Topic 14

Question 1

What’s a stimulus and how is it detected

Answer 1

Detectable change in the environment. Detected by cells which are called receptors

Question 2

What’s tropism? And what are the three types of stimulus

Answer 2

Plants respond to a stimulus via growth. It can be positive(growing towards) or negative(growing away).

The three different types are light, water and gravity.

Question 3

What is IAA?

Answer 3

Indoleacetic acid(IAA) - is a specific type of growth factor that controls tropism.
IAA is a type of auxin and can control cell elongation in the shoots and inhibits in the roots. It is made in the tips of roots and shoots but can diffuse to other cells

Question 4

What’s phototropism? How does it affect shoots and roots?

Answer 4

Phototropism is growth in response to light. In shoots - positive phototropism and in roots - negative phototropism

Question 5

How does positive phototropism in shoots work? How does it help the plant?

Answer 5

Light is need for LDR in photosynthesis so it grows and bends towards it.(helps plant)
1-The shoot produces IAA which causes cell elongation.
2-the IAA diffuses to the other cells, if light is from a unilateral source then IAA would diffuse to the shaded parts of the plant which caused the shaded parts to elongate more.
3- this causes the plant to bend towards light

Question 6

How does negative phototropism work? How’s it helpful for the plant

Answer 6

Roots don’t photosynthesise so doesn’t require light.
In roots a high concentration of IAA inhibits cell elongation.
Due to gravity the IAA gets deposited on the bottom part of the roots causing the part exposed to light elongate more as it doesn’t have/less of IAA causing the root to bend towards the ground and away from light.
This helps them seep deeper into the soils and potentially find a water source

Question 7

What is gravitropism? And when is it positive and negative

Answer 7

Gravitropism is growth in response to gravity. Shoots show negative gravitropism and roots - positive.

Question 8

How does negative gravitropism work

Answer 8

In shoots - the IAA diffuses from upper to the lower side of a shoot.
If the plant is vertical it causes cell elongation and the plant grows upwards
If the plant is on its side it cause the plant to bend upwards and away from gravity(Negative)

Question 9

How does positive gravitropism work? How does it help the plant

Answer 9

In the roots - IAA diffuses to the lower part and inhibits cell elongation and therefore the upper part elongates more and bends towards the ground(positive)
Helps the roots anchor the plant in.

Question 10

What is taxis?

Answer 10

An organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus. It’s classified according to whether the movement is towards the stimulus or away

Question 11

3 examples of taxis(algae, earthworms and bacteria

Answer 11

Single celled algae move towards light - positive phototaxis - increases chance of survival since being photosynthetic requires light to manufacture their food

Earthworms will move away from light - negative phototaxis - they’re better able to conserve water, find food and avoid some predators

Some species of bacteria - move towards a region where glucose if highly concentrated - positive chemotaxis - increases chance of survival as glucose is used as a source of food

Question 12

What is kinesis? Give an example? How does it react when it’s surrounded by negative stimuli?

Answer 12

Change of speed at which it moves and the rate at which it changes direction.
Woodlice starts moving rapidly and change direction more often when it’s in dry conditions - this increases the chance of it moving back into damp conditions

When it’s surrounded by a negative stimuli it’s rate of turns decreases to keep it moving in a straight line to increase the chance of it finding a new location with favourable conditions

Question 13

What’s a reflex arc

Answer 13

Reflex is a rapid, automatic response to protect you from danger
The reflex arc is made up of 3 neurons - sensory, relay and motor

Question 14

How do receptors work? What are the 3 receptors?

Answer 14

Receptors detect stimuli.
Each receptor only responds to specific stimuli. This stimulation causes the establishment of generator potential which causes a response.

The 3 receptors - Pacinian corpuscle, rods and cones

Question 15

What are the 2 major divisions of the nervous system? What does it consist of?

Answer 15

CNS (Central nervous system) and PNS (Peripheral nervous system)
CNS - Brain and spinal cordPNS - pairs of nerves which either originate from brain or spinal cord

Question 16

What are the 2 divisions of PNS? And function

Answer 16

Sensory neurones - carry impulse from receptor to CNS
Motor neurones - carry impulse away from CNS to effectors

Question 17

What are the 2 subdivisions of the motor nervous

Answer 17

Voluntary nervous system - carries nerve impulse to the body muscles and is under voluntary control
Autonomic nervous system - carries nerve impulse to the glands, smooth muscles and cardiac muscles that is involuntary

Question 18

What’s the spinal cord

Answer 18

Column of nervous tissue which runs along the back and inside the vertebral column for protection

Question 19

What’s the main stages of reflex arc? (7)

Answer 19

Reflex arc is also called spinal reflex as one of the neurones involved is inside the spinal cord.
Stimulus, receptor, sensory neurone, coordinator is the intermediate neurone(relay neurone) which links the sensory neuron to the motor neuron in the spinal cord, motor neurone, effector and response

Question 20

Importance of reflex

Answer 20

Involuntary and doesn’t involve the decision making powers of brain so it’s faster and doesnt overload the brain.
Protects the body and it’s effective from birth.
It’s fast because of the short neurone pathways because of very few, typically one or two synapses

Question 21

What’s the structure of pacinian corpuscle?

Answer 21

The single sensory neurone of the pacinian corpuscle is at the centre of many layers of tissue each separated by a gel

Question 22

Features of pacinian corpuscle

Answer 22

It’s specific to a single type of stimulus - mechanical pressure
Produces a generator potential by acting as a transducer

Question 23

What’s the role of a transducer

Answer 23

They convert the energy from the stimulus into nervous impulse known as generator potential.
The pacinian corpuscle transduces the mechanical energy of the stimulus into a generator potential

Question 24

Where are the pacinian corpuscle found abundantly (3)

Answer 24

Fingers, soles of feet, external genitalia and also found in joints, ligaments and tendons - which enables the organism to know which join is changing direction

Question 25

Tell me in detail about the structure of the pacinian corpuscle

Answer 25

The sensory neurone ending in the centre of the pacinian corpuscle has a special type of sodium channel in its plasma membrane called STRETCH MEDIATED SODIUM CHANNEL. These open and allow Na+ to enter only when it’s stretched and deformed

Question 26

How does the Pacinian corpuscle function in the resting potential

Answer 26

In the resting potential the stretch mediated Na+ channels are too narrow to let the Na+ ions diffuse along them - in this state the Pacinian corpuscle has a resting potential

Question 27

How does the pacinian corpuscle generate a action potential

Answer 27

1- When pressure is applied it deforms the neurone plasma membrane , stretches and widens the sodium channels and Na+ ions diffuse into the neurone
2- the influx of Na+ ions changes the potential of the membrane (becomes depolarised) and this produces a generator potential (when Na+ ions reach or become more than the threshold that’s when generator potential is created)
3- this in turn produces a action potential (nerve impulse) that passes along the neurone and then, via other neurones into the CNS

Question 28

What are the two receptor cells in the eyes? Where are they present

Answer 28

Rods and cone cells found in the retina.
They act as transducers and convert the light energy into electrical energy of the nerve impulse.

Question 29

Why do rod cells have low visual acuity?

Answer 29

1- With rod cells image can only be seen in black and white as it cannot distinguish between different wavelength of light and this is because - Many rod cells are connected to a single sensory neurone(bipolar cell)in the optic nerve (Retinal convergence).
2-This means only a single nerve impulse is generated regardless of how many neurones are stimulated and this means the brain cannot distinguish between the separate light sources and very low or no resolution - low visual acuity.
3- Rod cells can detect very low light intensity. A certain threshold has to be exceeded to produce a generator potential in the bipolar cells.
4- To produce a generator potential the pigment rhodopsin has to be broken down and there is enough energy in low light intensity to cause this breakdown.

Question 30

How many rod cells in each eye?

Answer 30

120 million

Question 31

How many done cells in each eye?

Answer 31

6 million

Question 32

How are the 3 cone cells different?

Answer 32

Each contains a specific type of iodopsin and therefore sensitive to a different specific range of wavelengths.
Depending on the proportions of each type stimulated we can perceive images in full colour.

Question 33

How does cone cells provide high visual acuity?

Answer 33

Each cone cell has its own connection to a single bipolar cells connected which means the brain receives separate nerve impulse for every cone cells stimulated - means it can resolve between 2 close points - high visual acuity - very accurate vision.
Stimulation of multiple cone cells will not help reach the individual threshold and therefore cone cells only respond to high light intensity.
The pigments iodopsin can only be broken down by high light intensity and only then it can produce a generator potential

Question 34

How are rod and cone cells distributed

Answer 34

Distribution of rod and cone cells is uneven.
Light is focused by lens on the part of retina opposite the pupil and this is called FOVEA. And this is where highest light intensity is received.
Therefore cone cells are found in the fovea. The concentration of cone cells diminishes around the peripheries of retina but rod cells increase as it has lowest light intensity.

Question 35

What are the two divisions of the autonomic nervous system

Answer 35

Sympathetic nervous system - stimulates effectors and therefore speeds up any activity. Looks after when we exercise or in any strenuous activity.
Parasympathetic nervous system - inhibits effectors and therefore slows down any activity- looks after the normal resting conditions
ACTIONS IF SYMPATHETIC AND PARASYMPATHETIC NERVOUS SYSTEM OPPOSE ONE ANOTHER AND ARE ANTAGONISTIC.

Question 36

Why are cardiac muscles called myogenic?

Answer 36

As the contraction is initiated within the muscle and not nervous impulse(neurogenic) unlike other muscles

Question 37

What’s SAN? What is it also called as?

Answer 37

Within the wall of right atrium of the heart are a distinct group of cells known as sinoatrial node (SAN) - This is where the initial stimulus for contraction originates.
The SAN has a basic rhythm which determines the heart beat - this is the reason it’s called the pacemaker of the heart

Question 38

What’s the sequence of events that controls the heart rate

Answer 38

1- a wave of electrical excitation is spread out from the SAN across both the atria causing them to contract.
2- the layer of nonconductive tissue (atrioventricular septum) prevents the wave from reaching the ventricles
3- The wave of excitation reaches the AVN (atrioventricular node) which lies between the atria.
4- The AVN, after a short delay, conveys a wave of electrical excitation between the ventricles along a series of special muscle fibres called Purkyne tissue which collectively forms the bundle of His.
5- The bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundle branches into the smaller fibres of purkyne tissue
6- The wave of excitation is it released from the purkyne tissue causing the ventricles to contract quickly at the same time from the bottom of the heart upwards.

Question 39

What’s the region in the brain that controls the changes in the heart rate? And what are the 2 centres

Answer 39

Medulla oblongata has 2 centres concerned with heart rate
1- centre that increases heart rate which is linked to SAN by the sympathetic nervous system.
2- centre that decreases heart rate which is linked to SAN by the parasympathetic nervous system

Question 40

What are the 2 stimuli they respond to

Answer 40

Which centre is stimulated depends on the nerve impulse they receive from two types of receptor, which respond to stimuli of either chemicals or pressure changes in the blood.

Question 41

How do chemoreceptors control the pH of the blood

Answer 41

When the rate of respiration increases from exercise the concentration of co2 increases in blood lowering the pH
This is detected by the chemoreceptors in the walls of the carotid artery and aorta and this increases the frequency of the impulses to the centre I the medulla oblongata which increases the heart rate.
This centre therefore increases the rate of impulse sent to the SAN via the sympathetic nervous system. And this increases the rate of production of electrical impulse by the the the SAN and this increases the heart rate.
Therefore more of the blood is oxygenated and the pH increases and comes back to normal.
This change is detected by the chemoreceptors in the carotid artery and aorta and reduces the frequency of impulses to the medulla oblongata and this reduces the frequency of impulses to the SAN and this reduces the heart rate