14: Response to stimuli

Question 1

Stimulus

Answer 1

a detectable change in the environment. these changes are detected by cells, which are called receptors
- organisms increase chance of survival by responding to stimuli via different response mechanisms

Question 2

Tropisms

Answer 2

when plants respond, via growth, to stimuli
- tropisms can be positive or negative, growing towards or away from a stimulus. plant respond to light or gravity

Question 3

one example of a specific growth factor tropisms are controlled by

Answer 3

• indoleacetic acid (IAA)
• a type of auxin which controls cell elongation in shoots and inhibit growth of cells in roots. its made in tip of roots or shoots but can diffuse to other cells

Question 4

Phototropism in shoots

Answer 4

tropisms where plant is responding to light
- shoots; light in LDR so plants grow and bend towards light - positive phototropism
1. shoot tip cells produce IAA, cell elongation
2. IAA diffuses to other cells
3. if unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there
4. so cells on shaded side elongate and plant bends towards light source

Question 5

Phototropism in roots

Answer 5

• roots dont photosynthesis, so dont require light. anchor plant deep in soil
• in roots, high conc of IAA inhibits cell elongation. so root cells on light side elongate more and root bends away from light to anchor into ground and reach water sources
• negative phototropism

Question 6

Gravitropism in shoots

Answer 6

• IAA will diffuse from the upper side to the lower side of a shoot
• if a plant is vertical, this causes plant cells to elongate and plant grows upwards
• if plant on its side it will cause the shoot to bend upwards. - negative gravitropism

Question 7

Gravitropism in roots

Answer 7

• IAA moves to lower side of the roots so that the upper side elongates and the roots bends down towards gravity and anchors the plant
• positive gravitropis

Question 8

Reflex

Answer 8

a rapid, automatic response to protect you from danger. a reflex arc is made up of three - neurones
- sensory, relay, motor

Question 9

Simple responses

Answer 9

Taxis and Kinesis which keep organisms within the favourable conditions of their environment (light, moisture, chemicals)

Question 10

Taxes

Answer 10

• organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus
• when organism moves a stimulus its positive taxis

Question 11

Kinesis

Answer 11

• an organism changes the speed of movement and the rate it changes direction
• if in harmful stimuli, its kinesis response will be to increase the rate it changes direction to return to favourable conditions quickly
• if organism is surrounded by negative stimuli, the rate of turning decreases

Question 12

Receptors

Answer 12

cells that detect stimuli
- each receptor responds to a specific stimuli which creates a generator potential which can cause a response

Question 13

Three examples of receptors

Answer 13

1. pacinian corpuscle
2. rods
3. cones

Question 14

Pacinian corpuscle

Answer 14

• pressure receptors located deep in skin, mainly in fingers and feet
• the sensory neuron in the pacinian corpuscle has special channel proteins in its plasma membrane
• the membranes of pacinian corpuscle have STRETCHED MEDIATED SODIUM CHANNELS
• these open and allow Na to enter sensory neurone only when stretched and deformed
• when pressures applied it deforms the membrane, stretches and widens the Na channels so Na diffuses in which leads to the establishment of a generator potential

Question 15

Rods

Answer 15

• type of photoreceptor
• images in black and white
• to create generator potential, pigment of rod cells (RHODOPSIN) must be broken down by light energy
• detects light at very low intensity as many rod cells connect to ONE sensory neurone (retinal convergence)
• so brain cant dinstinguish between separate sources of light that stimulated it - low visual acuity

Question 16

Cones

Answer 16

• processes images in colour
• three types that contain different types of iodopsin pigment (red, green and blue) which all absorb different wavelengths of light
• iodopsin only broken down if high light intensity, so action potentials can only be generated with enough light
• one cone cell connects to a bipolar cell. so cones can only respond to high light intensity - why we cant see colour in dark
• as each cone connects to one bipolar cell, brain can distinguish between seperate sources of light detected - high visual acuity

Question 17

Distribution of rod and cones

Answer 17

• distribution in retina is uneven
• lights focused by lens of fovea, which will recieve the highest intensity of light
• most cone cells are located near the fovea, so can only detect the colour light in high intensitys
• rod cells further away as can still detect light at low light intensitys

Question 18

differences between cone and rod cells

Answer 18

rod;
- rod shaped
- low visual acuity
- sensitive to low intensity light
- one type only
- greater number
- more at periphery rather than fovea of retina

cones;
- cone shaped
- high visual acuity
- not sensitive to low light intensity
- 3 types
- fewer
- more at fovea

Question 19

the nervous system

Answer 19

two major divisons;
CNS and PNS (divides into sensory neuron system and motor neuron system)
- the motor neuron system divides into voluntary ns (which is conscious to body muscles) and autonomic vs (which is unconscious to glands, smooth and cardiac muscle)

Question 20

Autonomic nervous system

Answer 20

divides into sympathetic and parasympathetic nervous system
- sympathetic = stimulates effectors so speeds up activity. fight or flight
- parasympathetic = inhibits effectors and slows down activity

Question 21

reflex arc (pathway of neurons)

Answer 21

1. stimulus
2. receptor
3. sensory neuron passes nerve impulses to spinal cord
4. coordinator neuron links sensory to motor neuron in spinal cord
5. motor neuron, nerve impluse to muscles from spinal cord
6. effector
7. response

Question 22

What is the cardiac muscle

Answer 22

its myogenic, which means that it contracts on its own accord
- but the rate of contraction is controlled by the wave of electrical activity

Question 23

Whats the sinoatrial node and where is it located

Answer 23

• its located in the right atrium and is the pacemaker

Question 24

Parts of the heart that relate to controlling heart rate

Answer 24

• sinoatrial node
• atrioventricular node (lies between atria)
• bundle of His (runs through the septum)
• purkyne fibres in the walls of ventricles

Question 25

Control of the heart process

Answer 25

- SAN releases a wave of depolarisation across atria causing it to contract - AVN releases another wave of depolarisation when the first reaches it. a non-conductive layer between the atria and ventricles prevents the waves of depolarisation travelling down to the ventricles. - instead the wave of depolarisation travels down the bundle of His down the septum and the purkyne fibres - as a result, there is a short delay before the apex and ventricle walls contract. this is so the atria has enough time to contract to pump all the blood into the ventricles before the ventricles contract - finally the cells repolarise and the cardiac muscles relaxes

Question 26

What triggers the san to release waves of depolarisation

Answer 26

in the autonomic nervous system; if more impulses are sent down the sympathetic nervous system to the SAN, SAN releases waves of depolarisation more frequently - if more impulses down parasympathetic to the SAN, less depolarisation waves from SAN

Question 27

Homeostatic control of the heart rate

Answer 27

- hr changes in repsonse to pH and blood pressure. stimuli detected by chemoreceptors and pressure receptors in the aorta and carotid artery

Question 28

heart rate in response to pH

Answer 28

- in high respiratory rates and increased muscular activity, the pH decreases due to production of co2 and lactic acid - excess acid must be removed from blood rapidly to prevent enzymes denaturing - achieved by increasing heart rate to diffuse co2 out of alveoli more rapidly - chemoreceptors in walls of carotid arteries and the aorta detect this and increase freq of nervous impulses to the centre in the medulla oblongata that speeds heart rate - this increases frequency of impulses to sinoatrial node via the sympathetic ns - this increases production of electrical waves by sinoatrial node = increases heart rate = blood flow = removing co2 from lungs

Question 29

heart rate in response to pressure receptors

Answer 29

- pressure receptors are in walls of carotid arteries and the aorta A) when blood pressure higher than normal = pressure receptors transmit more nervous impulses to centre in medulla oblongata that decrease heart rate. this centre sends impluses to the SNA via the parasympathetic nervous system B) when blood pressure is lower than normal. pressure receptors send more nervous impulses to the medulla oblongata centre to increase heart rate. this is via sympathetic nervous system to SNA