Response To Stimuli

Question 1

Define stimulus

Answer 1

A detectable change in the internal/ external environment of an organism that leads to a response

Question 2

What is response and what are the advantages of being able to respond to stimuli

Answer 2

it increases the chances of survival in an organism which increases the likelihood of reproducing and passing alleles onto the next generation. Examples: phototaxis (worms and algae moved towards light for food, chemotaxis where bacteria move towards glucose)

Question 3

Define taxis and the different types

Answer 3

The movement of a motile organism in response to a stimulus, a simple directional response & direction is determined by direction of stimulus (+= towards stimulus and -= away from stimulus)

Question 4

Define kinesis

Answer 4

A motile organism doesn’t move towards or away from a stimulus so movement is non directional and random. Organism changes speed and rate of turning in response to a stimulus to remain in favourable conditions. In unfavourable= rate of turning deceases and speed increases to move in straight line back to favourable

Question 5

How does taxis and kinesis increase organisms rate of survival

Answer 5

Taxis- move to positive stimuli such as food or humidity if dry out easily and move away from predators in light, low food source) kinesis- stay in more favourable Areas and move away from unfavourable

Question 6

Define tropism

Answer 6

The growth of plants in response to a directional stimulus, plant grows towards (positive) or away (negative) from stimulus

Question 7

Describe the stimuli plants respond to

Answer 7

Light (in shoots there is + phototropism), gravity (in roots + gravitropism), water ( positive hydrotropism). Also temp, touch, atmospheric gases, pathogens, chemicals from other plants

Question 8

Can you describe plant growth factors such as IAA

Answer 8

affect growth and diffuse down shoot. Main growth factor is IAA which increases flexibility of cell walls, allowing cell elongation. (This done by acid growth hypothesis where IAA stimulates H+ pump so H+ move into cellulose and weaken it)

Question 9

What did Darwin originally find when investigating phototropism

Answer 9

When there is a unilateral light source, the shoot bends towards the light but there is no growth when the tip is removed and when there is a cover over the tip

Question 10

How do roots and shoots respond differently to IAA

Answer 10

In shoots, IAA stimulates cell elongation so high concs of IAA promote growth but in the roots, IAA inhibits cell elongation so high concs limit growth

Question 11

Explain phototropism in flowering plants

Answer 11

1. Cells in tip produce IAA, which diffuses down the shoot 2. Light causes movement of IAA to shaded side of shoot 3. Increased IAA causes cell elongation on shaded side, so shoot bends towards unilateral light source

Question 12

Explain gravitropism in flowering plants

Answer 12

In horizontal roots, the tips produce IAA which diffuses to the lower side due to gravity 2. The IAA inhibits cell elongation on the lower side (less inhibition on upper side) so lower side grows less and upper side grows more, bending down 1. In horizontal shoots, tips produce IAA which diffuses to the lower side due to gravity 2. IAA stimulates cell elongation so lower side grows more as upper side less stimulated

Question 13

Explain how a simple reflex arc works

Answer 13

1. A stimulus is a change in the internal or external environment of an organism 2. Receptor cells detect a stimulus and the nerve impulse is passed along a sensory neurone into the spinal cord/cns/control system 3. The impulse is taken up by a relay neurone and then the impulse leaves the spinal cord via a motor neurone which carries the impulse to an effect or which produces a response

Question 14

Outline the importance of reflex arcs

Answer 14

They are an involuntary and rapid response to changes in the environment which prevents injury/damage to tissues. Reflexes are important for homeostasis, balance, to escape from predators or for finding food

Question 15

The main features of sensory reception

Answer 15

Stimuli are detected through receptors and each receptor responds to a single specific stimulus. Receptors are found at the end of sensory neurones and act as transducers which convert stimulus into an electrical signal (first a generator potential and then an action potential)

Question 16

3 examples of sensory receptors

Answer 16

Pacinian corpuscle (respond to mechanical pressure), photoreceptors (rods and cones in the eye and respond to light) and chemo and pressure receptors in the heart that control heart rate

Question 17

Structure of pacinian corpuscle

Answer 17

It is a tactile mechanoreceptor deep in the skin. Has a nerve ending that has layers of connective tissue with viscous gel around it / Swann cells-axon-lamellae/connective tissue

Question 18

How does pacinian corpuscle work

Answer 18

Increased pressure deforms/ changes membrane and stretch mediated sodium ion channels. Sodium ion channels open and Na+ diffuse in which causes depolarisation, producing a generator potential. With greater pressure, more channels open and more Na+ enter

Question 19

Where are light receptor cells found and what are the two types

Answer 19

Found in the retina and they are rod cells and cone cells

Question 20

Describe the distribution/position of rods and cones in the retina (describe and draw it out)

Answer 20

Cones are only found at the fovea, rods not at fovea and more in periphery. No rods or cones in blind spot

Question 21

What are the general differences between rods and cones

Answer 21

Rods are more numerous (12m compared to 6m cones), there’s only one type or rod cell but 3 types of cone cell. Rods cells can’t distinguish between wavelengths of light so image in black and white whereas cone cells are sensitive to different wavelengths (green red and blue) so image seen in colour.

Question 22

What are the difference between pigments in rods and cones

Answer 22

In rods: pigment is rhodopsin (1 type) it has a high sensitivity to light and the pigment is easily bleached. In cones: lodopsin (3 types) and has a low sensitivity to light

Question 23

What does bleaching/breakdown of pigments cause

Answer 23

Produces a generator potential, which triggers a change in NA+ permeability (causes action potential in sensory neurone)

Question 24

What are the specific ability differences between rod and cone cells

Answer 24

Rod cells used to see in low light intensities as rhodopsin has high light sensitivity whereas cone cells respond to high light intensities as lodopsins have low sensitivity to light. Multiple rod cells synapse with a single sensory neurone/ retinal convergence (means greater chance threshold reached/summation) whereas each cone synapses with its own separate sensory neurone/ no retinal convergence. Rod cells have low visual activity due to retinal convergence but cone cells have high visual acuity as no retinal convergence (brain receives separate impulses so can distinguish between separate sources of light)

Question 25

Describe the electrical conduction system in the heart

Answer 25

1. A wave of electrical activity spreads out from the sino atrial node (San) initiating heart beat/atrial contraction 2. Wave of excitation enters the atrioventricular node (avn) 3 . There is a short delay so blood flows into the ventricles 4. Wave of excitation continues along bundle of his (muscle fibres between ventricles) and branched further 5. Wave passed to base or ventricle above purkyne tissue fibres, causing ventricles to contract from base up

Question 26

Describe the autonomic nervous system

Answer 26

It controls involuntary unconscious activities and is part of the motor nervous system. It has 2 divisions ( sympathetic and parasympathetic) that work antagonistically

Question 27

How does the autonomic nervous system control heart rate

Answer 27

The medulla oblongata stimulates impulses which are sent to the SAN to either increase or decrease heart rate via sympathetic or parasympathetic nerve

Question 28

How do receptors control heart rate

Answer 28

Receptors Send impulses to medulla. Chemoreceptors detect changes in pH and baro receptors detect changes in pressure

Question 29

How do chemical receptors control heart rate

Answer 29

1. During exercise, respiration in muscles increases so co2 levels in blood increase and pH decreases 2. Chemoreceptors detect change and increase frequency of impulses to medulla 3. Medulla oblongata increases frequency of impulses through the sympathetic nerve to SAN in heart, which increases heart rate 4. More blood flows through lungs so more co2 is excreted and pH is raised

Question 30

How do pressure receptors control heart rate

Answer 30

1. When blood pressure is higher than normal, baroreceptors detect the change and increase frequency of impulses to the area in the medulla that decrease heart rate. Medulla increases frequency of impulses through parasympathetic nerve to SAN which decreases heart rate and heart rate and blood pressure decrease