Response To Stimuli (A Level Only)

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Survival & Response

Responding to changes in the environment aids survival by allowing organisms to find food, avoid predators and stay in suitable conditions.
A stimulus is detected by receptor cells, e.g. light is detected by photoreceptor cells in the eye, and changes in blood glucose are detected by β cells in the pancreas.
Information from receptors is processed by a coordinator, which may be part of the nervous system (e.g. brain or spinal cord) or the hormonal system (e.g. pancreas or pituitary gland).
An effector responds appropriately to the environmental change; effectors are either muscles, which allow movement, or glands, which secrete chemicals.
Signals between receptors, coordinators and effectors involve nervous communication (electrical impulses in neurones) or hormonal communication (chemicals in the blood).

Growth Factors in Flowering Plants

Plants survive by responding to changes in their environment, including directional stimuli.
A tropism is a directional growth response, e.g. a plant growing towards light.
Phototropism is a growth response to light; gravitropism is a growth response to gravity.
Tropisms can be positive (growth towards a stimulus) or negative (growth away from a stimulus).
Plant growth responses are controlled by growth factors (not hormones) released by plant cells; these act similarly to hormones in animals.
Growth factors are specific, produced in growing parts of plants (e.g. tips of roots and shoots), and move to other tissues to regulate growth.
Indoleacetic acid (IAA) is synthesised at the tips of roots and shoots but mainly affects growth in the region adjacent to the tip.

Indoleacetic Acid (IAA)

IAA is a plant growth factor belonging to a group called auxins.
IAA is synthesised in the growing tips of roots and shoots and moves by diffusion, active transport, and over longer distances via the phloem.
IAA controls cell elongation; the concentration of IAA determines the rate of elongation, and an uneven concentration causes uneven growth.
In roots, IAA inhibits cell elongation; in shoots, IAA causes cell elongation.
Gravitropism in roots: IAA moves to the lower side, setting up a gradient; amyloplasts gather at the bottom due to density; IAA inhibits elongation on the lower side, so the root bends towards gravity (positive gravitropism).
Phototropism in shoots: IAA moves to the shaded side; IAA causes elongation on the shaded side, so the shoot bends towards light (positive phototropism).
In roots, IAA moves to the shaded side and inhibits elongation there, causing the root to bend away from light (negative phototropism).

Investigating the Effect of IAA on Root Growth

Seedlings of the same age and species are used; root tips are cut into 1 cm sections and marked at 2 mm intervals.
Root tips are placed in test tubes of water to keep tissue alive, then divided into three groups.
Group A: root tips removed and placed on agar; illuminated equally on all sides. Expected: roots grow evenly because no IAA is produced, so no inhibition of elongation.
Group B: intact root tips on agar, covered with a light-proof container. Expected: roots grow less than Group A but evenly, because IAA inhibits elongation equally on both sides.
Group C: intact root tips on agar with directional light. Expected: root bends away from light because greater IAA concentration on shaded side inhibits elongation there, so illuminated side grows faster (negative phototropism).
Control variables include moisture, temperature, and light intensity; control treatments ensure results are due to IAA and not other factors.

Taxes & Kineses

Taxes and kineses are simple, innate responses that enable mobile organisms to stay in a favourable environment, responding to stimuli such as light, temperature, and humidity.
Kineses are non-directional responses involving random movement; the rate of movement or turning changes with stimulus intensity.
Example: flatworms show photokinesis; in bright light they move faster and change direction more often, increasing chance of leaving unfavourable conditions.
Taxes are directional responses involving movement towards or away from a stimulus.
Example: Euglena show positive phototaxis, swimming towards light using a flagellum to carry out photosynthesis.
Moths moving towards light show positive phototaxis; a mosquito moving away from an aerosol spray is negative chemotaxis.
Key difference: kineses are non-directional and random; taxes are directional and involve movement towards or away from a stimulus.

Reflex Arcs

Simple reflexes allow rapid, automatic responses to protect an organism from harm, e.g. pupil reflex prevents bright light damaging the retina; coughing reflex prevents obstructions entering airways.
Reflex arcs do not involve conscious parts of the brain, so reflexes are faster than other nervous responses.
The pathway of a reflex arc is: receptor → sensory neurone → relay neurone → motor neurone → effector.
Sensory neurones carry impulses from receptors to the central nervous system (CNS).
Relay neurones connect sensory and motor neurones and are found within the CNS.
Motor neurones carry impulses from the CNS to effectors.
Synapses ensure impulses travel unidirectionally because neurotransmitters are released from the presynaptic neurone and bind to receptors on the postsynaptic membrane only.

Required Practical: Investigating Animal Movement

Environmental factors influencing animal movement can be investigated using a choice chamber or maze.
Investigating moisture preference in woodlice: set up a choice chamber with one moist side (distilled water) and one dry side (drying agent).
Place a gauze platform to keep woodlice away from water and drying agent; place 10 woodlice in the centre.
Allow woodlice to move freely for 10 minutes, then record numbers in each section; repeat at least twice more.
Control variables include light intensity, temperature, and surface texture; use same individuals and clean chamber to avoid chemical traces.
Handle woodlice carefully with a soft paintbrush to minimise stress; return them to original location afterwards.
Observe movement to determine whether taxis or kinesis is shown; a variation can use light as the independent variable by covering half the chamber.

The Pacinian Corpuscle

Receptors are cells that detect stimuli and are specific, each type detecting a different stimulus.
Pacinian corpuscles are receptors found deep in the skin that respond to changes in pressure.
Structure: many layers of membrane (lamellae) separated by gel, surrounding the ending of a sensory neurone.
When stimulated by pressure, a generator potential is established: a change in potential difference across the membrane.
The size of the generator potential depends on the size of the stimulus; if large enough, it can initiate an action potential.

Pacinian Corpuscles: Generator Potential

Pressure distorts the layers of membrane, causing stretch-mediated sodium channels in the axon membrane to open.
Sodium ions enter the axon via facilitated diffusion.
The influx of positively charged sodium ions makes the inside of the axon more positive, establishing a generator potential.
Generator potentials only occur at the site of a stimulus, differ in size depending on stimulus size, and can lead to an action potential.
If the generator potential is large enough, an action potential is triggered; the membrane potential during an action potential is identical regardless of stimulus intensity once threshold is reached.

Investigating Touch and Temperature Receptors

Temperature receptors: use a thermometer from a 45°C water bath and one from iced water; touch different spots on the back of the hand and record temperature and whether warmth or only touch is felt.
Results show temperature receptors detect temperature only when it deviates significantly from body temperature; near body temperature, only touch is felt.
Limitations: results are subjective and only one subject is used; repeating with more subjects improves reliability.
Habituation of touch receptors: response to a stimulus decreases after repeated or prolonged exposure, e.g. becoming unaware of clothing.
Method: apply a standardised stimulus (e.g. cotton bud) to the same spot at regular intervals or steady pressure; record time when stimulus is no longer felt; after rest, sensitivity returns.
Resolution of touch receptors: the ability to distinguish two points of contact; areas with many receptors (fingers, soles) have high resolution, while back of hands and legs have lower resolution.
Method: use blunt scissors or paperclip at measured distances; randomly apply one or two points; find minimum distance at which two points are correctly identified.

The Human Retina

The retina contains rod cells (sensitive to light intensity only, distributed across entire retina) and cone cells (sensitive to different wavelengths, concentrated in the fovea).
Light causes breakdown of optical pigments in rods and cones, producing a generator potential; if large enough, a nerve impulse is sent along a bipolar neurone to the optic nerve.
Rod cells are very sensitive to light because multiple rods connect to a single bipolar cell, allowing spatial summation of weak generator potentials.
Cone cells are less sensitive to light because each cone connects to its own bipolar neurone, so a higher generator potential is needed to trigger an impulse.
Cone cells are sensitive to different wavelengths; stimulation of red-, blue-, and green-sensitive cones in different combinations generates colour images.
Visual acuity is the ability to distinguish two separate points; rods have low acuity because many rods synapse with one bipolar cell, so the brain cannot determine which rod generated which impulse.
Cones have high acuity because each cone synapses with a single bipolar cell, allowing separate impulses to be interpreted as separate points.

Myogenic Stimulation of the Heart

Heart muscle contraction is myogenic, meaning the heart beats at a baseline rate without input from the nervous system.
The sinoatrial node (SAN) is a group of cells in the wall of the right atrium, often called the pacemaker; it initiates a wave of depolarisation across the atria, causing them to contract.
The wave of depolarisation reaches non-conducting tissue between atria and ventricles, then passes to the atrioventricular node (AVN); this delay allows atria to empty into ventricles before ventricular contraction.
Electrical activity passes from the bundle of His to the Purkyne tissue, causing ventricles to contract from the base upwards.
The wave of depolarisation spreads in a coordinated manner from the SAN.

Heart Rate

Heart rate is controlled by the cardioregulatory centre in the medulla of the brain, connected to the SAN by nerves of the autonomic nervous system.
The autonomic nervous system controls unconscious activities and has two branches: sympathetic ('fight or flight') and parasympathetic ('rest and digest').
Chemoreceptors detect carbon dioxide concentration (and blood pH); pressure receptors detect blood pressure.
Low blood CO₂ or high blood pressure: impulses to medulla → parasympathetic neurones → acetylcholine released at SAN → SAN decreases frequency of heartbeat initiation → heart rate decreases.
High blood CO₂ or low blood pressure: impulses to medulla → sympathetic neurones → noradrenaline released at SAN → SAN increases frequency of heartbeat initiation → heart rate increases.
Cardiac output = heart rate × stroke volume; can be rearranged to find heart rate or stroke volume.
Investigating caffeine effect on heart rate: control group drinks water, experimental group drinks caffeine; measure resting heart rate, then after consumption at 15-minute intervals for 2 hours; control variables and large sample size improve reliability.

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1.What is the correct term for a growth response to a directional stimulus in plants?
Easy
- ATropism
- BTaxis
- CKinesis
- DReflex
2.Which of the following is the plant growth factor that controls cell elongation?
Easy
- AIndoleacetic acid (IAA)
- BGibberellin
- CEthene
- DAbscisic acid
3.Kineses are non-directional responses that involve random movement.
Easy
True or false?
4.In which part of a plant is indoleacetic acid (IAA) synthesised?
Easy
- AGrowing tips of roots and shoots
- BMature leaves
- CRoot hair cells
- DXylem vessels
5.What is the effect of IAA on cell elongation in shoots and roots?
Easy
- AIt promotes elongation in shoots and inhibits elongation in roots
- BIt inhibits elongation in shoots and promotes elongation in roots
- CIt promotes elongation in both shoots and roots
- DIt inhibits elongation in both shoots and roots
6.Which of the following correctly describes a taxis?
Medium
- AA directional response towards or away from a stimulus
- BA non-directional response involving random movement
- CA growth response to a directional stimulus
- DA rapid automatic response that protects from harm
7.Which of the following is an example of a negative taxis?
Medium
- AA mosquito moving away from an aerosol spray
- BA moth moving towards a light source
- CA flatworm moving faster in bright light
- DA plant shoot growing towards light
8.In a reflex arc, which neurone carries impulses from the receptor to the central nervous system?
Medium
- ASensory neurone
- BRelay neurone
- CMotor neurone
- DEffector neurone

Thêm câu hỏi

9.Which of the following is a correct sequence of components in a simple reflex arc?
Medium
- AReceptor → sensory neurone → relay neurone → motor neurone → effector
- BReceptor → motor neurone → relay neurone → sensory neurone → effector
- CEffector → sensory neurone → relay neurone → motor neurone → receptor
- DReceptor → relay neurone → sensory neurone → motor neurone → effector
10.What is the name of the receptor that detects pressure changes in the skin?
Medium
- APacinian corpuscle
- BRods and cones
- CChemoreceptors
- DNociceptors
11.Which of the following are true of rod cells? (Select all that apply.)
Medium
- AThey are sensitive to light intensity
- BThey are concentrated in the fovea
- CThey connect to bipolar neurones in a many-to-one arrangement
- DThey are sensitive to different wavelengths of light
- EThey provide low visual acuity
12.What is meant by the term myogenic?
Medium
- AThe heart muscle can contract and relax without nervous stimulation
- BThe heart muscle requires nervous stimulation to contract
- CThe heart muscle contracts in response to hormones only
- DThe heart muscle is controlled by the autonomic nervous system
13.Which structure in the heart is known as the pacemaker?
Medium
- ASinoatrial node (SAN)
- BAtrioventricular node (AVN)
- CBundle of His
- DPurkyne tissue
14.What is the effect of the parasympathetic nervous system on heart rate?
Medium
- AIt decreases heart rate by releasing acetylcholine
- BIt increases heart rate by releasing noradrenaline
- CIt decreases heart rate by releasing noradrenaline
- DIt increases heart rate by releasing acetylcholine
15.How does spatial summation in rod cells increase sensitivity to light?
Hard
- AMultiple rod cells connect to a single bipolar neurone, so weak generator potentials combine to trigger an impulse
- BEach rod cell connects to its own bipolar neurone, allowing individual impulses to be sent
- CRod cells contain more optical pigment than cone cells, so they break down more pigment
- DRod cells are concentrated in the fovea where light is focused most intensely
16.Why do cone cells provide higher visual acuity than rod cells?
Hard
- AEach cone cell connects to its own bipolar neurone, so the brain receives separate impulses from each cone
- BCone cells are more sensitive to light than rod cells
- CCone cells are distributed across the entire retina
- DCone cells contain more than one type of optical pigment
17.During the cardiac cycle, why is there a delay at the atrioventricular node (AVN)?
Hard
- ATo allow the atria to empty into the ventricles before ventricular contraction begins
- BTo allow the ventricles to contract before the atria
- CTo prevent the wave of depolarisation from reaching the ventricles
- DTo allow the bundle of His to initiate the heartbeat
18.Match each receptor to the stimulus it detects.
Easy
- Pacinian corpuscle
- Rod cell
- Cone cell
- Chemoreceptor
- Changes in pressure
- Light intensity only
- Different wavelengths of light
- Carbon dioxide concentration of the blood
19.Place the events of a simple reflex arc in the correct order, starting with the stimulus.
Medium
- Effector responds
- Stimulus detected by receptor
- Impulse passes along motor neurone
- Impulse passes along sensory neurone
- Impulse passes along relay neurone
20.Which of the following statements about taxes and kineses are correct? (Select all that apply.)
Medium
- AA kinesis is a non-directional response involving random movement.
- BA taxis is a directional response involving movement towards or away from a stimulus.
- CFlatworms show photokinesis by moving faster and changing direction more often in bright light.
- DEuglena show positive phototaxis by swimming towards a light source.
- EA beetle moving away from light is an example of a tropism.
21.Which of the following are features of rod cells? (Select all that apply.)
Medium
- AThey are sensitive to light intensity only.
- BThey are concentrated in the fovea.
- CMultiple rod cells connect to a single bipolar cell.
- DThey provide low visual acuity.
- EThey are sensitive to different wavelengths of light.
22.Which of the following are part of the myogenic control of the heartbeat? (Select all that apply.)
Medium
- ASinoatrial node (SAN)
- BAtrioventricular node (AVN)
- CBundle of His
- DPurkyne tissue
- EMedulla oblongata
23.Which of the following are controlled variables in the choice chamber experiment investigating moisture and woodlice movement? (Select all that apply.)
Medium
- ALight intensity
- BTemperature
- CSurface texture
- DNumber of woodlice in each section
24.Which of the following statements about IAA are correct? (Select all that apply.)
Hard
- AIAA is synthesised in the growing tips of roots and shoots.
- BIAA moves from cell to cell by diffusion and active transport.
- CIAA inhibits cell elongation in roots.
- DIAA causes cell elongation in shoots.
- EIAA moves toward the illuminated side of a shoot.
25.The sinoatrial node (SAN) is often referred to as the pacemaker of the heart.
Easy
True or false?
26.In roots, indoleacetic acid (IAA) causes cell elongation.
Easy
True or false?
27.A scientist investigates the effect of temperature on the uptake of a synthetic plant growth factor by roots. The rate of uptake increases as temperature rises from 10 °C to 30 °C then falls sharply above 40 °C. Which of the following best explains the fall in uptake above 40 °C?
Medium
- AThe carrier proteins involved in active transport are denatured.
- BThe growth factor is broken down by enzymes in the soil.
- CThe rate of diffusion increases at higher temperatures.
- DThe plant's stomata close, preventing uptake.
28.In an investigation into the effect of moisture on woodlice movement, a choice chamber was set up with one moist side and one dry side. After 10 minutes, 8 woodlice were on the moist side and 2 on the dry side. What is the most valid conclusion?
Medium
- AWoodlice show a taxis towards moisture.
- BWoodlice show a kinesis in response to moisture.
- CWoodlice prefer moist conditions because they show negative phototaxis.
- DWoodlice move randomly and the distribution is due to chance.
29.A student investigates the resolution of touch receptors on the fingertip and the back of the hand using a pair of blunt scissors. On the fingertip, the subject correctly identified two points when they were 2 mm apart, but on the back of the hand, two points had to be 20 mm apart. Which of the following best explains this difference?
Medium
- AThe fingertip has a higher density of touch receptors than the back of the hand.
- BThe back of the hand has more Pacinian corpuscles than the fingertip.
- CThe fingertip has fewer sensory neurones connecting to the brain.
- DThe back of the hand has a thicker epidermis, reducing sensitivity.
30.In an experiment on habituation of touch receptors, a cotton bud is gently applied to the same spot on a subject's forearm every 2 seconds. After 30 seconds, the subject reports no longer feeling the stimulus. Which of the following best explains this observation?
Hard
- AThe touch receptors have become desensitised to the repeated stimulus.
- BThe sensory neurones have stopped conducting action potentials permanently.
- CThe brain has stopped processing all sensory information from the arm.
- DThe touch receptors have been damaged by the repeated stimulus.
31.In an investigation into the effect of IAA on root growth, root tips were marked at 2 mm intervals and placed in three groups: A (tips removed), B (intact tips, no light), C (intact tips, directional light). After 3 hours, which group would show the greatest overall root growth?
Medium
- AGroup A, because removing the tip removes the source of IAA.
- BGroup B, because IAA is evenly distributed.
- CGroup C, because IAA accumulates on the shaded side.
- DAll groups would show equal growth.
32.A student sets up a choice chamber to investigate the effect of light on woodlice. Half of the chamber is covered with black paper. After 10 minutes, most woodlice are found in the covered half. Which of the following conclusions is best supported?
Medium
- AWoodlice show negative phototaxis.
- BWoodlice show positive phototaxis.
- CWoodlice show a kinesis in response to light.
- DWoodlice show positive gravitropism.
33.A student investigates the sensitivity of temperature receptors by touching a thermometer at 45 °C to different spots on the back of a subject's hand. At some spots the subject feels warmth, at others only touch. Which of the following best explains why warmth is not felt at every spot?
Medium
- ATemperature receptors are not evenly distributed across the skin.
- BThe thermometer cooled down before touching some spots.
- CTouch receptors inhibit temperature receptors.
- DThe subject's body temperature varied between spots.
34.A person has a cardiac output of 5.0 dm³ min⁻¹ and a heart rate of 70 beats per minute. What is their stroke volume?
Hard
- A71.4 cm³
- B0.071 cm³
- C350 cm³
- D14 cm³
35.During exercise, carbon dioxide concentration in the blood increases. Which of the following sequences correctly describes the resulting change in heart rate?
Medium
- AChemoreceptors detect high CO₂ → impulses to medulla → sympathetic neurones → SAN → noradrenaline released → heart rate increases
- BChemoreceptors detect high CO₂ → impulses to medulla → parasympathetic neurones → SAN → acetylcholine released → heart rate increases
- CPressure receptors detect high CO₂ → impulses to medulla → sympathetic neurones → SAN → noradrenaline released → heart rate decreases
- DChemoreceptors detect high CO₂ → impulses to SAN → parasympathetic neurones → medulla → acetylcholine released → heart rate decreases
36.A student investigates the effect of caffeine on heart rate. They measure resting heart rate, give volunteers a caffeinated drink, and measure heart rate every 15 minutes for 2 hours. Which of the following is the most important control in this experiment?
Medium
- AGiving a control group the same volume of water instead of the caffeinated drink.
- BEnsuring all volunteers are the same age.
- CMeasuring heart rate at the wrist rather than using a monitor.
- DAllowing volunteers to eat during the experiment.
37.In an investigation into the effect of IAA on shoot growth, a shoot is grown under a directional light source. Which of the following would you expect to observe?
Medium
- AThe shoot bends towards the light, with a higher concentration of IAA on the shaded side.
- BThe shoot bends away from the light, with a higher concentration of IAA on the illuminated side.
- CThe shoot grows straight upwards, with IAA evenly distributed.
- DThe shoot bends towards the light, with a higher concentration of IAA on the illuminated side.
38.A student investigates the effect of light intensity on the movement of maggots using a choice chamber. Which of the following would be valid control variables? (Select all that apply.)
Medium
- ATemperature
- BHumidity
- CLight intensity
- DNumber of maggots
- ESurface texture
39.Which of the following statements about the generator potential in a Pacinian corpuscle are correct? (Select all that apply.)
Hard
- AIt is a change in potential difference across the membrane of a nerve cell.
- BIts size depends on the size of the stimulus.
- CIt is always the same size regardless of stimulus intensity.
- DIt can lead to an action potential if it is large enough.
- EIt only occurs at the site of the stimulus.
40.In an investigation into the effect of IAA on root growth, removing the root tip causes the root to grow less because IAA is no longer produced.
Easy
True or false?
41.Complete the sentence about plant responses.
Easy
A growth response towards light is called positive ____, while a growth response away from gravity is called negative ____.
42.Complete the sentence about the control of heart rate.
Medium
The autonomic nervous system controls heart rate via the ____ branch, which releases noradrenaline to increase heart rate, and the ____ branch, which releases acetylcholine to decrease heart rate.
43.Name the group of plant growth factors to which indoleacetic acid (IAA) belongs.
Hard
44.What is the term for the change in potential difference across the membrane of a receptor cell in response to a stimulus?
Hard
45.The human retina contains approximately 125 million rod cells and 7 million cone cells. Calculate the ratio of rods to cones in the form X:1, giving your answer to 1 decimal place.
Easy
46.A person has a heart rate of 75 beats per minute and a stroke volume of 70 cm³. Calculate their cardiac output in cm³ per minute.
Medium
47.Complete the sentence about plant growth factors.
Easy
The plant growth factor IAA belongs to a group of growth factors known as ____.
48.A person has a heart rate of 80 beats per minute and a stroke volume of 75 cm³. Calculate their cardiac output in cm³ per minute.
Medium
49.The human retina contains approximately 125 million rod cells and 7 million cone cells. Calculate the ratio of rod cells to cone cells in the form X:1, giving your answer to the nearest whole number.
Medium
50.Complete the sentence about the effect of temperature on the uptake of a synthetic plant growth factor by roots.
Medium
As temperature increases from 10 °C to 30 °C, the rate of uptake increases because the kinetic energy of the molecules increases, but above 40 °C the rate falls because the ____ involved in active transport are ____.
51.Complete the sentence about the choice chamber experiment with woodlice.
Medium
In a choice chamber with one moist side and one dry side, woodlice will move towards the ____ side. This is an example of a ____, which is a directional response.
52.Name the type of receptor that detects changes in pressure in the skin.
Easy
53.Describe how the plant growth factor indoleacetic acid (IAA) allows plants to respond to light stimuli from their surroundings.
Hard
54.Describe how the stretch-mediated sodium ion channels facilitate the establishment of a generator potential when pressure is applied to the Pacinian corpuscle.
Hard
55.Explain why the membrane potential was identical despite different intensities of pressure being applied to the Pacinian corpuscle.
Hard
56.Suggest three advantages of simple reflexes.
Hard
57.Describe and explain two ways in which an axon may be adapted to conduct impulses at a faster rate in the nervous system.
Hard
58.The human retina contains approximately 125 million rod cells and 7 million cone cells. Calculate the ratio of rods to cones in the form X:1, giving your answer to 1 decimal place.
Easy
- A19.9
- B17.9
- C18.9
- D16.9
59.A person has a heart rate of 75 beats per minute and a stroke volume of 70 cm³. Calculate their cardiac output in cm³ per minute.
Medium
- A6300 cm³/min
- B5250 cm³/min
- C4725 cm³/min
- D5775 cm³/min
60.A person has a heart rate of 80 beats per minute and a stroke volume of 75 cm³. Calculate their cardiac output in cm³ per minute.
Medium
- A6600 cm³ min⁻¹
- B6000 cm³ min⁻¹
- C7200 cm³ min⁻¹
- D5400 cm³ min⁻¹
61.The human retina contains approximately 125 million rod cells and 7 million cone cells. Calculate the ratio of rod cells to cone cells in the form X:1, giving your answer to the nearest whole number.
Medium
- A19 :1
- B18 :1
- C20 :1
- D17 :1

Ghi chú ôn tập

## Survival & Response - Responding to change aids survival: find food, avoid predators, stay in suitable conditions. - **Stimulus → receptor → coordinator → effector → response**. - Receptors are specific (e.g. photoreceptors detect light; beta cells detect blood glucose). - Coordinators: nervous system (brain, spinal cord) or hormonal system (pancreas, pituitary). - Effectors: **muscles** (move) or **glands** (secrete chemicals). - Communication: nervous = electrical impulses in neurones; hormonal = chemicals in blood. ## Growth Factors in Flowering Plants - Plants respond to directional stimuli by growth responses. - **Tropism** = directional growth response. - **Phototropism** = growth response to light; **gravitropism** = growth response to gravity. - Positive = towards stimulus; negative = away. - Growth factors are specific, produced in growing parts, then move to other tissues. - Use A-level term **growth factor**, not "plant hormone". ## Indoleacetic Acid (IAA) - IAA is an auxin growth factor; synthesised in growing tips of roots and shoots. - Moves cell-to-cell by diffusion and active transport; long distance via phloem. - Controls **cell elongation**; concentration determines rate; uneven concentration → uneven growth. - **Gravitropism in roots (positive):** IAA produced in root tip → moves to lower side (amyloplasts gather at bottom due to density) → IAA **inhibits** elongation → lower side elongates more slowly → root bends towards gravity. - **Phototropism in shoots (positive):** IAA produced in shoot tip → moves to shaded side → IAA **causes** elongation → shaded side elongates faster → shoot bends towards light. - **Remember:** IAA inhibits elongation in roots; causes elongation in shoots. It affects elongation, not division. ## Taxes & Kineses - Simple, innate responses keeping mobile organisms in favourable conditions. - **Kinesis** = non-directional response involving random movement (e.g. flatworms in bright light move faster and change direction more often). - **Taxis** = directional response, moving towards or away from stimulus (e.g. Euglena positive phototaxis; mosquito negative chemotaxis). ## Reflex Arcs - Rapid, automatic protective responses (pupil reflex, coughing reflex). - Do not involve conscious brain → faster than other nervous responses. - Pathway: **receptor → sensory neurone → relay neurone → motor neurone → effector**. - Sensory: receptors → CNS. Relay: connect sensory and motor, in CNS. Motor: CNS → effectors. ## Required Practical: Investigating Animal Movement - Use choice chamber or maze; e.g. moisture effect on woodlice. - Set up moist vs dry sides, gauze platform, place 10 woodlice centrally, time 10 min, count each side, repeat ≥3 times, calculate means. - Control: light intensity, temperature, surface texture; same woodlice; clean chamber; minimise handling. - Observe movement to determine taxis or kinesis. ## The Pacinian Corpuscle - Receptor deep in skin responding to **changes in pressure**. - Structure: sensory neurone ending surrounded by many layers of membrane (lamellae/connective tissue) separated by gel. - Stimulation causes a **generator potential** (change in potential difference across membrane). ## Pacinian Corpuscles: Generator Potential - Pressure distorts membrane layers → **stretch-mediated sodium channels** open. - Sodium ions enter axon by facilitated diffusion → inside becomes more positive → generator potential. - Size of generator potential depends on size of stimulus; if large enough, initiates action potential. - Generator potentials: only at stimulus site, vary in size, can lead to action potential. ## Investigating Touch and Temperature Receptors - Temperature receptors detect temperature only when it deviates significantly from body temperature. - **Habituation**: response to a stimulus decreases after repeated or prolonged exposure. - **Resolution**: ability to distinguish two points of contact; high resolution = many touch receptors (fingers, soles); low = back of hands, legs. - Limitations: subjective, human error, single subject. ## The Human Retina - Retina contains **rod cells** (light intensity only; across entire retina) and **cone cells** (different wavelengths; concentrated in fovea). - Light → breakdown of optical pigments → generator potential → nerve impulse along bipolar neurone to optic nerve. - Rods: very sensitive (many rods connect to one bipolar cell → **spatial summation**); not wavelength sensitive; low visual acuity. - Cones: less sensitive (each cone connects to its own bipolar neurone); sensitive to different wavelengths; high visual acuity. - **Visual acuity** = ability to distinguish between two separate points when viewing an image. ## Myogenic Stimulation of the Heart - Heart muscle contraction is **myogenic**: beats at baseline rate without nervous input. - **SAN** (pacemaker) initiates wave of depolarisation across atria → atria contract. - Delay at non-conducting tissue before AVN allows atria to empty into ventricles. - **AVN** → **bundle of His** → **Purkyne tissue** → ventricles contract from base upwards. ## Heart Rate - Controlled by cardioregulatory centre in medulla, connected to SAN by autonomic nervous system. - **Sympathetic** ('fight or flight'): increases heart rate, breathing rate; diverts blood to muscles. - **Parasympathetic** ('rest and digest'): slows heart rate and breathing; digestion. - **Chemoreceptors** detect blood CO₂ concentration (affects pH); **pressure receptors** detect blood pressure. - Low CO₂ / high blood pressure → parasympathetic → **acetylcholine** → SAN decreases frequency → heart rate decreases. - High CO₂ / low blood pressure → sympathetic → **noradrenaline** → SAN increases frequency → heart rate increases. - **Cardiac output = heart rate × stroke volume**. - Heart rate = cardiac output ÷ stroke volume; stroke volume = cardiac output ÷ heart rate.

Tropism: A directional growth response in plants, e.g. phototropism (light) or gravitropism (gravity).
Indoleacetic acid (IAA): A plant growth factor (auxin) that controls cell elongation; inhibits elongation in roots and causes elongation in shoots.
Kinesis: A non-directional response to a stimulus involving random movement.
Taxis: A directional response to a stimulus involving movement towards or away from it.
Reflex arc: Nerve pathway: receptor → sensory neurone → relay neurone → motor neurone → effector.
Pacinian corpuscle: A receptor deep in the skin that responds to changes in pressure; made of layers of membrane separated by gel.
Generator potential: A change in the potential difference across the membrane of a nerve cell that occurs in response to a stimulus.
Habituation: A decrease in response to a stimulus after repeated or prolonged exposure.
Visual acuity: The ability to distinguish between two separate points when viewing an image.
Myogenic: Heart muscle contraction that beats at a baseline rate without any input from the nervous system.
Sinoatrial node (SAN): A group of cells in the wall of the right atrium, often referred to as the pacemaker, that initiates the heartbeat.
Atrioventricular node (AVN): Conductive tissue that links the atria and ventricles.
Cardiac output: Cardiac output = heart rate × stroke volume.

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