TOPIC 6A - Stimuli and Responses

Question 1

What is a stimulus?

Answer 1

A change in an organisms internal /external environment

Question 2

How do animals/plants increase their chances of survival?

Answer 2

By responding to external stimuli: e.g. avoiding harmful places that are too hot/cold

And

Internal stimuli: ensuring that the conditions are always optimal for their metabolism (chemical reasons that go on inside of them)

Question 3

What are receptors?

Answer 3

They are cells or proteins on cell surface membranes that detect different stimuli (different types of receptors detect different stimuli)

Question 4

What are effectors?

Answer 4

They are cells that bring about a response to a stimulus, to produce an effect.
- they include muscle cells and cells found in glands

Question 5

How do receptors communicate with effectors?

Answer 5

Via the nervous system or the hormonal system, or sometimes even both

Question 6

What do are the jobs of the 3 different types of neurones?

Answer 6

1. Sensory: transmits electrical impulses from receptors to the CNS
2. Motor neurones: transmits electrical impulses from CNS to effectors
3. Relay: transmits electrical impulses between sensory neurones and motor neurones

Question 7

What happens when an electrical impulse reaches the end of a neurone?

Answer 7

Chemicals called neurotransmitters take the info across to the next neurone, which then sends the electrical impulse.

Question 8

Name the 2 types of nervous system?

Answer 8

1. Central nervous system (CNS)

2. Peripheral nervous system (PNS)

Question 9

Draw a break down of the PNS

Answer 9

PNS
I
Somatic««<»»»Autonomic
I
Sympathetic«»parasympathetic

Question 10

What does the CNS do?

Answer 10

The CNS processes the information on how to respond to a stimulus

Question 11

What is a reflex?

Answer 11

A reflex is where the body responds to a stimulus without having to make a conscious decision to respond.

Question 12

How do simple reflexes help organisms?

Answer 12

As we don’t have to spend time deciding how to respond, information travels really fast down reflex arc.
- This can prevent us from severaly injuring ourselves

Question 13

What can override a reflex? (give example)

Answer 13

If there’s a relay neurone involved in the simple reflex arc, it’s possible to override the reflex, E.g when you touch something hot, your brain could tell your hand to withstand the heat (due to relay neurone involvement)

Question 14

Outline 3 words to describe nervous system communication.

Answer 14

1. Localised: when electrical impulse reaches the end of a neutron, neurotransmitters are secreted directly onto target cells - so nervous response is localised
2. Short-lived: neurotransmitters quickly removed once they do their job
3. Rapid

Question 15

Give 3 adaptations/properties that helps plants respond to stimuli e.g low light availability and gravity

Answer 15

Low light availability:

1. They can sense light direction and grow towards it to maximise light absorption for photosynthesis
2. Climbing plants have a sense of touch, so they can find things to climb up and reach the sunlight

Gravity:
1. They can sense gravity, so roots/shoots grow in right direction

Question 16

Define these key words

1. Tropism
2. Positive tropism
3. Negative tropism

Answer 16

1. Tropism: response of plant to a directional stimulus (stimulus from certain direction)
2. Positive tropism: growth towards stimulus
3. Negative tropism: growth away from stimulus

Question 17

Define phototropism. (include shoots and roots in your answer)

Answer 17

Phototropism: the growth of a plant in response to light

• Shoots: positively phototrophic and grow towards light
• roots: negatively phototrophic and grow away from light

Question 18

Define gravitropism. (include shoots and roots in your answer)

Answer 18

Gravitropism: the growth of plants in response to gravity

• Shoots: negatively gravitrophic and grow upwards
• roots: positively phototrophic and grow downwards

Question 19

Define growth factors and state where they’re produced.

Answer 19

They are hormone like-chemicals like speed up/slow down plant growth, helping plants respond to directional stimuli.
- they are produced in growing regions of the plant (E.g. Shoots, leaves) and they move to where they needed in other parts of the plant.

Question 20

What are auxins?

Answer 20

They are growth factors that stimulate the growth of shoots by cell elongation - this is where the cell walls become loose and stretchy, so the cells get longer.

Question 21

Fill in this gap in this sentence: “high conc of auxins ……. growth in roots”

Answer 21

Inhibit

Question 22

What is indoleacetic acid (IAA)?

Answer 22

IAA is an auxin that’s produced in the tips of shoots in flowering plants.

It moves around the plant to control tropisms via diffusion and active transport for short distances or via the phloem for long distances.
- This results in different parts of the plant having an uneven distribution of IAA, and therefore, there’s uneven growth of the plant.

Question 23

How does IAA react in shoots and roots during phototropism?

Answer 23

PHOTOTROPISM

• shoots: IAA conc increases on more shaded side, so cells elongate and shoot bends towards light ASK: do we assume that the shoot is shooting straight up initially
• roots: IAA conc increases on shaded side, growth is inhibited, so root bends away from light

Question 24

How does IAA react in shoots and roots during gravitropism?

Answer 24

GRAVITROPISM

• shoots: IAA conc increases on lower side so cells elongate so shoot grows upwards
• roots: IAA conc increases on lower side so growth is inhibited and root grows downwards

Question 25

What are simple responses?

Answer 25

They are responses that keep simple mobile organisms (e.g. woodlice and earthworms) in a favourable environment
- They can be either tactic or kinetic

Question 26

What are tactic responses (taxes) ?

GIVE AN EXAMPLE

Answer 26

When organisms move towards/away from a directional stimulus

EXAMPLE: woodlice show a phototaxis - they move away from a light source. This helps them survive as it keeps them concealed under stones during the day (where they’re safe from predators) and keeps them in damp conditions (which reduced water loss)

Question 27

What are kinetic responses (kineses)?

• GIVE AN EXAMPLE

Answer 27

When organisms movement is affected by a non-directional stimulus e.g. Humidity

EXAMPLE: woodlice show a kinetic response to humidity.

• in high humidity, they move slowly and turn less often so that they stay where they are. when air gets drier, they move faster and turn more often, so that they move into a new area.
• this response improves its chances of staying in an area with high humidity. High humidity = less water loss and helps keep lice concealed and hence, improves its survival rate.

Question 28

What are pacinian corpuscles?

Answer 28

They are mechanoreceptors (receptors that detect mechanical stimuli e.g pressure, vibrations) in the skin

Question 29

What do pacinian corpuscles look like?

*DIAGRAM-

Answer 29

They contain the end of a sensory neurone, called the sensory nerve ending.
- the sensory nerve ending is wrapped in loads of layers or connective tissue called lamellae.

Question 30

How do Pacinian Corpuscles work?

DIAGRAM

Answer 30

1. When they are stimulated e.g by a tap on the arm, the lamellae become deformed and press on sensory nerve ending
2. This causes sensory neurone’s cell membrane to stretch, deforming the stretch-mediated Na+ channels. The channels open and Na+ diffuse into cell, creating a generator potential.
3. If generator potential reaches threshold, action potential is triggered.

Question 31

Outline the role of these parts in the eye:

• pupil
• muscles of iris
• retina
• Fovea
• optic nerve
• blind spot

Answer 31

Pupil: where light enters eye.

Muscles of the Iris: control amount of light that enters eye.

Retina: contains photoreceptor cells, which detect light.

Fovea: an area of the retina where there are lots of photoreceptors.

Optic nerve: a bundle of neurones that carry nerve impulses from photoreceptor cells to brain.

Blind spot: Where the optic nerve leaves the eye. There are no photoreceptor cells, so it’s not sensitive to light

Question 32

Describe the process of our eyes reponding to light

HINT;

• photoreceptors
• generator potential

DIAGRAM

Answer 32

1. Light enters eye, hits photoreceptors and is absorbed by light-sensitive optical pigments.
2. Light bleaches pigments, causing a chemical change and altering membrane permeability to sodium ions.
3. Generator potential is created and if it reaches threshold, action potential sent along bipolar neurone.
4. Bipolar neurones connect photoreceptors to optic nerve, which takes impulses to brain.

Question 33

What are the 2 different types of photoreceptors found in the human eye and what do they contain?

Answer 33

Rods and cones

- they contain different optical pigments making them sensitive to different wavelengths of light.

Question 34

How do the ‘location’ and ‘type of info given’ differ in rods and in cones?

Answer 34

LOCATION: cones packed together in Fovea whilst rods mainly in peripheral part of retina

TYPE OF INFO: rods in black and white, whilst cones give info in colour
- there are 3 types of cones, each containing different optical pigment: red-sensitive, green-sensitive, blue-sensitive. When they’re stimulated in different proportions, you see different colours

Question 35

How does sensitivity differ in rods and in cones?

Answer 35

SENSITIVITY

rods: very sensitive to light so they work well in dim light. This is because many rods join 1 neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.

Cones: cones are less sensitive so they work best in bright light. This is because 1 cone joins 1 neurone, so it takes more light to reach the threshold and trigger an action potential. **

Question 36

How does visual acuity differ in rods and in cones?

DIAGRAM

Answer 36

VISUAL ACUITY (the ability to tell apart points that are close together)

Rods: give low visual acuity as many rods join same neurone, so light from 2 different points close together can’t be told apart

Cones: high visual acuity as cones are close together and one cone joins 1 neurone. When light from 2 points hits 2 cones, 2 impulses go to brain - you can distinguish 2 points close together as 2 separate points.

Question 37

What does it mean if we say ‘the heart if myogenic’ and why is this important?

Answer 37

It means that it can contract and relax without receiving signals from nerves
- this pattern of contractions is important as it controls the regular heartbeat

Question 38

Describe the process of how the cardiac muscle controls regular beating of the heart.

DIAGRAM

Answer 38

1. The SAN acts a pacemaker and sets rhythm of heart by sending out regular waves of electrical activity to atrial walls, causing right and left atria to contract at the same time.
   2- Band of non-conducting collagen tissue prevents waves of electrical activity being passed directly from atria to ventricles.
2. Instead, waves of electrical activity are transferred from SAN to AVN.
3. AVN sends electrical activity waves to bundle of His. There is a slight delay before AVN reacts to make sure atria have emptied before the ventricles contract.
   5) Bundle of His sends impulse down length of the heart, to finer muscle fibres in right and left ventricles, called Purkyne tissue.
   6) Purkyne tissue carries electrical activity waves into muscular walls of right and left ventricles, causing them to contract simultaneously, from bottom up.

Question 39

What controls the rate at which the SAN fires (the rate at which it sends out regular waves) ?

Answer 39

It is unconsciously controlled by the medulla oblongata.

Question 40

Why may animals need to alter their heart rate?

GIVE EXAMPLES

Answer 40

To respond to internal stimuli e.g to prevent fainting due to low blood pressure or to make sure the heart rate is high enough to supply the body with enough O2.

Question 41

What are:

• baroreceptors
• chemoreceptors

Answer 41

These are 2 types of receptors that can detect stimuli

BARORECEPTORS: pressure receptors in the aorta and carotid arteries (major arteries in the neck).
- they are stimulated by high and low blood pressure.

CHEMORECEPTORS: chemical receptors in aorta, carotid arteries and medulla.
-they monitor O2 level in blood and also CO2 and pH (which are indicators of O2 levels)

Question 42

COMPLETE THIS TABLE AS TO HOW THE HEART REACTS TO DIFFERENT STIMULI

STIMULUS: High blood pressure

RECEPTOR:

NEURONE AND TRANSMITTER:

EFFECTOR:

RESPONSE:

Answer 42

STIMULUS: High blood pressure

RECEPTOR: baroreceptors

NEURONE AND TRANSMITTER: impulses sent to medulla (sensory), which sends impulses along parasympathetic neurones to the SAN receptors (via neurotransmitters).

EFFECTOR : cardiac muscle

RESPONSE: heart rate slows down to reduce blood pressure back to normal

Question 43

COMPLETE THIS TABLE AS TO HOW THE HEART REACTS TO DIFFERENT STIMULI

STIMULUS: Low blood pressure

RECEPTOR:

NEURONE AND TRANSMITTER:

EFFECTOR:

RESPONSE:

Answer 43

STIMULUS: low blood pressure

RECEPTOR: baroreceptors

NEURONE AND TRANSMITTER: impulses sent to medulla (sensory) , which sends impulses along sympathetic neurones to the SAN receptors (via neurotransmitters).

EFFECTOR: Cardiac muscles

RESPONSE: heart rate speeds up to increase it back to normal

Question 44

COMPLETE THIS TABLE AS TO HOW THE HEART REACTS TO DIFFERENT STIMULI

STIMULUS: High blood O2, low CO2, high pH levels.

RECEPTOR:

NEURONE AND TRANSMITTER:

EFFECTOR:

RESPONSE:

Answer 44

STIMULUS: High blood O2, low CO2, high pH levels

RECEPTOR: chemoreceptors

NEURONE: impulses sent to medulla, which sends impulses along parasympathetic neurones to the SAN receptors (via neurotransmitters).

EFFECTOR: cardiac muscle

RESPONSE: heart rate decreases to return O2, CO2, and pH levels back to normal

Question 45

COMPLETE THIS TABLE AS TO HOW THE HEART REACTS TO DIFFERENT STIMULI

STIMULUS: low blood O2, high CO2, low pH levels.

RECEPTOR:

NEURONE AND TRANSMITTER:

EFFECTOR:

RESPONSE:

Answer 45

STIMULUS: low blood O2, high CO2, low pH levels.

RECEPTOR: chemoreceptors

NEURONE AND TRANSMITTER: impulses sent to medulla, which sends impulses along sympathetic neurones to the SAN receptors (via neurotransmitters).

EFFECTOR: cardiac muscles

RESPONSE: heart rate increases to return O2, CO2, and pH levels back to normal.