Topic 6: Organsims Respond To Changes In Their Internal And External Environment

Question 1

How is a generator potential established in a Pacinian corpuscle?

Answer 1

1) A mechanical stimulus (eg. Pressure) deforms the lamellae and stretch mediated sodium ion channels

2) The Na+ channels in the membrane open and sodium ions diffuse into the sensory neurone
- More pressure applied –> more Na+ channels open –> more Na+ enters

3) This causes depolarisation, leading to a generator potential
- If generator potential reaches threshold, and action potential is triggered

Question 2

What does the Pacinian corpuscle illustrate?

Answer 2

1) Receptors only respond to specific stimuli
- Pacinian corpuscle only responds to mechanical stimuli

2) Stimulation of receptor leads to establishment of generator potential
- When threshold is reached, an action potential is sent (all-or-nothing principle)

Question 3

Why are rods more sensitive to light?

Answer 3

1) Several rods are connected to a single neurone

2) This is spatial summation, which allows the threshold to be reached to generate an action potential

Question 4

Why are cones less sensitive to light?

Answer 4

1) Each cone is only connected to a single neurone
2) So no spatial summation occurs –> no action potential generated

Question 5

Where are rods and cones found?

Answer 5

In the retina of the eye

Question 6

Why do rods give lower visual acuity?

Answer 6

1) Several rods are connected to a single neurone
2) So several rods send a single set of nervous impulses to the brain (cant distinguish between separate light sources)

Question 7

Why do cones give higher visual acuity?

Answer 7

1) Each cone is connected to a single neurone
2) So cones sends separate impulses to the brain (can distinguish between different light sources)

Question 8

How can you remember that cones allow for coloured vision?

Answer 8

C for colour

Question 9

Why do cones allow for coloured vision?

Answer 9

1) There are 3 types of cones –> red-sensitive, blue-sensitive, green-sensitive
2) With different optical pigments which absorb different wavelengths of light
3) Stimulating different combinations of cones gives a range of colour perception

Question 10

What does SAN stand for?

Answer 10

Sinoatrial node

Question 11

What does AVN stand for?

Answer 11

Atrioventricular node

Question 12

Describe the myogentic stimulation of the heart and transmission of a subsequent wave of electrical activity?

Answer 12

1) SAN acts as pacemaker, sending regular waves of electrical activity across the atria –> makes the atria contract simultaneously

2) Non-conducting tissue between atria / ventricles prevents impulses passing directly to ventricles –> prevents immediate ventricle contraction

3) Waves of electrical activity reach AVN, delaying impulses -> allows atria to fully contract and empty before ventricles contract

4) AVN sends waves of electrical activity down the bundle of his, conducting wave between ventricles to apex where it branches to Purkyne tissue –> causes ventricles to contract simultaneously from the base up

Question 13

What is the Bundle of His?

Answer 13

A branch of nerve cells extending from the AVN

Question 14

What is purkyne tissue?

Answer 14

The Bundle of His divided into 2 conducting fibres

Question 15

Where are chemoreceptors located?

Answer 15

Aorta
Carotid arteries

Question 16

Where are pressure receptors found?

Answer 16

Aorta
Carotid arteries

Question 17

Role of chemoreceptors, pressure receptors, ANS and effectors in increasing heart rate?

Answer 17

1) Baroreceptors detect fall in blood pressure / chemoreceptors detect rise in blood CO2 concentration (so fall in pH)

2) Sending impulses to medulla

3) This sends more frequent impulses to the SAN along the sympathetic neurones

4) So more frequent impulses are sent from the SAN and to the AVN

5) causing cardiac muscles to contract more frequently

6) Increasing heart rate

Question 18

Roles of chemoreceptors, pressure receptors ANS and effectors in decreasing heart rate

Answer 18

1) Baroreceptors detect rise in blood pressure / chemoreceptors detect fall in blood CO2 concentration (rise in pH)

2) Sending impulses to the medulla

3) More frequent impulses are sent to the SAN along parasympathetic neurones

4) So less frequent impulses are sent from the SAN and to the AVN

5) Cardiac muscles contract less

6) Heart rate decreases

Question 19

What is a resting potential?

Answer 19

The inside of the axon has a more negative charge relative to the outside (more + ions on outside)

Question 20

How is a resting potential established across the axon membrane?

Answer 20

1) Na+ pump actively transports Na+ out of axon & K+ pump actively transports K+ into axon

2) Creates and electrochemical gradient –> higher K+ concentration inside & higher Na+ concentration outside

3) Membrane becomes more permeable to K+ (so it can move out by facilitated diffusion) and less permeable to Na+ (closed channels)

Question 21

First step of generating an action potential

Answer 21

Stimulus
–> Na+ channels open, increasing membrane permeability to Na+
–> Na+ diffuses into axon down an electrochemical gradien
–> Causing depolarisation

Question 22

Second step of generating an action potential

Answer 22

Depolarisation
–> If threshold is reached, action potential is generated
–> Because more voltage gated Na+ channels open
–> So more Na+ diffuses in rapidly

Question 23

Third step of generating an action potential

Answer 23

Repolarisation
–> Voltage gated Na+ channels close
–> Voltage gated K+ channels open so K+ diffuses out axon

Question 24

Fourth step of generating an action potential

Answer 24

Hyperpolarisation
–> K+ channels are slow to close, causing a slight overshoot
–> Too many K+ diffuse out axon

Question 25

Fifth step of generating an action potential

Answer 25

Resting potential --> Restored by Na+ / K+ pump

Question 26

Describe the all or nothing principle

Answer 26

1) Depolarisation must occur for an action potential to be reached 2) Action potentials produced are always the same size --> bigger stimuli increases frequency of action potentials

Question 27

How does the passage of an action potential along a non-myelinated axon result in nerve impulses?

Answer 27

1) Action potential passes as a wave of depolarisation 2) Influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open 3) So adjoining regions depolarise

Question 28

How does the passage of an action potential along a myelinated axon result ion nerve impulses?

Answer 28

1) Myelination provides electrical insulation 2) Depolarisation of axon at nodes of ranvier 3) Results in saltatory conduction 4) So no need for depolarisation long the whole length of the axon

Question 29

How does damage to the myelin sheath lead to slow response / jerky movement?

Answer 29

1) Less / no saltatory conduction --> depolarisation occurs along the entire length of axon 2) So nerve impulses take longer to reach neuromuscular junction --> delays muscle contraction 3) depolarisation may pass to other neurones 4) Causing wrong muscle fibres to contract

Question 30

Describe the nature of the refractory period

Answer 30

1) Time taken to restore axon to resting potential when no further cation potential can be generated 2) As Na+ channels are closed

Question 31

Importance of refractory period

Answer 31

1) Ensures discrete impulses are produced (action potentials don't overlap) 2) Limits frequency of impulse transmission at a certain intensity - Higher intensity stimulus causes higher frequency of action potentials - But only up to a certain intensity 3) Also ensures action potentials travel in one direction

Question 32

Effect of myelination on speed of conductance

Answer 32

1) Depolarisation at nodes of ranvier only --> causes saltatory conduction 2) Impulses don't travel along whole length of axon

Question 33

Effect of axon diameter on speed of conductance

Answer 33

Bigger diameter --> less resistance to flow of ions in cytoplasm

Question 34

Effect of temperature on speed of conductance

Answer 34

1) Increases rate of diffusion of Na+ and K+ --> due to more KE 2) However, proteins and enzymes could denature at a certain temperature

Question 35

What is a cholinergic synapse?

Answer 35

Synapse which uses acetylcholine (ACh) neurotransmitter

Question 36

Describe the process at the presynaptic neurone

Answer 36

1) Depolarisation of pre-synaptic membrane causes voltage-gated Ca2+ channels to open 2) Ca2+ diffuses into presynaptic neurone 3) Causing vesicles containing ACh to move and fuse with pre-synaptic membrane 4) ACh released into synaptic cleft by exocytosis

Question 37

What happens at the post-synaptic neurone?

Answer 37

1) ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane 2) Causing Na+ channels to open 3) Na+ diffuses into post-synaptic neurone causing depolarisation 4) If threshold is met, action potential is initiated

Question 38

What happens to acetylcholine after synaptic transmission?

Answer 38

1) Hydrolysed into acetylcholinesterse 2) Products are reabsorbed by presynaptic neurone 3) To stop overstimulation

Question 39

What happens if products aren't reabsorbed by pre-synaptic neurone?

Answer 39

Will keep building up and bind to receptors, causing depolarisation

Question 40

How do synapses result in unidirectional nerve impulses?

Answer 40

1) Neurotransmitters are only made in and released from pre-synaptic neurone 2) Receptors are only on post-synaptic membrane

Question 41

Explain summation by synapses

Answer 41

1) Addition of impulses converging on a single post-synaptic neurone 2) Causes rapid build up of neurotransmitters 3) So threshold is more likely to be reached to generate an action potential

Question 42

Describe spatial summation

Answer 42

1) Many pre-synaptic neurones share one synaptic cleft / post synaptic neurone 2) So collectively release sufficient neurotransmitters to reach threshold to trigger an action potential

Question 43

Describe temporal summation

Answer 43

1) One presynaptic neurone releases neurotransmitters many times over a short time 2) Sufficient neurotransmitters to reach threshold and generate action potential

Question 44

Inhibition by inhibitory synapses

Answer 44

1) Inhibitory neurotransmitters hyperpolarise post-synaptic membrane - Cl- channels open --> Cl- diffuses in - K+ channels open --> K+ diffuses out 2) More Na+ required for depolarisation 3) reduces likelihood of threshold being met / action potential forming at post-synaptic membrane

Question 45

Structure of neuromuscular junction

Answer 45

1) Receptors on muscle fibre (instead of post-synaptic membrane) & more receptors 2) Muscle fibres form clefts to store enzyme

Question 46

Transmission across cholinergic synapse

Answer 46

1) Neurone to neurone 2) Neurotransmitters can be excitatory or inhibitory 3) Action potential may be initiated in post-synaptic neurone

Question 47

Transmission across neuromuscular junction

Answer 47

1) Motor neurone to muscle 2) Always excitatory neurotransmitters 3) Action potential propagates along sarcolemma down T tubules

Question 48

Effect of excitatory drugs on synapse

Answer 48

1) Stimulate nervous system --> leads to more action potentials - Similar shape to neurotransmitter - Stimulates release for more neurotransmitter - Inhibit enzyme that breaks down neurotransmitter --> Na+ continues to enter

Question 49

Effect of inhibitory drugs on synapse

Answer 49

1) Inhibit nervous system --> fewer action potentials - Inhibits release of neurotransmitters (prevent Ca2+ channels opening) - Block receptors by mimicking shape of neurotransmitter

Question 50

How do muscles work?

Answer 50

1) In antagonistic pairs --> pull in opposite directions - One muscle contracts, pulling on the bone - One muscle relaxes (antagonistic) 2) Skeleton is incompressible so muscle can transmit force

Question 51

Advantage of antagonistic muscle

Answer 51

1) Second muscle is required to reverse the movement caused by first muscle 2) Contraction of both muscles helps maintain posture

Question 62

Importance of maintaining stable core temperature when temperature is too high

Answer 62

1) H bonds in tertiary structure of enzymes break 2) Enzymes denature --> active sites change shape and substrates can't bind 3) Fewer enzyme-substrate complexes

Question 63

Importance of maintaining stable core temperature when temperature is too low

Answer 63

Not enough KE so fewer enzyme-substrate complexes

Question 64

Importance of maintaining stable blood pH

Answer 64

1) Above / below optimal --> ionic & h bonds in tertiary structure break 2) Enzymes denature --> active sites change shape and substrate can't bind 3) Fewer enzyme-substrate complexes

Question 65

What is hypoglycaemia?

Answer 65

Blood glucose concentration too low

Question 66

Importance of maintaining stable blood glucose concentration with hypoglycaemia

Answer 66

1) Not enough glucose for respiration 2) So less ATP produced 3) Active transport can't happen --> cell death

Question 67

What is hyperglycaemia?

Answer 67

Blood glucose concentration too high

Question 68

Importance of maintaining stable blood glucose concentration for hyperglycaemia

Answer 68

1) Water potential of blood decreases 2) Water lost from tissue to blood via osmosis 3) Kidneys can't absorb all glucose --> more water lost in urine 4) Causing dehydration

Question 69

Role of negative feedback in homeostasis

Answer 69

1) Receptors detect change from optimum 2) Effectors respond to counteract change 3) Returning levels to optimum / normal

Question 70

Importance of conditions being controlled by separate mechanisms involving negative feedback

Answer 70

1) Departures in directions from original state can be controlled 2) Giving greater degree of control

Question 71

Describe positive feedback

Answer 71

1) Receptors detect change from normal 2) Effectors respond to amplify change 3) Producing greater deviation from normal

Question 72

Examples of positive feedback

Answer 72

Contractions for giving birth Blood clotting

Question 73

Structure of nephron

Answer 73

Nephron = basic structural and functional unit of kidneys With a network of blood vessels associated with each nephron

Question 74

Role of the Bowman's capsule / renal capsule

Answer 74

Ultrafiltration --> formation of glomerular filtrate

Question 75

Role of proximal convoluted tubule

Answer 75

Selective reabsorption --> reabsorption of water and glucose

Question 76

Role of Loop of Henle

Answer 76

Maintain gradient of sodium ions in the medulla

Question 77

Role of distal convoluted tubule and collecting duct

Answer 77

Reabsorption of water

Question 78

Describe ultrafiltration

Answer 78

1) High hydrostatic pressure in glomerulus --> arteriole diameter being wider than afferent arteriole diameter 2) Small substances (eg. Water, glucose, ions & urea) forced into glomerular filtrate --> Filtered by pores / capillary basement membrane / podocytes 3) Large proteins / blood cells remain in blood

Question 79

Describe selective reabsorption of glucose in PCT

Answer 79

1) Na+ actively transported out of epithelial cell to capillary 2) Na+ moves by facilitated diffusion into epithelial cells down concentration gradient --> brings glucose against its concentration gradient 3) Glucose moves into capillary by facilitated diffusion down its concentration gradient

Question 80

Describe reabsorption of water by PCT

Answer 80

1) Glucose in capillaries lowers water potential 2) Water moves by osmosis down water potential gradient

Question 81

How do features of the proximal convoluted tubule allow rapid reabsorption of glucose?

Answer 81

1) Microvilli --> provides large surface area 2) Many channel / carrier proteins --> for facilitated diffusion / co-transport 3) Many carrier proteins --> for active transport 4) Many mitochondria --> produce ATP for active transport 5) Many ribosomes --> produce carrier / channel proteins

Question 82

Why is glucose found in urine of an untreated diabetic person?

Answer 82

1) Blood glucose concentration too high --> not all glucose reabsorbed at PCT 2) As glucose carrier / co-transporter proteins are saturated

Question 83

Importance of maintaining gradient of sodium ions in medulla

Answer 83

1) So water potential decreases down medulla 2) So water potential gradient is maintained between medulla and collecting duct 3) To maximise reabsorption of water by osmosis from filtrate

Question 84

Role of the Loop of Henle in maintaining gradient of sodium ions in medulla

Answer 84

1) Ascending limb - Na+ actively transported out --> filtrate concentration decreases - Water remains as ascending limb is impermeable to water - Increasing concentration of Na+ in medulla --> lowers water potential 2) Descending limb - Water moves out by osmosis & reabsorbed by capillaries --> filtrate concentration increases - Na+ recycled / diffused back in

Question 85

Where on the loop of henle does Na+ move out?

Answer 85

Ascending limb

Question 86

Where in the loop of henle does Na+ move in?

Answer 86

Descending limb

Question 87

Why do animals, that need to conserve water, have a long loop of henle?

Answer 87

1) More Na+ moved out --> Na+ gradient maintained for longer in medulla 2) So water potential is maintained longer 3) So more water can be reabsorbed from collecting duct

Question 88

Describe reabsorption of water by distal convoluted tubule & collecting ducts

Answer 88

1) Water moves out distal convoluted tubule & collecting duct by osmosis down water potential gradient 2) Controlled by ADH which increases permeability of DCT and CD

Question 89

What is osmoregulation?

Answer 89

Control of water potential by blood

Question 90

Role of hypothalamus in osmoregulation

Answer 90

1) Contains osmoreceptors that detect increase / decrease in blood water potential 2) Produces more ADH when water potential is low 3) produces less ADH when water potential is high

Question 91

Role of posterior pituitary gland

Answer 91

Secrete ADH into blood due to signals from hypothalamus

Question 92

What does ADH stand for?

Answer 92

Antidiuretic hormone

Question 93

Role of ADH in osmoregulation

Answer 93

1) Attaches to receptors on collecting duct (& DCT) 2) Stimulating addition of channel proteins into cell surface membranes 3) So increases permeability of cells of collecting duct and DCT to water 4) So increases water reabsorption from collecting duct / DCT by osmosis 5) So decreases volume & increases concentration of urine produced

Question 95

Homeostasis in animals

Answer 95

1) Maintain stable internal environment within restricted limits 2) By physiological control systems