3.6 Organisms Respond to Changes in Their Internal and External Environments

Question 1

Define stimulus

Answer 1

A detectable change in the environment that can be detected by cells and trigger a response

Question 2

How do plants respond to stimuli?

Answer 2

Via tropisms where they release chemicals to respond to stimuli through growth

Question 3

What is IAA?

Answer 3

A type of auxin that controls cell elongation in shoots and inhibits growth of roots.

Question 4

Explain why shoots show positive phototropism.

Answer 4

Plants need light for the LDR therefore the plants bend and grow towards the light. The shoot tip produces IAA which causes cell elongation. IAA diffuses to the side of the shoot that has less light incident on it which causes it to bend and grow towards the light therefore increasing how much the shoot can photosynthesise.

Question 5

Explain why roots show negative phototropism.

Answer 5

Roots don’t need light for photosynthesis but need to anchor the plant deep into the soil. So the root tips produce IAA which in high concentration, inhibits growth. The IAA diffuses to the parts of the root tips which has less light which inhibits growth and causes the plant to turn and grow downwards, away from the light.

Question 6

Describe gravitropism in shoots.

Answer 6

IAA diffuses down from upper to lower side.
Causes plant to grow upwards.
Negative geotropism.

Question 7

Describe gravitropism in roots.

Answer 7

IAA diffuses down to the lower side of the root.
Inhibiting root growth of the lower side causing the root to grow downwards.
Positive geotropism.

Question 8

Gravitropism/geotropism and phototropism are types of responses that plants give in response to stimuli such as gravity and light respectively. Name two other types of stimuli that trigger a response in plants and their respective tropism names.

Answer 8

Chemicals - chemotropism
Water - osmotropism

Question 9

Describe roughly what a reflex is.

Answer 9

A rapid, automatic response to protect an organism from danger.

Question 10

Describe the two types of simple response and why they are necessary.

Answer 10

Taxis and kinesis. They are simple responses which keep the organism within favourable conditions.

Question 11

Explain the difference between a taxis and a kinesis.

Answer 11

Taxes are directional responses to stimuli which cause an organism to move directly towards favourable conditions. Kineses are non directional responses in that the organism carries out random movement until the conditions are favourable.

Question 12

Describe the properties of a taxis.

Answer 12

Organism moves entire body towards favourable stimuli/conditions or away from unfavourable. It is a directional response to stimuli towards favourable conditions.

Question 13

Describe the properties of kineses.

Answer 13

An organism changes the speed of movement and the rate it changes direction dependent on intensity of stimuli. Non-directional response to stimuli. Random movement until favourable conditions are met.

Question 14

Briefly describe receptors and their purpose.

Answer 14

Receptors are cells that detect stimuli. Each type of receptor is specific to the stimuli that they detect. If the stimulus is intense enough, a generator potential is produced which can stimulate a response.

Question 15

What are pacinian corpuscles and how do they work?

Answer 15

They are pressure receptors. The receptors are wrapped in many layers of plasma membranes which have many stretch-mediated sodium channel proteins embedded in them. When pressure is applied to the receptor, the membranes are stretched and deformed which opens the channel protein to allow sodium ions to enter the neurone. If the stimulus is intense enough, the Na+ ion influx will be sufficient to depolarise the cell above the threshold potential therefore triggering an action potential.

Question 16

Describe the function of rod cells and how they work.

Answer 16

Rod cells process the intensity of light, regardless of the wavelength/colour. For the generator potential to be met, the rhodopsin must be broken down enough by the light energy.

Question 17

Describe the function of cone cells and how they work.

Answer 17

Cone cells process images in colour.
Three types of rod cells: red, green and blue (between the three types, all wavelengths of visible light can be seen.
Iodopsin is only broken down in high light intensities which is why we don’t see colour in the dark.

Question 18

Explain how between cone and rod cells, one has a higher visual acuity than the other.

Answer 18

Multiple rod cells are connected to the same bi-polar neurone whereas each cone cells is connected to its own bi-polar neurone. So the message that is sent to the brain from rod cells doesn’t allow the rod cells to differentiate between

Question 19

What are plant growth factors?

Answer 19

Chemicals that regulate plant growth in response to directional stimuli.

Question 20

Compare and contrast mammalian hormones and plant growth factors.

Answer 20

Mammalian hormone - Plant growth factor
-> response not always dependent on concentration - response proportional to concentration
-> binds to complimentary proteins on target cells - affects all cells
-> specialised glands produce the hormones - various tissues in growing regions produce growth factors
-> hormones transported via circulatory system - growth factors diffuse from cell to cell or move via phloem translocation
-> faster acting - slower acting

Question 21

Outline what happens in a simple reflex arc.

Answer 21

Receptor detects stimulus
Sensory neurone
Relay neurone in CNS coordinates response
Motor neurone
Response by effector

Question 22

Describe the advantages of a simple reflex arc.

Answer 22

Rapid response to potentially dangerous stimuli since only 3 neurones involved
Instinctive

Question 23

State a suitable statistical test to determine whether a factor has a significant effect on the movement of an animal.

Answer 23

Chi-squared

Question 24

Describe the distribution of rod and cone cells in the retina.

Answer 24

Rod cells are evenly distributed around the periphery but not in the central fovea.
Cone cells are mainly found in the central fovea but no photoreceptors are found in front of the optical nerve “blind spot”.

Question 25

Define what is meant by the term “myogenic”.

Answer 25

Contraction of the heart is initiated within the muscle itself rather than by external nerve impulses.

Question 26

State the name and location of the two nodes that control heart contraction.

Answer 26

Sinoatrial node (SAN): within the wall of the right atrium
Atrioventricular node (AVN): near the lower end of the right atrium in the call that separates the 2 atria.

Question 27

Describe how heartbeats are initiated and coordinated.

Answer 27

SAN initiates a wave of depolarisation.
Depolarisation spreads across both atria (atrial systole)
Layers of fibrous, non-conducting tissue prevents the wave of depolarisation travelling directly to the ventricles
AVN detects depolarisation and sends a secondary wave of depolarisation down the septum via Bundle of His which branches into the Purkinje fibres along the ventricles
This causes the ventricles to contract from the apex upwards.

Question 28

Explain how the coordination of the heart contraction creates a delay between the two contractions and its benefit.

Answer 28

Non-conducting tissue between atria and ventricles ensures that the wave of depolarisation first has to travel through the AVN.
The AVN then sends the impulse down the Bundle of His and up the Purkinje fibres.
This delay ensures the atria contract and then the ventricles which forces unidirectional blood flow.

Question 29

What are the sounds heard when the heart contract and explain why there are two?

Answer 29

The sounds are the valves in the heart closing. Due to the delay between the atria contracting and then the ventricles, the tricuspid and biscuspid valves close first (causing the first thump), and then open as the atrioventricular valves close (causing the second thump).

Question 30

What is the autonomic nervous system?

Answer 30

A system that controls the involuntary actions of glands and muscles.
Made up of the sympathetic and parasympathetic divisions

Question 31

State the difference between the sympathetic and parasympathetic nervous system.

Answer 31

Sympathetic involved in ‘fight or flight’ response: stimulates effectors to speed up activity.
Parasympathetic involved in normal resting conditions: inhibits effectors to slow down activity.

Question 32

What are the two internal environment conditions that stimulate change in heart rate when they change?

Answer 32

Blood pressure
Blood pH

Question 33

Name the receptors involved in changing the heart rate and state their location.

Answer 33

Baroreceptors (detect changes in blood pressure): carotid artery
Chemoreceptors (detect changes in pH): carotid artery and aortic body

Question 34

Explain how the body responds to an increase in blood pressure.

Answer 34

Baroreceptors send impulses to the cardioinhibitory centre in the medulla oblongata. This causes impulses to be send down to the SAN via the parasympathetic nervous system. This causes a decrease in heart rate.

Question 35

How does the body respond to a decrease in blood pressure?

Answer 35

Baroreceptors send more impulses to the cardioacceleratory centre in the medulla oblongata. This causes impulses to be sent to the SAN via the sympathetic nervous system. This causes an increase in heart rate.

Question 36

Explain the benefits of maintain blood pressure at a good level.

Answer 36

High blood pressure could cause damage to arterial walls.
Low blood pressure causes insufficient tissue oxygenation or removal of waste products.

Question 37

Name the components of the heart that contribute to the control of heart rate and name their location.

Answer 37

Sinoatrial node (SAN): located in the right atrium
Atrioventricular node (AVN): located in the border between the right and left ventricle towards the upper part of the septum
Bundle of His: tissue that runs down the septum
Purkinje fibres: in the walls of the ventricles

Question 38

How does the body respond to an increase in carbon dioxide concentration?

Answer 38

The chemoreceptors detect an decrease in pH due to the carbonic acid that is formed which causes them to send impulses to the cardioacceleratory centre of the medulla oblongata. Impulses are then sent down to the SAN via the sympathetic nervous system which causes an increase in heart rate. This increases blood flow to the lungs therefore allowing a faster rate of gas exchange and an increase in ventilation rate.

Question 39

Describe the general structure of a motor neurone.

Answer 39

Cell body that contains organelles and a high proportion of RERs.
Dendrons branch into dendrites which carry impulses towards the cell body.
Axon: long, unbranched fibre that carries nerve impulses away from the cell body.

Question 40

Describe the features of a myelinated motor neurone.

Answer 40

Schwann cells: cells that wrap around the axon.
Myelin sheath: made from myelin-rich membranes of Schwann cells
Nodes of Ranvier: very short gaps between neighboring Schwann cells where there is no myelin sheath.

Question 41

Describe how an action potential travels along an unmyelinated neurone.

Answer 41

The stimulus creates an influx of Na+ ions. The first section of the membrane depolarises which changes the potential difference of the neurone. This causes the sodium voltage-gated channels further along the membrane to open. This process continues until the other side of the neurone is reached.

Question 42

Explain why myelinated axons conduct impulses faster than unmyelinated axons.

Answer 42

Saltatory conduction: Impulses jump from one node of Ranvier to another. Depolarisation cannot occur where the myelin sheaths are as they act as an electrical insulator. This removes the need to depolarise the entire axon therefore increasing the speed that the impulse travels.

Question 43

What is meant by the term resting potential?

Answer 43

When a neurone isn’t conducting an impulse, there is a difference in charge between the inside of the neurone and the outside of the neurone. This is due to the difference in concentrations of positive ions such as K+ or Na+. The difference in charge creates a potential difference of -70mV which is known as the resting potential.

Question 44

Describe how a resting potential is established.

Answer 44

Sodium-potassium pumps use ATP to carry out active transport. For every 2 potassium ions they transport in, 3 sodium ions are transported out. This creates an electrochemical gradient which causes potassium ions to diffuse in and sodium ions to diffuse out. Since the membrane is more permeable to potassium ions, more of them are moved out therefore resulting in the -70mV potential.

Question 45

What is meant by the term action potential?

Answer 45

The change in potential difference of a neurone as it is depolarised due to a stimulus.

Question 46

Describe how an action potential is generated.

Answer 46

A stimulus causes a change in potential, typically via opening gated-sodium channels. If the stimulus is strong enough, enough sodium channels will be opened to increase the potential above the threshold potential. An increase in potential greater than the threshold potential of -55mV will always cause the depolarisation of a neurone which generates an action potential of +35 to +40mV.

Question 47

Describe what happens after an action potential is generated.

Answer 47

Voltage gated sodium channels close at the peak voltage of +35mV so no more positive ions are able to move into the neurone. Potassium ions move out causing a decrease in voltage which leads to more potassium channels to open until the resting potential has been restored.

Question 48

What is meant by the term “refractory period”?

Answer 48

A refractory period occurs when the voltage is decreased far beyond the resting potential “hyperpolarisation”.

Question 49

Describe the all-or-nothing principle when talking about neurones.

Answer 49

There is a threshold potential of -55mV and if the stimulus isn’t strong enough, then the potential of the cell will not increase above the threshold potential therefore no action potential is generated. If a stimulus does cause the potential to exceed the threshold potential, then an action potential will always be generated.

Question 50

Explain the importance of the all-or-nothing principle.

Answer 50

Ensures that animals only respond to large enough stimuli rather than responding to every slight change in environment.

Question 51

Explain the importance of a refractory period.

Answer 51

It ensures that discrete impulses are produced and each one is separate.
Ensures that action potential only travels in one direction.
Limits the number of impulses that are transmitted therefore preventing an overreaction to a stimulus.

Question 52

What factors affect the speed of conduction of an impulse? How does this factor affect the speed?

Answer 52

Myelination & saltatory conduction - if myelinated, then faster conduction
Axon diameter - the wider the axon, the faster the conductance
Temperature - higher temp, faster diffusion of ions therefore faster conduction

Question 53

What is a synapse?

Answer 53

The gaps between the end of the axon of one neurone and the dendrites of another.

Question 54

What is the purpose of a synapse?

Answer 54

To transmit the action potential between neurones via the neurotransmitters that diffuse across the synapse.