6. RESPONSE TO STIMULI

Question 1

What is a Stimuli?

Answer 1

a change in the internal or external environment

Question 2

Why do Organisms need to Respond to Stimuli?

Answer 2

for survival (predator/prey awareness homeostasis)

Question 3

How do Simple Organisms Respond to Stimuli?

Answer 3

Taxis and Kinesis

Question 4

What is Taxis?

Answer 4

directional response to a stimuli (towards or away from)

Question 5

What is Kinesis?

Answer 5

• non-directional movement from an unfavourable area to a favourable area
• organism moves rapidly and randomly in unfavourable area until they reach favourable area where they move slowly and less randomly
• so spends more time in favourable area, less time in unfavourable area

Question 6

Example of Response to Stimuli in Plants?

Answer 6

Tropism

Question 7

What is Tropism?

Answer 7

• directional growth in plants in response to a stimuli
• towards = positive, away = negative
• light = photo, water = hydro, gravity = geo
• shoot shows positive phototropism and negative geotropism
• root shows positive geotropism and positive hydrotropism
• controlled by a Plant Growth Factor = Indoleacetic Acid (IAA) - auxin

Question 8

What is a Plant Growth Factor?

Answer 8

• equivalent to animal hormones
• difference: made by cells throughout the plant, only affects cells locally, affects growth

Question 9

What are the affects of IAA?

Answer 9

promotes growth in the shoot, inhibits growth in the root

Question 10

How does positive phototropism in the shoot take place?

Answer 10

• normally: shoot tip produces IAA, sending it down both sides causing the shoot to grow forwards
• if light is present on one side, the IAA redistributes to the opposite side (shaded side)
• this causes the opposite side to grow faster
• so the shoot bends towards the light

Question 11

How does negative geotropism in the shoot take place?

Answer 11

• if gravity is present on one side, the IAA redistributes to the same side
• this causes the same side to grow faster
• so the shoot bends away from gravity towards the light

Question 12

How does positive geotropism/hydrotropism in the root take place?

Answer 12

• if gravity/water is present on one side, the IAA redistributes to the same side
• this causes the same side to grow slowly, so the opposite side grows faster
• so the root bends towards the gravity/water

Question 13

Evidences for Tropism (positive phototropism in shoot)?

Answer 13

• removing or covering shoot tip prevents tropism [tip causes tropism]
• placing micin (prevents movement of chemicals e.g. IAA) across shoot inhibits tropism [tropism caused by movement of chemicals]
• placing gelatine (prevents movement of electrical signals) across shoot does not affect tropism [tropism not caused by movement of electrical signals]
• if shoot tip is moved to one side, that side grows faster and the shoot bends the other way [IAA promotes growth in shoot]
• when in light or darkness the overall levels of IAA remain the same [light does not inhibit or breakdown IAA but rather redistributes it]

Question 14

Response to Stimuli in Mammals?

Answer 14

uses Nervous System, Hormonal System (nervous and hormonal systems coordinate response to stimuli)

Question 15

Job of Nervous System?

Answer 15

coordinate response to certain stimuli – response is fast, short acting, localised

Question 16

Pathway of Nervous System?

Answer 16

stimuli to receptor to sensory neurone to spinal cord to brain to spinal cord to motor neurone to effector for response

Question 17

What does a Receptor do?

Answer 17

• detects stimuli & converts stimuli energy into nerve impulse (acts as a transducer – converts one type of energy into another)
• each type of stimuli has a specific receptor
• uses stimuli energy to send Na+ ions into the start of the sensory neurone
• 2 examples of receptors: Pacinian Corpuscle, Retina of Eye

Question 18

What does a Pacinian Corpuscle do?

Answer 18

• touch receptor
• found in skin, fingers and toes
• responds to pressure/touch
• structure = corpuscle (several layers of tissue) wrapped around the start of a sensory neurone
• process = pressure applied, corpuscle compressed, stretch-mediated Na+ channels opened, Na+ ions move into the start of the sensory neurone

Question 19

How does the Retina of the Eye work?

Answer 19

• detects light so the brain can generate an image
• detected by retina (located at back of eye)
• made of Cone and Rod cells
• Cone Cells detect high light intensity only, produces colour image, with high visual acuity
• Rod Cells can detect low light intensity, produces black and white image, with low visual acuity
• Cone Cells located in centre of retina (fovea) – site of high light intensity
• Rod Cells located in periphery of retina

Question 20

Properties of Cone Cells in Retina?

Answer 20

• made of Iodopsin Pirgment which is only broken down at high light intensity
• one cone cell connects to one bipolar neurone which connects to one sensory neurone (therefore no summation of light can take place so only detects high light intensity)
• but because one cone cell connects to one bipolar neurone which connects to one sensory neurone, each stimuli can be distinguished = high visual acuity

Question 21

Properties of Rod Cells in Retina?

Answer 21

• made of Rhodopsin Pigment which can be broken down at low light intensity
• a few rod cells connect to one bipolar neurone which connects to one sensory neurone (therefore summation of light can take place so can detect low light intensity)
• but because a few rod cells connect to one bipolar neurone which connects to one sensory neurone, the stimuli will be merged together = low visual acuity

Question 22

What is the Central Nervous System (CNS)?

Answer 22

• made of brain and spinal cord
• brain = analyses and coordinates response to stimuli
• spinal cord = connects brain to sensory and motor neurones

Question 23

What is the Peripheral Nervous System (PNS)?

Answer 23

• made of the sensory and motor neurone
• a neurone transmits a nerve impulse
• sensory neurone takes nerve impulse from receptor to CNS
• motor neurone takes nerve impulse from CNS to effector
• sensory neurone has its cell body in the middle and has a dendron and axon
• motor neurone has its cell body at the start and only has a long axon

Question 24

What are the 2 different types of Motor Neurone?

Answer 24

• Voluntary (Somatic) and Involuntary (Autonomic) Motor Neurones
• Somatic supplies skeletal muscle = under conscious control
• Autonomic supplies cardiac muscle, smooth muscle, glands = under subconscious control
• Autonomic can be divided into Sympathetic and Parasympathetic (have opposite effects)

Question 25

What is a Nerve Impulse?

Answer 25

• movement of an action potential along a neurone
• action potential = change in membrane potential (charge’ in one section of the neurone
• changes from negative (polarised) to positive (depolarised) back to negative (repolarised /hyperpolarised)

Question 26

What is Resting Potential?

Answer 26

• membrane potential of neurone at rest
• is -65mV
• polarised
• caused by having more positive ions outside neurone compared to inside
• involves Na+/K+ pump, pumping 3 Na+ ions out, 2 K+ ions in
• K+ channel allowing K+ ions to diffuse out
  (K+ ions will eventually stop diffusing out due to a positive potential outside)

Question 27

What happens during an Action Potential?

Answer 27

• stimuli causes Na+ ions to enter the start of the neurone
• makes membrane potential less negative
• if it reaches threshold (-50mV), Na+ channels open
• therefore more Na+ ions diffuse into the neurone, therefore membrane potential becomes positive (depolarised)
• the membrane potential reaches +40mV
• then the Na+ channels close, the K+ channels open
• therefore K+ ions diffuse out, therefore membrane potential becomes negative (repolarised)
• too many K+ ions move out, so the membrane potential becomes more negative than normal (hyperpolarised)
• one action potential = depolarisation, repolarisation, hyperpolarisation

Question 28

How does an Action Potential move along a Neurone?

Answer 28

• by local currents
• if the stimuli energy is large enough and enough Na+ ions enter the start of the neurone, threshold will be reached and an AP will occur
  (the 1st AP is called a Generator Potential)
• Na+ ions that move in during depolarisation of the generator potential diffuse along the neurone causing the next section to reach threshold and an AP to occur
• this process continues along the neurone
  • an AP will always move along the neurone to the end
  • why does AP not move back? because previous section has just finished an AP, therefore it is in refractory period (Na+ channels cannot be opened) and is hyperpolarised (therefore threshold cannot be reached)

Question 29

How does the Size of Stimuli affect a Nerve Impulse?

Answer 29

• does not affect size of AP
  (AP is all or nothing – reach threshold = get AP [all]
  do not reach threshold = no AP [nothing])
• larger stimuli increases the frequency (number) of APs

Question 30

What affects Speed of Nerve Impulse?

Answer 30

• temperature = higher temp, higher kinetic energy, faster rate of diffusion of ions (faster nerve impulse)
• axon diameter = wider diameter, neurone less leaky (faster nerve impulse)
• myelination = schwann cells wrap around axon, insulates axon preventing AP, therefore AP only occurs in gaps – called node of ranvier, so AP jumps from node to node = saltatory conduction (faster nerve impulse)

Question 31

What is a Synapse?

Answer 31

• connection between 2 different neurones
• sends nerve impulse across the gap (synaptic cleft) using neurotransmitters (e.g. acetylcholine)
• AP arrives in end of presynaptic neurone
• Ca2+ channels open
• Ca2+ ions enter presynaptic neurone
• causes vesicles containing neurotransmitter to move to presynaptic membrane
• vesicle binds to membrane releasing neurotransmitter into cleft
• neurotransmitter diffuses across cleft
• binds to complementary receptors on postsynaptic membrane
• Na+ channels open, Na+ ions enter
• if threshold is reached, AP occurs
  (to return to rest: enzyme used to breakdown neurotransmitter, e.g. acetylcholinesterase breaksdown acetylcholine into ethanoic acid and choline, diffuses back into presynaptic neurone, ATP used to reform neurotransmitter into vesicle and actively transport Ca2+ ions out)

Question 32

What are the Properties of Synapses?

Answer 32

• unidirectionality = AP/nerve impulse travels in one direction, from pre to post, pre has the neurotransmitter, post has the receptors
• filters out low level stimuli = low level stimuli do not release enough neurotransmitter, therefore not enough Na+ ion channels open, therefore not enough Na+ ions enter postsynaptic neurone for threshold to be reached, therefore no AP produced
• summation = low level stimuli add together to release enough neurotransmitter to produce an AP in postsynaptic neurone, can be temporal or spatial
  temporal = low level stimuli present for extended period of time
  spatial = a low level stimuli from a few presynaptic neurones add together
• inhibitory = normal synapses are excitatory (cause AP), some can be inhibitory – prevent action potential from occurring by making postsynaptic neurone hyperpolarised

Question 33

What is a Reflex?

Answer 33

• a rapid involuntary response to a stimuli
• does not use the brain
• the sensory neurone connects directly to motor neurone
  (stimuli to receptor to sensory neurone to relay neurone to motor neurone to effector for response)
• ensures less damage done and does not require learning

Question 34

How is Heart Rate controlled?

Answer 34

• the heart is myogenic, its heart beat is initiated by the SAN
• the Medulla Oblongata in the brain can increase or decrease heart rate
• receives nerve impulse from chemoreceptors (respond to blood pH) in the carotid arteries and pressure receptors (respond to blood pressure) in the carotid arteries and aorta
• sends impulse in sympathetic nerves to SAN to increase HR and sends impulse in parasympathetic nerves to SAN to decrease HR

Question 35

How does Exercise affect Heart Rate?

Answer 35

• exercise = muscle contraction, which requires respiration
• therefore, waste product CO2 is released into blood
• this lower pH of blood (acidic)
• this is detected by chemoreceptors in carotid arteries
• sends impulses to medulla oblongata
• then medulla oblongata sends impulses to SAN via the sympathetic nerves causing the heart rate to increase
• benefit = increase blood flow to lungs to remove CO2 and take in O2

Question 36

How does Low Blood Pressure affect Heart Rate?

Answer 36

• if a person moves from lying/sitting to standing, blood pressure falls (reducing blood flow to the brain)
• this is detected by pressure receptors in the carotid arteries and aorta
• sends impulses to medulla oblongata
• then medulla oblongata sends impulses to SAN via the sympathetic nerves causing the heart rate to increase
• benefit = increasing heart rate leads to an increase in blood pressure (so enough blood can reach the brain)

Question 37

What are the different types of Muscles?

Answer 37

• Skeletal
• Smooth
• Cardiac

Question 38

What is the job of the Skeletal Muscle?

Answer 38

• moves the body skeleton
• when the muscle contracts (shortens) the tendon pulls on joints causing movement

Question 39

Structure of Skeletal Muscle?

Answer 39

• basic structure = sarcomeres
  made up of actin and myosin, actin is thin and has tropomysosin wrapped around it, myosin is thick and has heads, when the sarcomere contracts the whole muscle contracts, contracts/shortens by the sliding filament mechanism
• many sarcomeres = myofibril
• many myofibrils = muscle fibre surrounded by a membrane called sarcolemma contains myofibrils, fluid called sarcoplasm and tubes called sarcoplasmic reticulum
• many muscle fibres = bundle
• many bundles = whole muscle

Question 40

Locations in a Sarcomere?

Answer 40

• A band = location of myosin [no change in contraction]
• I band = location between the myosin [shortens in contraction]
• H zone = location between the actin [shortens in contraction]
• Z line = end line of sarcomere [moves closer together in contraction]

Question 41

What occurs in Sliding Filament Mechanism?

Answer 41

• how the sarcomere shortens
• the myosin heads pull the actin inwards
• the somatic motor neurone connects to the skeletal muscle via a neuro-muscular junction
  (one motor neurone connects to a few muscle fibres = motor unit
  (benefit = simultaneous muscle contraction and can control strength of contraction))
• releases acetylcholine that binds to complementary receptors on the muscle fibre membrane (sarcomere)
• Na+ channels open, Na+ ions enter the muscle fibre causing depolarisation
• wave of depolarisation travels through sarcoplasmic reticulum
• causes release of Ca2+ ions into the sarcoplasm (fluid surrounding sarcomeres/myofibril)
• this moves the tropomyosin on the actin
• exposes binding sites on the actin
• myosin heads now bind to the actin (form actin-myosin cross bridge)
• a power stroke occurs, the myosin pulling the actin inwards
• ATP attaches to myosin head so it detaches
• ATP brokendown by ATPase to release energy
• causes myosin head to go back to its original position
• so it reattaches, pulling the actin further inwards

Question 42

Role of Ca2+ ions and ATP in muscle contraction?

Answer 42

• Ca2+ ions causes the tropomyosin to move exposing binding sites on actin
• Ca2+ ions stimulate ATPase
• ATP causes myosin head to detach
• ATP releases energy so myosin head returns to original position
• ATP actively transports Ca2+ ions back into sarcoplasmic reticulum when the muscle is relaxed

Question 43

What are the 2 types of Muscle Fibres?

Answer 43

Fast Twitch and Slow Twitch

Question 44

How does Fast Twitch Muscle Fibres work?

Answer 44

• provide powerful but short lasting contractions
• found in biceps and sprinters
• adapted for anaerobic respiration
• has thicker myosin for powerful contractions
• contains more enzymes for anaerobic respiration
• contains phosphocreatine, provides phosphate to ADP to reform ATP

Question 45

How does Slow Twitch Muscle Fibres work?

Answer 45

• provide less powerful but long lasting contractions
• found in thigh muscles and marathon runners
• adapted for aerobic respiration
• has a rich blood supply
• contains many mitochondria
• contains glycogen
• contains myoglobin (stores oxygen)

Question 46

Job of the Hormonal System?

Answer 46

• coordinates the response to certain stimuli
• involves chemical messengers released by endocrine glands into the blood (exocrine glands release substance into open spaces e.g. salivary gland), travels to target cells causing changes
• protein hormones bind to complementary receptors on target cells, activates enzymes that convert ATP into Cyclic AMP in the cell, the Cyclic AMP then makes changes in the cell (=2nd messenger system) e.g. insulin
• lipid hormones enter cells by simple diffusion and cause direct changes e.g. oestrogen

Question 47

Control of Blood Glucose Levels?

Answer 47

• if high = should be in cells for respiration, also lowers blood water potential
• if low = not enough to supply cells of the brain, also increases blood water potential
• controlled by the Pancreas
• contains the Islets of Langerhans
• made of alpha and beta cells
• alpha cells produce glucagon
• beta cells produce insulin

Question 48

What happens with High Blood Glucose Levels?

Answer 48

• occurs after a meal
• insulin is released
• most cells in the body have complementary receptors (particularly muscle, liver, brain cells)
• causes increase in glucose channels and carriers
• glucose taken up and used in respiration
• in muscle and liver cells, glucose also converted into glycogen for storage (glycogenesis)
• in liver cells, glucose also converted into fat

Question 49

What happens with Low Blood Glucose Levels?

Answer 49

• occurs after starvation or exercise
• glucagon is released
• only liver cells have complementary receptors
• converts glycogen into glucose (glycogenolysis)
• converts fats and amino acids into glucose (gluconeogenesis)
• glucose is released into blood

Question 50

Diabetes?

Answer 50

• person loses control of blood glucose levels
• normally high (hyperglycaemia)
• 2 types: type 1 and type 2
• type 1 starts at young age, person does not make insulin, beta cells damaged by an autoimmune disorder (treatment = insulin injections)
• type 2 starts at middle age, person makes insulin but cells are less sensitive, caused by obesity and diet high in simple sugars (treatment = diet and exercise, drugs, insulin injection)
• symptoms = tiredness, increase urination, thirst
• diagnosis = high blood glucose levels on random testing & blood glucose levels remain high following a fasting blood glucose test (person fasts for a number of hours, then consumes a drink of glucose, should normally rise then decrease due to insulin)

Question 51

What is Homeostasis?

Answer 51

• maintenance of a constant internal environment (the blood and tissue fluid) in animals
• control body temperature, blood pH, blood glucose levels, blood water levels, blood salt levels, blood pressure

Question 52

Homeostasis and Negative Feedback?

Answer 52

the response to the change is to oppose the change to bring levels back to normal (e.g. body temperature increases – response is to bring it down to normal,blood glucose levels decrease – response is to increase it back to normal)

Question 53

What is Positive Feedback?

Answer 53

the response to the change is to continue the change (e.g. Na+ ions entering a neurone stimulating more to enter in depolarisation)

Question 54

Why do organisms need to Maintain a Constant Body Temperature?

Answer 54

maintain optimum temperature for enzyme activity

Question 55

What are Endotherms and Ectotherms?

Answer 55

• endotherms = animals that maintain a strict constant internal body temperature irrespective of external environmental temperature (e.g. mammals)
• ectotherms = animal’s internal body temperature maintained more generally and varies with changes in external environmental temperature (e.g. reptiles)

Question 56

Benefit of being an Endotherm?

Answer 56

• can maintain activity over a range of settings e.g. early morning or winter

Question 57

Benefit of being an Ectotherm?

Answer 57

• require less food/energy

Question 58

How is internal body temperature controlled?

Answer 58

• anatomical, behavioural, physiological changes
• ecotherms mainly rely on behavioural changes
• endotherms mainly rely on physiological changes

Question 59

Anatomical adaptations in organisms in warm areas?

Answer 59

• small body size = large surface area to volume ratio (lose heat)
• less fur
• less fat
• large extremities e.g. ears/hand/feet (lose heat)

Question 60

Anatomical adaptations in organisms in cold areas?

Answer 60

• large body size = small surface area to volume ratio
• more fur
• more fat
• small extremities

Question 61

Behavioural/Physiological changes in Ectotherms?

Answer 61

• warming up = expose to sun, press on warm surface, darker skin colouration to absorb heat, more respiration in liver, less breathing
• cooling down = shade from sun, press on cold surface, lighter skin colouration, less respiration in liver, more breathing

Question 62

Control of Body Temperature in Endotherms?

Answer 62

• controlled by Hypothalamus in the brain
• receives nerve impulse from peripheral thermoreceptors in the skin and central thermoreceptors in the hypothalamus
• peripheral thermoreceptors monitor changes in external environmental temperature
• central thermoreceptors monitor changes in core body temperature (blood supplying major organs)

Question 63

How an Endotherm warms itself up?

Answer 63

• reduce blood flow to the skin surface = vasoconstriction, smooth muscle in arterioles to the skin contract, lumen narrows, less blood to skin surface, less heat lost from blood by radiation
• hair on skin stands up = hair erector muscles contract, hairs stand up, traps in air particles, forms an insulating layer, reduces heat loss
• shivering = involuntary contraction of muscles – friction in sliding filament mechanism generates heat and respiration generates heat
• increase respiration in liver = generates heat

Question 64

How an Endotherm cools itself down?

Answer 64

• increase blood supply to skin surface = vasodilation, smooth muscle in arterioles to the skin relax, lumen widens, more blood to skin surface, more heat lost from blood by radiation
• sweating = evaporation of water particles from the skin surface using the heat in the blood

Question 65

Structure of Kidneys?

Answer 65

Outer region called Cortex, Middle region called Medulla

Question 66

Role of Kidneys?

Answer 66

filters blood (removes urea, excess salts, excess water – combined known as urine)

Question 67

Why remove urea?

Answer 67

toxic waste product made from excess amino acids

Question 68

Why remove excess salts and water?

Answer 68

maintain correct water potential and pressure in blood

Question 69

How do Kidneys filter?

Answer 69

made up of millions of nephron (each nephron filters the blood producing urine)

Question 70

Structure of Nephron?

Answer 70

1st part = Bowmans Capsules
2nd part = Proximal Convoluted Tubule
3rd part = Loop of Henle
4th part = Distal Convoluted Tubule
5th part = Collecting Duct

Question 71

Bowmans Capsule?

Answer 71

• start of nephron
• site of ultrafiltration (where blood is filtered)
• occurs between specialised capillaries called Glomerulus and Bowmans Capsule
• glomerulus located in the middle of an arteriole
• afferent arteriole before glomerulus is wide, efferent arteriole after glomerulus is narrow
• so build up of hydrostatic pressure in the glomerulus pushes fluid and small substances from the glomerulus into the bowmans capsule
• small substances filtered = glucose, amino acids, salts, urea
• only small substances can pass through the 3 layers
  (endothelium of glomerulus, basement membrane, podocytes of bowmans capsule)
• results in glomerular filtrate in bowmans capsule
  (water + glucose/amino acids/salts/urea)
• the job of the rest of the nephron is to send all the glucose/amino acids and some of the salts/water back into the blood [reabsorption]

Question 72

Proximal Convoluted Tubule?

Answer 72

• second part of the nephron
• site of selective reabsorption
• all the glucose/amino acids and some of the salts/water are sent back into blood (from lumen of PCT, through cells lining PCT, into blood)
  how:
• salts (sodium ions) are actively transported from cells lining the PCT into the blood
• this lowers sodium ion concentration in the cells, so sodium ions diffuse from the lumen of the PCT into the cells
• as sodium ions move, they pull in glucose and amino acids with them via co-transport
• glucose and amino acids build up in the cell, then diffuse into the blood
• the movement of salt/glucose/amino acids into the blood, lowers it’s water potential, so water follows into blood by osmosis

Question 73

Loop of Henle?

Answer 73

• third part of the nephron
• site of further water reabsorption
• occurs by hairpin countercurrent multiplier
  how:
• sodium and chloride ions are actively transported out of the ascending limb of the loop of henle into the surrounding medulla of kidney
• this lowers water potential of medulla
• so water moves out of the descending limb of loop of henle (and collecting duct) by osmosis into the medulla
• this water then moves into the blood
• the sodium and chloride ions then diffuse into the descending limb of loop of henle so the above process can be repeated

Question 74

Distal Convoluted Tubule?

Answer 74

• fourth part of nephron
• site of further salt reabsorption
• corrects required salt balance between blood and urine

Question 75

Collecting Duct?

Answer 75

• final part of nephron
• site of further water reabsorption and osmoregulation
• end up being left with urine that is sent into the ureter to the bladder
• water reabsorption occurs by the hairpin countercurrent multiplier
• amount of water being reabsorbed is controlled at this stage, this is known as osmoregulation
• osmoregulation is the process by which the hypothalamus controls water potential of the blood (an example of homeostasis)
  if water levels become low (dehydration):
• osmoreceptors in hypothalamus shrink
• this stimulates the release of ADH from the posterior part of the pituitary gland
• ADH stimulates the cells lining the collecting duct to increase the number of aqauporins (water channels)
• so more water moves from the collecting duct back into blood
• so less water is lost in the urine
  if water levels become high (overhydration):
• less ADH released
• less aquaporins in collecting duct
• less water moves from collecting duct into blood
• more water lost in urine (reduces overhydration)