Download AQA Biology: Energy transfers and changes in

WORKBOOK ANSWERS
AQA A-level Biology
Sections 5 and 6
Energy transfers in and between
organisms  Organisms respond to
changes in their internal and external
environments
This Answers document provides suggestions for some of the possible answers that might be
given for the questions asked in the workbook. They are not exhaustive and other answers
may be acceptable, but they are intended as a guide to give teachers and students feedback.
Section 5
Energy transfers in and between
organisms
Photosynthesis
1
Stage of
photosynthesis
Substances
required
Products
Location in cell
Light-dependent
stage
Light
Chlorophyll
Water
ATP
Reduced
NADP/NADPH
Oxygen
Grana/thylakoids (of
chloroplast)
Light-independent
stage
ATP
Reduced NADP
Carbon dioxide
Carbohydrate (triose
phosphate)
Stroma (of
chloroplast)
1 mark for each correct column
2 It needs ATP and reduced NADP/NADPH; produced during light-dependent stage.
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3 Splitting of water using light/photons; to provide electrons (to replace those lost by
chlorophyll).
4 (Photon of) light causes electron to be lost/‘excited’; from chlorophyll.
5 Electrons pass along series of carriers/electron transfer chain; at decreasing energy
levels/energy released; used to make ATP.
6 ATP provides the energy to convert GP to TP; reduced NADP reduces GP to TP.
7 Ribulose bisphosphate/RuBP combines with CO2; forms two molecules of GP; GP is
reduced to TP by reduced NADP; ATP provides energy.
8 Any two: e.g. amino acids/proteins/starch/cellulose/fatty acids/lipids
9a
i
Temperature, because A has faster rate and only temperature is different.
ii
Carbon dioxide concentration, because A has faster rate and only CO 2
concentration is different.
iii
Light intensity, because increasing light intensity increases rate of photosynthesis.
9b An internal factor such as availability of chlorophyll.
10 It is only worth increasing temperature if that is the limiting factor; on a dull day light
intensity is likely to be the limiting factor.
11a
i
Pen/ink would dissolve in solvent and disappear.
ii
To produce a concentrated spot; to ensure that enough of each component is
present (to produce a spot at the end).
11b
Distance travelled by spot divided by distance travelled by solvent.
Respiration
1
Stage
Location
Glycolysis
Cytoplasm
Link reaction
Matrix of mitochondrion
Krebs cycle
Matrix of mitochondrion
Oxidative phosphorylation
Cristae/inner mitochondrial membrane
2 Oxidises NADH/recycles NADH/NAD; so NAD available for glycolysis to continue (and
produce ATP).
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3
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4 Any three: e.g. protein synthesis/DNA replication/active transport/muscle contraction/to
phosphorylate glucose in glycolysis/spindle formation in mitosis/meiosis
2 marks for three uses; 1 mark for two uses
5 Substrate level phosphorylation is when ATP is formed as one intermediate is converted to
another; oxidative phosphorylation is when ATP is produced using oxygen/as electrons
pass through electron transfer chain.
Examples: for substrate level, any from glycolysis/link reaction/Krebs cycle; and for
oxidative phosphorylation, reference to electron transfer.
1 mark for two correct examples
6 Reduced NAD passes its hydrogen to a carrier in electron transfer chain; electrons pass
through series of carriers; hydrogen ions/protons pass into intermembrane space; final
electron acceptor is oxygen; ATP made.
Maximum 4 marks
7 Reference to leaving apparatus to equilibrate/closing syringe to outside.
Note position of liquid on scale.
Note distance moved in specified time.
Find mass of organism(s).
Note distance liquid has moved in tube to find volume of oxygen, using radius of
tube/using πr 2l.
Divide volume by mass of organism in g and by time in minutes.
Maximum 6 marks
8a Any two: oxygen/pyruvate/phosphate/ADP/reduced NAD
8b Any of: carbon dioxide/ATP/NAD
9
Event or description
Glycolysis
Stage A
✓
Stage B
Takes place inside the
mitochondrion
✓
Acetyl coenzyme A is involved
✓
ATP is synthesised from ADP
Reduced NAD is re-oxidised
✓
✓
✓
1 mark for each correct row
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Exam-style questions
1a
Statement
Glycolysis
Krebs
cycle
Light-dependent
reaction of
photosynthesis
NAD is reduced
✓
✓
NADP is reduced
✗
✗
✓
ATP is produced
✓
✓
✓
ATP is required
✓
✗
✗
✗
1b
i
Pyruvate or acetyl coenzyme A
ii
Oxygen required as final electron acceptor; to produce ATP from ADP; ADP
required to produce ATP.
i
Because each molecule of RuBP joins with a carbon dioxide molecule to form two
molecules of GP.
ii
There is not enough carbon dioxide so not all the RuBP can combine with carbon
dioxide to form GP.
i
So that it is possible to trace what happens to it; products containing 14C will be
radioactive.
ii
No light-dependent stage to produce ATP and NADPH; therefore GP not
reduced/converted to TP.
iii
Used in respiration.
2a
2b
Energy and ecosystems
1 May not land on chloroplast/chlorophyll/photosynthetic part of plant; not all wavelengths
used; may be transmitted through leaf; may be converted to heat energy.
2a The amount of chemical energy stored as biomass, which primary producers create in a
given length of time.
2b Energy per mass per time, e.g. kJ g–1 y–1
3 Amount of chemical energy stored as biomass in primary consumers in a given time after
respiratory losses are taken into account.
4 GPP = NPP + respiration or NPP = GPP – respiration
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5a Energy per area per time, e.g. kJ ha–1 y–1.
5b
20810
´100 = 1.2%
1700000
2 marks for correct answer, 1 mark for correct method with arithmetic error
5c Respiration
5d There is not enough energy left (in tertiary consumers) to support another trophic level.
5e
1478
´100 = 16.7%
18833
2 marks for correct answer, but allow 1 mark for correct method with arithmetic error
6 N = I – (F + R) where N = net production of animal, I = chemical energy store in ingested
food, F = energy lost in faeces and R = energy lost in respiration
7 N = 250 – (157 + 83) = 10 kJ
8 (Reduce respiratory losses by) restricting animal movement; keeping animals indoors;
(reduce energy lost in faeces by) feeding food that is more digestible.
9 The mass of organisms (usually dry mass) in a given area or volume, e.g. kg ha–1
10 Place several 0.25 m2 quadrats at random on the field; remove all grass from each area
individually, including roots; place in oven at 100°C; dry to constant mass; find mean dry
mass per 0.25 m2; find total area of field and use figures to calculate biomass in field.
Maximum 5 marks
11 Use a calorimeter; burn known dry mass of grass; find temperature rise of known volume
of water; calculate energy released from grass; based on 1 J heats 1 cm3 water by 1°C.
Maximum 4 marks
Nutrient cycles
1 Nitrogen — two examples, such as amino acids/proteins/DNA/RNA/urea
Phosphorus — two examples, such as DNA/RNA/ATP/ADP/AMP/GP/TP/RuBP
2 Organisms A = decomposers
Process B = death/excretion
Process C = eating/ingestion/digestion
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3
Microorganisms
Role
Nitrogen-fixing bacteria
Convert nitrogen to ammonia/ammonium/nitrate
Decomposers/ammonifying
bacteria
Release ammonia/ammonium from dead organic matter
Denitrifying bacteria
Convert nitrates/ammonium to nitrogen gas/oxides of
nitrogen
Nitrifying bacteria
Convert ammonium to nitrite then nitrate
4 Saprobiotic organisms secrete (extracellular) enzymes that hydrolyse/digest dead organic
matter; absorb products of digestion/use products of digestion in respiration.
5 Nutrients in soil enter grass and are taken in by cattle; lost in milk/calves that are produced
and sold.
6 Nutrients/fertiliser dissolves (in soil water); washed out of soil/into waterways.
7 Natural, advantages (maximum 2 marks):

improves soil texture/water holding ability/adds humus

contains trace nutrients

nutrients released slowly/less likely to leach out

cheaper only if qualified, e.g. farmer has to get rid of it anyway
Natural, disadvantages (maximum 2 marks):

unknown/variable composition

heavy to transport
Artificial, advantages (maximum 2 marks):

known composition/can choose a variety most suitable for crops

less bulky/lighter to transport
Artificial, disadvantages (maximum 2 marks):

more soluble/more prone to leaching

lacks trace nutrients

does not improve soil quality
8 When crops are harvested/animals moved elsewhere.
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9 Nutrients/nitrates/phosphates leach into river and cause growth of plants/algae on surface;
blocks light from plants lower down; plants can’t photosynthesise and so die; bacteria
digest dead plant material; use up oxygen in respiration; lack of oxygen, so fish/other
organisms die.
Maximum 5 marks
10 Fungi that live in close association/symbiosis with plant roots; increase water uptake;
increase ion uptake.
11a Fresh mass is variable/varied water content; dry mass allows comparison.
11b Mycorrhizal plants grow (about three times) taller than non-mycorrhizal; mycorrhizal
plants have approximately double the mass; roots increase in mass rather more than
tops/ratio of roots to tops a little higher in mycorrhizal.
11c Mycorrhizae increase water uptake, increasing its availability for growth; increase ion
uptake, stimulating growth.
Exam-style questions
1a A = ammonifying
B = nitrifying
C = denitrifying
D = nitrogen fixing
2 marks for four correctly identified, 1 mark for three correct, no marks for two or one correctly
identified
1b
i
Any energy per area per time, e.g. kJ ha–1 y–1
Allow 1 mark if two units correct
ii
4.24 ´104
´100 = 1.41%
3 ´106
2 marks for correct answer, but give 1 mark for correct method with arithmetic error
iii
Some does not land on chlorophyll/photosynthetic part of plant; some is transmitted
through leaf; some converted to heat; some is the wrong wavelength.
Any two points for 2 marks
1c
i
420
´100 = 0.99%
4.24 ´104
Allow 1 mark for correct method with arithmetic error
ii
Lost in heat/respiration; not all the phytoplankton/zooplankton digestible by next
trophic level; migration out of food chain; some lost as waste products/passes to
decomposers.
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Any three points for 3 marks
2a Adds nitrates/nitrogen; adds phosphate; adds humus/organic matter.
Allow ‘adds nutrients’ for 1 mark if no named nutrients given
2b
i
Leaching/dissolves in soil water and drains out; taken up by plants; denitrifying
bacteria.
Any two points for 2 marks
ii
Takes up water so less water drains out of field (carrying nitrates); roots bind soil so
less likely to wash away; plant cover reduces impact of rainfall; plants take up
nitrates from soil.
Maximum 2 marks
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Section 6
Organisms respond to changes in
their internal and external
environments
Stimuli, both internal and external, are
detected and lead to a response
1 Growth (movement) of part of a plant; directional response to directional stimulus.
2 (Root grows) towards gravity; away from light.
3a Intact coleoptile grows towards light; (cut off apex does not grow to light), therefore apex
detects stimulus or produces chemicals causing growth; (shielded apex doesn’t grow to
light) confirms that the apex detects stimulus.
3b
i
The diagram shows that mica on shaded side prevents passage of signal/chemical,
therefore no bending; mica on light side doesn’t impede signal/chemical on shaded
side, so bending occurs.
ii
Must be chemical not electrical signal; as signal passes through gelatin, and
electrical signal would not; bending normal as chemical produced by apex can
diffuse through gelatin.
4 Curvature to the right; more chemical/IAA on left side; chemical/IAA causes cell
elongation/growth.
5a Root grows towards gravity, shoot grows away from gravity.
5b In both root and shoot, IAA accumulates on side towards gravity; in shoot higher auxin
concentration stimulates growth; in root lower auxin concentration stimulates cell
elongation; root grows downwards and shoot grows upwards.
6 As cell takes in water it becomes turgid; applies pressure against cell wall; if fibres are
more loosely bonded, the cell wall expands.
7a Movement of an organism towards/away from/at an angle to a directional stimulus.
7b Rate of random movement/turning is proportional to intensity of stimulus.
8 Phototaxis
9a Kinesis
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9b Ensures it stays in dark conditions/out of light, where unlikely to be seen by predator.
10 Phototaxis; Euglena all moved towards the light.
11a
1 mark for neurones in right place
1 mark for correct labels
11b Very fast; so prevents damage.
12
Stimulus
High temperature
Receptor
Temperature/pain receptor in skin
Coordinator
Spinal cord
Effector
Arm/hand muscle
Response
Removal of hand
13 Membrane deformed; opens stretch-mediated sodium channels; sodium ions move in;
depolarises membrane.
14a
i
Cones have individual connections to optic nerve; can detect exactly where stimulus
came from.
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ii
Spatial summation; several rods all stimulated a little ‘add up’ to trigger an action
potential.
14b Arrow pointing vertically upwards.
15
Rods
Cones
Contain rhodopsin
Contain three kinds of iodopsin, each
sensitive to red, green or blue light
Low visual acuity
High visual acuity
Detect black and white only
Can detect colour
Sensitive to low light intensities
Only detect high light intensities
Most numerous cells in retina, mainly
around periphery
Mainly at fovea
1 mark for each correct row
16 Heart generates its own impulses/does not need stimulation by nerves to contract.
17a A = sinoatrial node; B = atrioventricular node; C = bundle of His
17b SAN initiates impulses; spread across atria, causing atrial systole; slight time delay at
AVN to allow atria to empty completely; impulses pass very quickly down bundle of His;
allows ventricles to contract from bottom up.
17c (Fibrous tissue) prevents impulses passing directly to ventricles; ensures impulses only
pass via AVN.
18a In aorta; carotid artery; medulla of brain.
18b Detect fall in pH; due to increased CO2 concentration; send impulses to cardiac centre in
medulla.
19a Increased respiration; increases carbon dioxide concentration; fall in pH detected by
chemoreceptors; in carotid artery/aorta/medulla; (at least two locations) send impulses to
cardiac centre in medulla; more impulses along sympathetic nerve to SAN.
19b pH rises as CO2 concentration falls; detected by chemoreceptors; impulses to cardiac
centre in medulla; more impulses via parasympathetic nerve to SAN.
20 Baroreceptors send impulses to cardiac centre in medulla; increases heart rate; more
impulses along sympathetic nerve to SAN; impulses to muscle in wall of arterioles,
causing vasoconstriction.
21 Greater volume of blood returning to right side of heart stretches ventricle; contracts more
forcefully, increasing stroke volume; at rest, required volume of blood can be pumped
round body with fewer beats, as stroke volume is higher.
22 CO = HR × SV; SV = CO/HR = 5600/80 = 70 cm3
2 marks for right answer, 1 mark for correct method but arithmetic error
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23 CO = HR × SV = 70 × 72 = 5040 cm3 = 5.04 dm3
2 marks for correct answer in dm3, 1 mark for correct answer in cm3 or right method but
arithmetic error
Nervous coordination
1a
Structure
Letter
Myelin sheath
G
Cytoplasm
E
Axon
C
Dendrite
A
Node of Ranvier
H
Synaptic knobs
D
Nucleus
B
Cell body
F
All right = 3 marks, lose 1 mark for each wrong answer.
1b Arrow going from dendrites to synaptic knobs;
2 Sodium/potassium pump; exchanges three sodium ions out for two potassium ions in; idea
that more potassium ions leak out than sodium ions leak in; more positive ions outside cell
than inside.
3 Depolarisation; sodium channels open; sodium ions move into axon down electrochemical
gradient; potassium channels open; potassium ions move out down electrochemical
gradient; causing repolarisation; hyperpolarisation; sodium/potassium pump restores
resting potential.
Maximum 7 marks (reference to electrochemical gradient needs mentioning once only)
4 Period when axon cannot conduct another impulse.
5 Action potentials are all the same size; if stimulus is above threshold then an action
potential occurs.
6 Ensures that action potentials travel in one direction only; means there is always a gap
between impulses/impulses cannot merge together.
7 Ion exchange only at nodes; saltatory conduction/sodium ions ‘jump’ from node to node.
8a The wider the diameter, the faster impulses pass; more efficient ion exchange as lower
surface area:volume ratio (in wider axons).
8b Higher temperature gives faster impulses; ions have more kinetic energy.
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9 Sodium channels open; sodium ions enter axon; threshold reached; potassium channels
open; potassium ions leave axon.
Maximum 4 marks
10 C G E A F H B D
11 The number/concentration of sodium ions that must enter/degree of depolarisation of the
neurone necessary to set up an action potential.
12a If several action potentials arrive in a very short time at the synapse; their effects may
‘add up’ to cause an action potential in the postsynaptic neurone.
12b One presynaptic neurone is not enough to overcome threshold value of postsynaptic
neurone; but several neurones stimulating the synapse at the same time can ‘add up’ to
cause an action potential.
13 Makes postsynaptic neurone less likely to carry an action potential; makes postsynaptic
neurone even more negative inside (compared with outside).
14 Synapse can be inhibitory, neuromuscular junction (nmj) always excitatory; synapses can
use various neurotransmitters, nmj always acetylcholine; synapse is junction between two
neurones, nmj is junction between a neurone and a muscle.
Maximum 2 marks
15 (By fitting into receptors) stops acetylcholine binding; sodium ion channels do not open; no
depolarisation; no calcium ions released into muscle/from T-tubules.
Maximum 3 marks
16 Postsynaptic neurone does not release transmitter substance; synaptic knob of
presynaptic neurone does not have sodium channels.
Exam-style questions
1a
i
Several rods connect with one sensory neurone; spatial summation.
ii
Each cone has individual connection to optic nerve; can pinpoint where stimulus
was located.
1b Rod cells mainly located round periphery of retina/not in fovea; low light intensity from dim
star insufficient to stimulate cones; looking straight at an object causes image to fall on
fovea/mostly cones.
Maximum 2 marks
2a Nodes of Ranvier larger; sodium ions only depolarise a short section of membrane; more
ion exchange needed; (at wider synapse) acetylcholine has to diffuse further; greater
delay before action potential occurs in postsynaptic neurone.
Maximum 4 marks
2b Sodium channels open; sodium ions enter neurone; potassium channels open; potassium
ions leave neurone.
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2c
i
By the frequency of action potentials.
ii
The greater the stimulus, the greater the frequency of action potentials.
2d Several action potentials in quick succession can ‘add up’ to cause action potential in
postsynaptic neurone.
Skeletal muscles are stimulated to
contract by nerves and act as effectors
1 Muscles can only contract and relax, not push; one muscle needed to bend a joint,
another to straighten it.
2a A band stays the same. I band gets narrower. H zone gets narrower.
All three right for 1 mark
2b 1 is through H zone (myosin filaments only); 2 is through I band (actin filaments only);
3 is through the part of the A band where both actin and myosin filaments are present.
3 ACEDHBFG
4 Produces ATP very quickly; by adding phosphate to ADP; allows muscle contraction to
occur for a little longer.
Maximum 2 marks
5 Slow: B C F G J K L N; fast: A D E H I M O P
6a Slow fibres
6b Fast fibres
Exam-style questions
1a
i
Darkest band has both actin and myosin, I band has actin only and H zone has
myosin only; myosin thicker than actin, therefore darker.
ii
H zone and I band narrower; A band stays the same.
1b 0.5 µm = 500 nm, 500/40 = 12.5
To move 40 nm requires 1 ATP, therefore to move 0.5 µm needs 13 ATP.
2 marks for correct answer, 1 mark for correct working with arithmetic error
1c Respiration no longer occurs, therefore no ATP produced; calcium ions no longer
pumped/actively transported out of muscle fibre; therefore bind to tropomyosin, allowing
muscle contraction.
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Homeostasis is the maintenance of a
stable internal environment
1 Stimulus = low pH/high CO2 concentration
Receptor = chemoreceptors
Coordinator = (cardiac centre in) medulla
Effector = SAN
Response = heart rate increases
2 marks for all correct, 1 mark for 1 error, 0 for 2 or more errors
2a (Positive feedback) because more sucking leads to more milk production; changes further
from norm.
2b As baby grows/needs more milk; milk production increases.
3
Term
Meaning
Glycogenesis
Glycogen is formed from glucose
Glycogenolysis
Glycogen is hydrolysed to release glucose
Gluconeogenesis
Glucose is formed from lipids and proteins
4 Fits into specific receptors; only found in liver and muscle cells.
5 Fits into specific receptors and stimulates enzymes that convert glucose to glycogen;
activates enzymes in adipose tissue that convert glucose to (fatty acids and glycerol that
can be stored as) fat; stimulates number of channel/carrier proteins (in liver and muscle
cells) that take in glucose by facilitated diffusion.
6 Attaches to receptor proteins in cell membrane of liver and muscle cells; activating
enzymes that convert glycogen back into glucose; activating enzymes that convert
proteins/fats into glucose.
7 Causes conversion of glycogen to glucose; binds to receptor proteins in liver and muscle
cells; activates enzymes.
8 Binds to specific protein in cell membrane; activates adenyl cyclase/enzyme in cell
membrane; removes two phosphate groups from ATP, producing cyclic AMP; activates
protein kinase; converts glucose to glucose phosphate.
Maximum 4 marks
9 Type 1 diabetics do not produce enough insulin, whereas type 2 diabetics usually do
produce insulin but their cells fail to respond to it.
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10 Use exercise to reduce blood glucose concentration; stimulates use of glucose in
respiration; eat diet low in sugar/high in starch (and fibre); glucose released slowly; small,
frequent meals.
Maximum 4 marks
11a So blood glucose concentration not affected by food/drink recently consumed.
11b Insulin released; causes glucose to be stored as glycogen.
11c Blood glucose concentration continues to rise after meal, and only decreases slowly;
produces insufficient insulin.
11d Curve rises very high and decrease is very slow; insulin not released/body does not
respond to insulin.
12a 10 cm3 sucrose solution and 10 cm3 water
12b 16 cm3 sucrose and 4 cm3 water
13 Make series of glucose solutions of known concentrations; add same volume of Benedict’s
to same volume of solution and heat; measure transmission/absorbance of each solution;
plot graph of concentration vs. transmission/absorbance; repeat with unknown solution
and find transmission/absorbance; read concentration off calibration curve.
Maximum 5 marks
14 Efferent vessel narrower than afferent; pumping of left ventricle of heart.
15 Water — by osmosis; water potential in proximal convoluted tubule (pct) lower than in
nephron.
Glucose — by active transport/cotransport; specific carriers in cells of pct.
16 Microvilli increase surface area — for absorption; many mitochondria for ATP — for active
transport.
17 Person with diabetes has high concentration of glucose in filtrate; not enough carriers in
pct to absorb all glucose.
18 In thin ascending limb sodium chloride diffuses out of filtrate (into medulla); in thick
ascending limb sodium chloride pumped out into medulla; water leaves filtrate in
descending limb by osmosis; filtrate very concentrated at base of loop of Henlé; ascending
limb impermeable to water.
19 ADH increases permeability of distal convoluted tubule and collecting duct of kidney; by
stimulating aquaporins to enter cell membranes; allows more water to pass through
membrane by osmosis; since water potential in medulla lower than in filtrate.
20 When blood water potential is low, osmoreceptors detect this; stimulate ADH to be
released by pituitary gland; causes more water to be reabsorbed, so water potential of
blood rises; therefore less ADH secreted.
21 Keeps blood water potential approximately the same; so cells don’t burst/shrink by
osmosis; does not cause large changes in blood volume/blood pressure.
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Exam-style questions
1a When blood glucose concentration rises, insulin released; causes reduction in blood
glucose concentration, therefore less insulin released.
1b
i
Blood glucose concentration rises steeply then falls very slowly/remains high for a
long time; little/no insulin secreted.
ii
Used up in respiration.
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