6.1 - Detection of Stimuli

Question 1

What is tropism?

Answer 1

Tropism is the term given when plants respond, via growth to stimuli.

Question 2

What is tropism controlled by?

Answer 2

Tropism is controlled by specific growth factors e.g Indoleacetic Acid (IAA)

Question 3

What is IAA?

Answer 3

IAA is a type of auxin and can control cell elongation in shoots and inhibit growth of cells in the roots. It is made in the tip of the roots and shoots and can diffuse to other cells.

Question 4

What is positive phototropism?

Answer 4

Shoots need light for the LDR in photosynthesis which is why plants grow and then bend towards the light. (Controlled by IAA)

Question 5

What is negative phototropism?

Answer 5

Roots do not photosynthesise and do not require light and are more able to anchor the plant if they are deep in the soil away from light. In roots, a high concentration of IAA inhibits cell elongation, causing root cells to elongate more on the lighter side, and so the root bends away from light.

Question 6

What is negative gravitropism?

Answer 6

IAA diffuses from the upper side to the lower side of the shoot. If a plant is vertical, this causes the plant cells to elongate and the plant grows upwards. If a plant is on its side, it will cause the shoot to bend upwards.

Question 7

What is positive gravitropism?

Answer 7

IAA moves to the lower side of roots so that the upper side elongates and the root bends down towards gravity and anchors the plant in.

Question 8

What is a stimulus?

Answer 8

A stimulus is a detectable change in the environment.

Question 9

What are changes in environment detected by?

Answer 9

The changes are detected by receptors.

Question 10

How do organisms increase their chances fo survival?

Answer 10

Organisms increase their chances of survival by responding to stimuli via different response mechanisms.

Question 11

What is taxis?

Answer 11

A simple response in which an organism will move its entire body towards a favourable stimulus or away from an unfavorable stimulus (directional).

Question 12

What is kinesis?

Answer 12

When an organism changes the speed of movement and the rate it changes direction (non-directional).

Question 13

What three neurones does a reflex arc involve?

Answer 13

Sensory, Relay and Motor

Question 14

What is a reflex?

Answer 14

An automatic response to a stimulus which helps organisms respond quickly to a harmful stimulus.

Question 15

Describe the process of a reflex in the reflex arc

Answer 15

1) A receptor detects the stimulus.

2) A sensory neurone transmits changes in frequencies of impulses in the motor neurone.

3) A relay neurone acts as the coordinator and causes nerve impulses in the motor neurone.

4) A motor neurone to the effector.

5) An effector that brings about the response, such as rapid movement.

Question 16

What is the role of a receptor in a reflex?

Answer 16

A recptor detects the stimulus.

Question 17

What is the role of a sensory neurone in a reflex?

Answer 17

A sensory neurone transmits changes in frequencies of impulses into the CNS.

Question 18

What is the role of a relay neurone in a reflex?

Answer 18

A relay neurone acts as the coordinator and causes nerve impulses in the motor neurone.

Question 19

What is the role of a motor neuron in a reflex?

Answer 19

Motor neuronsends electrical impulses to an effector.

Question 20

What is the role of an effector in a reflex?

Answer 20

An effector brings about the response, such as rapid movement.

Question 21

What are receptors?

Answer 21

Receptors detect changes in the environment.

Question 22

What are the three receptors in the body?

Answer 22

1) Pacinian Corpuscle
2) Rods
3) Cones

Question 23

What is the PNS?

Answer 23

The Peripheral Nervous System (PNS) includes the receptors, sensory and motor neurones. It is the part of your nervosu system that lies outside your brain and spinal cord.

Question 24

What is the CNS?

Answer 24

The Central Nervous System (CNS) is the coordination centres such as the brain and spine.

Question 25

What does the pacinian corpuscle respond to?

Answer 25

Responds to pressure changes.

Question 26

Where are the pacinian corpuscle found?

Answer 26

These receptors occur deep in skin, mainly in fingers and feet.

Question 27

What does the plasma membrane of the pacinian corpuscle contain?

Answer 27

Plasma membrane contain channel proteins which allows ion transportation and the membranes surrounding the sensory neurone have stretch mediated sodium ion channels/

Question 28

How does the pacinian corpuscle change when pressure is applied?

Answer 28

1) In the resting state, Na+ channels are too narrow for Na+ to diffuse into the sensory neurone therefore the resting potential is maintained. 2) When pressure in applied it deforms the neurone plasma membrane, stretches and widens the Na+ channels so Na+ diffuses in which leads to the establishment of a generator potential. 3) If the generator potential reaches the threshold, an action potential will be generated causing an impulse to be passed down the sensory neurone to the brain.

Question 29

What kind of receptors are rods and cones?

Answer 29

Photoreceptors

Question 30

Do rods have a high or low visual acuity?

Answer 30

Low visual acuity

Question 31

What colour do rods process images in?

Answer 31

Rod cells cannot distinguish between different wavelengths of light and processes images in black and white.

Question 32

How can rods detect light of very low intensity?

Answer 32

Rods can detect light of very low intensity though because many rod cells connect to one sensory neurone (retinal convergence)

Question 33

How is a generator potential established in a rod cell?

Answer 33

To create a generator potential, the pigment of rod cells (rhodopsin) must be broken down by light energy. There is enough energy from low intensity light to cause the breakdown.

Question 34

How can the threshold still be met at low light intensities in rod cells?

Answer 34

The threshold can be reached even in low light because so many rod cells are connected to a single bipolar cell (summation)

Question 35

Do cones have a high or low visual acuity?

Answer 35

High visual acuity

Question 36

What are the three types of cone cell?

Answer 36

There are red, green and blue cone cells that contain different types of iodopsin which all absorb different wavelengths of light. Depending on the proportion of each cone cell that is stimulated we percieve colour images.

Question 37

When is iodopsin broken down in cone cells?

Answer 37

Iodopsin is only broken down if there is a high light intensity, so action potentials can only be generated with enough light.

Question 38

How many cone cell conect to each bipolar cell?

Answer 38

Usually only one cell connects to a bipolar cell. Therefore no spatial summation occurs and cones can only respond to high light intensity, which is why we can't see colour when it is dark.

Question 39

What is the distribution of rods and cones in the retina?

Answer 39

Distribution of rods and cones in the retina in uneven.

Question 40

Where in the eye recieves the highest intensity of light?

Answer 40

Light is focused by the lens on the fovea, which will receive the highest intensity of light.

Question 41

Where are the rod and cone cells located in the eye?

Answer 41

Most cone cells are located near the fovea as they only respond to high light intensities and rod cells further away as these can respond at lower light intensities.

Question 42

What does myogenic mean?

Answer 42

Cardiac muscle is myogenic, which means that the contraction of cardiac muscle initiates within the heart itself.

Question 43

Where is the SAN located?

Answer 43

In the right atrium and is known as the pacemaker.

Question 44

Where is the AVN located?

Answer 44

Located near the border of the right and left ventricle within the atria still.

Question 45

Where is the bundle of his located?

Answer 45

The bundle of his runs through the septum

Question 46

Where are the purkyne fibres located?

Answer 46

The purkyne fibres are in the walls of the ventricles.

Question 47

In what order does the wave of electrical excitation go throught the heart?

Answer 47

1) Sinoatrial Node (SAN) - Initiates the heartbeat by stimulating the atria to contract. 2) A layer of collagen fibres - Prevents direct electrical flow from atria to ventricles. 3) Atrioventricular node (AVN) - Picks up the electrical activity from the SAN and imposes a slight delay. 4) Bundle of His - Receives electrical activity from the AVN and conducts the wave of excitation to the apex (base) of the heart. 5) Purkyne Fibres - These branch off the bundle of His, causing the right and left ventricles to contract from the bottom upwards.

Question 48

Describe the process of how the wave of electrical excitation goes throught the heart

Answer 48

1) The SAN will release a wave of depolarisation across the atria, causing it to contract. 2) The AVN will release another wave of depolarisation when it first reaches it. There is a non-conductive layer between the atria and ventricles will prevent the wave of depolarisation travelling down the ventricles. 3) Instead, the Bundle of His, running through the septum can conduct and pass the wave of depolarisation down the septum and the purkyne fibres in the walls of the ventricles. 4) As a result, the apex and then walls of the ventricles contract. There is a short delay before this happens whilst the AVN transmits the second wave of depolarisation. 5) This allows enough time for the atria to pump all the blood into the ventricles. Finally, the cells repolarise and the cardiac muscle relaxes.

Question 49

What part of the brain controls the heart rate?

Answer 49

The medulla oblongata in the brain controls the heart rate, via the autonomic nervous system.

Question 50

What are the two receptors that detect changes in the heart?

Answer 50

1) Baroreceptors - blood pressure receptors. 2) Chemoreceptors - chemical receptors.

Question 51

What happens when high blood pressure is detected?

Answer 51

Receptors that detect the stimulus - baroreceptors. Autonomic pathway - parasympathetic. Neurotransmitter used - acetylcholine. Effect on SAN - decreased electrical activity. Change in heart rate - slows down.

Question 52

What happens when low blood pressure is detected?

Answer 52

Receptors that detect the stimulus - baroreceptors. Autonomic pathway - sympathetic. Neurotransmitter used - noradrenaline. Effect on SAN - increased electrical activity. Change in heart rate - speeds up.

Question 53

What happens when high O2, low CO2, or high pH is detected?

Answer 53

Receptors that detect the stimulus - chemoreceptors. Autonomic pathway - parasympathetic. Neurotransmitter used - acetylcholine. Effect on SAN - decreased electrical activity. Change in heart rate - slows down.

Question 54

What happens when low O2, high CO2 or low pH is detected?

Answer 54

Receptors that detect the stimulus - chemoreceptors. Autonomic pathway - sympathetic. Neurotransmitter used - noradrenaline. Effect on SAN - increased electrical activity. Change in heart rate - speeds up.

Question 55

How is heart rate decreased?

Answer 55

1) Impulses from the cardioinhibitory centre pass down neurones to the heart. 2) These impulses causes the release of a substance called acetylcholine at the end of the neurones in the SAN and AVN. 3) Acetylcholine inhibits the production of impulses by the SAN and the transmission of impulses by the AVN. 4) The inhibition of these impulses slows down the rate of heartbeat and the strength of each contraction.

Question 56

How is heart rate increased?

Answer 56

1) Impulses from the cardio acceleratory centre pass down neurones to the heart. 2) These impulses causes the release of a hormone called noradrenaline at the end of the neurones in the SAN. 3) Noradrenaline causes the SAN to increase the rate at which it produces impulses. 4) These impulses cause an increase in both rate of heartbeat and the strength of each contraction. 5) During exercise so much noradrenaline is released that some passes into the bloodstream to other parts of the body.