Stimuli and response - A

Question 1

How to an organisms response to the environment help them survive?

Answer 1

Organisms increase their chance of survival by responding to changes in their external environment
They also respond to changes in their internal environment, ensuring conditions are always optimal for metabolism
A change in the environment is a stimulus

Question 2

What is a receptor

Answer 2

Proteins on cell surface membranes that can detect a stimuli
Receptors are specific to one type of stimulus

Question 3

What is an effector?

Answer 3

Cells that bring about a response to a stimulus
Effectors include muscle cells and cells found in glands

Question 4

What are the components of a reflex arc?

Answer 4

Stimulus
Receptor
Sensory neurone
A coordinator (intermediate/relay neurone)
A motor neurone
An effector
The response

Question 5

What are the two major divisions of the nervous system?

Answer 5

The CNS which consists of the brain and spinal chord

The peripheral nervous system which consists of the neurones that originate from either the brain or the spinal chord and connect the CNS to the rest of the body

Question 6

What do sensory neurones do?

Answer 6

Carry nerve impulses from receptors to the CNS

Question 7

What do motor neurones do?

Answer 7

Carry nerve impulses from the CNS to effectors

Question 8

What do relay neurones do?

Answer 8

Transmit nerve impulses between sensory and motor neurones

Question 9

Subdivisions of the peripheral nervous system:

Answer 9

The autonomic nervous system which controls unconscious activities

The somatic nervous system which controls conscious activities

Question 10

Divisions of the autonomic nervous system:

Answer 10

The sympathetic nervous system

The parasympathetic nervous system

They’re antagonistic

Question 11

What is the sympathetic nervous system?

Answer 11

Stimulates effectors so speeds up any activity

Controls effectors during strenuous exercise or strong emotions

Heightens awareness preparing us for activity (fight or flight)

Question 12

What is the parasympathetic nervous system?

Answer 12

Inhibits effectors and slows down activity
Controls activity under normal resting conditions
Concerned with conserving energy and replenishing the bodies reserves

Question 13

Why are reflex arcs important?

Answer 13

Protect the body from harm
Involuntary therefore complex responses can be carried out without conscious decision from the brain
Though, if there’s a relay neurone involved it’s possible to override the reflex
Neurone pathway is short so fast reflex
The absence of decision making also makes the action rapid

Question 14

What is a reflex?

Answer 14

Where the body responds to a stimulus without making a conscious decision

Question 15

What is a tropism?

Answer 15

A plant’s response to a directional stimulus
A positive tropism is growth towards the stimulus
A negative tropism is growth away from the stimulus
Phototropism is the growth of a plant in response to light
Gravitropism is the growth of a plant in response to gravity

Question 16

What are growth factors in plants?

Answer 16

Hormones that speed up or slow down the growth of a plant
Produced in shoot tips/leaves - growing regions of the plant
Growth factors called auxins stimulate the growth of shoots by cell elongation
High concentrations of auxins inhibit growth in roots

Question 17

Where is indoleacetic acid (IAA) found in plants?

Answer 17

An important auxin produced in the tips of shoots in flowering plants
Moves around the plants to control tropisms by diffusion and active transport over short distances and via the phloem over long distances
The results in different concentrations of IAA around the plant resulting in uneven growth

Question 18

The role of IAA in phototropism

Answer 18

IAA concentration increases in the shaded side of roots and shoots
In shoots, cells elongate and bend towards the light
In roots, growth is inhibited so the root bends away from the light

Question 19

The role of IAA in gravitropism

Answer 19

IAA moves to the underside of shoots and roots
In shoots, IAA concentration increases on the lower side so cells elongate and the shoot grows upwards
In roots, IAA concentration increases on the lower side and growth is inhibited so the roots grow downwards

Question 20

What is a tactic response for a simple mobile organism?

Answer 20

Taxes is where the organisms move towards or away from a directional stimulus (e.g. light which is phototaxis)

Question 21

What is a kinetic response for a simple mobile organism?

Answer 21

Kinesis is where the organism’s movement is effected by a non-directional stimulus (E.g. humidity)

Question 22

What type of receptor is the Pacinian Corpuscles?

Answer 22

A mechanoreceptor found in the skin
This means they only detect mechanical pressure
Contain the end of a sensory neurone which is wrapped in layers of connective tissue called lamellae which are separated by gel

Question 23

What happens when a Pacinian Corpuscle is stimulated?

Answer 23

The lamellae become deformed and apply pressure on the sensory neurone
This causes the cell membranes in the sensory neurone to deform and so stretch-mediated sodium ion channels open
Sodium ions diffuse into the sensory neurone
The charge inside the neurone is now more positive than the outside. This creates a generator potential
If the generator potential reaches the threshold value an action potential is triggered

Question 24

What is potential difference?

Answer 24

When a receptor is at it’s resting state there’s a difference in charge between the inside and outside of the cell
This results in a voltage (or potential difference) across the cell membrane
When a cell is at rest the potential difference is called its resting potential
A stimulus changes the potential difference as the membrane becomes more permeable and ions move in and out of the cell
This change is called the generator potential
If the threshold level is reached this results in an action potential

Question 25

Where are light receptor cells in the mammalian eye located?

Answer 25

The retina
Which consists of rod and cone cells
Both act as transducers by conserving light energy into the electrical energy of a nerve impulse

Question 26

Rod cells

Answer 26

Can’t distinguish different wavelengths of light therefore images are seen in black and white
Are more abundant than cone cells
Many are connected to a single bipolar cell so they’re high sensitivity
Detect low intensity light
Found at the peripheries of the retina where light intensity is low
A certain threshold value must be exceeded before a generator potential is created in the bipolar cells

Question 27

How is a generator potential created in a rod cell?

Answer 27

Light hits photoreceptors and is absorbed by rhodopsin - the pigment in rod cells
The energy from low intensity light causes rhodopsin to break down
This chemical change alters membrane permeability to sodium ions
A generator potential is created and if it reaches the threshold a nerve impulse is sent along a bipolar neurone which connect photoreceptors to the optic nerve

Question 28

Why do rod cells give low visual acuity?

Answer 28

The brain doesn’t receive separate information about two close points as many rods are connected to the same bipolar neurone/cell
So regardless of how many rod cells are stimulated only a single impulse will be generated and sent to the brain

Question 29

Cone cells

Answer 29

Three different types which respond to different wavelengths of light - red, green and blue. Depending on the proportion of each type that’s stimulated we see images in full colour
Cones are less sensitive than rods as one cone joins to one bipolar cell - so they work best in high light intensity
Found in the fovea - the concentration diminishes further away from the fovea

Question 30

Why do cone cells give high visual acuity?

Answer 30

The brain receives separate impulses as each cone cell has it’s own connection to a bipolar cell
The brain can therefore distinguish between the sources of light that stimulated the cone cells

Question 31

What is the sinoatrial node (SAN) and where is it located?

Answer 31

The SAN is a group of cells in the wall of the right atrium
This is where the initial stimulus for contraction originates
It acts as a pacemaker

Question 32

Why is the heart muscle described as myogenic?

Answer 32

It’s contraction is initiated from within the muscle itself rather than by external nervous impulses (neurogenic)

Question 33

What are the sequence of events that control basic heart rate?

Answer 33

An electrical impulse generated by the SAN spreads across both atria causing them to contract
The atrioventricular septum is a layer of non-conductive collagen tissue which prevents the impulse from crossing to the ventricles
The waves of electrical impulse enters the atrioventricular node (AVN) which conveys the impulse along the bundle of His. There’s a slight delay before the AVN reacts to ensure the atria have emptied before the ventricles contract
The bundle of His are a group of specialised muscle tissue which conduct the waves of electrical activity through the atrioventricular septum to the apex of the heart.
The bundle branches into smaller muscle fibres called Purkyne tissue
The Purkyne tissue releases the impulse causing the ventricles to contract from the apex upwards

Question 34

How is resting heart rate modified?

Answer 34

The rate at which the SAN sends electrical impulses is unconsciously controlled by the medulla oblongata

There’s a centre that increases heart rate which is linked to the SAN by the sympathetic nervous system

And another centre which decreases heart rate which is linked to the AVN by the parasympathetic nervous system

The stimulation of either of these centres depends on the nerve impulses received from receptors

Question 35

The role of pressure receptors in heart rate

Answer 35

Baroreceptors are found in the aorta and carotid arteries
They’re stimulated by high or low blood pressure

Question 36

The role of chemical receptors in heart rate

Answer 36

Chemoreceptors are found in the aorta, the carotid arteries and the medulla
They’re stimulated by changes in pH or CO2 or oxygen concentration

Question 37

What happens if baroreceptors detect a high blood pressure?

Answer 37

Impulses are sent to the medulla oblongata which sends impulses along parasympathetic neurones

These secrete the neurotransmitter acetylcholine which binds to receptors on the SAN

Heart rate slows down to reduce blood pressure back to normal

Question 38

What happens if baroreceptors detect a low blood pressure?

Answer 38

Impulses are sent to the medulla oblongata which sends impulses along sympathetic neurones

These secrete the neurotransmitter noradrenaline which binds to receptors on the SAN

Heart rate increases to increase blood pressure back to normal

Question 39

What happens if chemoreceptors detect high pH levels or high oxygen, low CO2 concentration in the blood?

Answer 39

Impulses are sent to the medulla oblongata which sends impulses along parasympathetic neurones

These secrete the neurotransmitter acetylcholine which binds to receptors on the SAN

Heart rate decreases to return oxygen, CO2 and pH levels back to normal

Question 40

What happens if chemoreceptors detect a low pH or low oxygen high CO2 concentration in the blood?

Answer 40

Impulses are sent to the medulla oblongata along sympathetic neurones which secrete the neurotransmitter noradrenaline

Noradrenaline binds to receptors on the SAN

Heart rate increases to return O, CO2 and pH levels back to normal