Unit 6 Response To Changes

Question 1

Give one advantage of taxis to organisms

Answer 1

to avoid competition, to find a mate, increase dispersal, to avoid predators

Question 2

Suggest two advantages of simple reflexes.

Answer 2

1. Rapid;
2. Protect against damage to body tissues;
3. Do not have to be learnt;
4. Help escape from predators;
5. Enable homeostatic control.

Question 3

Use your knowledge of indoleacetic acid (IAA) to explain the curved growth of a shoot

Answer 3

1. Tip produces IAA;
   Accept source/release for produces but ignore contains/stores IAA.
2. IAA diffuses (into shoot);
   Accept auxin for IAA.
   Accept IAA diffuses down.
3. (More) elongation of cells on one side (than other);

Question 4

Explain how the uneven distribution of IAA causes a root to bend.

Answer 4

1. (IAA) at bottom of root/where AII concentration high inhibits
   expansion/elongation (of cells);
2. (IAA) at the top of the root/where IAA concentration low leads to
   expansion/elongation (of cells)

Question 5

Describe the role of receptors and of the nervous system in the increasing of heart rate.

Answer 5

1. Chemoreceptors detect rise in CO2 / H+ / acidity / carbonic acid / fall in pH
   OR
   Baro / pressure receptors detect rise in blood pressure;
2. Send impulses to cardiac centre / medulla;
3. More impulses to SAN;
4. By sympathetic (nervous system for chemoreceptors / CO2)
   OR
   By parasympathetic (nervous system for baro / pressure receptors / blood
   pressure);

Question 6

When the heart beats, both ventricles contract at the same time.
Explain how this is coordinated in the heart after initiation of the heartbeat by the SAN.

Answer 6

1. Electrical activity only through Bundle of His / AVN;
2. Wave of electrical activity passes over / through both ventricles at the same time;
   For ‘electrical activity’ accept impulses / depolarisation / action potential
   Reject messages/signals/information once only

Question 7

Describe how a heartbeat is initiated and coordinated.

Answer 7

1. SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
   Accept excitation
2. Non-conducting tissue prevents immediate contraction of ventricles /
   prevents impulses reaching the ventricles;
3. AVN delays (impulse) whilst blood leaves atria / ventricles fill;
4. (AVN) sends wave of electrical activity / impulses down Bundle of His;
5. Allow Purkyne fibres / tissue
6. Causing ventricles to contract from base up;

Question 8

The heart controls and coordinates the regular contraction of the atria and ventricles.
Describe how.

Answer 8

1. SAN → AVN → bundle of His / Purkyne fibres;
2. Mark for correct sequence
3. Impulses / electrical activity (over atria);
4. Atria contract;
5. Non-conducting tissue (between atria and ventricles);
6. Delay (at AVN) ensures atria empty / ventricles fill before ventricles
   contract;
7. Ventricles contract from apex upwards;

Question 9

The cardiac cycle is controlled by the sinoatrial node (SAN) and the
atrioventricular node (AVN).
Describe how.

Answer 9

1. SAN initiates heartbeat / acts as a pacemaker / myogenic;
   Q Must be in context
2. (SAN) sends wave of electrical activity / impulses (across atria) causing
   atrial contraction;
   Reject: signals / electronic / messages / nerve impulses once only3. AVN delays (electrical activity / impulses);
   Neutral: reference to non-conducting tissue delaying impulses instead of the AVN
3. (Allowing) atria to empty before ventricles contract / ventricles to fill
   before they contract;
4. (AVN) sends wave of electrical activity / impulses down Bundle of His /
   Purkyne fibres;
5. (Causing) ventricles to contract (from base up) / ventricular systole;

Question 10

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

Answer 10

1. Higher concentration of potassium ions inside and higher
   concentration of sodium ions outside (the neurone) OR potassium ions diffuse out
2. (Membrane) more permeable to potassium ions (leaving than sodium ions entering)
   OR
   (Membrane) less permeable to sodium ions (entering than potassium
   ions leaving);
   Accept for ‘less permeable to sodium ions’ is ‘impermeable to sodium
   ions’ or ‘sodium gates/channels are closed’ (alternative explanation).
3. Sodium ions (actively) transported out and potassium ions in;
   reference to ions or Na+ and K+ is required. If mentioned once allow for
   all mark points.
   If an answer provides two or three of these mark points without any
   reference to ions – award one maximum mark.
   Accept 3 Na+ out and 2 K+ in but reject if numbers used are incorrect.

Question 11

Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon.

Answer 11

1. Myelination provides (electrical) insulation;
   Reject thermal insulation.
   Accept description of (electrical) insulation.
2. (In myelinated) saltatory (conduction)
   OR
   (In myelinated) depolarisation at nodes (of Ranvier);
3. In non-myelinated depolarisation occurs along whole/length (of axon);

Question 12

Describe the sequence of events involved in transmission across a cholinergic synapse.
Do not include details on the breakdown of acetylcholine in your answer.

Answer 12

1. Depolarisation of presynaptic membrane;
   Accept action potential for depolarisation.
2. Calcium channels open and calcium ions enter (synaptic knob);
   Accept Ca2+
3. (Calcium ions cause) synaptic vesicles move to/fuse with
   presynaptic membrane and release acetylcholine/neurotransmitter;
   Accept abbreviations for acetylcholine as term is in the question.
4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
   Accept abbreviations for acetylcholine as term is in the question.
5. (Acetylcholine attaches) to receptors on the postsynaptic membrane;
6. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Question 13

Describe how the influx negatively charged ions can inhibit postsynaptic
neurones.

Answer 13

1. (Inside of postsynaptic) neurone becomes more negative/hyperpolarisation/inhibitory postsynaptic potential;
   Ignore K+
   Accept -75mV or any value below this as equivalent to more negative
   Accept ‘decrease in charge’
2. More sodium ions required (to reach threshold)
   OR
   Not enough sodium ions enter (to reach threshold);
   Accept Na+ for sodium ions
3. For depolarisation/action potential;
   Context must covey idea that depolarisation / action potential is less
   likely

Question 14

Explain how blocking the calcium ion channels at some synapses can reduce impulses at the post synaptic membrane.

Answer 14

1. No/fewer calcium ions enter synaptic knob OR No/less calcium enter synaptic knob via calcium ion channels;
2. No/fewer synaptic vesicles move to/fuse with presynaptic membrane and no/less glutamate is released;
3. No/less neurotransmitter diffuses across (synaptic cleft);
4. No/less (neurotransmitter attaches) to receptors on the postsynaptic membrane;
5. No/fewer sodium ions enter (postsynaptic neurone) so no/ fewer impulses (sent to brain);

Question 15

Suggest and explain how the interaction between the circular and radial muscles could cause the pupil to constrict (narrow)

Answer 15

1. Circular muscle contracts;
2. Radial muscle relaxes;

Question 16

Explain how a fovea allows an organism to see in detail

Answer 16

1. High (visual) acuity;
2. (Each) cone is connected to a single neurone;
   Accept no retinal convergence.
   Accept ‘bipolar/nerve cell’ for neurone.
3. (Cones send) separate (sets of) impulses to brain;

Question 17

Explain how a high density of rod cells allows an organism to have good night vision.

Answer 17

1. High (visual) sensitivity;
   Accept retinal convergence.
2. Several rods connected to a single neurone;
   Accept ‘bipolar/nerve cell’ for neurone
   Accept 2, ‘many’ or
3. Enough (neuro)transmitter to reach/overcome threshold
   OR
   Spatial summation to reach/overcome threshold; more for ‘several

Question 18

Describe how stimulation of a Pacinian corpuscle produces a generator potential

Answer 18

1. (There are) stretch-mediated sodium ion channels (in the membrane);
2. (Increased pressure) deforms/changes (sensory neurone/axon)
   membrane/lamella(e) OR (Increased pressure) deforms/changes sodium ion channels;
3. Sodium ion channels open;
4. Sodium ions diffuse in;
5. Depolarisation (leading to generator potential);

Question 19

Describe the roles of ATP in muscle contraction.

Answer 19

1. To break actinomyosin (bridges);
2. To move/bend the myosin head/arm;
3. (So) actin (filaments) are moved (inwards);
4. For active transport of calcium ions (into the sarcoplasmic/endoplasmic reticulum);

Question 20

Explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contraction.

Answer 20

1. (Less/No) tropomyosin moved from binding site
   OR
   Shape of tropomyosin not changed so binding site not exposed/available;
   Ignore troponin.
   Reject active site only once.
2. (Fewer/No) actinomyosin bridges formed;
   Accept actin and myosin do not bind.
   Reject active site only once.
3. Myosin head does not move
   OR
   Myosin does not pull actin (filaments)
   OR
   (Less/No) ATP (hydrol)ase (activation);
   Reject ATP synthase.
   Do not penalise reference to calcium rather than calcium ions.
   Credit all mark points even if context relates to what happens when calcium ions are present.

Question 21

Describe the role of glucagon in gluconeogenesis.

Answer 21

1. (Attaches to receptors on target cells and) activates/stimulates
   enzymes;
   Reject ‘produces enzymes’.
2. Glycerol/amino acids/fatty acids into glucose;
   Reject ‘glucagon converts’ as context suggests enzyme action.
   Reject occurs in pancreas.

Question 22

Explain how inhibiting adenylate cyclase may help to lower the blood glucose concentration.

Answer 22

1. Less/no ATP is converted to cyclic AMP/cAMP;
2. Less/no kinase is activated;
3. Less/no glycogen is converted to glucose
   OR
   Less/no glycogenolysis;
   If no indication of less/no for any of the mark points award max 2 marks

Question 23

Using your knowledge of the kidney, explain why glucose is found in the urine of a person with untreated diabetes.

Answer 23

1. High concentration of glucose in blood/filtrate;
   Accept tubule for filtrate.
2. Not all the glucose is (re)absorbed at the proximal convoluted tubule;
   Reject no glucose is (re)absorbed.
3. Carrier/co-transport proteins are working at maximum rate
   OR
   Carrier/co-transport proteins/ are saturated;

Question 24

Explain why a pancreas transplant would not be a suitable treatment for type 2 diabetes.

Answer 24

1. Type II produces insulin
2. Cells/receptors are less sensitive/responsive (to insulin)
   OR Faulty insulin receptors;
3. (treated/controlled by) diet/exercise

Question 25

Adrenaline binds to receptors in the plasma membranes of liver cells. Explain how this causes the blood glucose concentration to increase.

Answer 25

1. Adenylate cyclase activated / cAMP produced / second messenger
   produced;
2. Activates enzyme(s) (in cell so) glycogenolysis / gluconeogenesis occurs
   / glycogenesis inhibited;

Question 26

Describe and explain how three features of the cells in the proximal convoluted tubule allow the rapid reabsorption of glucose into the blood.

Answer 26

1. Microvilli provide a large surface area OR Folded (cell-surface) membrane provides a large surface area;
2. Many channel/carrier proteins for facilitated diffusion;
3. Many carrier proteins for active transport;
4. Many channel/carrier proteins for co-transport;
5. Many mitochondria produce ATP OR Many mitochondria for active transport;
6. Many ribosomes to produce carrier/channel proteins;

Question 27

Describe the effect of ADH on the collecting ducts in kidneys.

Answer 27

1. (Stimulates) addition of channel proteins into membrane;
   Accept aquaporins for channel proteins.
   Accept movement for addition.
   Accept (stimulates) opening of channel proteins in membrane.
2. Increases permeability to water
   OR
   (More) water (re)absorbed;
   Accept for reabsorbed ‘enters blood’ or ‘leaves collecting duct’.
3. By osmosis;

Question 28

Describe how ultrafiltration occurs in a glomerulus.

Answer 28

1. High blood/hydrostatic pressure;
   Ignore references to afferent and efferent arterioles
   Ignore ‘increasing/higher blood pressure’ as does not necessarily mean high
2. Two named small substances pass out eg water, glucose, ions, urea;
   Accept correct named ions
   Accept mineral ions/minerals
   Accept amino acids/small proteins
   Ignore references to molecules not filtered
3. (Through small) gaps/pores/fenestrations in (capillary) endothelium;
   Accept epithelium for endothelium
4. (And) through (capillary) basement membrane;

Question 29

How does the inhibition of the absorption of sodium and chloride ions from the filtrate cause an increase in the volume of urine produced.

Answer 29

1. Water potential of filtrate/tubule decreased;
   Accept correct reference to water potential gradient
   Accept maintains low water potential.
   Accept nephron for filtrate/tubule.
2. Less water (reabsorbed) by osmosis (from filtrate/tubule);
   Accept nephron for filtrate/tubule.
   Accept no water (reabsorbed) for ‘less’
   Accept (more) water (absorbed) by osmosis (into filtrate/tubule)
3. Collecting duct (is where osmosis occurs);
   Accept proximal convoluted tubule or distal convoluted tubule or
   (descending) loop of Henle
   Ignore PCT, DCT