Organisms Respond To Internal And External Environments

Question 1

What is a stimulus?

Answer 1

change in an organisms internal or external environment.

Question 2

Why is it important that organisms can respond to stimuli?

Answer 2

Organisms increase their chance of survival by responding to stimuli

Question 3

What is a tropism (+ve and -ve)

Answer 3

Growth of a plant in response to a directional stimulus
Positive tropism = towards a stimulus;
negative tropism = away from stimulus

Question 4

Summarise the role of growth factors in flowering plants

Answer 4

● Specific growth factors (hormone-like growth substances) eg. Auxins (such as IAA) move
(via phloem or diffusion) from growing regions eg. shoot / root tips where they’re produced
● To other tissues where they regulate growth in response to directional stimuli (tropisms)

Question 5

Describe how indoleacetic acid (IAA) affects cells in roots and shoots

Answer 5

● In shoots, high concentrations of IAA stimulates cell elongation
● In roots, high concentrations of IAA inhibits cell elongation

Question 6

Explain gravitropism in flowering plants
*hint - are 5 steps

Answer 6

1. Cells in tip of shoot / root produce IAA
2. IAA diffuses down shoot / root (evenly initially)
3. IAA moves to lower side of shoot / root (so concentration increases)
4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
5. So shoots bend away from gravity whereas roots bend towards gravity

Question 7

Explain phototropism in flowering plants

Answer 7

1. Cells in tip of shoot / root produce IAA
2. IAA diffuses down shoot / root (evenly initially)
3. IAA moves to shaded side of shoot / root (so conc. ↑)
4. In shoots this stimulates cell elongation whereas in
   roots this inhibits cell elongation
5. So shoots bend towards light
   whereas roots bend away from ligh

Question 8

List the 2 simple responses that can maintain a mobile organism in a
favourable environment

Answer 8

Taxes
Kinesis

Question 9

Describe taxes

Answer 9

Taxes (tactic response)
○ Directional response
○ Movement towards or away from a directional stimulus

Question 10

Describe kinesis

Answer 10

Kinesis (kinetic responses)
○ Non-directional response
○ Speed of movement or rate of direction change
changes in response to a non-directional stimulus
○ Depending on intensity of stimulus

Question 11

Example of taxis and kinesis in woodlice in response to light

Answer 11

Examples: taxis- woodlice moving away from light to avoid predators;
kinesis- woodlice moving faster in drier
environments to increase their chance of moving to an area with higher humidity to prevent drying out.

Question 12

Explain the protective effect of a simple (eg. 3 neurone) reflex

Answer 12

● Rapid as only 3 neurones and few synapses (synaptic transmission is slow)
● Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt
● Protects from harmful stimuli eg. escape predators / prevents damage to body tissues

Question 13

Describe the basic structure of a Pacinian corpuscle

Answer 13

Lamellae (layers of connective tissue)
Stretch mediated sodium ion channel (closed)
Sensory neurone ending
Sensory neurone axon
Myelin sheath (Schwann cells)

Question 14

Describe how a generator potential is established in a Pacinian corpuscle

Answer 14

1. Mechanical stimulus eg. pressure deforms lamellae and stretch- mediated sodium ion
   (Na+) channels
2. So Na+ channels in membrane open and Na+ diffuse into sensory neurone
   ○ Greater pressure causes more Na channels to open and more Na+ to enter
3. This causes depolarisation, leading to a generator potential
   ○ If generator potential reaches threshold
   it triggers an action potential

Question 15

Explain what the Pacinian corpuscle illustrates

Answer 15

● Receptors respond only to specific stimuli
○ Pacinian corpuscle only responds to mechanical pressure
● Stimulation of a receptor leads to the establishment of a generator potential
○ When threshold is reached, action potential sent (all-or-nothing principle)

Question 16

Explain the differences in sensitivity to light for rods & cones in the retina

Answer 16

Rods are more sensitive to light
● Several rods connected to a single neurone
● Spatial summation to reach / overcome threshold (as enough
neurotransmitter released) to generate an action potential

Cones are less sensitive to light
● Each cone connected to a
single neurone
● No spatial summation
Explain

Question 17

Explain the differences in visual acuity for rods & cones in the retina

Answer 17

Rods give lower visual acuity
● Several rods connected to a single neurone
● So several rods send a single set of impulses
to brain (so can’t distinguish between
separate sources of light)

Cones give higher visual acuity
● Each cone connected to a single neurone
● Cones send separate (sets of) impulses to brain
(so can distinguish between 2 separate sources
of light)

Question 18

Explain the differences in sensitivity to colour for rods & cones in the retina

Answer 18

Rods allow
monochromatic vision
● 1 type of rod / 1
pigment

Cones allow colour vision
● 3 types of cones - red-
, green- and blue-sensitive
● With different optical pigments → absorb different wavelengths
● Stimulating different combinations of cones gives range of colour perception

Question 19

Cardiac muscle is myogenic. What does this mean?
It

Answer 19

It can contract and relax without receiving electrical impulses from nerves.

Question 20

Structures in the heart

Answer 20

Sinoatrial node (SAN)
Atrioventricular node (AVN)
Bundle of his
Purkyne tissue

Question 21

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

Answer 21

1. Sinoatrial node (SAN) acts as pacemaker → sends regular waves of electrical activity across atria
   ○ Causing atria to contract simultaneously
   2 . Non-conducting tissue between atria / ventricles prevents impulse passing directly to ventricles
   ○ Preventing immediate contraction of ventricles
2. Waves of electrical activity reach atrioventricular node (AVN) which delays impulse
   ○ Allowing atria to fully contract and empty before ventricles contract
3. AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissue
   ○ Causing ventricles to contract simultaneously from the base up

Question 22

Where are chemoreceptors and pressure receptors located?

Answer 22

Chemoreceptors and pressure receptors are located in the aorta and carotid arteries.

Question 23

Describe the roles of chemoreceptors, pressure (baro) receptors, the autonomic
nervous system and effectors in controlling heart rate

Answer 23

1. Baroreceptors detect [fall / rise] in blood pressure and / or chemoreceptors detector [fall/ rise] in blood pH
2. Send impulses to medulla
3. Which send more frequent impulses to SAN along [sympathetic / parasympathetic] neurones
4. So [more / less] frequent impulses sent from SAN and to / from AVN
5. So cardiac muscle contracts [more / less] frequently
6. So heart rate [increases / decreases]

Question 24

List the structures in a myelinated motor neurone

Answer 24

Dendrite
Cell body/soma
Axon
Myelin sheath made of Schwann cells
Node of ranvier
Axon terminal

Question 25

Describe resting potential

Answer 25

Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside). -70mv

Question 26

Explain how a resting potential is established across the axon membrane in a neurone

Answer 26

● Na +/K+ pump actively transports: ○ (3) Na+ out of axon AND (2) K+ into axon ● Creating an electrochemical gradient: ○ Higher K+ conc. inside AND higher Na+ conc. outside ● Differential membrane permeability: ○ More permeable to K+ → move out by facilitated diffusion ○ Less permeable to Na+ (closed channels)

Question 27

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

Answer 27

1. Stimulus ● Na+ channels open; membrane permeability to Na+ increases ● Na+ diffuse into axon down electrochemical gradient (causing depolarisation) 2. Depolarisation ● If threshold potential reached, an action potential is generated ● As more voltage-gated Na+ channels open (positive feedback effect) ● So more Na + diffuse in rapidly 3. Repolarisation ● Voltage-gated Na + channels close ● Voltage-gated K+ channels open; K+ diffuse out of axon 4. Hyperpolarisation ● K+ channels slow to close so there’s a slight overshoot – too many K+ diffuse out 5. Resting potential ● Restored by Na +/K+ pump

Question 28

Describe the all-or-nothing principle

Answer 28

● For an action potential to be produced, depolarisation must exceed threshold potential ● Action potentials produced are always same magnitude at same potential ○ Bigger stimuli instead increase frequency of action potentials

Question 29

Explain how the passage of an action potential along non-myelinated axons results in nerve impulses

Answer 29

● Action potential passes as a wave of depolarisation ● Influx of Na + in one region increases permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open so adjoining region depolarises

Question 30

Explain how the passage of an action potential along myelinated axons results in nerve impulses

Answer 30

● Myelination provides electrical insulation ● Depolarisation of axon at nodes of Ranvier only ● Resulting in saltatory conduction (local currents circuits) ● So there is no need for depolarisation along whole length of axon

Question 31

Suggest how damage to the myelin sheath can lead to slow responses and / or jerky movement

Answer 31

● Less saltatory conduction; depolarisation occurs along whole length of axon ○ So nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction ● depolarisation may leak to other neurones ○ Causing wrong muscle fibres to contract

Question 32

Describe the nature of the refractory period

Answer 32

● Time taken to restore axon to resting potential when no further action potential can be generated ● As Na+ channels are closed

Question 33

Explain the importance of the refractory period ●

Answer 33

● Ensures discrete impulses are produced (action potentials don’t overlap) ● Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus) ○ Higher intensity stimulus causes higher frequency of action potentials ○ But only up to certain intensity ● Also ensures action potentials travel in one direction– can’t be propagated in a refractory region In the second half of the refractory period an action potential can be produced but requires greater stimulation to reach threshold.

Question 34

List the factors that affect speed of conductance

Answer 34

Myelination Axon diameter Temperature

Question 35

Describe how myelination affects the speed of conductance

Answer 35

● Depolarisation at Nodes of Ranvier only → saltatory conduction ● Impulse doesn’t depolarise whole length of axon

Question 36

Describe how axon diameter affects the speed of conductance

Answer 36

● Bigger diameter means less resistance to flow of ions in cytoplasm

Question 37

Describe how temperature affects the speed of conductance

Answer 37

● Increases rate of diffusion of Na+ and K+ as more kinetic energy ● But proteins / enzymes could denature at a certain temperature

Question 38

Describe the structure of a synapse

Answer 38

Pre synaptic neurone Axon Vesicle containing neurotransmitter Voltage gated calcium ion Channel Axon terminal Synaptic cleft Receptor and sodium ion Channel

Question 39

What are cholinergic synapses

Answer 39

Synapses that use the neurotransmitter acetylcholine (ACh).

Question 40

Describe transmission across a chlolingergic synapse

Answer 40

1. Depolarisation of the pre synaptic membrane causes opening of voltage gated ca2+ channels - Ca2+ diffuse into pre synaptic neurone/knob 2. Causing vesicles containing ACH to move and fuse with pre synaptic membrane - releasing ACh into synaptic cleft (by exocytosis) 3. ACh diffuses across the synaptic cleft to bind to receptors on post synaptic membrane 4. Causing Na+ channels to open - Na+ diffuse into post synaptic knob causing depolarisation - if threshold is met an action potential is generated

Question 41

Explain what happens to acetylcholine after synaptic transmission

Answer 41

● It is hydrolysed by acetylcholinesterase ● Products are reabsorbed by the presynaptic neurone ● To stop overstimulation- if not removed it would keep binding to receptors, causing depolarisation

Question 42

Explain how synapses result in unidirectional nerve impulses

Answer 42

● Neurotransmitter only made in / released from pre-synaptic neurone ● Receptors only on post-synaptic membrane

Question 43

Explain summation by synapses

Answer 43

● Addition of a number of impulses converging on a single post-synaptic neurone ● Causing rapid buildup of neurotransmitter (NT) ● So threshold more likely to be reached to generate an action potential Importance- low frequency action potentials release insufficient neurotransmitter to exceed threshold.

Question 44

Describe spatial summation

Answer 44

● Many pre-synaptic neurones share one synaptic cleft / post-synaptic neurone ● Collectively release sufficient NT to reach threshold to trigger an action potential

Question 45

Describe temporal summation

Answer 45

● One pre-synaptic neurone releases neurotransmitter many times over a short time ● Sufficient NT to reach threshold to trigger an action potential

Question 46

Describe inhibition by inhibitory synapses

Answer 46

● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as: ○ Cl- channels open → Cl- diffuse in ○ K+ channels open → K+ diffuse out ● More Na+ required for depolarisation ● Reduces likelihood of threshold being met / action potential formation at post-synaptic membranes Importance- both excitatory and inhibitory neurones forming synapses with the same post-synaptic membrane gives control of whether it ‘fires’ an action potential.

Question 47

Describe the structure of a neuromuscular junction

Answer 47

Very similar to a synapse except: ● Receptors are on muscle fibre instead of postsynaptic membrane and there are more ● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter

Question 48

Compare transmission across cholinergic synapses and neuromuscular junctions

Answer 48

- In both, transmission is unidirectional. In a cholergernic synapse transmission occurs from neurone to neuron (or effectors or glands) In neuromuscular junction transmission occurs from motor neurone to muscle - in cholinergic synapse neurotransmitters can be excitatory or inhibitory, in neuromuscular junction its always excitaotrory In cholenergic synapse Action potential may be initiated in postsynaptic neurone, in neuromuscular junction Action potential propagates along sarcolemma down T tubules

Question 49

Use examples to explain the effect of drugs on a synapse

Answer 49

● Some drugs stimulate the nervous system, leading to more action potentials, eg.: ○ Similar shape to neurotransmitter ○ Stimulate release of more neurotransmitter ○ Inhibit enzyme that breaks down neurotransmitter → Na + continues to enter ● Some drugs inhibit the nervous system, leading to fewer action potentials, eg.: ○ Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels ○ Block receptors by mimicking shape of neurotransmitter

Question 50

Describe how muscles work

Answer 50

● Work in antagonistic pairs → pull in opposite directions eg. biceps / triceps ○ One muscle contracts (agonist), pulling on bone / producing force ○ One muscle relaxes (antagonist) ● Skeleton is incompressible so muscle can transmit force to bone

Question 51

Advantage of muscles working in antagonistic pairs

Answer 51

the second muscle required to reverse movement caused by the first (muscles can only pull) and contraction of both muscles helps maintain posture.

Question 52

Describe the gross and microscopic structure of skeletal muscle

Answer 52

● Made of many bundles of muscle fibres (cells) packaged together ● Attached to bones by tendons ● Muscle fibres contain: ○ Sarcolemma (cell membrane) which folds inwards (invagination) to form transverse (T) tubules ○ Sarcoplasm (cytoplasm) ○ Multiple nuclei ○ Many myofibrils ○ Sarcoplasmic reticulum (endoplasmic reticulum) ○ Many mitochondria

Question 53

Describe the structure of a mmyofibril

Answer 53

● Made of two types of long protein filaments, arranged in parallel ○ Myosin- thick filament ○ Actin- thin filament ● Arranged in functional units called sarcomeres ○ Ends – Z-line / disc ○ Middle – M-line ○ H zone– contains only myosin Explain

Question 54

Explain the banding pattern to be seen in myofibrils

Answer 54

● I-bands- light bands containing only thin actin filaments ● A-bands- dark bands containing thick myosin filaments (and some actin filaments) ○ H zone contains only myosin ○ Darkest region contains overlapping actin and myosin

Question 55

Give an overview of muscle contraction

Answer 55

● Myosin heads slide actin along myosin causing the sarcomere to contract ● Simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract ● When sarcomeres contract (shorten)... ○ H zones get shorter ○ I band get shorter ○ A band stays the same ○ Z lines get closer

Question 56

Describe the role of actin myosin calcium ions tropmyosin and atp in muscle contraction

Answer 56

1 Depolarisation spreads down sarcolemma via T tubules causing Ca2+ release from sarcoplasmic reticulum, which diffuse to myofibrils 2 Calcium ions bind to tropomyosin, causing it to move → exposing binding sites on actin 3 Allowing myosin head, with ADP attached, to bind to binding sites on actin → forming an actinomyosin crossbridge 4 Myosin heads change angle, pulling actin along myosin, (ADP released), using energy from ATP hydrolysis 5 New ATP binds to myosin head causing it to detach from binding site 6 Hydrolysis of ATP by ATP(hydrol)ase (activated by Ca2+) releases energy for myosin heads to return to original position 7 Myosin reattaches to a different binding site further along actin Process is repeated as long as calcium ion conc. is high

Question 57

During muscle relaxation:

Answer 57

1. Ca2+ actively transported back into the endoplasmic reticulum using energy from ATP 2. Tropomyosin moves back to block myosin binding site on actin again → no actinomyosin cross bridges

Question 58

Describe the role of phosphocreatine in muscle contraction

Answer 58

● A source of inorganic phosphate (Pi)→ rapidly phosphorylates ADP to regenerate ATP ○ ADP + phosphocreatine → ATP + creatine ● Runs out after a few seconds → used in short bursts of vigorous exercise ● Anaerobic and alactic

Question 59

General properties of slow vs fast switch

Answer 59

Slow twitch is ● Specialised for slow, sustained contractions (eg. posture, long distance running) ● Obtain ATP mostly from aerobic respiration → release energy slowly ● Fatigues slowly Fast twitch is ● Specialised for brief, intensive contractions (eg. sprinting) ● Obtain ATP mostly from anaerobic respiration → release energy quickly ● Fatigues quickly due to high lactate conc.

Question 60

Compare the, location of slow and fast skeletal muscle fibres

Answer 60

Slow twitch are ● High proportion in muscles used for posture eg. back, calves ● Legs of long distance runners Fast twitch are ● High proportion in muscles used for fast movement eg. biceps, eyelids ● Legs of sprinters

Question 61

Compare the structure of slow and fast skeletal muscle fibres _

Answer 61

For slow twitch ● High conc. of myoglobin → stores oxygen for aerobic respiration ● Many mitochondria → high rate of aerobic respiration ● Many capillaries → supply high conc. of oxygen / glucose for aerobic respiration and to prevent build-up of lactic acid causing muscle fatigue For fast twitch ● Low levels of myoglobin ● Lots of glycogen → hydrolysed to provide glucose for glycolysis / anaerobic respiration which is inefficient so large quantities of glucose required ● High conc. of enzymes involved in anaerobic respiration (in cytoplasm) ● Store phosphocreatine