Stimuli and Response

Question 1

Acid Growth Hypothesis

Answer 1

IAA increases the plasticity of cells by acid growth hypothesis. Hydrogen ions are actively transported from the cytoplasm into spaces in the cell wall. This activates expansions, loosening the cellulose causing it to become ‘more plastic’ and so allowing cells to elongate by expansion.

Question 2

How Auxin moves through cells

Answer 2

The negatively charged auxin encounters the acidic environment of the cell wall, so picks up a Hydrogen ion. This means it is now a small, non-charged molecule which can diffuse directly through the plasma membrane. Inside the cell the Auxin is ionised, which temporarily traps the Auxin inside the cell. The cell is maintained at a pH of 7 through an ATP driven proton pump. Auxin exits the cell at the Basal end where there are specific carrier proteins in the plasma membrane.

Question 3

Experiment to show that the light stimulus is detected by the tip of the root

Answer 3

Place a light proof cover on the intact tip of the shoot, there is no response.

Question 4

Experiment to show that it is a chemical not electrical signal in plants

Answer 4

Tip removed, gelatine block inserted and then the tip is replaced. The tip bends towards the light, because gelatine allows chemicals to pass through it but not electrical signals.

Question 5

Peripheral nervous system

Answer 5

made up of pairs of nerves that originate from either the brain or the spinal chord (can be divided into sensory and motor neurones).

Question 6

Autonomic nervous system

Answer 6

carries nerve impulses to glands, smooth muscle and cardiac muscle. It is involuntary.

Question 7

What is the spinal chord

Answer 7

A column of nervous tissue that runs along the back and lies inside the vertebral column for protection.

Question 8

Importance of reflex arcs

Answer 8

They are involuntary and therefore do not require the decision making powers of the brain, so leaving it free to carry out more complex responses. This also means that the brain is not overloaded with situations where the response is always the same.
They are effective from birth and do not have to be learnt so protect the body from harm
They are fast because the neurone pathway is short and has few synapses. The absence of any decision making process also means that their action is rapid.

Question 9

Cell Body

Answer 9

This contains all of the usual organelles including the nucleus and a large amount of rough ER (for producing proteins and neurotransmitters).

Question 10

Schwann Cells

Answer 10

These surround the axon and provide protection and electrical insulation. They also carry out phagocytosis removing any cell debris, and they play an important role in nerve regeneration. They wrap themselves around the axon many times so that layers of membrane build up around it forming the myelin sheath.

Question 11

Nodes of Ranvier

Answer 11

Gap between two adjacent Schwann cell

Question 12

What is the position of the cell body in relation to the axon in a sensory neurone

Answer 12

cell body is at a 90 degree angle to the axon

Question 13

Type of Sodium Channel in the Pacinian Corpuscle

Answer 13

Stretch mediated Sodium Channel

Question 14

Generator potential

Answer 14

if the influx of sodium ions is too small to reach threshold value no action potential is generated. This is the all or nothing principle.

Question 15

Resting Potential

Answer 15

1. Sodium Potassium pump transports 2 Potassium ions into the axon for every 3 sodium ions that it pumps out. This means that it will be more positive outside the axon and more negative inside the axon.
2. This creates a twofold gradient (of charge and concentration), so Sodium ions try to move into the axoplasm, however, the Sodium ion gated channels are closed so they can’t.
3. So there is a potential difference across the membrane and so the axon is polarised.

Question 16

Sodium potassium pump in the axon at resting potential

Answer 16

2 potassium ions into the axon for every 3 sodium ions out. More positive outside the axon and more negative inside.

Question 17

Potential difference at resting potential

Answer 17

-65mV

Question 18

Where are the rod cells found

Answer 18

on the retina, particularly at the periphery (none on the fovea)

Question 19

Where are cone cells found

Answer 19

concentrated on the fovea

Question 20

Rod cells key points

Answer 20

cannot distinguish between different wavelengths of light and therefore lead to black and white images.
They respond to low light intensity.
A number of rod cells are connected to a single bipolar cell.
Give low visual acuity
Only one type
there are more rod cells than cone cells.

Question 21

what is the pigment in rod cells

Answer 21

Rhodopsin. There is enough energy in low light intensity light to break this down.

Question 22

why are many rod cells connected to a single sensory neurone in the optic nerve

Answer 22

As a certain threshold has to be exceeded in order to create an action potential. Due to retinal convergence, there is a much greater chance that the threshold will be exceeded than If only a single rod cell were connected to each bipolar cell. This is due to Summation. However, it means that only a single impulse will be generated travelling to the brain regardless of how many of the neurones have been stimulated. This means that, in perception, the brain cannot distinguish between the separate sources of light that stimulated them, and so two dots close together cannot be resolved and will appear as a single blob. = low visual acuity.

Question 23

What is retinal convergence

Answer 23

a number of rod cells are connected to a single bipolar cell

Question 24

Cone cells main points

Answer 24

there are three different types each responding to a different range of wavelengths
Each have their own separate bipolar cell connected to a sensory neurone in the optic nerve.
Respond to high light intensity.
Concentrated in the fovea (this is where light is focused by the lens and so receives the highest concentration of light).

Question 25

Pigment in Cone cells

Answer 25

Iodopsin requires a higher light intensity for its breakdown. the three different types of cone cell each has a specific type of Iodopsin and so each cone cell is sensitive to a different range of wavelengths.

Question 26

Each cone cell is connected to its own bipolar cell and sensory neurone

Answer 26

This means that cone cells only respond to high light intensity. This means that if two adjacent cone cells are stimulated, the brain receives two separate impulses and so it can therefore distinguish between the two separate sources of light. This means that two dots close together can be resolved and will appear as two dots, so they have high visual acuity.

Question 27

Sympathetic Nervous system

Answer 27

Stimulates effectors and so speeds up any activity. eg. it controls effectors when we exercise strenuously or experience powerful emotions. It helps us cope with stressful situations by heightening our awareness and preparing us for activity.

Question 28

Parasympathetic Nervous system

Answer 28

This inhibits effectors and so slows down any activity. It controls activities under normal resting conditions and is concerned with conserving energy and replenishing the bodies reserves.

Question 29

Heart beat (myogenic stimulation)

Answer 29

Myogenic means contraction is initiated within the muscle itself.
1. wave of excitation initiated in the SAN (Sino atrial node) and spreads to both atria, causing them to contract.
2. a layer of non-conductive tissue prevents the wave crossing into the ventricles (atrioventricular septum)
3. Wave of excitations reaches the AVN (atrioventricular node)
wave of excitation passes down the Purkyne fibres to the bundle of His. This conducts the wave through the atrioventricular septum to the base of the ventricles and up through the ventricles causing them to contract from the bottom of the heart upwards.

Question 30

Changes in heart rate

Answer 30

controlled by a region of the brain called the medulla oblongata. This has 2 centres for heart rate: a centre that increases heart rate linked to the SAN by the Sympathetic nervous system. and the centre that decreases heart rate linked to the SAN by the parasympathetic nervous system.

Pressure and chemo receptors are found in the wall of the carotid arteries (the arteries that serve the brain).

If blood pressure is higher than normal = slow down heart rate
If blood pressure is lower than normal = speed up heart rate

The chemoreceptors detect changes in pH. More Oxygen in the blood is directly proportional to there being less carbon dioxide (which is acidic). So a more acidic blood increases heart rate.

the chemo and pressure receptors detect this and send a nervous impulse to correct centre in the medulla oblongata.

if you want to increase heart rate, the sympathetic nervous system will increase the frequency of signals sent to the SAN and therefore more frequent electrical impulses will be generated.

Question 31

Comparison of the nervous and hormonal system (7 points)

Answer 31

1. communication by nerve impulses vs chemicals
2. transmission by the blood stream vs neurones
3. hormones = relatively slow as opposed to nervous which is rapid
4. hormones travel to all parts of the body, but only target cells respond whereas nerve impulses travel only to specific parts of the body.
5. Hormones: response is widespread but in nervous the response is localised.
6. Hormones often is a long lasting response whereas in nervous the response is short lived
7. hormonal responses may be permanent and irreversible whereas nervous responses are usually temporary and reversible

Question 32

Ageing in neurones

Answer 32

dendrites become longer with age. However, with Alzhiemers the dendrites are shorter and less branched and theres fewer of them.

Question 33

Cholingeric Synapse

Answer 33

Acetylcholine (made up of acetyl which is ethanoic acid and choline)

Question 34

Synaptic knob

Answer 34

swollen portion at the end of the axon of the presynaptic neurone. It contains lots of mitochondria and endoplasmic reticulum (for the production of the neurotransmitter which is stored in vesicles).

Question 35

Action potential

Answer 35

1. Stimulus causes sodium ion gated channel to open.
2. Sodium ions diffuse into the axoplasm by facilitated diffusion.
3. as they’re positively charged they reverse the potential difference across the membrane and this creates a localised circuit.
4. the localised circuit causes the voltage gated sodium channels further along the membrane to open.
5. When the Sodium channels in the first section of the membrane close, the voltage gated potassium ion channel opens and potassium diffuse OUT of the axoplasm by facilitated diffusion until potential difference returns to resting state. However, the potassium ions continue to diffuse out even after resting state has been established causing hyperpolarisation.
6. The potassium ion channel then closes and the potassium sodium pump begins to establish a resting potential again. = repolarisation.

Question 36

Importance of the refractory period / repolarisation

Answer 36

1. ensures that action potentials are propagated in one direction only.
2. It produces discrete impulses, as it means that new action potentials cannot be formed immediately behind the first one. This ensures that impulses are separate from each other.
3. By separating out each action potential from the rest, it limits the number of action potentials that can pass along the axon at any one time and so limits the strength of the stimulus that can be detected.

Question 37

Myelinated axons and Saltatory Conduction.

Answer 37

Mylein is impermeable to sodium ions and so acts as an electrical insulator, preventing action potentials. Every 1-3mm there are breaks in the Mylein sheath called Nodes of ranvier. Action potentials can occur here. This means that localised circuits occur between adjacent nodes of ranvier and the action potentials essentially jump from node to node. this means that action potentials pass along the axon faster and is called Saltatory Conduction.

Question 38

Factors that effect the speed that impulses travel at

Answer 38

1. saltatory conduction / myelination
2. Diameter of the axon
3. temperature (rate of diffusion of ions and enzymes for respiration for the ATP for any active transport).

Question 39

How does the diameter of the axon effect the speed at which the impulses travel at

Answer 39

Larger diameter = faster. This is because lager axons have less leakage and so can maintain potential differences easier.

Question 40

How can an organism perceive the size of a stimulus

Answer 40

1. the number of impulses in a given time. the larger the stimulus, the more impulses that are generated in a given time.
2. by having different neurones with different threshold values.

Question 41

temporal summation vs spatial summation

Answer 41

spatial: two or more nuerones release neurotransmittersmitter to the same post synaptic neurone.

Temporal: a single presynaptic neurone releases neurotransmitter many times in the space of a short amount of time.

Both increase the concentration of neurotransmitter released and therefore make it more likely that a action potential occurs as threshold value is reached.

Question 42

transmission across a synapse

Answer 42

1. action potential in the presynaptic neurone causes calcium ion protein channels to open and calcium ions enter the presynaptic knob.
2. the influx of calcium ions causes the synaptic vesicle to fuse with the presynaptic membrane causing Acetylcholine to be released into the synaptic cleft.
3. Acetylcholine diffuses across the synaptic cleft and binds with receptors on the sodium ion channels in the postsynaptic membrane.
4. Sodium ion channels open and an action potential is generated.
5. Acetylcholinesterase enzyme hydrolyses the acetylcholine into ethanoic acid (acetyl) and choline. the acetyl and choline diffuse back across to the presynaptic neurone where ATP is released from the mitochondria and used to recombine the acetyl and choline to be stored in vesicles.

Question 43

the name of the enzyme that hydrolyses acetylcholine and why this must occur.

Answer 43

acetylcholinesterase.
this is recycling and the rapid breakdown prevents the acetylcholine from continuing to produce action potentials as the sodium ion channels close when not in the presence of acetylcholine.

Question 44

inhibitory synapses

Answer 44

this makes it less likely for a new action potential to be created.

1. different type of neurotransmitter is released and this binds to the chlorine ion protein channels, which open allowing chlorine ions into the axoplasm by facilitated diffusion.
2. This binding also causes the potassium ion channels to open, and so potassium ions leave the axoplasm.
3. this combined effect of negatively charged ions moving in and positively charged ions moving out means that the membrane is hyper polarised and so a larger influx of sodium ions is needed to create an action potential and this is less likely.

Question 45

excitatory synapse

Answer 45

creates a new action potential

Question 46

Effects of drugs on synapses

Answer 46

A drug can stimulate the nervous system by creating more action potentials in postsynaptic neurones. A drug could do this by mimicking a neurotransmitter, or by inhibiting an enzyme that breaks down the neurotransmitter. This enhances the bodies responses to impulses, eg. if a neurone transmits impulses from sound receptors, a person will perceive the sound as being louder.

They can also inhibit the nervous system by creating fewer action potentials in the post synaptic membrane. It could inhibit the release of a neurotransmitter or block receptors on the post synaptic membrane.

eg. endorphins are neurotransmitters which block the sensation of pain. Drugs such as morphine and codeine bind to specific receptors in the brain used by endorphins and so mimic the effects of endorphins.

Question 47

what are the three types of muscle

Answer 47

1. cardiac muscle
2. smooth muscle (walls of blood vessels and the gut)
3. skeletal muscle