Topic 6: Environmental Changes

Question 1

define stimulus

Answer 1

• a detectable change in the environment
• which can be detected by receptors
• Organisms increase their chance of survival by responding to stimmuli via different response mechanisms.

Question 2

give examples of simple responses to stimuli

Answer 2

• taxes
• kineses

Question 3

What are taxis?

give examples

Answer 3

• a directional response to a stimulus
• the whole organism moves directly away from or toward the stimulus
• move towards: positive taxis
• moves away: negative taxis

worms: show negative phototaxis to prevent dehydration and predators.
bacteria: show positive chemotaxis

Question 4

what are kineses?

example

Answer 4

• non directional response to a stimulus
• when an organism** changes the speed of movement** and the rate it changes direction
• the rate of movement is impacted by the intensity of the stimulus
• ## If an organism moves from an area where there are benefical stimuli to an area with harmful sitmuli , its kinesis response will be to increase the rate it changes direction** to return to the favourable** conditions quickly.

woodlice respond to water and must be in damp areas to prevent water loss so in a dry area they would turn rapidly to increase the probabilit that it will end up back in the damp area

Question 5

Investigating taxes and kineses

Answer 5

• Choice chambers and mazes are common pieces of apparatus that are used

Question 6

define tropism

Answer 6

• term given to when plants respond, via growth, to stimuli.
• it can be positive or negative. Responding to light, gravity and wate.
• Phototropism - growth response to light
• Gravitropism - growth response to gravity

Question 7

Give one similarity and one difference between a taxis and a tropism.
(2)

Answer 7

1. Similarity − directional response (to a stimulus) / movement towards / away from a stimulus;
   2.** Difference** − taxis (whole) organism moves and tropism a growth (response).
   - Taxis occurs in animals / motile organisms and tropism occurs in plants

Question 8

What is the role of growth factors in plants?

• e.g. indoleacetic acid (IAA)

Answer 8

• Growth factors in plants control and regulate growth responses to environmental stimuli.
• in roots high conc of IAA inhibits cell elongation
• in shoots, high conc of IAA stimulates cell elongation

Question 9

Describe Indoleacetic acid’s role as a growth factor.

Answer 9

• IAA is synthesised at the tips of roots and shoots i.e. in the meristems and it affects the elongation of cells in a plant.
• it binds to the protein receptors on the cell membranes
• lowers the pH causing cell wall to loosen and and allows the cells to be more easily stretched when the turgor of cell increases.

Question 10

Describe how IAA causes elongation of cells

Answer 10

• bind to a receptor protein on the cell surface membrane
• stimulates ATPase to pump hydrogen ions from the cytoplasm into the cell wall
• acidifies the cell wall (lowers the pH of the cell wall)
• This activates expansins, which loosen the bonds between cellulose microfibrils
• At the same time, potassium ion channels are stimulated to open
• This leads to an increase in K+ concentration in the cytoplasm, decreasing the water potential
• This causes the cell to absorb water by osmosis (water enters the cell through aquaporins) which is then stored in the vacuole
• This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins)
• The cell elongates

Question 11

Describe phototropism in a plant

in shoots and in roots

Answer 11

• cells in the tip of shoot to produce IAA.
• IAA diffuses down shoot/root (evenly intially)
• IAA moves to the shaded side of shoot/root
• In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation/
• so shoots bend towards light whereas roots bend towards light

root atipical meristem and shoot

Question 12

Explain gravitropism in flowering plants (4)

Answer 12

• Cells in tip of shoot/ root produce IAA.
• IAA diffuses down shoot/root evenly initally
• IAA moves to lower side of shoot/root (so conc increases)
• In shoots , this stimulates cell elongation whereas in roots this inhibits cell elongation.
• So shoots bend away from gravity whereas roots bend towards gravity

Question 13

Use your knowledge of indoleacetic acid (IAA) to explain the growth curvature shown in Figure 1. (3)

Answer 13

1. Tip produces IAA;
2. IAA diffuses (into shoot);
   Accept auxin for IAA.
   Accept IAA diffuses down.
3. (More) elongation of cells on one side (than other);

Accept (more) elongation of cells on left side.
Reject any reference to shaded/dark side or away
from light

Question 14

Using the procedure in Figure 2 and the calibration curve in Figure 3, describe how you could compare the IAA concentration in shoot tips from
two different plant species.
In your answer you should refer to all the variables that should be controlled to produce a valid comparison (5)

Answer 14

1. Size of shoot/tip;
2. Number of shoot tips;
3. Size/type of agar (block);**
   Accept ‘amount of agar’.
4. (Shoots) at same stage of growth/development;
   Accept (Shoots/plants) are same age.
5. Time (period) tips kept on agar
   OR
   Time (period shoots) kept in dark;
6. Temperature;
7. (Repeat several times and) calculate a mean;
8. Compare/read degree of curvature (on calibration curve) to determine (IAA) concentration
   OR
   Higher the degree of curvature the higher the IAA concentration;

5 max

Mark points 1 to 6 = max 3.
Ignore pH, species, carbon dioxide, humidity,
nutrients, water and light.

Question 15

Explain the protective effecct of a simple reflex (3)

Answer 15

• rapid as only 3 neurones and few synapses
• Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt.
• protects from harmful stimuli e.g. escape prefators/prevents damage to body tissues

Question 16

Suggest 2 advantages of simple reflexes (2)

Answer 16

1. Rapid;
2. Protect against damage to body tissues;
3. Do not have to be learnt;
4. Help escape from predators;
5. Enable homeostatic control.

max 2 points

Question 17

what are three main types of neurones and their roles

Answer 17

1. Sensory neurones carry impulses from receptors to the Central Nervous System (CNS - the brain or spinal cord)
   2. Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones
2. Motor neurones carry impulses from the CNS to effectors (muscles or glands)

Question 18

What is a reflex arc?

Answer 18

• A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain

Question 19

Describe a reflex response

example of a pin

Answer 19

1. A pin (the stimulus) is detected by a pain receptor in the skin.
2. The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
3. Electrical impulses are passed on to relay neurone in the spinal cord
4. The relay neurone connects to the motor neurone and passes the impulses on
5. The motor neurone carries the impulses to the muscle in the leg (the effector)
6. The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)

Question 20

describe the structure of the pacinian corpuscle

draw it

Answer 20

include:
- lamellae
- stretch mediated NA+ ion channel (closed)
- Gel
- Sensory neurone axon
- myelin sheath
- sensory neuron ending.

Question 21

What does the Pacinian Corpuscle do and where is it found?

Answer 21

• Pacinian corpuscles are a type of receptor found deep in the skin
• They are present in the skin of fingers, soles of the feet as well as in joints, tendons and ligaments.
• They respond to changes in pressure
• When these receptors are stimulated by pressure on the skin it leads to the establishment of a generator potential

Question 22

Describe how a generator potential is established in a Pacinian corpuscle (4)

Answer 22

1. Mechanical stimulus e.g. pressure deforms lamellae and stretch mediated channels (Na+) channels
2. So Na+ channels in membrane open and Na+ diffuse into sensory neurone. Greater pressure causes more NA+ channels to open and more Na+ to enter.
3. This causes depolarisation, leading to a generator potential.
4. If generator potential reaches threshold it triggers an action potential.

Question 23

what is the eye and the main receptor cells inside of it?

Answer 23

• The eye **is a sense organ **containing receptors sensitive to light intensity and colour.
• contains:
• Rod cells which are sensitive to light intensity
• Cone cells which are sensitive to different wavelengths of visible light (colour)

Question 24

what is the structure of the eye?

Answer 24

label it:
- cornea
- iris
- lens
- retina
- optic nerve
- pupil

Question 25

Name and describe the function of strcutures in the eye

Answer 25

cornea: transparent lens that refracts light as it enters the eye
iris: controls how much light enters the pupil
lens: transparent disc that can change shape to focus light onto the retina
retina: contains light receptor cells - rods which detect light intensity and cones that detect colour.
optic nerve: sensory neurone that carries impulses between the eye and the brain.
pupil: hole that allows light to enter the eye

Question 26

what is the role of optical pigments in the eye

Answer 26

• Rod cells contain rhodopsin
• Cone cells contain iodopsin
• The breakdown of optical pigments results in a generator potential being produced
• The pigments within the receptors are broken down by different conditions
• Rhodopsin within rods breaks down in dim light
• Iodopsin within cones breaks down in bright light only

Question 27

Explain the difference in sensitivity to light for rods and cones in the retina

Answer 27

• Rod cells are more sensitive to light intensity and are primarily responsible for vision in low-light conditions.
• cone cells are sensitive to different wavelengths of visible light and enable color vision.
• Rods allow humans to distinguish between light and dark, while cones allow the perception of color in brighter light.

Question 28

define visual acuity

Question 29

Explain how the structure of rod and cone cells contributes to differences in visual acuity.

Answer 29

• Rods give lower visual acuity. This is because several rods are connected to a single neurone. So several rods send a single set of impulses, to the brain.
• Cone cells give a higher visual acuity, each cone is connected to a single neurone. Cones send seperate sets of impulses to brain, so can distinguish between 2 seperate sources of light.

Question 30

Why do rod cells enable better vision in dim light compared to cone cells?

Answer 30

• Rod cells contain the pigment rhodopsin, which breaks down in dim light, making them highly sensitive to low light levels.
• Cone cells contain iodopsin, which only breaks down in bright light, making them less effective in dim conditions.

Question 31

Explain the difference in sensitivity to colour for rods and cones in the retina.

Answer 31

• rods allow monochromatic vision: 1 type of rod/1 pigment.
• Cone cells contain three different types of iodopsin pigments that are sensitive to specific wavelengths of light—red, blue, and green. These pigments allow the brain to interpret colors.

Question 32

What is summation, and how does it enhance vision in low-light conditions?

Answer 32

• a single stimulated rod is unlikely to produce a large enough generator potential to stimulate the bipolar cell for the conduction of nerve impulses.
• Thus when a group of rods are stimulated at the same time, the combined generator potentials ( spatial summation) are sufficient to reach the threshold.
• To generate an action potential.

Question 33

Suggest and explain how the interaction between the muscles labelled in the diagram above could cause the pupil to constrict (narrow). (2)

muscles labelled: radial , pupil, iris , circular muscle

Answer 33

1. Circular muscle contracts;
2. Radial muscle relaxes;

Question 34

The fovea of the eye of an eagle has a high density of cones. An eagle focuses the image
of its prey onto the fovea.
Explain how the fovea enables an eagle to see its prey in detail. (3)

Answer 34

1. High (visual) acuity;
2. (Each) cone is connected to a single neurone;
   Accept no retinal convergence.
   Accept ‘bipolar/nerve cell’ for neurone.
3. (Cones send) separate (sets of) impulses to brain;

Question 35

The retina of an owl has a high density of rod cells.
Explain how this enables an owl to hunt its prey at night. (3)

Do not refer to rhodopsin in your answer

Answer 35

1. High (visual) sensitivity;
   Accept retinal convergence
2. Several rods connected to a single neurone;
   Accept ‘bipolar/nerve cell’ for neurone
3. Enough (neuro)transmitter to reach/overcome threshold
   OR
   Spatial summation to reach/overcome threshold; more for ‘several’

Question 36

Explain what causes vision using the fovea.
(i) to be in colour;

(1)
(ii) to have high visual acuity

Answer 36

(i) (Three) different types of (cone) cells / types 6 and 7 sensitive to different wavelengths / different frequencies / different colours;
(ii) Impulses along separate neurone from each receptor cell / each receptor cell connects to separate neurone;

Question 37

define myogenic and its relation to the heart

Answer 37

• it can contract and relax without recieiving electrical impulses from nerves.

Question 38

Explain the role of the sinoatrial node in a heartbeat.

Answer 38

• The sinoatrial node (SAN) is a group of cells in the wall of the right atrium. The SAN initiates a wave of depolarisation that causes the atria to contract

Question 39

Explain the role of the non-conducting tissue Annulus fibrosus in a heartbeat.

Answer 39

• prevents the excitation from spreading to the ventricles and so this ensures that atria and ventricles don’t contract at the same time.

Question 40

Explain the role of the atrioventricular node in a heartbeat

Answer 40

• The Atrioventricular node then sends the wave of excitation to the ventricles after a short delay of (around 0.1 - 0.2 seconds), ensuring that the atria have time to empty their blood into the ventricles.

Question 41

Explain the role of the Purkyne fibres in a hearbeat

Answer 41

• conduct the excitation down the septum of the heart and to the apex, before the excitation is carried upwards in the walls of the ventricles.
• the blood contracts from its base and blood is pushed upwards and outwards.

Question 42

Describe the myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity

Answer 42

1. SAN acts as a pacemaker - senes regular waves of electrical activity across atria. Causing atria to contract simultaneously.
2. Non-conducting tissue between atria/ventricles prevents imupulse passing directly to ventricles. Preventing immediate contraction of ventricles. This delay allows blood movement.
3. Waves of electircala ctivity reach atrioventricular which delays impulse. Allowing atria to fully contract and empty before ventricles contract.
4. AVN sends waves of electrical activity down bundle of His, conducting wave between ventricles to apex when it branches into Purkyne tissue.
5. Causing ventricles to contract simultaneously from the base up.

Question 43

what is the medulla responsible for ?

Answer 43

• the medulla is locted at the base of the brain near the top of the spinal cord.
• it is the cardioregulatory centre in the brain regulates heart rate)
• it is made up of two distinct parts ; the accelatory centre ( speeds up the heart), the inhibitory centre. slows it down).
• These are connected to the SAN via nerves. They make up the autonomic nervous system.

Question 44

What are autonomic nervous system?

Answer 44

• system that controls involuntary actions of glands and muscles.
• 2 subdivisions: sympathetic + parasympathetic.
• Sympathetic: responsible for fight or flight. Parasympathetic: rest and digest.

Question 45

Name the receptots involved in changing heart rate and state their location.

Answer 45

• chemoreceptors and pressure receptors are located in the aorta and carotid arteries.
• baroreceptors: detect changes in blood pressure ; carotid body.
• chemoreceptors: detect changes in ph ( due to increase in CO2 concentration. : Carotid body and aortic body.

Question 46

How does the body respond to an increase in blood pressure?

Answer 46

1. Baroreceptors detect rise in blood pressure.
2. Send impulses to the cardioinhibitory centre in the medulla oblongata
3. which send more frequent impulses to SAN down vagus nerve via the parasympathetic nervous system
4. This stimulates the release of acetylcholine so cardiac muscle contracts frequently.
5. So heart rate decreases /increases

Question 47

How does the body respond to a decrease in blood pressure?

Answer 47

1. Baroreceptors detect a decrease in blood pressure.
2. Then sends more impulses to cardioacceleratory centre in the medulla oblongata.
3. More impulses to SAN via sympathetic nervous system
4. Stimulates release of noradrenaline which increases heart rate and strength of contraction.

Question 48

Describe how the body respond to an increase in CO2 concentration?

Answer 48

1. Chemoreceptors detect pH decrease
2. It sends more impulses to cardioacceleratory centre of medulla oblongata.
3. Which send more frequent impulses to SAN along sympathetic neurones.
4. So more frequent imulses sent from SAN to and from AVN.
5. So cardiac mucle contracts more frequently.
6. So heart rate increases.

Question 49

How does the body respond to a decrease in CO2 concentration?

Answer 49

1. Chemoreceptors detect rise in blood pH.
2. Send impulses to medulla / cardiac control centre.
3. Which send more frequent impulses to SAN along parasympathetic neurones.
4. So less frequent impulses sent from SAN and to / from AVN.
5. So cardiac muscle contracts less frequently.
6. So heart rate decreases