Responding To The Environment

Question 1

What are effectors?

Answer 1

Effectors are cells which bring about a response to a stimulus, e.g. Muscle cells and cells found in glands.

Question 2

What are the 3 types of neurones?

Answer 2

Sensory neurones, motor neurones, relay neurones.

Question 3

What is a sensory neurones?

Answer 3

The sensory neurones transmits electrical impulses from receptors to the central nervous system.

Question 4

What is a motor neurone?

Answer 4

The motor neurone transmits electrical impulses from the central nervous system to effectors.

Question 5

What is a relay neurone?

Answer 5

The relay neurone transmits electrical impulses between sensory neurones and motor neurones

Question 6

What response does your nervous system make when you detect a stimulus?

Answer 6

The stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone. When the electrical impulse reaches the end of a neurone, chemicals called neurotransmitters take the information to the next neurone. The CNS processes the information and sends impulses along motor neurones to an effector.

Question 7

What is a gland?

Answer 7

A gland is a group of cells that are specialised to secrete a useful substance, such as a hormone. E.g. The pancreas.

Question 8

What are hormones?

Answer 8

Hormones are chemical messengers, e.g. Insulin.

Question 9

How does the hormonal system send information?

Answer 9

When a gland is stimulated, it secretes hormones. Hormones diffuse into the blood, where they are taken around the body by the circulatory system. They diffuse out of the blood and bind to specific receptors for that hormone. The hormones trigger a response in the target cells.

Question 10

How can glands be stimulated?

Answer 10

Glands are stimulated by a change in concentration of a substance, e.g. A hormone, and they can also be stimulated by electrical impulses.

Question 11

What are the properties of the hormonal communication system?

Answer 11

Hormones travel in the blood to reach their target organs, so hormone communication is slower than electrical communication. Hormones aren’t broken down as quickly as neurotransmitters so the effects of hormones last for longer. Hormones are transported all over the body, so the response may be widespread.

Question 12

What a the properties of the nervous communication system?

Answer 12

When an electrical impulse reaches the end of a neurone, neurotransmitters are secreted onto cells so the nervous response is localised. Neurotransmitters are quickly removed once they are secreted so the response is short lived. Electrical impulses are rapid, so the response is rapid.

Question 13

Why do organisms respond to changes in their environment?

Answer 13

Animals respond to changes in their external environment by avoiding harmful places which are too hot or too cold. They respond to changes in their internal environment by making sure that the conditions are always optimal for their metabolism.

Question 14

How do receptor cells communicate information via the nervous system?

Answer 14

When a receptor is not being stimulated, there is a difference in charge between the inside and outside of the cell - so there is a potential difference across the membrane. When a stimulus is detected, the cell membrane becomes more permeable, so more ions move in and out of the cell- altering the potential difference. The bigger the stimulus, the more ions move in and out of the cell. More ions creates a larger generator potential. If the generator potential is large enough it triggers an action potential.

Question 15

What is a generator potential?

Answer 15

The generator potential is the change in potential difference across the receptor membrane.

Question 16

How is a bigger generator potential produced?

Answer 16

When there is a bigger stimulus the membrane is excited more, causing a bigger movement of ions and a bigger change in potential difference.

Question 17

How does an action potential get produced?

Answer 17

If a generator potential is big enough it triggers an action potential - an electrical impulse along a neurone. The action potential is only triggered if the generator potential reaches a certain level called the threshold level.

Question 18

What is the threshold level?

Answer 18

The threshold level is the level which the potential difference has to change by to create and action potential.

Question 19

What is a pacinian corpuscle?

Answer 19

Pacinian corpuscles are mechanoreceptors, which detect mechanical stimuli, e.g. Pressure and vibrations. They are found in your skin.

Question 20

What is the structure of a pacinian corpuscle?

Answer 20

A pacinian corpuscle contains the end of a sensory neurone which is wrapped in layers of conductive tissue called lamellae.

Question 21

What happens when a pacinian corpuscle is stimulated?

Answer 21

When a pacinian corpuscle is stimulated the lamellae are deformed and press on the sensory nerve ending. This causes deformation of stretch mediated sodium channels in the cell membrane of the sensory neurone. The sodium ion channels open and sodium ions diffuse into the cell, creating a generator potential. If the generator potential reaches the threshold level an action potential is triggered.

Question 22

How do photoreceptors in your eye detect light?

Answer 22

Light enters your eye through the pupil. Your pupil is made smaller or larger to control the amount of light entering your eye, by the muscles of your iris. Light rays are focused by the lens onto the retina. The retina contains photo receptor cells.

Question 23

What is the fovea?

Answer 23

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

Question 24

Why is the blind spot not sensitive to light?

Answer 24

The blind spot is not sensitive to light because there aren’t any photoreceptors there.

Question 25

How do photoreceptors convert light to an electrical impulse?

Answer 25

Light enters your eye and is absorbed by light sensitive pigments. Light bleaches the pigments and causes a chemical change which alters the membrane permeability to sodium. This creates a generator potential and if it reaches the threshold a nerve impulse is sent across a bipolar neurone which takes the impulse to the brain.

Question 26

What are the two types of photoreceptors?

Answer 26

Rod cells and cone cells.

Question 27

Why are rod cells very sensitive to light?

Answer 27

Rod cells are very sensitive to light because many rod cells join one neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.

Question 28

Where are the rod cells?

Answer 28

Rod cells are found in the peripheral parts of the retina.

Question 29

Where are cone cells found?

Answer 29

Come cells are packed together in the fovea.

Question 30

Why are cone cells only sensitive to bright light?

Answer 30

Cone cells are less sensitive to rod cells because one cone cell joins one neurone do it takes more light to reach the threshold and trigger an action potential.

Question 31

Why do rod cells have low visual acuity?

Answer 31

Rods cells have low visual acuity because many rod cells join the same neurone. Therefore, light from 2 objects close together can't be told apart.

Question 32

Why do cone cells have high visual acuity?

Answer 32

Cone cells have high visual acuity because cones are close together and one cone joins one neurone. Therefore when light from 2 points hits 2 cone cells 2 action potentials are made so you can distinguish the 2 points which are close together.

Question 33

What is the resting potential?

Answer 33

The resting potential is the voltage across the membrane when it is at rest.

Question 34

What does the sodium potassium pump do?

Answer 34

The sodium potassium pump actively transports 3 sodium ions out the neurone for every 2 potassium cells moved in.

Question 35

What does the potassium ion channel do?

Answer 35

Potassium ion channels allow facilitated diffusion of potassium ions out of the neurone.

Question 36

How is the resting potential established ?

Answer 36

Sodium ions are actively transported out of the axon and potassium ions move into the axon through potassium ion channels by facilitated diffusion. This makes the outside of the axon positively charged compared to the inside.

Question 37

What happens when a stimulus is detected?

Answer 37

The neurone cell membrane gets excited, which causes sodium channels to open. Then the membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone. The inside of the neurone becomes less negative.

Question 38

What is depolarisation?

Answer 38

Depolarisation is when the potential difference reaches the threshold (around -55mV) more sodium ion channels open and more sodium ions diffuse into the neurone.

Question 39

What is repolarisation?

Answer 39

At a potential difference of around +30mV the sodium ion channels close and potassium ion channels open. This makes the membrane more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient.

Question 40

What is hyperpolarisation?

Answer 40

Hyperpolarisation is when the potassium ions are slow to close so too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential.

Question 41

What is the resting potential?

Answer 41

When the ion channels are reset. The sodium potassium pump maintains the resting potential until the membrane is excited by another stimulus.

Question 42

Why can a neurone not be excited straight away after an action potential?

Answer 42

The ion channels are recovering and the can't be made to open. Sodium ion channels close during repolarisation and potassium ion channels close during hyperpolarisation. The recovery Period is called the refractory period.

Question 43

How does an action potential travel along a neurone?

Answer 43

When an action potential happens, some sodium ions diffuse sideways so the sodium ion channels in the next part of the neurone open and sodium ions diffuse into that part. This causes a wave of depolarisation to travel along a neurone.

Question 44

What does the refractory period do?

Answer 44

In the refractory period the ion channels are closed. The refractory period limits the number of action potentials and makes sure that action potentials don't overlap. Therefore, action potentials pass along as discrete (separate) impulses. The refractory period also makes sure action potentials are unidirectional (that they only travel in one direction).

Question 45

What is the all or nothing principle?

Answer 45

When the threshold is reached an action potential will always fire with the same change in voltage, no matter how big the stimulus is. If the threshold isn't reached an action potential won't fire.

Question 46

What are the 3 features of axons which speed by conduction of action potentials?

Answer 46

Axon diameter, temperature and if the axon is myelinated.

Question 47

How does the axon diameter affect the speed of the nerve impulse?

Answer 47

The larger the diameter of the axon, the faster the action potential. This is because there is less resistance to the flow of ions in the cytoplasm of a large axon, so depolarisation reaches other parts of the neurone cell membrane quicker.

Question 48

How does temperature affect the speed of the nerve impulse?

Answer 48

The speed of conduction increases as temperature increases because ions diffuse faster. The speed only increases up to a maximum temperature of 40°C. After 40°C the proteins begin to denature and the speed decreases.

Question 49

How does the myelin sheath increases the speed of the nerve impulse?

Answer 49

The myelin sheath is an electrical insulator so action potentials cannot form in the area covered by the sheath. Depolarisation only happens at the nodes of ranvier ( where sodium ions can get through the membrane). The action potential can jump from one node to the next. This is called saltatory conduction.

Question 50

What is a synapse?

Answer 50

A synapse is a junction between a neurone and another neurone.

Question 51

What is the synaptic cleft?

Answer 51

The synaptic cleft is the tiny gap between the cells at a synapse.

Question 52

How is an impulse transferred across a synapse?

Answer 52

The presynaptic neurone contains synaptic vesicles which contain neurotransmitters. When the action potential reaches the end of the neurone it causes neurotransmitters to be released into the synaptic cleft. They diffuse across to the postsynaptic membrane and bind to receptors. This triggers an action potential, causes muscles contraction or releases a hormone.

Question 53

How do synapses make sure that impulses are unidirectional?

Answer 53

Synapses ensure impulses are unidirectional because the receptors which neurotransmitters bind to are only found on the postsynaptic membrane.

Question 54

How does acetylcholine (ACh) transmit the nerve impulse across a cholinergic synapse?

Answer 54

An action potential arrives at the presynaptic neurone. It stimulates voltage gated calcium ions in the presynaptic neurone to open. Calcium ions diffuse into the synaptic knob. This causes the synaptic vesicles to fuse with the presynaptic membrane and release the neurotransmitter ACh into the synaptic cleft. ACh diffuses across the synaptic cleft and binds to cholinergic receptors on the postsynaptic membrane. This causes sodium ion channels in the postsynaptic neurone to open. The influx of sodium ions into the postsynaptic neurone causes an action potential. ACh is removed from the synaptic cleft so the response doesn't keep happening.

Question 55

How are neurotransmitters removed from the synaptic cleft?

Answer 55

Neurotransmitters can be taken back into the presynaptic membrane or they are broken down by enzymes.

Question 56

What are receptors?

Answer 56

Receptors are cells or proteins on cell surface membranes, which detect stimuli. Receptors are specific, so they only detect one kind of stimulus, e.g. Pacinian corpuscles detect pressure.

Question 57

What is a neuromuscular junction?

Answer 57

Neuromuscular junctions are synapses between neurones and muscles.

Question 58

What is the difference between excitatory and inhibitory neurotransmitters?

Answer 58

Excitatory neurotransmitters bind to receptors on the postsynaptic membrane and cause an action potential to be fired if the threshold is reached. Inhibitory neurotransmitters prevent the postsynaptic neurone from firing an action potential.

Question 59

What is summation?

Answer 59

Sometimes if there is a weak stimulus there will only be a small amount of a neurotransmitter released. This might not be enough to excite the postsynaptic membrane to the threshold level. So an action potential won't be triggered. Summation is the when the effect of neurotransmitters released from many neurones is added together to create an action potential.

Question 60

What are the 2 types of summation?

Answer 60

Spatial summation and temporal summation.

Question 61

What is spatial summation?

Answer 61

Sometimes many presynaptic neurones connect to one postsynaptic neurone. Each presynaptic neurone only releases a small amount of neurotransmitter but this can combine to trigger an action potential in the postsynaptic neurone if the threshold is reached.

Question 62

What is temporal summation?

Answer 62

Temporal summation is when one presynaptic neurone releases neurotransmitters many times in a short period. Therefore, an action potential is more likely because there is more neurotransmitter in the synaptic cleft.

Question 63

What is the sarcolemma?

Answer 63

The sarcolemma is the cell membrane of muscle fibre cells.

Question 64

What are transverse tubules?

Answer 64

Transverse tubules are bits of the sarcolemma that fold inwards across the muscle fibre and stick into the sarcoplasm. They help to spread electrical impulses to all parts of the muscle fibre.

Question 65

Describe the structure of myofibrils. (A-bands and I-bands).

Answer 65

Myofibrils contain thick and thin myofilaments. The thick myofilaments are made of myosin and the thin myofilaments are made of actin. If you look at myofibrils under a microscope you can see alternating dark and light bands. The dark bands contain thick myosin filaments and some overlapping actin filaments. These dark bands are called A-bands. The light bands only contain actin filaments. These are called I-bands.

Question 66

Describe the structure of myofibrils. | Z-line, H-zone and M-line.

Answer 66

Myofibrils are made up of lots of short units called sarcomeres. At the end of each sarcomere there is a Z-line. In the middle of each sarcomere there is an M-line, which is in the middle of the myosin filaments. Around the M-line there is the H-zone which only contains myosin filaments.

Question 67

What is the sliding filament theory?

Answer 67

The myosin and actin filaments slide over each other, which makes the sarcomeres contract simultaneously. This causes the myofibrils and muscle fibres to contract. The A-bands stay the same length, the I-band gets shorter and the H-zone gets shorter.

Question 68

What happens to a sarcomere when a muscle relaxers?

Answer 68

When a muscle relaxes the sarcomeres return to their original length.

Question 69

Describe the structure of myosin filaments.

Answer 69

Myosin filaments have globular heads that are hinges. The globular heads can move back and forth. The myosin heads have a binding site for actin and a binding site for ATP. Actin filaments have binding sites for the myosin heads, called actin-myosin binding sites. There are 2 other proteins called tropomyosin and troponin which are found between actin filaments. They help the myofilaments move past each other.

Question 70

What happens when muscles are relaxed?

Answer 70

In a resting muscles the actin-myosin binding site is blocked by tropomyosin. Therefore, the myofilaments cannot slide past each other because the myosin heads cannot bind to actin filaments.

Question 71

What happens during muscle contraction?

Answer 71

Initially, tropomyosin is blocking the myosin binding sites on actin. Calcium ions bind to the actin, which displaces the tropomyosin and exposes the binding sites. Now myosin heads can attach to the actin filaments (which forms a cross bridge). Then the myosin heads change angle, causing a ratchet mechanism that slides actin relative to myosin. ATP binds to myosin heads which cause them to detach from the actin. The ATP is hydrolysed which releases energy and allows the myosin head to return to its normal position.

Question 72

What happens when muscles are not being stimulated?

Answer 72

When a muscle stops being stimulated he calcium ions do not hind to the actin filaments. Therefore, the tropomyosin can block the actin-myosin binding sites again.

Question 73

What are slow twitch muscle fibres and what are they useful for?

Answer 73

Slow twitch muscle fibres contract slowly and less powerfully over a long period. They are good for endurance activities, e.g. Long distance running. They are adapted to their roles because they contain myoglobin, which stores oxygen, glycogen and lots of mitochondria. Also they have a good blood supply.

Question 74

What are fast twitch muscle fibres and what are they useful for?

Answer 74

Fast twitch muscle fibres contract very quickly and powerfully for a short period. They are good for fast movements, e.g. Sprinting. They are adapted to their role because they have thick and lots of myosin filaments. They have lots of enzymes involved in aerobic respiration and they have phosphocreatine stores.

Question 75

How is ATP provided for muscle contraction?

Answer 75

ATP can be provided by the electron transport chain in aerobic respiration, when there is oxygen. ATP can be provided by anaerobic respiration by glycolysis. This produces lactate. ATP is also provided by phosphocreatine. ATP can be made by phosphorylation of ADP by adding a phosphate from phosphocreatine( PCr). PCr is stored in cells and it generates ATP very quickly. It doesn't require oxygen.

Question 76

How is high blood pressure controlled by the autonomic nervous system?

Answer 76

Pressure receptors in the blood vessels detect high blood pressure. Impulses are sent to the medulla which sends impulses along parasympathetic neurones, which secrete acetylcholine which binds to receptors on the SAN. This causes your heart rate to slow down to reduce blood pressure back to normal.

Question 77

How is low blood pressure controlled by the autonomic nervous system?

Answer 77

Pressure receptors in blood vessels detect low blood pressure. Impulses are sent to the medulla which sends impulses along sympathetic neurones. They release a neurotransmitter which binds to receptors on the SAN, which causes heart rate to speed up and increase blood pressure back to normal.

Question 78

How is high blood oxygen, low CO2 and high pH levels controlled by the autonomic nervous system?

Answer 78

Chemoreceptors detect chemical changes in the blood. Impulses are sent to the medulla which sends impulses along parasympathetic neurones, which secrete acetylcholine, which binds to receptors on the SAN. This causes the heart rate to decrease and return oxygen, CO2 and pH levels to their normal level.

Question 79

How is low blood oxygen, high CO2 and low pH levels controlled by the autonomic nervous system?

Answer 79

Chemoreceptors detect chemical changes in the blood. Impulses are sent to the medulla which sends impulses along sympathetic neurones. These secrete a neurotransmitter which binds to receptors on the SAN. This causes the heart rate to increase and return oxygen, CO2 and pH levels back to normal.

Question 80

Why are reflex arcs important?

Answer 80

Reflex arcs are important because they are automatic, it reduces damage to the tissues, they help us to escape from predators or to find food.

Question 81

How do reflex arcs work?

Answer 81

A receptor detects a stimulus. The sensory neurone carries impulses to the relay neurone. The relay neurone connects to a motor neurone. The motor neurone sends impulses to an effector which carriers out a response.

Question 82

What is taxis?

Answer 82

Taxis is when an organism moves toward some away from a directional stimulus, e.g. A woodlouse moves away from light.

Question 83

What is kinesis?

Answer 83

Kinesis is when an organisms movement is affected by a non directional stimulus, e.g. A woodlouse moves slowly in high humidity so he stays in the high humidity area to reduce his water loss.

Question 84

What are chemical mediators?

Answer 84

Chemical mediators are chemicals that are released from mammalian cells and they have an affect on cells in their immediate vicinity.

Question 85

What is histamine and what does it do?

Answer 85

Histamine is a chemical mediator that is released in response to your body being injured or infected. It increases permeability of your capillaries so allow more immune system cells to leave the capillaries and go to the injured area. It causes redness and swelling in the infected area.

Question 86

What is prostaglandins and what do they do?

Answer 86

Prostaglandins are a group of chemical mediators that are released following an injury. They increase the permeability of the capillaries.

Question 87

What is a tropism?

Answer 87

A tropism is the response of a plant to a directional stimulus. A positive tropism is growth towards a stimulus. A negative tropism is grown away from a stimulus.

Question 88

What is phototropism?

Answer 88

Phototropism is the growth of a plant in response to light. Shoots are positively phototrophic so they grow towards light. Roots are negatively phototrophic so they grow away from light.

Question 89

What is geotropism?

Answer 89

Geotropism is the growth of a plant in response to gravity. Shoots are negatively geotropic so they grow away from gravity. Roots are positively geotropic so they grow towards gravity.