6 Organisms respond to changes: 14 Response to Stimuli

Question 1

How do organisms increase their chance of survival?

In general

Answer 1

By responding to changes in their environment.

Question 2

How do flowering plants regulate growth?

Answer 2

Specfic growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli.

Question 3

What is IAA?

Answer 3

Indoleacetic acid.

An auxin produced in the tips of shoots in flowering plants.

Question 4

What is the effect of IAA on cell elongation in the roots of flowering plants?

Answer 4

Gravitropism:
Concentration of IAA increases on the lower side, which inhibits growth so the root bends downwards.

Question 5

What is the effect of IAA on cell elongation in the shoots of flowering plants?

Answer 5

Phototropism:
Concentration of IAA increases on the shaded side, stimulating cell elongation so the shoot bends towards the light.

Question 6

What are taxes?

Answer 6

Simple responses where organisms move towards or away from a directional stimulus.

Question 7

Why do woodlice show phototaxis?

Answer 7

Woodlice move away from a light source, helping them survive. This is because it keeps them concealed from predators.

Question 8

What are kineses?

Answer 8

Simple responses where an organisms’ movement is affected by a non-directional stimulus.

Question 9

Why do woodlice show a kinetic response to humidity?

Answer 9

In high humidity, woodlice move slower and turn less often, so they remain in the favourable environment.
In low humidity, woodlice move faster and turn more often, to increase the chance of moving to an area with higher humidity. This reduces their water loss.

Question 10

What does a receptor do?

Answer 10

Detect stimuli.

Question 11

What does an effector do?

Answer 11

Bring about a response to a stimulus.

Question 12

What are the three main types of neurones?

Where does each transmit from and to?

Answer 12

Sensory - from receptors to CNS.
Motor - from CNS to effectors.
Relay - between sensory and motor neurones.

Question 13

What is the general route of an electrical impulse?

Answer 13

Stimulus, receptor, sensory neurone, CNS, motor neurone, effector, response.

Question 14

What is the peripheral nervous system?

What are the two different systems in it?

Answer 14

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

Somatic nervous system, and autonomic nervous system.

Question 15

What does the somatic nervous system control?

Answer 15

Conscious activities.

Question 16

What does the autonomic nervous system control?

What are its two divisions?

Answer 16

Unconscious activities.

Sympathetic nervous system, and parasympathetic nervous system.

Question 17

What does the sympathetic nervous system control?

Answer 17

The ‘flight or fight’ response, getting the body ready for action.

Question 18

What are reflexes?

Answer 18

Rapid, automatic responses to stimuli.

Question 19

What does the parasympathetic nervous system control?

Answer 19

The ‘rest and digest’ system, calming the body down.

Question 20

What is the process of a simple reflex arc?

Answer 20

Stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response.

Question 21

What is the resting potential of a cell?

How is it generated?

Answer 21

The potential difference of a cell at rest.

There’s a difference in charge between the inside and outside of the cell, generated by ion pumps and ion channels.

Question 22

What is a generator potential?

How is it generated?

Answer 22

The change in potential difference due to a stimulus.

When a cell detects a stimulus, the cell membrane becomes excited and more permeable, allowing more ions to move between.

Question 23

What is an action potential?

How is it generated?

Answer 23

An electrical impulse.

When the generator potential reaches the threshold level.

Question 24

What is a Pacinian corpuscle?

Where are they found?

Answer 24

A mechanoreceptor which detect mechanical stimuli, e.g. pressure.

Found in the skin.

Question 25

What is the basic structure of a Pacinian corpuscle?

Answer 25

They contain a sensory nerve ending, wrapped in lamellae (layers of connective tissue).

Question 26

How is a generator potential produced in a Pacinian corpuscle?

Answer 26

When a Pacinian corpuscle is stimulated, the lamellae are deformed and press on the sensory nerve ending.
This causes the sensory neurone’s cell membrane to stretch, deforming the stretch-mediated sodium ion channels.
The channels open and sodium ions diffuse into the cell, creating a generator potential.

Question 27

What are photoreceptors?

Answer 27

Light receptors on the retina.

Question 28

How does light get detected by photoreceptors?

Answer 28

Light enters the eye through the pupil (controlled by muscles of the iris). The lens focuses light rays onto the retina.

Question 29

What is the fovea?

Answer 29

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

Question 30

Where do nerve impulses go from the photoreceptor cells?

Answer 30

They are carried from the retina to the brain by the optic nerve.

Question 31

How do photoreceptors convert light into an electrical impulse?

Answer 31

Light is absorbed by light-sensitive optical pigments in the photoreceptors.

Light bleaches the pigments, altering the membrane permeability to sodium ions.

A generator potential is created.

If the generator potential reaches the threshold, an action potential is sent along a bipolar neurone.

Bipolar neurones connect photoreceptors to the optic nerve which sends it to the brain.

Question 32

Where are rods and cones found in the eye, respectively?

Answer 32

Rods - peripheral parts of the retina.
Cones - packed together in the fovea.

Question 33

Why are rods and cones sensitive to different wavelengths of light?

Answer 33

They contain different optical pigments.

Question 34

Why are rods more sensitive to light than cones?

Answer 34

Many rods join one bipolar neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential. Dim light is enough.

One cone joins to one bipolar neurone, so it takes more light to reach the threshold and trigger an action potential.

Question 35

Why do rods give lower visual acuity than cones?

Answer 35

Many rods join the same bipolar neurone, so light from two points close together can’t be told apart.

Cones are close together and one cone joins to one bipolar neurone. When light from two points hits two cones, two action potentials go to the brain, so you can distinguish the two points.

Question 36

What is visual acuity?

Answer 36

The ability to tell apart points that are close together.

Question 37

What is the process of the heart beating?

Answer 37

1. The sinoatrial node (in the wall of the right atrium) sends out regular waves of electrical activity to the atrial walls, setting the rhythm of the heartbeat.

• this causes the right and left atria to contract at the same time.
• A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.

2. Instead, the waves of electrical activity are transferred from the SAN to the atrioventricular node.
3. There’s a slight delay before the AVN passes the waves of electrical activity to the bundle of His; this is to make sure the atria have emptied before the ventricles contract.
4. The bundle of His (a group of muscle fibres) is responsible for conducting the waves of electrical activity between the ventricles to the bottom of the heart. The bundle splits into Purkyne tissue (finer muscle fibres) in the walls of the ventricles.
5. The Purkyne tissure carries the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously.

Question 38

What is the rate at which the sinoatrial node generates electrical impulses controlled by?

Answer 38

Medulla oblongata.

Question 39

What happens to the heart rate when you have high blood pressure?

The process

Answer 39

Pressure receptors detect high blood pressure.

Impulses are sent to the medulla oblongata, which sends impulses along the parasympathetic nervous system. Acetylcholine (a neurotransmitter) is secreted, which binds to receptors on the sinoatrial node.

The heart rate is slowed to reduce blood pressure.

Question 40

What happens to the heart rate when you have low blood pressure?

The process

Answer 40

Pressure receptors detect low blood pressure.

Impulses are sent to the medulla oblongata, which sends impulses along the sympathetic nervous system. Noradrenaline (a neurotransmitter) is secreted, which binds to receptors on the sinoatrial node.

The heart rate is sped up to increase blood pressure.

Question 41

What happens to the heart rate when you have high O2/ low CO2/ high pH levels?

The process

Answer 41

Chemoreceptors detect chemical changes.

Impulses are sent to the medulla oblongata, which sends impulses along the parasympathetic nervous system. Acetylcholine is secreted, which binds to receptors on the sinoatrial node.

The heart rate is slowed to return levels to normal.

Question 42

What happens to the heart rate when you have low O2/ high CO2/ low pH levels?

The process

Answer 42

Chemoreceptors detect chemical changes.

Impulses are sent to the medulla oblongata, which sends impulses along the sympathetic nervous system. Noradrenaline is secreted, which binds to receptors on the sinoatrial node.

The heart rate speeds up to return levels to normal.

Question 43

Why is the heart described as ‘myogenic’?

Answer 43

It can contract and relax without receiving signals from nerves.

Question 44

Where are pressure receptors found?

Answer 44

Aorta and carotid arteries.

Question 45

Where are chemoreceptors found?

Answer 45

Aorta, carotid arteries, and the medulla oblongata.