors

Answer 44

they are sensory receptors found in the wall of the carotid arteries that detect changes in the concentration of chemicals in the blood -when the blood has increased levels of co2, the concentration of H+ ions also increases which causes the pH levels to fall -chemoreceptors detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata -this centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node, this increases the rate of production of electrical waves by the SAN, increasing heart rate -the increased blood flow, leads to more co2 being removed by the lungs and so the co2 concentration in the blood returns to normal -pH levels rise back to normal, chemoreceptors reduce the frequency of nerve impulses to the medulla oblongata -which reduces the frequency of impulses to the SAN therefore a reduction in heart rate

Question 45

control by baroreceptors

Answer 45

pressure receptors found within the walls of the carotid arteries -when blood pressure is higher than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate. this centre sends impulses down the parasympathetic NS to the SAN which decreases heart rate -when blood pressure is lower than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that increases heart rate. centre sends signals down the sympathetic NS to the SAN which increases heart rate

Question 46

two main forms of coordination

Answer 46

nervous system- uses nerve cells to pass electrical impulses along their length. they stimulate their target cells by secreting neurotransmitters directly onto them. hormonal system- produces hormones that are transported in the blood plasma to their target cells, which have specific receptors on their cell surface membranes and the change in hormone concentration stimulates them

Question 47

comparisons of hormonal and nervous systems

Answer 47

hormonal: -communication is by chemicals (hormones) -transmission is by the blood stream -transmission is usually relatively slow -hormones travel to all parts of the body, but only target cells respond -response is slow and often long lasting -effect may be permanent and irreversible nervous: -communication is by nerve impulse -transmission is by neurons -transmission is very rapid -nerve impulses travel to specific parts of the body -response is rapid and short lived -effect is usually temporary and reversible

Question 48

neurons

Answer 48

specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another

Question 49

cell body

Answer 49

contains all the usual cell organelles, including a nucleus and large amounts of rough ER. this is associated with the production of proteins and neurotransmitters

Question 50

dendrons

Answer 50

extensions of the cell body which subdivide into smaller branched fibers, called dendrites, that carry nerve impulses towards the cell body

Question 51

axon

Answer 51

single long fibre that carries nerve impulses away from the cell body

Question 52

Schwann cells

Answer 52

surround the axon, protecting it and providing electrical insulation. they also carry out phagocytosis

Question 53

myelin sheath

Answer 53

forms a covering to the axon and is made up of the membranes of the Schwann cells. neurons with a myelin sheath are called myelinated neurons

Question 54

nodes of Ranvier

Answer 54

constrictions between adjacent Schwann cells where there is no myelin sheath

Question 55

sensory neurons

Answer 55

nerve impulses from a receptor to an intermediate/motor neuron -one long dendron and one axon -cell body in the middle

Question 56

motor neurons

Answer 56

nerve impulses from an intermediate/relay neuron to an effector -long axon and many short dendrites -cell body at one end

Question 57

intermediate/relay neurons

Answer 57

impulses between neurons with numerous short processes

Question 58

define resting potential

Answer 58

when the inside of the axon is negatively charged relative to the outside of the axon, when this occurs the axon is polarised

Question 59

establishing a resting potential

Answer 59

-sodium ions ae actively transported out of the axon by the sodium potassium pumps -potassium ions are actively transported into the axon by the sodium potassium pumps -active transport of sodium is greater than that of potassium because 3 sodium ions move out for every 2 potassium ions in -as a result, there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, more potassium ions are in the cytoplasm than in the tissue fluid. this creates an electrochemical gradient -sodium ions begin to diffuse back naturally into the axon while the potassium ions diffuse back out of the axon -most of the gated channels for diffusion of sodium ions are closed but for potassium are open, maintaining the electrochemical gradient causing resting potential

Question 60

define action potential

Answer 60

when the negative charge inside the membrane becomes the positive charge, depolarising the axon, occurs when transmitting a nerve impulse -simply means that the axon membrane is transmitting a nerve impulse

Question 61

establishing an action potential

Answer 61

-at resting potential, some potassium voltage gated channels are open but the sodium voltage gated channels are closed -the energy of the stimulus causes some of the sodium voltage gated channels in the axon membrane to open so sodium ions diffuse into the axon along their electrochemical gradient -sodium ions are +ively charged, so this diffusion causes a reversal in the potential difference across the membrane -as more ions diffuse, more sodium ion channels open, causing a greater influx of the ions -once the action potential of around +40mV exists, voltage gates close on the sodium ion channels and those on potassium ion channels open -the electrical gradient which was preventing further further outward movement of the potassium ion is now reversed so even more channels open, starting repolarisation of the axon due to movement of potassium ions -this causes hyperpolarisation of the axon due to the inside being more negative than usual -potassium ion channel gates close and the sodium potassium pump moves sodium out and potassium's in -this re-establishes the resting potential so the axon is repolarised