6-14 Response to stimuli

Question 1

What is a stimulus?

Answer 1

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

Question 2

What is a receptor?

Answer 2

Detects stimuli

Specific to one type of stimulus

Question 3

What is a coordinator?

Answer 3

It formulates a response to a stimulus

This may be at the molecular level or involve a large organ such as the brain

Question 4

What does an effector do?

Answer 4

It produces a response

Question 5

What is the general sequence of events for the reaction to a stimulus?

Answer 5

Stimulus
Receptor

Coordinator
Effector
Response

Question 6

What is a taxis?

Answer 6

A simple response whose direction is determined by the direction of the stimulus
Towards stimulus = positive taxis

Away from the stimulus = negative taxis

Question 7

What is kinesis?

Answer 7

A form of response where the organism does not move towards or away from a stimulus
Instead it changes the speed at which it moves and the rate at which it changes direction

Important when the stimulus is less directional e.g. humidity and temperature

Question 8

What is a tropism?

Answer 8

The growth of a plant in response to a directional stimulus
Towards stimulus = positive

Away from stimulus = negative

Question 9

What is the name for the response of plants to a light stimulus?

Answer 9

Phototropic

Question 10

What is the name for the response of plants to a gravity stimulus?

Answer 10

Gravitropic

Question 11

What is the name for the response of plants to a water stimulus?

Answer 11

Hydrotropic

Question 12

What are plant growth factors?

Answer 12

Hormone-like substances which are involved in the response to external stimuli
They affect growth

They affect tissues which release them rather than a target organ

Question 13

What is the process of phototropism?

Answer 13

Cells in the tip of the shoot produce IAA which is transported down the shoot
IAA is initially transported evenly as it moves through the shoot

Light causes the movement of IAA from the light to the dark side of the shoot
This creates a greater concentration on the shaded site
IAA causes the cells on the shaded side to elongate more
The shaded side elongates which causes the root tip to bend towards the light

Question 14

What does IAA do?

Answer 14

It causes elongation of the shoot cells
It controls the bending of the shoots and roots in response to light

High concentrations inhibit cell growth in the roots and increase elongation in the shoots

Question 15

What is the process of gravitropism?

Answer 15

Cells in the root tip produce IAA which are transported along the root
IAA is initially transported to all sides

Gravity influences IAA to move from the upper to lower side of the roots
IAA inhibits elongation of root cells so it inhibits the cells on the lower side
This causes the root to bend downwards towards gravity

Question 16

Why does IAA only affect young cells?

Answer 16

IAA increases plasticity of the cell walls
The response only occurs in young cell walls where cells are able to elongate

Older cells develop greater rigidity

Question 17

What is the acid growth hypothesis?

Answer 17

Hydrogen ions are actively transported by active transport from the cytoplasm into spaces in the cell wall, causing the cell wall to become more plastic
This allows the cells to elongate by expansion

Question 18

What is a behaviour?

Answer 18

The response of an organism to the environment which increases the chance of survival

Question 19

What is a reflex?

Answer 19

An involuntary response that follows a specific pattern to a given stimulus

Question 20

What is a cholinergic synapse?

Answer 20

A synapse with the neurotransmitter acetylcholine

Question 21

What is acetylcholine composed of?

Answer 21

Acetyl and choline

Question 22

What are receptors?

Answer 22

Cells or proteins which detect stimuli (energy)
Can only detect a specific stimulus e.g. light

Can create generator potentials in sensory neurones
Can act as transducers that convert one form of energy to another

Question 23

What is the Pacinian corpuscle?

Answer 23

A receptor which can detect mechanical stimuli

The corpuscles wrap around the sensory nerve ending in layers of connective tissue called lamellae

Question 24

What does compression of the lamellae cause?

Answer 24

A sodium influx (due to stretch-mediated Na+ channels) which causes an action potential

Question 25

Where are Pacinian corpuscles found?

Answer 25

Abundant in the hands and feet | Also found in ligaments and tendons to determine which joints are changing direction

Question 26

What are the steps of the compression of the Pacinian corpuscle?

Answer 26

Shape of the corpuscle changed Stretch-mediated Na+ channels open Na+ diffuses into the neurone The membrane is depolarised A generator potential is produced The greater the pressure, the greater the generator potential The action potential develops if the threshold is reached

Question 27

What is the process of photoreceptors?

Answer 27

Light enters the eye through the pupil The amount of light entering is controlled by the muscles in the iris The lens focuses the light rays onto the retina The retina contains the photoreceptor cells

Question 28

What is the forea?

Answer 28

An area of the retina with loads of receptors

Question 29

How do nerve impulses from the photoreceptor cells get to the brain?

Answer 29

They are carried from the retina to the brain via the optic nerve (a bundle of nerves)

Question 30

What is the blind spot caused by?

Answer 30

The spot where the optic nerve leaves the eye

Question 31

What is the process of information from the eyes reaching the brain?

Answer 31

Light sensitive optical pigments absorb light and chemically breakdown Na+ permeability changes in the photoreceptor (Na+ channels close) and neurotransmitter release stops A generator potential can be created in the bipolar neurone A signal is transmitted to the brain

Question 32

What do rod and cone cells act as?

Answer 32

Transducers by conserving light energy into the electrical energy of a nerve impulse

Question 33

What are rod cells?

Answer 33

Lead to black and white imagery More numerous than cone cells Many connected to a single sensory neurone Certain threshold has to be exceeded before a generator potential is created Three connected to one bipolar cell which creates a greater chance of exceeding the threshold due to summation Pigment must be broken down to create a generator potential Give low visual acuity Found in higher concentrations further away from the fovea

Question 34

What are cone cells?

Answer 34

Three types each responding to a different range of wavelength of light The proportion of each causes images to be in full colour One cone cell connects to one bipolar cell Only respond to high light intensity due to no summation Requires a higher light intensity to break down the pigment High visual acuity Found at the forea

Question 35

What are the differences between rods and cones in the eye?

Answer 35

Rods vs cones Mainly located in the retina vs fovea Black and white vs colour Many rods connected to one bipolar neurone vs one cone High sensitivity to light vs low sensitivity Low visual acuity vs high

Question 36

Where is the sinatorial node found?

Answer 36

At the connection of the right atrium and the superior vena cava

Question 37

Where is the atrioventricular node found?

Answer 37

Between the walls of the two ventricles next to the semi lunar valves

Question 38

What can be increased or decreased in the heart?

Answer 38

The heart rate The force of ventricular contractions The stroke volume

Question 39

What does the sinatorial node do?

Answer 39

Causes the heart to speed up or slow down | Causes the atria to contract

Question 40

What does the atrioventricular node do?

Answer 40

Carries impulses from the sinatorial node down to the bottom of the heart Ventricles contract from the base

Question 41

What determines the speed of the heart?

Answer 41

Receptors which detect activity

Question 42

What connects with the sinatorial node?

Answer 42

The brain

Question 43

What are the receptors involved in controlling the heart rate?

Answer 43

Chemoreceptors: detect changed in the pH due to increasing carbonic acid Stretch receptors in the muscles Stretch receptors which monitor blood pressure

Question 44

What are the two receptors connecting the heart and the brain?

Answer 44

Vagus and accelerans

Question 45

What is the process of controlling heart rate?

Answer 45

A wave of electrical excitation spreads out from the sinatorial node across both atria, causing them to contract A layer of non conductive tissue between the atria prevents the wave from crossing to the ventricles The wave of excitation enters the atrioventricular node which lies between the atria The atrioventricular node, after a short delay, conveys a wave of electrical excitation between the ventricles along a series of specialised muscle fibres called Purkyne tissue which makes up a structure called the bundle of His The bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundle branches into smaller fibres The wave of excitation is released from the Purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottom of the heart upwards