3.6.1 Stimuli, both internal and external, are detected and lead to a response

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How do organisms increase their chance of survival? (1)

By responding to changes in their environment

what is a stimulus

a change in the external environment (i.e. temperature, light intensity and pressure)

What are taxes and kineses? (1)

Simple, innate responses that can maintain a mobile organism in a favourable environment

What is a taxes (tactic response) ? (2)

- Directional movement

- Towards (positive) or away (negative) from a stimulus

What is a common example of taxes? (1)

Woodlice moving away from a light source to avoid predators

What is a kineses (kinetic response) ? (2)

- Non-directional (random) response to a stimulus

- Resulting in organisms changing their speed or rate of direction

- to stay in/move to a favourable environment

What is an example of a non-directional stimulus? (1)

The conditions of an environment

What is a common example of kinesis? (3)

- Woodlice move faster when in drier environment

- To increase chances of moving to an environment with a higher humidity

- To prevent drying out

What is tropism? (1)

the growth response of a plant to a directional stimulus (light or gravity)

What is positive tropism? (1)

growth TOWARDS a stimulus

What is negative tropism? (1)

growth AWAY from a stimulus

What is photo tropism? (1)

growth of a plant in response to light

Describe phototropism in plant shoots and roots (6)

1. Cells in the tip of the shoot or root produce IAA

2. IAA diffuses down the shoot or root

3. IAA moves to shaded side of the shoot or root

3. In shoots, this stimulates cell elongation

4. Whereas in roots, it inhibits cell elongation

5. So shoots bend towards light

6. Whereas, roots bend away from light

Describe gravitropism in plant shoots and roots (7)

1. Cells in the tips of the shoot or root produce IAA

2. IAA diffuses down the shoot or root

3. IAA moves to the lower side of the shoot or root

4. In shoots, this stimulates cell elongation

5. In roots, this inhibits cell elongation

6. So shoots bend away from gravity

7. Whereas, roots bend towards gravity

What is the role of growth factors in flowering plants? (3)

1. Hormone like chemicals that allow the plant to respond to directional stimuli by speeding up or slowing down plant growth

2. Move from growing regions, e.g., shoot and root tips (where they are produced)

3. To other tissues where they regulate growth in response to directional stimuli

Describe auxins in flowering plants (4)

• produced in the tips of shoots

• diffuse backwards to stimulate the cell just behind the tips to elongate

• If the tip of a shoot is removed, no auxin will be available and the shoot stops growing.

• Auxins stimulate growth in shoots but high concentrations inhibit growth in roots.

what are other types of growth factors apart from auxins and their functions in plants

• Gibberellins stimulate seed germination and flowering.

• Abscisic acid (ABA) helps plants respond to environmental stress and is involved in stomatal closure.

• Cytokinins stimulate cell division and cell differentiation.

• Ethene stimulates flowering and fruit ripening.

EXAMPLE - An experiment was carried out to investigate the role of IAA in shoot growth.

Eight shoots, equal in height and mass, had their tips removed.

Sponges soaked in glucose and either IAA or water were then placed where the tip should be.

Four shoots were then placed in the dark (experiment A) and the other four shoots were exposed to a light source, directed at them from the right (experiment B) — see Figure 8.

After two days the amount of growth (in mm) and direction of growth was recorded. The results are shown in the table on the right.

a) Explain the data.

b) suggest why this experimental design was good

c) why were all plants soaked in glucose

a)

• The results show how the movement of IAA controls phototropism in plant shoots.

• In experiment A shoot A, the IAA diffused straight down from the sponge into the left-hand side of the shoot. This stimulated the cells on this side to elongate, so the shoot grew towards the right.

• In shoot B, the opposite occurred, making the shoot grow towards the left.

• In shoot C, equal amounts of IAA diffused down both sides, making all the cells elongate at the same rate.

• In experiment B, the shoots were exposed to a light source. The IAA diffused into the shoot and accumulated on the shaded side (left-hand side) regardless of where the sponge was placed.

• Shoots A, B and C all grew towards the right because most IAA accumulated on the left, stimulating cell elongation there.

b) In this experiment the negative control treatment was the sponge soaked in water (and glucose) which was included to show that it was the IAA causing the observed effects and nothing else.

c) to provide energy for growth of shoots.

Photosynthesis can’t take place in the dark so the growth of seedlings in experiment A might have been limited if they weren’t provided with glucose (an external energy source).

What is Indoleacetic acid (IAA)

• an important auxin that’s produced in the tips of shoots and roots in flowering plants.

• It’s moved around the plant to control tropisms — it moves by diffusion and active transport over short distances, and via the phloem over long distances.

• This results in different parts of the plant having different concentrations of IAA.

• The uneven distribution of IAA means there’s uneven growth of the plant.

EXAMPLE

Q1 Thigmotropism is a plant growth response to touch.

a) In the diagram on the right, does the shoot display positive or negative thigmotropism?

b) Is the concentration of auxins, such as IAA, likely to be highest at the point labelled X or Y? Explain why.

a) positive. The shoot is bending towards the stimulus.

b) Y because this is where cell elongation is taking place, causing the shoot to bend towards the opposite side.

what is the role of a receptor

detect stimuli — they can be cells, or proteins on cell surface membranes.

receptors are specific to one type of stimulus

example of a receptor

e.g. baroreceptors are a type of receptor that detect changes in blood pressure,

what is the role of an effector

cells that bring about a response to a stimulus, to produce an effect.

example of an effector

muscle cells and cells in glands

how do receptors and effectors communicate

the nervous system or the hormonal system, or sometimes using both.

what are the 3 types of neurones

Sensory neurones

Motor neurones

Relay neurones

what is the role of a sensory neurone

transmit electrical impulses from receptors to the central nervous system (CNS)

what is the role of a relay neurone

transmit electrical impulses between sensory neurones and motor neurones.

what is the role of motor neurones

transmit electrical impulses from the CNS to effectors.

what is the role of neurotransmitters

take the information across the gap (called a synapse) to the next neurone, where another electrical impulse is generated

what is the role of the CNS

processes the information from the sensory neurone and sends impulses along motor neurones to an effector

what is a coordinator giving an example

- an organ that receives messages from receptors and uses the message to coordinate the activities in the body.

- For example, the brain or spinal cord (CNS)

What is the order of the protective effect of a simple reflex arc? (7)

1. Stimulus

2. Receptor

3. Sensory neurone

4. Relay neurone

5. Motor neurone

6. Effector

7. Response

What are the important features of the protective effect of a simple reflex arc? (3)

- Rapid (only 3 neurones and few synapses are involved)

- Automatic (It doesn't have to be learnt) - goes through the spinal cord but not conscious parts of the brain

- Offers protection from harmful stimuli - as the response happens so rapidly

what is a simple reflex

a rapid, involuntary response to a stimulus.

where are relay neurons located

in CNS

what part of the CNS do reflexes pass to

only spinal cord as faster

draw a simple reflex arc

Explain why nervous communication leads to a localised and short-lived response.

localised → neurotransmitters are secreted directly onto cells

short-lived → neurotransmitters are quickly removed once they have done their job.

EXAMPLE The knee-jerk reflex involves lightly tapping a person on the patellar tendon (just below the knee-cap) with a tendon hammer. When this happens, the quadriceps muscle (in the thigh) immediately contracts, causing the person’s lower leg to jerk forward.

a) This response is a reflex. Suggest one way in which the response would differ if it was not a reflex.

b) Name the stimulus and the effector in the knee-jerk reflex.

c) The knee-jerk reflex is unusual because the sensory neurone synapses directly onto the motor neurone in the spinal cord.

i) Describe how this differs from a simple reflex, such as the hand-withdrawal response to heat.

ii) Suggest what effect tapping the patellar tendon might have in someone with a spinal cord injury. Explain your answer.

a) Any one from, e.g. the response would be slower /

the response would be voluntary.

b) Stimulus — light tap/touch.

Effector — quadriceps muscle.

c) i) The knee-jerk reflex doesn’t involve a relay neurone

in the spinal cord. / There are usually three neurones

involved in a simple reflex.

ii) E.g. the quadriceps muscle may not contract/there may be no response. If the spinal cord is damaged then the sensory neurone may not be able to transmit nervous impulses to the motor neurone / the motor neurone may not be able to transmit nervous impulses to the leg muscle.

EXAMPLE

Many nociceptors (pain receptors) are located in the skin.

a) Describe the pathway of nervous communication that would take place in a healthy person if they pricked their finger with a pin.

Congenital insensitivity to pain is a condition where the body does not feel physical pain. The condition is a result of non-functional nociceptors. The ability of sufferers to feel a light touch is usually normal.

b) Suggest why people with this condition are able to feel a light touch even though they’re unable to feel pain.

c) Suggest why it’s beneficial to an organism to be able to detect and respond to pain.

a) The nociceptors detect the stimulus and impulses are passed to a sensory neurone. This passes the electrical impulses to a relay neurone in the spinal cord/CNS which carries the impulse to a motor neurone. The motor neurone carries impulses to an effector (e.g. a biceps muscle).

b) Particular receptors are specific to a particular stimulus. This means that it’s possible that while their pain receptors aren’t functional (so pain isn’t felt), their touch receptors are functional allowing light touches to be felt.

c) It helps to protect the body by reacting to situations/environments that could cause the body harm.

What is a pacinian corpuscle? (2)

mechanoreceptors in the skin

contain the end of a sensory neurone, (sensory nerve ending) wrapped in loads of connective tissue called lamellae

what type of stimuli do Pacinian corpuscles respond to

mechanical stimuli (i.e. pressure and vibrations), not to any other type of stimulus as receptors only respond to specific stimuli.

Draw the basic structure of a Pacinian corpuscle

- Lamellae

- Nerve ending

- Sensory neurone

- Stretch-mediate sodium ion channels

- Gel

What does the pacinian corpuscle illustrate? (3)

- Receptors respond only to a specific stimuli - a mechanical pressure

- Stimulation of a receptor leads to the establishment of a generator potential

- When the threshold is reached, action potential is sent (all-or-nothing principle)

what is a generator potential

Electrical signal generated in sensory receptors

created when sodium ions (Na+)

move across the axon membrane of Pacinian corpuscles in response to mechanical pressure.

Describe how a generator potential is established in a pacinian corpuscle (5)

1. A mechanical stimulus (e.g., pressure) deforms the lamellae and causes the sensory neurons cell membrane to stretch deforming the stretch-mediated sodium (Na+) channels

2. This causes stretch mediated Na+ channels in the membrane to open, allowing Na+ to diffuse into the sensory neuron

3. Greater pressure causes more Na+ channels to open, allowing more Na+ to enter

4. This influx of Na+ causes depolarisation, leading to a generator potential

5. If the generator potential reaches the threshold, it triggers an action potential

what is depolarisation

when a cell's membrane potential becomes less negative

leading to the generation of an action potential

What is meant by resting potential of receptors and how is it maintained

the inside of the cell is more negatively charged than the outside

The resting potential is generated by ion pumps and ion channels

What leads to a larger generator potential in receptors

A bigger stimulus excites the cell membrane more, making it more permeable causing a bigger movement of ions into and out of the cell and a bigger change in potential difference

action potential in receptors

If the generator potential is big enough to go over the threshold it’ll trigger an action potential — an electrical impulse along a neurone.

The strength of the stimulus is measured by the frequency of action potentials (the number of action potentials triggered during a certain time period).

If the stimulus is too weak the generator potential won’t reach the threshold, so there’s no action potential (all or nothing principle).

EXAMPLE Suggest how a person’s perception of touch might be affected by drugs that block stretch-mediated sodium ion channels in cell membranes.

pressure from touch would normally deform the stretch‐mediated sodium ion channels in Pacinian corpuscles, leading to a generator potential. However, by blocking sodium ion channels the drug would stop sodium ions from diffusing into the cell and generating an action potential. This would mean the person wouldn’t be able to perceive that they were being touched.

what are photoreceptors

receptors in your eye that detect light.

what is the pathway of light that enters the eye

Pupil (amount of light that enters is controlled by the muscles of the iris) → lens → retina → optic nerve

what is the forvea

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

where are no photoreceptors located in the eye

in the blind spot - where the optic nerve leaves the eye

label the eye

How photoreceptors work

1. Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments.

2. Light bleaches the pigments, altering the membrane permeability to sodium ions, producing a generator potential.

3. if it reaches the threshold, an action potential is produced which is sent along a bipolar (sensory) neurone.

4. Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain

Draw a diagram to show how photoreceptors work

What are the two photoreceptor cells found in the retina of the eye? (2)

- Rods

- Cones

where are most rod cells found

the peripheral parts of the retina,

where are most cone cells found

fovea

What is the differences in colour vision for rods and cones? (2)

- Rods only allow monochromatic vision (black and white)

- Cones allow coloured vision

Explain how there are differences in colour vision for rods and cones? (1)

Rods and cones contain different optical pigments making them sensitive to different wavelengths of light.

How do cones allow for coloured vision? (4)

- Three different types of cones

- With different optical pigments

- So will absorb different wavelengths

- Stimulation of different combinations / proportions of cones gives a range of colour perception

What are the three different types of cones? (3)

- Red Sensitive

- Blue sensitive

- Green sensitive

Why are rods more sensitive to light than cones? (3)

- Several rods are connected to a single bipolar (sensory) neurone

- many weak generator potentials combine (via spatial summation) to reach/overcome the threshold

- and trigger an action potential.

Why are cones less sensitive to light than rods? (2)

- Each cone connected to a single bipolar (sensory) neurone

- There is no spatial summation occurring

is this diagram pointing to rods or cones

rods

is this diagram pointing to rods or cones

cones

What is visual acuity? (1)

The clarity or sharpness of vision

low visual acuity = blurry vision

What is the difference in visual acuity between rods and cones? (1)

Cones give a higher visual acuity than rods

Why do cones give a higher visual acuity? (3)

- Each cone is connected to a single bipolar (sensory) neurone

- Cones send separate sets of impulses to the brain

- So they can distinguish between 2 separate sources of light

Why do rods give a lower visual acuity? (3)

- Several rods are connected to a single bipolar (sensory) neurone

- Several rods send a single set of impulses to the brain

- Therefore, it cannot distinguish between separate sources of light

Why do rods only allow for monochromatic vision? (2)

- There is only one type of rod

- That only contains one pigment

what type of photo receptor works best in dim light and why

rods - contain light sensitive pigment called rhodopsin

what type of photo receptor works best in bright light and why

cones - contain light sensitive pigments called photopsins that allow them to detect different wavelengths of light

summarise the differences between a rods and cones

mneumonic to remember where rods and cones are located

cones are packed closely together in the fovea.

what are the 2 divisions of the nervous system

Central

Peripheral

what are the 2 divisions of the peripheral nervous system

• autonomic

• somatic

what are the 2 divisions of the autonomic nervous system

• parasympathetic

• sympathetic

what is the CNS

is made up of the brain and spinal cord,

what is the PNS

made up of the neurones that connect the CNS to the rest of the body.

what is the autonomic nervous system

controls unconscious activities inside the body i.e. digestion

what is the somatic nervous system

system controls conscious activities and processes sensory information from the environment.

what is the sympathetic nervous system

‘fight or flight’ system that gets the body ready for action.

what is the parasympathetic nervous system

the ‘rest and digest’ system that calms the body down.

what part of the nervous system is involved in the control of heart rate

autonomic

what is myogenic stimulation

the process by which heart beat is controlled (via contracting and relaxing without receiving signals from the nerves)

EXAM TIP REGARDING AVN

there’s a delay before the AVN reacts. Don’t write in the exam that there is a delay in the wave of electrical activity reaching the AVN.

what is the sinoatrial node (SAN)

a small mass of tissue in the wall of the right atrium, that acts as a pacemaker — it sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls.

what is the bundle of His and Purkyne tissue

a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex (bottom) of the heart.

The bundle splits into finer muscle fibres in the right and left ventricle walls, called the Purkyne tissue.

Describe the myogenic stimulation of the heart

1. The sinoatrial node (SAN) acts as a pacemaker, sending regular waves of electrical activity across the atria, causing them to contract simultaneously

2. Non-conducting tissue between the atria and ventricles prevents the impulse from passing directly to the ventricles, preventing immediate contraction of the ventricles

3. Waves of electrical activity reach the atrioventricular node (AVN), which delays the impulse, allowing the atria to fully contract and empty before the ventricles contract

4. The AVN sends a wave of electrical activity down the bundle of His, which branches into Purkyne tissue which carries the waves of activity into the walls of the right and left ventricles causing them to contract simultaneously from the base up.

label the areas of the heart needed for myogenic stimulation

what is the rate at which the SAN fires caused by

unconsciously caused by the medulla in the brain

why may animals need to alter their heart rate

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 oxygen.

how are internal stimuli detected in the body

by pressure receptors (baroreceptors) and chemical receptors (chemoreceptors)

where are baroreceptors located and what are they stimulated by

pressure receptors

in the aorta and carotid arteries.

Stimulated by high and low blood pressure.

where are chemoreceptors located and what are they stimulated by

• Chemical receptors

• In aorta, the carotid arteries and in the medulla.

• Detect changes in the oxygen level in the blood and carbon dioxide and pH (which are indicators of O2 level).