6.1 Stimulus and Response & Receptors

What is a stimulus?

A change in internal or external environments that leads to a response.

Why is it important that organisms respond to a stimulus?

Organism increase their chances of survival by responding to stimuli.

State what is meant by a tropism and the two types

A tropism is the growth of a plant in respose to a directional stimulus.

The two types include:

1. Positive tropism - towards a stimulus.

2. Negative tropism - away from a stimulus.

State the categories of tropism

There are three categories of tropsim:

1. Hydrotropsim

2. Gravitropsim

3. Phototropsim

Summarise the role of growth factors in flowering plants

Specific growth factors e.g, Auxins such as IAA, move from growing regions e.g, shoot/root tips where they’re produced, to other tissues where they regulate growth in response to directional stimuli (tropisms).

State the name of the specific auxin

Indoleacetic Acid: IAA.

Describe how IAA effects elongation in shoots and roots

In shoots, high concentrations of IAA stimulates cell elongation.

In roots, high concentrations of IAA inhibits cell elongation.

Explain gravitropism in flowering plants

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

2. IAA diffuses down the shoot/root.

3. IAA moves/accumulates to the lower side of the shoot/root due to gravity.

4. This stimulates cell elongation in shoots and inhibits cell elongation in roots.

5. This leads to the shoot bending away from gravity and the root bending towards gravity.

This is negative gravitropsim for the shoot and positive gravitropism for the root.

Explain phototropism in flowering plants

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

2. IAA diffuses down the shoot/root.

3. IAA moves/accumulates to the shaded side of the shoot/root due to light.

4. This stimulates cell elongation in shoots and inhibits cell elongation in roots.

5. This leads to the shoot bending towards the light and the root bending away from the light.

This is positive phototropism for the shoot and negative phototropism for the root.

What is the general definition for Taxes and Kineses?

They are simple responses that can maintain a mobile organism in a favourable environment.

State the differences between Taxes and Kineses

Taxes:

• Directional response.

• Movement towards or away from a directional stimulus.

Example - woodlice moving away from light to avoid predators.

Kineses:

• Non-directional stimulus.

• Speed of movement or rate of direction change.

• Depends on intensity of stimulus.

Example - woodlice moving faster in drier environemtns to increase their chance of moving to an area with higher humidity.

Describe the pathway of an electrical impulse

Receptor → Sensory Neurone → Relay Neurone (CNS) → Motor Neurone → Effector

Explain the protective effects of a simple reflex (3 neurone)

The protective effects of a simple reflex are:

1. Rapid/immediate - only a few neurones and synapses.

2. Autonomic - doesn’t have to be learnt.

These traits enable protection, allowing the organism to escape predators or prevent damage to body tissues.

Describe the basic structure of a Pacinian corpuscle

Pacinian corpuscles consist of:

• Lamellae

• Stretch-mediated sodium ion channels.

• Sensory neurone ending and axon

• Gel

• Myelin sheath (schwann cells)

What does the Pacinian corpuscle illustrate?

The Pacinian courpuscle illustrates that receptors only respond to specific stimuli.The stimulation of a receptor then leads to a generator potential which can trigger an action potential if the threshold is reached.

Describe how an action potential is established in a Pacinian corpuscle

1. Mechanical stimulus, such as pressure, deforms the lamellae and stretch-mediated sodium ion channels.

2. Na+ channels open and Na+ diffuses into the neurone.

3. The greater pressure causes more Na+ channels to open and more Na+ to move in.

4. This causes depolarisation, leading to a generator potential.

5. If the threshold is reached, it will trigger an action potential.

What is meant by visual acuity?

Visual acuity is the ability to see objects apart and see fine detail.

What is meant by visual sensitivity?

Visual sensitivity is the ability to see detail even in low light.

Summarise the location and function of rod cells

Rod cells are located on the outer parts of the retina, away from the fovea:

• Several rod cells are connected to a single neurone.

• They have higher sensitivity.

• They have lower visual acuity.

• They allow us to see in the dark.

• They don’t allow us to see in detail or in colour.

What triggers depolarisation?

A pigment in rods, called rhodopsin, breaks down in light. If enough breaks down, it triggers depolarisation.

Describe convergence in rod cells

1. Light stimulus triggers depolarisation in rod cells.

2. Many rods converge and are connected to a single sensory neurone.

3. Depolarisation undergoes spatial summation to trigger an action potential.

4. In low light, there is still enough light to lead to an action potential but visual acuity is reduced.

Summarise the location and function of cone cells

Cone cells are found at high concentrations at the fovea:

• One cone cell is attached to one neurone.

• They have higher visual acuity.

• They have lower sensitivity.

• They can’t see in the dark.

• They can see in detail and in colour.

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

Rods are more sensitive to light:

• Several rod cells are connected to a single neurone.

• Spatial summation occurs to reach the threshold so an action potential can be triggered.

Cones are less sensitive to light:

• A single cone cell is connected to a single neurone.

• No spatial summation occurs.

Explain the differences in visual acuity for rods and cones in the retina

Rods give lower visual acuity:

• Several rod cells are connected to a single neurone.

• Several rods send a single set of impules to the brain - can’t distinguish between separate sources of light.

Cones give higher visual acuity:

• A single cone cell is connected to a single neurone.

• Cones send separate impulses to the brain - they can be distinguished from eachother.

Explain the differences in sensitivity to colour for rods and cones in the retina

Rods allow monochromatic vision:

• There is one type of rod → one type of pigment.

Cones allow colour vision:

• There are three types of cones → red, green, and blue sensitive. They have different optical pigments so absorb different wavelengths.

• Stimulating different combinations of cones gives a range of colour perception.

Define myogenic in terms of the heart

Myogenic means that it can contract and relax without receiving electrical impulses from the neurones.

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

1. Sinoatrial node (SAN) initiates a wave of electrical impulses across both atria, causing them to contract.

2. Non-conductive tissue prevents the impulses going straight to the ventricle. This prevents the immediate contraction of ventricles.

3. Atrioventricular node (AVN) delays the impulses so that the ventricles can fill.

4. Atrioventricular node (AVN) sends a wave of electrical impulses down the bundle of His.

5. Ventricles contract from bottom up.

State the location of chemoreceptors and baroreceptors

Chemoreceptors (pH) and baroreceptors (pressure) are located in the aorta and carotid arteries. Chemoreceptors are also found in the medulla.

What is meant by sympathetic and parasympathetic in general terms?

Sympathetic means to “speed things up”.

Parasympathetic means to “slow things down”.

Describe the roles and locations of chemoreceptors and baroreceptors and the roles of the autonomic nervous system and effectors in controlling heart rate

Baroreceptors detect fall/rise in blood pressure and/or chemoreceptors detect the rise/fall in CO₂ or the fall/rise in blood pH:

1. Sends impulses to the medulla.

2. Medulla sends more frequent impules to the SAN along the sympathetic/parasympathetic neurones.

3. More/less frequent impulses are sent from the SAN to the AVN.

4. Cardiac muscle contracts more/less frequently.

5. Heart rate increases/decreases.