StemUp: AQA A level Biology 3.6.1 Stimuli, both internal and external, are detected and lead to a response

What is tropism? (1)

Growth responses of a plant to a directional stimulus

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

By responding to changes in their environment

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

1. Specific growth factors (hormone-like growth substances), e.g., auxins (such as IAA)

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 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

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

What are taxes and kineses? (1)

Simple 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) a stimulus

NOTE: This does not include growth movements, that would be tropism

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 response

- Speed or rate of direction movement

- Changes in response to to a non-directional stimulus

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 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)

- Offers protection from harmful stimuli

What is a pacinian corpuscle? (1)

Nerve endings in the skin

Draw the basic structure of a Pacinian corpuscle

- Lamellae

- Nerve ending

- Sensory neurone

- Stretch-mediate sodium ion channels

- Gel

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

- Each cone connected to a single neurone

- There is no spatial summation occurring

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)

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

1. A mechanical stimulus (e.g., pressure) deforms the lamellae and stretch-mediated sodium (Na+) channels

2. This causes 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 are the two photoreceptor cells found in the retina of the eye? (2)

- Rods

- Cones

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

- Several rods are connected to a single neurone

- Spatial summation is required to reach/overcome threshold

- To generate an action impulse

What is visual acuity? (1)

The clarity or sharpness of vision

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

Cones give a higher visual acuity than rods

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

- Several rods are connected to a single neurone

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

- Therefore, it cannot distinguish between separate sources of light

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

- Each cone is connected to a single neurone

- Cones send separate sets of impulses to the brain

- So they can distinguish between 2 separate sources of light

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

- Rods only allow monochromatic vision (black and white)

- Cones allow coloured vision

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

- There is only one type of rod

- That only contains one pigment

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

- Red

- Blue

- Green

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

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 and causes the ventricles to contract simultaneously from the base up.

What acts as the pacemaker in the heart? (1)

The sinoatrial node (SAN)

What does the SAN do? (1)

Sends regular waves of electrical activity across the atria

What effect do the waves of electrical activity from the SAN have on the atria? (1)

Causes the atria (LA & RA) to contract simultaneously

What is the role of the non-conducting tissue between the atria and ventricles? (1)

Prevents the impulse from passing directly to the ventricles

Why is it important that the impulse does not pass directly to the ventricles? (1)

Prevents the immediate contraction of the ventricles

What happens when the waves of electrical activity reach the atrioventricular node (AVN)? (3)

- AVN delays the impluse

- Allowing the atria to fully contract and empty

- Before the ventricles contract

What does the AVN do after delaying the impulse? (1)

The AVN sends a wave of electrical activity down the bundle of His

What happens after the wave of electrical activity is sent down the bundle of His? (3)

- Wave branches into Purkyne tissue

- Causing the ventricles to contract simultaneously

- From the base up

Where are the chemoreceptors and pressure receptors in our heart located? (2)

- Aorta

- Carotid arteries

Describe how heart rate is controlled when either:

- Baroreceptors detect a rise in blood pressure

- Chemoreceptors detect a fall in blood CO2 concentration

- Chemoreceptors detect an increase in blood pH

(5)

1. An impulse is sent to the medulla / cardiac control centre

2. Which sends more frequent impulses to the SAN along the parasympathetic neurons

3. As a result, less frequent impulses are sent from the SAN to the AVN

4. This causes the cardiac muscle to contract less frequently

5. Therefore, the heart rate decreases

Describe how heart rate is controlled when either:

- Baroreceptors detect a fall in blood pressure

- Chemoreceptors detect a rise in blood CO2 concentration

- Chemoreceptors detect a fall in blood pH

(5)

1. An impulse is sent to the medulla / cardiac control centre

2. Which sends more frequent impulses to the SAN along the sympathetic neurons

3. As a result, more frequent impulses are sent from the SAN to the AVN

4. This causes the cardiac muscle to contract more frequently

5. Therefore, the heart rate increases