Biology - Chapter 14: Response to Stimuli

Stimulus

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

What detects stimuli?

Receptors

What do receptors do?

They detect a stimulus

How many different stimuli can a receptor detect?

One - they are specific to one type of stimulus

What do coordinators do?

Formulate a suitable response to a stimulus

What do effectors do?

Produce a response to a stimulus

Summarise the sequence of events in chemical control / nerve cells

stimulus → receptor → coordinator → effector → response

Taxis

A simple response whose direction is determined by the direction of the stimulus

Positive and negative taxis meaning

Positive - moving towards the stimulus
Negative - moving away from the stimulus

Kinesis

A form of response that changes the speed at which the organism moves and the rate at which it changes direction

How do kinesis responses help organisms to survive? (4)

• Organism moves at a greater speed and with a higher rate of changing direction

• This increases its chance of a quick return into a favourable environment

• If the organism moves a considerable distance into an unfavourable environment, the rate of turning slowly decreases

• This causes it to move in long straight lines, before turning sharply, bringing the organism into a new region with favourable conditions

Tropism

The growth of part of a plant in response to a directional stimulus

Postiive and negative tropism meaning

Positive - grows towards stimulus
Negative - grows away from stimulus

3 factors plants respond to

1. Light (shoots grow towards)

2. Gravity (shoots grow towards)

3. Water (roots grow towards)

Give an example of a plant growth factor

IAA (Indoleacetic acid)

Describe phototropism in flowering plants (4)

1. Cells in tip of shoot produce IAA

2. Light causes IAA to diffuse from light side of shoot to shaded side, so IAA accumulates on shaded side, resulting in a greater concentration of IAA

3. In shoots, this causes the cells on the shaded side to elongate more, so the shoot tip bends towards light

4. In roots, IAA inhibits cell elongation, so the cells on the shaded side elongate less (light side cells elongate more), so the roots bend away from light

Describe gravitropism in flowering plants (4)

1. Cells in tip of root produce IAA

2. Light causes IAA to diffuse from upper side of shoot to lower side, so IAA accumulates on lower side, resulting in a greater concentration of IAA

3. In roots, IAA inhibits cell elongation, so the cells on the lower side elongate less (upper side cells elongate more), so the roots bend downwards, towards force of gravity

4. In shoots, this causes the cells on the lower side to elongate more, so the shoot tip bend upwards, away from force of gravity

Does IAA affect old or young cells and why?

Only young cells - It increases their plasticity
In older cells, the cell walls are more rigid, so cannot respond

Neurones

A nerve cell, comprising a cell body, axon and dendrites, which are adapted to conduct action potentials

Two major divisions of the nervous system

The central nervous system (CNS) and the peripheral nervous system (PNS)

What makes up the CNS?

The brain and spinal cord

What makes up the PNS?

Pairs of nerves that originate from either the brain or spinal cord - sensory neurones and motor neurones

What do sensory neurones do?

Carry nerve impulses from receptors to the CNS/relay neurone

What do motor neurones do?

Carry nerve impulses away from the CNS/relay neurone to effectors

Effectors

An organ that responds to stimulation by a nerve impulse, resulting in a change or response

What are the two divisions of the motor nervous system?

The voluntary nervous system and the autonomic nervous system

What does the voluntary nervous system do?

Carries nerve impulses to body muscles and is under voluntary control

What does the autonomic nervous system do?

Carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control (i.e. it is subconscious)

Describe the response produced by reflex arcs (4)

Rapid, short-lived, localised and involuntary

Reflex

An involuntary response to a sensory stimulus

Reflex arc

The pathway of neurones involved in a reflex

Describe the process of a reflex arc (6)

1. A receptor detects the stimulus, generating nerve impulses in the sensory neurone

2. The sensory neurone passes nerve impulses to the spinal cord

3. The coordinator - relay neurone - links the sensory neurone to the motor neurone in the spinal cord

4. The motor neurone carries nerve impulses from the spinal cord to the effector

5. The effector (a muscle/gland) is stimulated to contract / release a substance, carrying out the response

5 reasons why reflex arcs are important

1. Rapid

2. Protect against damage to body tissues

3. Do not need to be learnt

4. Help escape from predators

5. Enable homeostatic control

What do Pacinian corpscules respond to?

Changes in mechanical pressure

2 features of sensory receptors

1. They’re specific to a single type of stimulus (e.g. mechanical pressure)

2. They produce a generator potential by acting as a transducer (convert energy from stimulus e.g. mechanical energy into a generator potential / nerve impulse)

Describe the structure of a Pacinian Corpuscle

• Neurone ending at centre of layers of connective tissue with viscous gel between

• Neurone ending has a stretch-mediated sodium channel

• Structure is covered by capsule

Why are stretch-mediated sodium channels called that?

When they are deformed, their permeability to sodium changes

Describe how Pacinian corpuscles produce an action potential (5)

1. In resting state, stretch-mediated sodium channels are too narrow, so Na+ ions cannot pass through them - neurone has resting potential

2. Pressure causes membrane to be deformed

3. Sodium channels are deformed and open, causing Na+ ions to diffuse in

4. The influx of Na+ ions causes the membrane to be depolarised, producing a generator potential

5. The generator potential creates an action potential (nerve impulse) which passes along the neurone to the CNS

Transducer cells

Cells that convert a non-electrical signal, such as light or sound, into an electrical single and vice-versa

Two main types of light receptor cells in the retina and what they detect

1. Rod cells - light

2. Cone cells - colour (between different wavelengths of light)

Generator potential

Depolarisation of the membrane of a receptor cell as a result of a stimulus

How do rod cells detect light? (3)

1. Light hits rod cells, causing rhodopsin to be broken down

2. Many rod cells are connected to a single sensory neurone / bipolar cell - retinal convergence

3. Due to spatial summation, this increases the chance that the threshold value is exceeded, creating a generator potential.

What light intensity do rod cells detect?

Low light intensity

What visual acuity do rod cells have and why? (1+3)

Low visual acuity

1. Many rod cells are linked to a single bipolar cell

2. The light received by the rod cells only generates a single impulse.

3. This means the brain cannot distinguish between the separate sources of light

How many different types of cone cells are there?

3

Why are there different types of cone cells?

They each respond to a different range of wavelengths of light

What light intensity do cone cells detect?

High light intensity

How do cone cells detect light?

1. Light hits cone cells, causing iodopsin to be broken down - this needs a high light intensity

2. Each cone cell type contains a specific type of iodopsin, so each cone cell is sensitive to a different specific range of waelengths.

3. When the threshold value is exceeded, a generator potential is created in the bipolar cells

What visual acuity do cone cells have and why?

High - each cone cell is connected to a single neurone, so they send separate impulses to the brain

Where are rod cells found and why?

At the peripheries of the retina - this is where light intensity is lowest

Where are cone cells found and why?

At the fovea, where light intensity is highest

What are the two divisions of the autonomic nervous system?

The sympathetic nervous system and the parasympathetic nervous system

What does the sympathetic nervous system do?

It stimulates effectors, speeding up any activity

What does the parasympathetic nervous system do?

It inhibits effectors, slowing down any activity

Why is the cardiac muscle described as myogenic?

Its contraction is initiated from within the muscle itself, rather than by nervous impulses from outside

What controls the contraction of the heart?

The sinoatrial node (SAN)

Where is the SAN found?

The wall of the right atrium

Where is the AVN found?

Between the atria

What controls changes to the heart rate?

The medulla oblongata

Describe the two centres of the medulla oblongata

1. Centre that increases heart rate, linked to the SAN by the sympathetic nervous system

2. Centre that decreases heart rate, linked to the SAN by the parasympathetic nervous system

What are the two types of receptors that contribute to changing the heart rate? Where are they found?

Chemoreceptors (wall of the carotid arteries) and pressure receptors (walls of the carotid arteries and aorta)

What are chemoreceptors sensitive to?

Changes in the pH of the blood due to changes in the CO2 concentration of the blood

How do chemoreceptors increase the heart rate? (4)

1. Chemoreceptors detect rise in CO2 due to fall in pH of blood

2. They increase frequency of nervous impulses to medulla oblongata

3. Medulla oblongata increases the frequency of impulses via the sympathetic system to the SAN

4. This then increases the rate of production of electrical waves by the SAN, increasing the heart rate

How do pressure receptors control the heart rate when blood pressure is higher than normal? (4)

1. Pressure receptors detect rise in blood pressure

2. They increase frequency of nervous impulses to medulla oblongata

3. Medulla oblongata increases the frequency of impulses via the parasympathetic system to the SAN

4. This then decreases the rate of production of electrical waves by the SAN, decreasing the heart rate

How do pressure receptors control the heart rate when blood pressure is lower than normal? (4)

1. Pressure receptors detect fall in blood pressure

2. They increase frequency of nervous impulses to medulla oblongata

3. Medulla oblongata increases the frequency of impulses via the sympathetic system to the SAN

4. This then increases the rate of production of electrical waves by the SAN, increasing the heart rate

When investigating the effect of IAA concentration on seedlings, why should you remove the shoot tip from each seedling? (2)

1. The tip produces IAA - affects the concentration of IAA, affecting the elongation of the shoots

2. Mitosis occurs in the shoot tips - affects the elongation of the shoots

How do chemoreceptors decrease the heart rate? (4)

1. Chemoreceptors detect fall in CO2 due to increase in pH of blood

2. They increase frequency of nervous impulses to medulla oblongata

3. Medulla oblongata increases the frequency of impulses via the parasympathetic system to the SA node

4. This then decreases the rate of production of electrical waves by the SAN, lowering the heart rate

Where is IAA produced in plants?

The tips of shoots

After the SAN initiates the heartbeat, how do both ventricles contract at the same time? (2)

1. Electrical activity present only in AVN and/or Bundle of His

2. Wave of electricity/impulses passes over both ventricles at the same time

What do relay neurones do?

Transmit impulses between neurones

How do the SAN and AVN control the cardiac cycle? (6)

1. SAN initiates heartbeat

2. SAN sends wave of electrical activity/impulse across the atria, causing the atria to contract

3. Non-conducting tissue prevents immediate contraction of ventricles

4. AVN delays the electrical activity/impulse, allowing the atria to fully empty or the ventricles to fill before the ventricles contract

5. AVN then sends the wave of electrical activity/impulse down the Bundles of His

6. This causes the ventricles to contract from the apex upwards

Why might one’s estimated speed of impulse transmission from an experiment be lower than the highest speed? (5)

1. Synaptic transmission

2. Time for muscle contraction

3. Time for coordination by brain

4. Student might have been distracted

5. Time for stretch-mediated sodium ion channels to open