3.6 Chapter 14- Response to Stimuli

Question 1

What is a stimulus?

Answer 1

A change in the internal or external environment of an organism that leads to a response.

Question 2

What is different about the way animals and plants respond to stimulus?

Answer 2

Animals are motile whereas plants have to respond through growth.

Question 3

How is response important?

Answer 3

• Organisms increase their chance of survival by responding to changes in their environment.
• Organisms respond to changes in external regions to move towards more favourable environments.
• Organisms respond to changes in internal environment to maintain optimal conditions for their metabolism (homeostasis).
• These organisms therefore have a greater chance of producing offspring and passing their alleles to the next generation- selection pressures favour organisms with the best responses.

Question 4

Give some suggestions of favourable and unfavourable conditions an organism needs to respond to to survive.

Answer 4

• Detect and move away from harmful stimuli such as predators or extreme weather.
• Move towards favourable stimulus such as food.

Question 5

Give an overview of the process of stimulus and response.

Answer 5

1. Receptor detects stimulus- can be cells or proteins on the cell-surface membrane- specific to one type of stimulus.
2. Coordinator- formulates a suitable response to a stimulus- molecular or involves organs such as the brain. (glands/ CNS). Can use the nervous or the hormonal system or sometimes both.
3. Effector- produces a response to a stimulus- uses muscles or cells in glands- response can be molecular, changes in organs or changes in the overall behaviour of an organism.

Question 6

Give the simplified version of the process of stimulus and response.

Answer 6

Stimulus -> Receptor -> Coordinator -> Effector -> Response

Question 7

What are hormones?

Answer 7

Chemicals used in large multicellular organisms to stimulate responses.

Question 8

What controls responses in plants and animals?

Answer 8

• Animals- Hormones and the nervous system.
• Plants- hormone-like growth substances

Question 9

Compare the hormonal and nervous systems and give e.g.s:

Hint: 11 points (including e.g.s)

Answer 9

• Chemicals called hormones communicate information. Secreted by glands. Nerve impulses communicate information. Generated by receptors.
• Mammalian hormones stimulate their target cells via the blood system- transported in blood plasma to target cells Nerve cells pass electrical impulses along their length to stimulate their target.
• Changes in concentration pf hormones stimulates receptors. Neurotransmitters stimulate effectors.
• Specific to the tertiary structure of receptors on the cell-surface membrane of target cells. Specific to a target cells as they secrete a chemical messenger (neurotransmitter) directly onto.
• Travel to all parts of the body, but only target cells respond. Travel to specific parts of the body.
• Slower communication. Rapid communication between specific parts of an organism.
• Longer-lasting Shorter-lived
• More widespread but less specific.
• Localised (restricted to one area of the body)
• Allows a full- body response (e.g. adrenaline stimulates the breakdown of glucose and increases heartrate to enable increased respiration). Allows animals to react quickly and precisely to stimuli.
• Effects may be permanent and irreversible (e.g. growth) Effects are usually temporary but reversible.
• E.g. Control of blood glucose conc.- slower response- more long term and widespread effect. E.g. reflex action- withdrawal of hand- needs to be short-lived, rapid and restricted.

Question 10

What is important to note about the nervous and hormonal systems.

Answer 10

Although they are different, both systems work together and interact with one another to coordinate responses.

Question 11

What is the nervous system composed of?

Answer 11

• Central Nervous system (CNS)- brain and spinal cord.
• Peripheral nervous system (PNS)- pairs of nerves that originate from the brain or spinal cord- connect the CNS to the rest of the body- contains different types of neurones.

Question 12

What are the different types of neurones in the Peripheral Nervous System?

Answer 12

• Sensory neurones- carry nerve impulses (electrical signals) from receptors to the CNS.
• Relay neurones (coordinator or intermediate neurone)- transmit electrical impulses between sensory and motor neurones.
• Motor neurones- carry nerve impulses away from CNS to effectors. These effectors can be in the voluntary or autonomic nervous system.

Question 13

How many systems do the motor neurones in the peripheral nervous system have and what are they?

Answer 13

• Two
• Voluntary (somatic) nervous system- carries impulses to body muscles under voluntary (conscious) control e.g. running.
• Autonomic nervous system- controls involuntary (subconscious) activities - carries impulses to glands, smooth muscle and cardiac muscle- e.g. digestion. Involves two antagonistic systems (they have opposite effects).

Question 14

What are the antagonistic systems in the autonomic nervous system?

Answer 14

• The sympathetic nervous system- stimulates effectors to speed up an activity- helps us cope with stressful situations and prepare for strenuous activity (e.g. the flight or fight response).
• Parasympathetic nervous system- inhibits effectors- slows down activity- controls activity under normal resting conditions- conserves energy.

Question 15

Why is the antagonistic autonomic nervous system important?

Answer 15

• Internal systems within the body need to automatically adapt to meet changing conditions and this requires the coordination of large amounts of information that comes from monitoring our internal environment continuously.
• The activity of internal muscles and glands are regulated by the balance of the sympathetic and parasympathetic systems.

Question 16

Draw a diagram to reflect how the different divisions of the nervous system link together.

Answer 16

Answer on revision card.

Question 17

Describe the spinal cord.

Answer 17

• Column of nerves that run along the back, lies in vertebral column for protection.
• Pairs of nerves emerge from the spinal cord at intervals.

Question 18

Describe the process of nervous communication.

Answer 18

• The nervous system has many different specific receptors.
• Stimulus are detected by receptors and electrical impulse sent along sensory neurone.
• Chemicals called neurotransmitters take the electrical impulse across the synapse (gap) from the first neurone to the second neurone.
• Receptors and effectors are linked by a central coordinator (in this case the CNS) which connects information from each receptor to the appropriate effector.
• The electrical impulse is sent to the CNS (coordinator) which processes it and sends impulses along the motor neurones to an effector.

Question 19

Describe the process of nervous response

Answer 19

• Electrical impulses reach the end of the motor neurone.
• Chemical neurotransmitters are secreted directly onto cells (e.g. muscle cells)- nervous response is localised.
• Neurotransmitters- quickly removed- response short-lived.

Question 20

What are muscles of the heart called?

Answer 20

Cardiac muscles

Question 21

What type of muscle is the heart and what does this mean?

Answer 21

Myogenic- contraction is initiated within the muscle itself rather than by nervous impulses like other muscles (neurogenic).

Question 22

What is responsible for the stimulation of heart contraction and regulation of the heartbeat?

Answer 22

• Sinoatrial node (SAN) in the wall of right atrium.
• Sometimes known as the pacemaker- it has a regular rhythm of stimulation that determines the heartbeat.

Question 23

What does nervous stimulation of the heart ensure?

Answer 23

• The right and left atria, and right and left ventricles contract at the same time.
• The atria contract from top-down, whereas the ventricles contract from bottom-up.

Question 24

How does nervous stimulation of the heart occur?

Answer 24

1. A wave of electrical activity (excitation) spreads from the sinoatrial node across both atria, causing them to contract.
2. Atrioventricular septum- non- conductive tissue- prevents the wave from crossing to the ventricles. The wave of excitation instead enters the atrioventricular node (AVN) between the atria.
3. The AVN, after a short delay to ensure the atria have emptied, sends a wave of electrical excitation between the ventricles along a group of Purkyne tissues (specialised muscle fibres), known as the bundle of His.
4. The bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles (the apex), where it branches into smaller Purkyne tissue fibres in the let and right ventricle walls.
5. The wave of excitation is released from the Purkyne tissues, causing the ventricles to contract simultaneously from the bottom of the heart upwards.

Question 25

Draw a diagram to illustrate the movement of nervous impulses around the heart.

Answer 25

Answer on revision card.

Question 26

What is the average resting heartrate in humans?

Answer 26

70bpm

Question 27

Why does heart rate need to be altered?

Answer 27

• To meet varying demands of oxygen for aerobic respiration (e.g. during excercise).
• To prevent blood pressure becoming too high and damaging the arteries.

Question 28

What controls changes in the heart rate (the rate at which the SAN produces electrical activity)?

Answer 28

• The autonomic nervous system and effectors.
• This includes the medulla oblongata.

Question 29

Which part of an organ is responsible for the subconsious control of the SAN?

Answer 29

• The medulla oblongata with two centres:
• One centre is linked to the SAN by the sympathetic nervous system and increases the heart rate.
• One centre is linked to the SAN by the parasympathetic nervous system- decreases heart rate.

Question 30

How are changes in heart rate controlled?

Answer 30

• Heart rate is controlled by chemoreceptors and pressure receptors, which are linked the medulla oblongata by sensory neurones.
• The medulla oblongata then uses motor neurones (parasympathetic and sympathetic) to control the SAN’s impulses.

Question 31

How do the parasympathetic and sympathetic nervous systems alter heartrate?

Answer 31

• Parasympathetic neurones secrete acetylcholine neurotransmitters, which bind to the receptors on the SAN and cause the heart rate to slow down.
• Sympathetic neurones secrete noradrenaline neurotransmitters, which bind to the receptors on the SAN and cause the heart rate to speed up.

Question 32

Describe the features of chemoreceptors:

Answer 32

• Found within the walls of the carotid arteries and aorta.
• Sensitive to changes in blood pH as a result of CO2 concentration (CO2 forms acid in solution).
• Control heart rate.

Question 33

What is the response when CO2 concentration is higher then normal?

Answer 33

1. Blood has a lower pH due to a higher concentration of CO2 (usually due to high respiration).
2. Chemoreceptors detect this and increase the frequency of nervous impulses to the centre of the medulla oblongata that increases heart rate.
3. This centre increases the frequency of impulses via the sympathetic nervous system, which secretes noradrenaline, which binds to receptors on the sinoatrial node.
4. This increases the rate at which the SAN produces electrical waves, and therefore increases the heart rate.
5. Increased blood- more CO2 removed by the lungs- CO2 concentration returns to normal.
6. pH rises to normal- chemoreceptors reduce frequency of nerve impulses to medulla oblongata centre.
7. Medulla oblongata reduces frequency of impulses via sympathetic nervous system to SAN- reduces heart rate.

Question 34

What is the response when CO2 concentration is lower then normal?

Answer 34

1. Blood has a higher pH due to a higher concentration of CO2.
2. Chemoreceptors detect this and increase the frequency of nervous impulses to the centre of the medulla oblongata that decreases heart rate.
3. This centre increases the frequency of impulses via the parasympathetic nervous system to the SAN, which secretes acetylcholine, binding to receptors on the SAN, leading to a decrease in the rate at which the SAN produces electrical waves, and therefore decreases the heart rate.
4. This causes the CO2 concentration to return to normal as less CO2 is released by the lungs. (This is important for the affinity of haemoglobin)/
5. The pH decreases back to normal and the medulla oblongata reduces frequency of impulses via parasympathetic nervous system to SAN.

Question 35

Describe the features of pressure receptors.

Answer 35

• Aka. baroreceptors
• In walls of carotid arteries and aorta.
• Monitor blood pressure.

Question 36

Describe the response when blood pressure is higher than normal.

Answer 36

1. Pressure receptors transmit more nervous impulses along sensory neurones to the medulla oblongata centre that decreases heart rate.
2. Centre increases impulses via the parasympathetic nervous system, which secretes acetylcholine, binding to receptors on the SAN, leading to a decrease in heartrate to bring blood pressure back to normal.

Question 37

Describe the response when blood pressure is lower than normal.

Answer 37

1. Pressure receptors transmit more nerve impulses along sensory neurones to medulla oblongata centre that increases heart rate.
2. Centre increases impulses via the sympathetic nervous system, which secretes noradrenaline, which binds to receptors on the sinoatrial node- increases heartrate to bring blood pressure back to normal.

Question 38

What are reflexes?

Answer 38

Reflexes are a simple form of nervous response.

Question 39

How do reflexes differ from conscious responses?

Answer 39

The response goes through the spinal cord but not through conscious parts of the brain.

Question 40

What are other names for a simple reflex?

Answer 40

• Three-neurone simple reflex.
• Spinal Reflex

Question 41

What are reflex arcs?

Answer 41

The pathways of neurones involved in the reflex and involve only three neurones

Question 42

Describe the process of a reflex arc:

Answer 42

1. Stimulus.
2. Receptor.
3. Sensory neurone.
4. Coordinator or intermediate neurone (sometimes a relay neurone) in spinal cord- CNS - links sensory neurone to motor neurone.
5. Motor neurone.
6. Effector.
7. Response.

Question 43

Why are reflexes important?

Answer 43

• Protect the body from harm from birth.
• Provide a rapid, short lived, localised and involuntary response that enables animals to react quickly to danger to help protect them from damage.
• Involuntary- don’t require decision making- leaves the brain able to carry out more complex responses- means the brain isn’t overloaded with too many responses.
• Fast- neurone pathway is short- very few synapses (usually one or two)- synapses are the slowest part of a neurone pathway.
• Rapid response- The conscious parts of the brain are not used- responses happen automatically- no time needed to make decisions.
• Relay neurones can be overridden by the brain if necessary.

Question 44

How does the CNS receive information about the environment?

Answer 44

The CNS receives information about the internal and external environment through many different receptors that respond to different specific stimuli (e.g. light, glucose concentration, pressure).

Question 45

What is the difference between sensory reception and sensory perception?

Answer 45

• Receptors are responsible for sensory reception.
• Sensory perception (processing information from the receptors) is done by coordinators (mostly the brain).

Question 46

Where does the sensory information the receptors pass on to the CNS come from?

Answer 46

• Can come from receptors inside the body responding the internal stimuli requiring an autonomic response.
• Can come from external stimuli, requiring a voluntary response.

Question 47

What do all stimuli involve?

Answer 47

A change in a form of energy (e.g. heat, light, sound).

Question 48

What do receptors act as in the nervous system?

Answer 48

Receptors act as transducers, converting the energy of the stimulus into nervous impulses (electrical signals) to be understood by the body.

Question 49

When is a generator potential established?

Answer 49

When a receptor is stimulated.

Question 50

Where can receptors be found?

Answer 50

• Some receptors can be cells e.g. photoreceptors.
• Some receptors can be proteins on the cell-surface membrane e.g. glucose receptors in pancreatic cells.
  *

Question 51

How do receptors establish a generato/ action potential?

Answer 51

1. A receptor is initially in resting potential- there is no difference in charge inside or outside the cel. The inside is negatively charged relative to the outside. There is a voltage across the membrane (potential difference) known as the resting potential. This is generated by ion pumps and channels.
2. When a stimulus is detected- the cell membrane becomes excited and more permeable- allowing ions to move in and out of the cell, altering the potential difference (creating a generator potential). A bigger stimulus excites the membrane more, causing more movement of ions and a bigger change in potential difference (a bigger generator potential).
3. If the generator potential is big enough to reach threshold level, it triggers an action potential, creating an electrical impulse along the neurone.
4. If the stimulus is too weak, the generator potential won’t reach threshold and no action potential is triggered.
5. Action potentials are all the same size, so the strength of the stimulus is measured by the frequency of action potentials.

5 steps

Question 52

What must you be careful of with regards to potential?

Answer 52

Be careful of potential vs. potential difference as potential difference can only be used when describing the difference between two areas.

Question 53

What do Pascinian Corpuscles demonstrate?

Answer 53

The function of receptors in responding to specific stimuli and establishing a generator potential

Question 54

Which receptors respond to mechanical stimuli?

Answer 54

Pacinian corpuscles

Question 55

What do Pacinian corpuscles detect?

Answer 55

• They are mechanoreceptors.
• Detect only mechanical stimuli e.g. pressure (they are specific to a single type of stimulus)-

Question 56

What is the role of Pacinian corpuscles?

Answer 56

Mechanical energy into a generator potential.

Question 57

Where can Pacinian Corpuscles be found?

Answer 57

• Deep in the skin (most abundant in fingers, feet and genitalia).
• Occur in joints, ligaments and tendons so organisms can fell movement.

Question 58

Describe the structure of a Pacinian Corpuscle.

3 Points

Answer 58

• Contain the end of a sensory neurone- sensory nerve ending.
• This is at the centre of layers of connective tissue called lamellae, separated by a gel.
• The sensory nerve ending contains stretch-mediated sodium ion channels in its plasma membrane. Their permeability to sodium changes when they are deformed.

Question 59

What is it important to note when thinking about nerves?

Answer 59

Protien channels are specific to different ions.

Question 60

How do Pacinian Corpuscles work?

5 steps

Answer 60

1. In its resting state, the stretch mediated sodium ions in the membrane of the sensory neurone are too narrow to allow sodium ions to pass through- the sensory neurone is at resting potential.
2. When pressure is applied, the lamellae are deformed and stretch the sensory nerve endings membrane.
3. This deforms the stretch-mediated sodium channels, and sodium ions diffuse into the neurone.
4. This influx of sodium ions depolarises the membrane, producing a generator potential (the membrane is no longer all negatively charged on the inside and all positively charged on the outside).
5. This generator potential, if it reaches threshold, creates an action potential that can pass along the neurone the CNS.

Question 61

Why are bigger stimulus more likely to be noticed?

Answer 61

The bigger the stimulus, the more likely the neurone is likely to reach threshold as more sodium channels open.

Question 62

Descrieb the features of receptors in the eye.

Answer 62

• Receptors in the eye are photoreceptors (detect light).
• They are found in the eye’s innermost layer- the retina.
• There are two types of photoreceptor in human eye- rod and cone cells.
• Rods and cones transduce light energy into nerve impulses.

Question 63

Describe the distribution of rod and cone cells.

Answer 63

• Uneven.
• Fovea- the part of the retina opposite the pupil light is focused on- receives highest intensity of light- only cone cells but no rod cells.
• Number of cone cells decreases further away from the fovea.
• At peripheral parts, where light intensity is at lowest- only rod cells are found.

Question 64

Describe how light enters the eye.

Answer 64

• Light enters the eye through the pupil and is focussed by lenses.
• The amount of light that enters is controlled by the iris muscles.
• Nerve impulses from photoreceptors are carried from the retina to the brain by the optic nerve. This is known as the blind spot because there aren’t any photoreceptors around the optic nerve.

Question 65

What do the differences in rod and cone cells allow?

Answer 65

• Enables mammals to have all round vision in the day and night due to differences in sensitivity and visual acuity.
• These differences are due to the arrangement of rod and cone cells and their connections in the optic nerve.

Question 66

How do photoreceptors work?

Answer 66

• Light hits photoreceptors and is absorbed by light-sensitive optical pigments.
• Light bleaches the pigment causing chemical changes and altering the membrane permeability to sodium ions.
• This creates a generator potential in the bipolar neurone. If it reaches threshold a nerve impulse is sent along a bipolar neurone.
• Bipolar neurones connect to photoreceptors in the optic nerve which take impulses to the brain.

Question 67

Describe the features of rod cells.

Answer 67

• Only see in black and white- can’t distinguish different wavelengths.
• More numerous than cone cells.
• Mainly found in the peripheral parts of the retina.
• Many rod cells are synapsed to one sensory neurone in the optic nerve.
• The rhodopsin pigment in rod cells breaks down in the presence of low- intensity light, so they can respond to low intensity light.

Question 68

What does retinal convergence mean?

Answer 68

Many rod cells are connected to one bipolar neurone connected to a sensory neurone in the optic nerve.

Question 69

What features do retinal convergance give rods?

Answer 69

• High sensitivity- rod cells can detect light at low intensities (although in black and white) as there is a much greater chance that threshold value will be reached due to summation (weaker generator potentials combine to reach the threshold and trigger an action potential)
• Low visual acuity (resolution)- only one impulse is generated by rod cells linking to the same bipolar cell, regardless of how many rod cells are stimulated, so the brain cannot distinguish between the two separate close together points of light that stimulated them.

Question 70

Describe the features of cone cells.

Answer 70

• 3 different types of pigment- each respond to a different wavelength of light (red, green and blue)
• Mainly packed in the fovea.
• Help to see in colour (depends on proportion of each type stimulated).
• Less than rod cells.
• Each cone cell has a separate bipolar neurone connected to a sensory neurone in the optic nerve.

Question 71

What does each cone cell having a separate bipolar neurone connected to a sensory neurone in the optic nerve mean.

Answer 71

• Lower sensitivity- the stimulation of separate cone cells can’t be combined to help reach threshold and create a generator potential- more light needed to reach threshold and trigger action potential- only respond to high light intensity.
• High visual acuity (resolution)- If two adjacent cone cells are stimulated by light from two separate close together points, the brain receives two separate action potentials to distinguish between.

Question 72

What type of language must you use when comparing rods and cones?

Answer 72

Comparative language.

Question 73

What are taxes and kineses?

Answer 73

• Simple responses that can keep a mobile organism in a favourable environment.
• Often occur in simple mobile organisms e.g. woodlice.

Question 74

Describe taxis and give e.gs.

Answer 74

• Directional movement in response to a stimulus.
• The direction of the stimulus affects the response.
• Motile organisms respond directly to environmental changes by either:
• Positive Taxis- moving towards a favourable stimulus.
• Negative taxis- moving away from an unfavourable stimulus.
• E.g. Bacteria- positive chemotaxis towards high concentrations of glucose- increased access to respiratory substrate.
• E.g. Woodlice- negative phototaxis- keeps them in damp conditions to reduce water loss.

Question 75

Give an overview of kinesis.

Answer 75

• Non-directional movement in response to a stimulus.
• The intensity of the stimulus affects the response.
• Changes in the speed at which and organism moves and the rate at which it changes direction.
• Important when a stimulus is less directional e.g. humidity and temperature.

Question 76

Describe the kineses of organisms in different environments and give an e.g.

Hint: 4 points including e.g.

Answer 76

• If an organism crosses line between favourable and unfavourable environment it’s rate of turning increases to increase chance of returning quickly to a favourable environment.
• If an organism moves for long time into unfavourable environment- rate of turning decreases to move in long straight lines then turns sharply to bring organisms into a region with favourable conditions.
• If in favourable conditions an organism may move slowly and change direction less to stay in favourable area.
• E.g. woodlice- exhibit these movements in favourable areas (damp) and unfavourable (dry) to increase chances of survival by spending more time in favourable humid areas to prevent them from drying out.

Question 77

How can a maze be used ot investigate the effect of an environmental variable on the movement of an animal.

Answer 77

Maze- can investigate movement by seeing which direction the woodlice choose to go to in response to e.g. light or humidity. Woodlice often alternate their movements to increase the chances of finding more favourable conditions by moving to different areas.

Question 78

What is a choice chamber and how can it be used to investigate the effect of an environmental varibale on the movement of an animal?

Answer 78

• Container with different compartments to create different environmental conditions.
• Can investigate how animals e.g. woodlice or maggots, respond to different conditions e.g. light intensity, humidity.

Question 79

Describe the process of using a choice chamber to investigate the movement of woodlice.

Hint: 5 steps

Answer 79

1. Take a choice chamber: For light intensity- cover one half of the lid with black paper. Put damp filter paper in both sides to create a constant humidity. Then put mesh over the filter paper.
2. Place 10 woodlice onto the mesh in the centre and add the lid.
3. After 10 minutes, record the number of woodlice on each side of the chamber. (Woodlice ideally move towards the dark side)
4. Repeat after moving the woodlice back into the centre.
5. For humidity: Put damp filter paper of one side, and dessicating agent on the other. Leave for 10 minutes before putting the woodlice in to stabilise the environmental conditions.

Question 80

What safety/ ethics need to be considered when using a choice chamber to investigate woodlice.

Answer 80

• Handle woodlice carefully and return them to their natural habitat.
• Wash hands after handling woodlice.
• Be careful not to touch dessicating agent or get it wet as they generate heat and act as irritants, wear eye protection to be careful.

Question 81

What is a tropism?

Answer 81

Growth of a plant in response to a directional stimulus.

Question 82

Why are tropisms important and what stimuli are they used to respond to?

Answer 82

• In order to survive, flowering plants have to respond to changes in external and internal environments.
• Stimuli include light, gravity and water.

Question 83

Describe the responses of plants to different stimuli.

Answer 83

• Plant shoots have positive phototropism (grow towards light) but negative gravitropism (away from gravity) so their leaves are in the most favourable position to capture light for photosynthesis.
• Plant roots have negative phototropism (grow away from light) but positive gravitropism (towards gravity) so that they have an increased chance of growing in the soil- better able to absorb water and mineral ions and anchor the plant.
• Water- roots positively hydrotropic (grow towards it) to absorb it for photosynthesis and other metabolic processes and to support the plant.

Question 84

How do plants control their response to stimuli?

Answer 84

Plants control their response using hormone-like plant growth factors (aka. auxins) as they have no nervous system.

Question 85

What are the features of growth factors and give an e.g.?

Hint: 5

Answer 85

• Regulate growth in response to directional stimulus (tropisms).
• Produced in small quantities in the growing regions of the plant (shoot and root tips).
• In flowering plants, specific growth factors move from growing regions to other tissues.
• Sometimes affect the growth of the tissues that release them, whereas animal hormones target distant organs.
• Indoleacetic acid (IAA).

Question 86

What does IAA stand for?

Answer 86

Indoleacetic acid

Question 87

How does IAA affect plants?

Answer 87

• Different concentrations of indoleacetic acid (IAA) affect cell elongation in the roots and shoots of flowering plants, allowing plants to respond to stimuli.
• In shoots, IAA stimulate cell elongation.
• In roots, IAA inhibits cell elongation.
• To do this, IAA increases the plasticity of plant cell walls- only occurs in young cell walls where cells are able to elongate- mature cells have greater rigidity so aren’t able to respond.

Question 88

What is the theory of how IAA increases plasticity.

Answer 88

The acid growth hypothesis

Question 89

Describe the acid growth hypothesis.

Hint: 3 steps

Answer 89

1. IAA stimulates protein carrier pumps to actively transport H+ ions from the cytoplasm into the cell wall.
2. The high H+ concentration activates ‘expansin’ enzymes that loosen the cellulose microfibrils by breaking hydrogen crosslinks.
3. The cell wall increases in plasticity, allowing it to stretch under pressure from water, causing elongation.

Question 90

Where is IAA produced, and how and where is it transported.

Answer 90

• IAA is only produced in the tip of roots and shoots (growing regions of the plant), if they are removed, no IAA will be available and the shoot will stop growing.
• IAA is moved around plants to control tropisms- short distances by diffusion and active transport, long distances by phloem- results in different plants having different concentration- uneven distribution leads to changes in elongation.
• IAA is transported by diffusion in one direction, away from the tip of the roots and shoots where it is produced to stimulate growth in other regions. Stimuli (e.g. gravity and light) cause an uneven distribution of IAA as it moves from the tip to a stem or root.

Question 91

What causes flowering plants to grow towards certain stimuli?

Answer 91

Elongation of cells on one side of a stem can lead to bending- this explains gravitropism and phototropism in flowering plants.

Question 92

What happens when light is applied directly to one side of the shoot (unilaterally)?

Answer 92

It produces a positive phototropism:
1. Cells in the tip of the shoot produce IAA which is transported down the shoot, initially evenly throughout all regions.
2. Light causes the movement of IAA from the light side to the shaded side, resulting in a greater concentration of IAA on the shaded side.
3. The greater concentration of IAA causes cell elongation to occur faster on the shaded side than the light side, due to IAA encouraging cell elongation.
4. This causes the shoot tip to bend towards the light.
5. The reverse happens in the roots.

Question 93

How do horizontally growing roots grow?

Answer 93

Have a positive gravitropism:
1. Cells in the tip produce IAA which is transported along the root, initially to all sides.
2. Gravity causes IAA to move to the lower side of the root, creating an increased concentration.
3. The greater concentration of IAA on the lower side compared to the upper side causes less cell elongation to occur on the lower side than the upper side as IAA inhibits the elongation of root cells.
4. This causes the root to bend towards gravity.
5. The reverse happens in the shoots.

Question 94

How can the effects of gravity be shown on a plant?

Answer 94

A clinostat with no gravity can be used to show how roots grow horizontally without gravity.

Question 95

What should you remember with regards to investigations on the effects of IAA.

Answer 95

• Students could design and carry out investigations into the effects of indoleacetic acid on root growth in seedlings.
• Remember growth factors are only produced in the tips.
• Remember growth factors diffuse straight down unless as stimulus is at play.
• If asked about designing an experiment, remember controls- like sponges soaked in water- show that the observed effects were caused by IAA and nothing else.
• Glucose needs to be provided for plants in the dark as they can’t photosynthesise.

Question 96

How should you approach data with regards to the effects of IAA?

Answer 96

Describe the data:
* Shoot A has IAA on the right side, shoot B has IAA on the left side, shoot C has IAA across the tip, and shoot D has no IAA.
* Experiment A is in the dark, Experiment B is in the light.
* Being in the light changes the direction of growth in shoots B and C, but not shoots A and D.
* Being in the light increases growth across all the shoots.

Explain the data:
* Data shows how the movement of IAA controls phototropism in plant shoots.
* In Experiment A, the IAA in shoot A diffused straight down from the sponge into the left hand side of the shoot. This stimulated cells to elongate, so the shoot grew to the right. The same happened in shoot B except it grew to the left as the IAA was on the opposite side. As equal amount of IAA diffused down both sides, the cells elongated at the same rate, making the shot grow straight.
* In Experiment B, IAA diffused into the shoot and accumulated on the shaded side, no matter where the sponge was placed. Shoots A,B and C all grew to the right because most IAA accumulated to the left, stimulating cell elongation.