Organisms Response To Environment: Response To Stimulis

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 response?

Answer 2

A physical/ chemical reaction to a stimulus

Question 3

What are an organisms advantages to responding to stimuli?

Answer 3

• ability to carry out homeostasis

- allows organisms to be better suited to adapt to the changing environment

Question 4

What are the three forms of stimulus response organisms carry out?

Answer 4

-taxis = the directional response to a directional stimulus through the movement of motile organisms
-kineses = the speed of movement seen in motile organisms. It’s non directional and involves change in the rate of turning
Tropism= growth of plants in response to a directional stimulus

Question 5

How does each stimulus response increase an organisms chances of survival?

Answer 5

• taxis: the motile organism can move its whole body either towards a favourable stimulus (positive taxis) or away from a unfavourable stimulus (negative taxis) e.g. algae are photosynthetic so move towards light to increase their chances of survival as more light = photosynthesis
• kineses= if an organism crosses a sharp dividing line between favourable and non favourable conditions, it’s rate of turning increases to raise the chances of a quick return to a favourable environment
• tropisms: plant shoots grow towards light so their leaves are in most favourable conditions to capture light for photosynthesis. Plant shoots grow away from light to increase the probability roots grow into the soil where they are better able to absorb water and mineral ions

Question 6

What are the stimuli that plants respond to?

Answer 6

• light for photosynthesis
• gravity as the plants need to be firmly anchored to the soil
• water to use for photosynthesis and other metabolic processes

Question 7

Give an example of a plant growth factor

Answer 7

IAA which is a type of auxin

Question 8

Give the sequence of events in positive phototropism of flowering plants

Answer 8

1. Cells in the tip of the shoot produce IAA which is transported down the shoot throughout all regions as it moves down
2. Light causes the movement of IAA from the light side to the shaded side
3. A greater concentration of IAA builds up on the shaded side of the shoot
4. The cells on the side of greater concentration of IAA elongate more
5. The shaded side of the shoot elongates faster than the light side causing the shoot tip to bend downwards towards the light

Question 9

Give the sequence of events detailing positive gravitropism in flowering plants

Answer 9

1. Cells in the tip of the root produce IAA which is then transported along the root
2. The IAA is initially transported to all sides of the root
3. Gravity influences the movement of IAA from the upper side to the lower side of the root
4. A greater concentration of IAA builds up on the lower side of the root than the upper side
5. IAA inhibits the elongation of root cells so the cells on the lower side which have a lower concentration of IAA elongate less than those on the upper side
6. The greater elongation of cells on the upper side cause the root to bend downwards towards gravity

Question 10

Give a use of IAA in the plant cells properties

Answer 10

It increases the plasticity of the cell wall

Question 11

What are the two major divisions of the nervous system?

Answer 11

• the CNS = brain and spinal chord

- the PNS = pairs of nerves that originate from either the brain or the spinal chord

Question 12

What is the peripheral nervous system made up of?

Answer 12

• Sensory neurones: carry nerve impulses from receptors towards the CNS
• motor neurones: carry impulses away from yen cns to effectors

Question 13

Give the pathway of a general reflex arc

Answer 13

1. the stimulus causes an internal or external detectable change
2. The receptors act as transducers by converting the stimulus into a series of nerve impulses which are stimulus specific.
3. Sensory neurones carry the nerve impulses to the spinal chord in the CNS
4. Relay neurones receive the impulses from the sensory neurones and pass them to motor neurones
5. The motor neurone passes the nerve impulse to the effector
6. The effector is usually a muscle or gland that stimulates the response e.g. muscle contraction

Question 14

Give four reasons why reflex arcs are important

Answer 14

• involuntary = no decision making so brain isn’t overloaded with situations where the same response is needed
• protect the body from harm
• fast as the neurone pathway is short with only one or two synapses
• absence of the decision making process make the action rapid

Question 15

Give 3 differences between nervous and hormonal coordination

Answer 15

• hormones are transmitted through the blood while nerve impulses are transmitted through neurones
• hormonal response is widespread while nervous response is localised
• hormonal response is slow transmission while nervous response is fast transmission
• hormonal response is long lasting while the nervous response is short lived

Question 16

What is the structure of a motor neurone and describe each component it is made up of

Answer 16

• cell body: contains the cell organelles and larger amounts of rough er due to the neurones production of proteins and neurotransmitters
• dendrons: extensions of the cell body which subdivide into dendrites that carry nerve impulses towards the cell body
• axon: single long fibre that carries nerve impulses away from the cell body
• Schwann cells: wrap around the axon many times so layers of there membrane build up around it. They protect the axon, provide electrical insulation, carry out phagocytosis.
• myelin sheath: the covering over the axon is made of Schwann cell membranes which contain the lipid myelin
• nodes of ranvier: constrictions between adjacent Schwann cells where there is no myelin sheath

Question 17

Name the three types of neurones and what their function is

Answer 17

• sensory neurones transmit nerve impulses from a receptor to a relay or motor neurone. Their dendron carries impulses towards cell body and the axon carries impulses away
• motor neurones transmit nerve impulses from a relay neurone to an effector. They have long axons and many small dendrites
• relay neurones transmit impulses between neurones e.g. from sensory neurones to motor neurones and have numerous short processes

Question 18

What are the features of sensory receptors using the pacinian corpuscle example

Answer 18

• specific to a single type of stimulus: responds only to changes in mechanical pressure
• produce a generator potential by acting as a transducer: receptors convert the energy transferred by mechanical pressure into the generator potential

Question 19

Describe the structure of the pacinian corpuscle

Answer 19

Check biology album

Question 20

How does the pacinian corpuscle work?

Answer 20

1. At resting state, the stretch mediated sodium channels of the membrane around the neurone of the pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the pacinian corpuscle has a resting potential
2. When pressure is applied to the pacinian corpuscle, it’s deformed and the membrane around it becomes stretched
3. The stretching widens the sodium ion channels in the membrane and sodium ions diffuse into the neurone
4. Influx of sodium ions changes the potential of the membrane as it becomes depolarised, hence producing a generator potential as the threshold is breached
5. The breeches threshold means the generator potential creates an action potential that is triggered at the first node of ranvier

Question 21

What kind of light is seen by rod cells

Answer 21

Black and white because they work in low light intensity

Question 22

How many rod cells are connected to each bipolar cell?

Answer 22

3 rod cells for every one bipolar cell hence creating retinal convergence as this allows a greater chance for the threshold value to be exceeded as a result of spacial summation

Question 23

Why do rod cells respond to low intensity light?

Answer 23

Because the generator potential is created by the breakdown of the pigment in rod cells and there is enough energy in low intensity light to trigger this breakdown

Question 24

Give a consequence of many rod cells connected to one bipolar neurone

Answer 24

Light received by the rod cells connected to one bipolar neurone only generates one action potential / impulse so brain can’t distinguish between separate light sources leading to low visual acuity

Question 25

How are come cells connected to the bipolar neurone?

Answer 25

One cone cell for every one bipolar neurone

Question 26

Explain what intensity of light cone cells respond to

Answer 26

High because each cone cell is connected to one bipolar neurone so stimulation of many cone cells can’t be combined to help exceed threshold and create generator potential and because the pigment in cone cells requires high energy to be broken down to trigger an action potential. This also means each cone cell is sensitive to a different specific wavelength of light

Question 27

What is the visual acuity like of cone cells and why?

Answer 27

Good visual acuity because cone cell is connected to its own bipolar neurone

Question 28

Where are cone and rod cells located?

Answer 28

Cone cells are concentrated in middle of the retina at the fovea where light intensity is highest and rods in the peripheries of the retina where light intensity is lowest

Question 29

What are the two parts of the autonomic nervous system?

Answer 29

The sympathetic nervous system- stimulates effectors and speeds up activity, heightens awareness and prepares for activity

The parasympathetic nervous system- inhibits effectors so slows down activity, controls activities under normal resting conditions

Question 30

How is the heart rate electrically controlled?

Answer 30

1. SAN initiates heartbeat/ cardiac cycle.
2. Wave of electrical excitation spreads out from SAN and through atrial walls causing atrial systole.
3. Av valves open and blood flows through them to fill the ventricles
4. Short delay then AVN sends wave of electrical excitation down bundle of his to the base of the heart allowing the ventricles to fill up fully
5. Wave of electrical excitation spreads through perkyne fibres in the walls of the ventricles from the bottom up in ventricular systole as the av valves close and semi lunar valves open so blood flows under pressure into the arteries
6. Diastole semi lunar valves close and av valves open so blood passively refills the ventricles

Question 31

What do chemoreceptors and what is their process an example of?

Answer 31

They are sensitive to changes in pH due to changes in the concentration of co2 in the blood. their process is an example of negative feedback

Question 32

How do chemoreceptors respond when the blood has a higher concentration of co2 than normal?

Answer 32

1. They detect the decrease in pH
2. Chemoreceptors in the cartoid arteries increase frequency of impulses to the medulla oblongata
3. The Medulla oblingatas centre that controls heart rate in the CNS increases frequency of impulses to the sinoatrial node via the sympathetic nervous system
4. The SA node increases heart rate and stroke volume (cardiac output = heart rate x stroke volume )
5. Increased blood flow removes co2 faster via the pulmonary system
6. Co2 concentration returns to normal

Question 33

How do pressure receptors respond when blood pressure is higher than normal?

Answer 33

They transmit a higher frequency of impulses to the CNS in the medulla via the parasympathetic nervous system to the SAN which decreases heart rate

Question 34

How do pressure receptors respond when blood pressure is lower than normal?

Answer 34

Pressure receptors increase the frequency of impulses to the CNS in the medulla via the sympathetic nervous system to the SAN which increases heart rate

Question 35

Give the steps in the molecular mechanism for muscle contraction

Answer 35

1. Nerve impusle / action potential in a motor neurone initiates skeletal muscle contraction
2. The nerve impulses releases acetylcholine from the synaptic knob/ synapse into the neuromuscular junction
3. Acetylcholine binds to membrane receptors on the sarcolemma depolarising the sarcolemma / t tubles
4. In relaxed state, ca 2+ ions remain in the sarcoplasmic reticulum
5. Depolarisation of the sarcolemma triggers the calcium ions to be released from the sarcoplasmic reticulum
6. Myosin is in a cocked state as its got adp and pi bound to it
7. Ca2+ ions diffuse into the sarcoplasm and bind to tropin protein which moves the tropomyosin so that the binding sites on actin are exposed
8. The myosin globular head forms an attachment / binds to the actin forming a cross bridge
9. ADP and pi are released from the myosin pulling the actin over the myosin so it slides over in the power stroke
10. ATP molecule binds to the mysoin releasing it from the actin
11. ATP then gets hydrolysed by ATPase causing the myeosin to recock