MOCK 1

Question 1

Energy in living organisms needed for:

Answer 1

 Anabolic reactions:
 Protein synthesis / DNA replication / glycogenesis / polymerisation
 Cellular work:
 Active transport / movement of chromosomes / sliding filaments /
movement of vesicles
 Movement
 Maintenance of body temperature in endotherms

Question 2

Glucose is stable due to

Answer 2

its activation energy – lowered by enzymes and raising the energy
level of glucose by phosphorylation

Question 3

Answer 3

Question 4

Features of ATP that make it suitable as the universal energy currency:

Answer 4

 Loss of phosphate / hydrolysis, leads to energy release
 Small packets of energy
 Small / water-soluble, so can move around cell
 Immediate energy donor
 Acts as link between energy-yielding and energy-requiring reactions
 High turnover

Question 5

Excess energy during transfer and reactions are converted into

Answer 5

thermal energy

Question 6

Answer 6

Question 7

glycolysis happens in

Answer 7

cytoplasm

Question 8

Glycolysis

Answer 8

 Glucose phosphorylated by ATP
 Raises energy level / overcomes activation energy to form fructose bisphosphate
 Lysis / splitting of glucose / hexose
 Breaks down to two TP (triose phosphate)
 6C (hexose bisphosphate) into 2 3C (triose phosphate) which is then
dehydrogenated; hydrogen transferred to NAD
 2 reduced NAD formed from each TP
 4 ATP produced; final net gain of 2 ATP
 Pyruvate produced

Question 9

link reaction happens in

Answer 9

mitochondrial matrix:

Question 10

link reaction

Answer 10

 Pyruvate passes by active transport from the cytoplasm through the outer and
inner membranes of a mitochondrion
 Undergoes decarboxylation, dehydrogenation (hydrogen transferred to NAD) and
combined with coenzyme A (CoA) to give acetyl coenzyme A

Question 11

role of CoA in link reaction

Answer 11

 Combines with acetyl group in the link reaction
 Delivers acetyl group to the Krebs cycle
 Acetyl group combines with oxaloacetate

Question 12

krebs cycle happens in

Answer 12

mitochondrial matrix

Question 13

krebs cycle

Answer 13

 Reactions are catalysed by enzymes
 Acetyl CoA combines with a four-carbon compound (oxaloacetate) to form a six
carbon compound (citrate)
 Citrate is decarboxylated and dehydrogenated – through intermediate compounds – to yield CO2 (waste gas) and hydrogens are accepted by hydrogen carriers (NAD
and FAD) to form reduced NAD and reduced FAD
 Oxaloacetate is regenerated to combine with another acetyl CoA
 Two CO2 are produced
 One FAD and three NAD molecules are reduced
 One ATP molecule is generated (substrate-level phosphorylation)

Question 14

oxidative phosphorylation happens in

Answer 14

inner mitochondrial membrane

Question 15

Oxidative phosphorylation

Answer 15

 Reduced NAD / FAD are passed to the electron transport chain (ETC) on the inner
membrane of the mitochondria (cristae)
 Hydrogen released from reduced NAD / FAD and splits into electron and proton
 Electrons are passed along the electron carriers on the ETC
 Energy released from the electrons, pumps protons into the intermembrane space
 Proton gradient is set up
 Protons diffuse back through the membrane – through ATP synthase – down the
potential gradient
 Oxygen acts as the final electron acceptor; acts as proton acceptor to form water;
allows ETC to continue and ATP to be produced

Question 16

Answer 16

Question 17

Answer 17

Question 18

Answer 18

Question 19

Answer 19

NAD

Question 20

Comparison between the structures of ATP and NAD:

Answer 20

 Both have ribose sugars
 ATP has 1 ribose, while NAD has 2
 Both have adenine base
 NAD has nicotinamide base
 ATP has three phosphates

Question 21

Function of NAD in the cytoplasm of a cell:

Answer 21

 Acts as a hydrogen carrier
 Acts as a coenzyme / enables dehydrogenases to work
 Used in glycolysis / anaerobic respirations

Question 22

 Anaerobic respiration:

Answer 22

Alcoholic fermentation (conversion of glucose to ethanol):
 Lactic fermentation (conversion of glucose to lactate):

Question 23

Alcoholic fermentation (conversion of glucose to ethanol):

Answer 23

 In various microorganisms (e.g. yeast) and in some plant tissues
 The hydrogen from reduced NAD is passed to ethanol (CH3CHO); releasing the NAD
and allows glycolysis to continue
 Pyruvate is decarboxylated into ethanal, which gets reduced to ethanol (C2H5OH) by
the enzyme ethanol dehydrogenase
 Irreversible reaction
 NAD regenerated, hence glycolysis can continue

Question 24

 Lactic fermentation (conversion of glucose to lactate):

Answer 24

 In mammalian muscles when deprived of oxygen
 Pyruvate and reduced NAD formed by glycolysis
 Pyruvate is decarboxylated by pyruvate decarboxylase into ethanal – which acts as
a hydrogen acceptor from reduced NAD
 NAD regenerated, hence glycolysis can continue
 Reversible reaction

Question 25

Answer 25

Question 26

Answer 26

Question 27

The structure of the mitochondrion is related to its function:

Answer 27

 Double membrane
 Inner membrane is folded / cristae has a large surface area; has ATP synthase /
stalked particles; has carrier proteins / cytochromes for the site of ETC /
chemiosmosis
 Mitochondrial matrix contains enzymes; is the site of link reaction and the Krebs
cycle
 Outer membrane has protein carriers for pyruvate and reduced NAD
 Intermembrane space has low pH due to high concentration of protons from ETC,
creating a proton gradient between intermembrane space and matrix, resulting to
the synthesis of ATP

Question 28

Answer 28

 The post-exercise uptake of extra oxygen is called the oxygen debt

Question 29

 Respiratory substrates:
 Most energy liberated in aerobic respiration comes from the

Answer 29

oxidation of hydrogen to
water, hence the greater the number of hydrogens in the structure the greater the energy
value

Question 30

The energy value of a substrate us determined by

Answer 30

burning a known mass of the substance
in oxygen in a calorimeter, where the energy liberated can be determined from the rise in
temperature of a known mass of water in the calorimeter

Question 31

Answer 31

Question 32

Lipids have a higher energy value than carbohydrates due to the

Answer 32

higher number of C-H
bonds, hence yields more reduced NAD, so produces more ATP per gram, thus more
aerobic respiration / oxidative phosphorylation / chemiosmosis; fats can only be broken
down aerobically

Question 33

 Respiratory quotient:
 Shows the

Answer 33

substrate used in respiration and whether or not anaerobic respiration is
occurring

Question 34

RQ=

Answer 34

Question 35

For the aerobic respiration of glucose:

Answer 35

Question 36

For the aerobic respiration of fatty acid oleic acid:

Answer 36

Question 37

Answer 37

Question 38

For anaerobic respiration (e.g. alcoholic fermentation):

Answer 38

Question 39

Or yields a high value (> 1.0), as some of the

Answer 39

respiration might still be aerobic
 No RQ can be calculated for muscle cells using lactate pathway, as no CO2 is
produced:

Question 40

Answer 40

Question 41

he respiration of glucose in anaerobic conditions

Answer 41

oxygen is not available as a final
electron acceptor, hence oxidative phosphorylation on the ETC, where most ATP are
produced – produces less ATP than in aerobic conditions as only glycolysis (substrate-linked
phosphorylation) occurs (only produces a net gain of 2 ATP); pyruvate converted to lactate
which is energy rich

Question 42

Oxygen debt is needed to

Answer 42

Convert lactate to pyruvate in the liver cells, re-oxygenate
haemoglobin, and to meet demands of continued increased in metabolic rate

Question 43

Adaptations of rice to grow with its roots submerged in water in terms of tolerance to
ethanol from respiration in anaerobic conditions and the presence of aerenchyma:

Answer 43

 Aerenchyma in stem and roots which help oxygen to, move / diffuse, to the roots ;
 Shallow roots
 Air film trapped on underwater leaves
 Has fast internode growth
 Modified growth regulated by gibberellin
 Anaerobic respiration underwater
 Tolerance to high ethanol concentration
 Ethanol dehydrogenase switched on in anaerobic conditions
 Carbohydrates conserved

Question 44

Respirometers

Answer 44

 To measure rate of oxygen consumption during respiration
 CO2 produced absorbed by concentrated solution of KOH / NaOH
 Oxygen consumption in unit time read through the level of the manometer fluid
against the scale, decrease or increase
 Changes in temperature and pressure alter the volume of air in the apparatus,
hence needed to be kept constant (e.g. electronic water bath & a control tube –
with equal volume of inert material to the volume of the organisms to compensate
for changes in pressure)
 Can be used to measure the RQ of an organism done by noting down the found
oxygen consumptions; set up with the same controls, but with no CO2 absorbing
chemicals

Question 45

DCPIP / methylene blue – investigating the rate of respiration using a redox dye

Answer 45

 Turns from blue to colourless; the rate of change from blue to colourless is a
measure of the rate of respiration; can be used to investigate effects of
temperature / different substrate concentrations

Question 46

Photosynthesis is the

Answer 46

the fixation of CO2 and its subsequent reduction to carbohydrate, using
hydrogen from water, taking place in the chloroplast; where two reactions are involved:
light dependent reactions and light independent reactions

Question 47

Answer 47

Question 48

Answer 48

Question 49

The photosynthetic pigments involved fall into two categories:

Answer 49

primary pigments
(chlorophylls) and accessory pigments (carotenoids) of which are arranged in light
harvesting clusters called photosystems (I and II), where several hundred accessory
pigment molecules surround a primary pigment molecule to pass absorbed light energy
towards the primary pigment – the reaction centre

Question 50

Answer 50

Question 51

light dependent reactions happens in

Answer 51

thylakoids (holds ATP synthase):

Question 52

Light energy is necessary for the synthesis of

Answer 52

ATP in photophosphorylation and the
splitting of water (photolysis – photosystem II) into hydrogen ions (combine with a
carrier molecule NADP to make reduced NADP) and oxygen – waste product

Question 53

Photophosphorylation of ADP to ATP are of two types:

Answer 53

cyclic and non-cyclic

Question 54

Cyclic phosphorylatio

Answer 54

 Only involves photosystem I
 Light absorbed by photosystem I passed to the primary pigment resulting to
the excitation of an electron for which is emitted from the chlorophyll
molecule (photoactivation) and captured by an electron acceptor to be
passed onto the electron transport chain
 Protons from photolysis pumped into the membrane space to synthesise
ATP from ADP and an inorganic phosphate group (Pi) by the process of
chemiosmosis for which goes to the light independent stage (Calvin cycle) to
produce complex organic molecules

Question 55

Non-cyclic phosphorylation

Answer 55

 Involves both the photosystems
 Light is absorbed by both photosystems resulting to excited electrons
emitted from the primary pigments of both reaction centres, which are then
absorbed by electron acceptors and pass along the ETC
 The primary pigment of photosystem I absorbs electrons from photosystem
II for which receives replacement electrons from the splitting of water
(photolysis)
 Protons from photolysis pumped into the membrane space to synthesise
ATP from ADP and an inorganic phosphate group (Pi) by the process of
chemiosmosis for which goes the light independent stage (Calvin cycle) to
produce complex organic molecules

Question 56

Light independent reaction (Calvin cycle) happens in

Answer 56

stroma

Question 57

Light independent reaction (Calvin cycle) – stroma :

Answer 57

 Carbon dioxide reaches the inside of a palisade mesophyll cell from the external
atmosphere through the stomata by diffusion down a concentration gradient, and
passes through air spaces; dissolves in film of water on cell surface then diffuses
through cell wall / surface membrane of palisade cells
 Using a series of enzyme-controlled reactions:
 Fixation of carbon dioxide by combination with RuBP, a 5C compound,
(carboxylation) – using Rubisco enzyme – to form an unstable 6C
intermediate, resulting to 2 molecules of GP, a 3C compound; using ATP and
reduced NADP from the light dependent reaction reduces GP to TP for
which most of it regenerates to form RuBP while others undergo
rearrangement of carbons to form pentose sugars / lipids / amino acids /
hexose sugars; ATP is required for the phosphorylation of ribulose
phosphate into ribulose bisphosphate

Question 58

Answer 58

Question 59

Answer 59

Question 60

Role of accessory pigment in photosynthesis:

Answer 60

passes energy to primary pigment; absorb
light wavelengths that primary pigment does not; forms part of the light-harvesting cluster
of pigments (photosystem)

Question 61

Chlorophyll absorb mainly in the

Answer 61

red and blue-violet regions of the line spectrum and
reflects green light; whereas carotenoids absorb mainly in the blue-violet region of the
spectrum

Question 62

An absorption spectrum

Answer 62

is a graph that shows the, absorbance / absorption, of different
wavelengths of light by chloroplast pigments

Question 63

An action spectrum

Answer 63

is a graph of the rate of photosynthesis at different wavelengths of
light, showing the effectiveness of different wavelengths related to their absorption and
energy content

Question 64

Answer 64

Question 65

Answer 65

action spectrum

Question 66

nvestigation to determine the effect of light intensity or light wavelength on the rate of
photosynthesis using a

Answer 66

redox indicator (e.g. DCPIP) and a suspension of chloroplasts (the
Hill reaction):

Question 67

The main external factors that affects the rate of photosynthesis:

Answer 67

light intensity and
wavelength, temperature, and carbon dioxide concentration

Question 68

The rate of any process which depends on a series of reactions is limited by

Answer 68

the slowest
reaction in the series

Question 69

Answer 69

Question 70

Answer 70

Question 71

Chromatography – to

Answer 71

separate and identify chloroplast pigments and carry out an
investigation to compare the chloroplast pigments in different plants (reference should be
made to Rf values in identification):

Question 72

chromatography process

Answer 72

 Usage of a chromatogram
 Place spot of pigments on pencil mark at base of the paper
 Dry and repeat to concentrate spot
 Dip the paper / chromatogram in the solvent (ethanol) to travel up the paper
 Measure distance travelled by solvent (front) and pigment (spot)
 Calculate the Rf value = distance travelled by pigment spot / distance travelled by
solvent front
 Look up / compare results with known Rf values to identify pigments

Question 73

Answer 73

Question 74

Chloroplasts can move within

Answer 74

palisade cells to maximise the amount of light absorption of
light and to avoid damage by high light intensities

Question 75

In the light independent stage of photosynthesis, carbon dioxide combines with RuBP

Answer 75

to
form a six-carbon compound, which immediately splits to form two three-carbon
molecules (GP), these plants are called C3 plants; However, maize and sorghum plants –
and most other tropical grasses – do something different, as their first compound that isproduced in the light independent reaction contains four carbon atoms, therefore are
called C4 plants

Question 76

Rubisco catalyses

Answer 76

he reaction of both carbon dioxide and RuBP; oxygen with RuBP
(photorespiration), causing less photosynthesis to take place as less RuBP available to
combine with carbon dioxide; occurs readily in high temperatures and light intensity

Question 77

C4 plants keep

Answer 77

RuBP and Rubisco away from high oxygen concentrations in the bundle
sheath cells around the vascular bundles

Question 78

Carbon dioxide is absorbed by the

Answer 78

tightly mesophyll cells (so O2 cannot reach the bundle
sheath cells), which contains PEP carboxylase enzyme (has high optimum temperature and
does not accept O2) catalysing the combination of CO2 with a 3C compound, PEP, resulting
to oxaloactetate, 5C, which is converted into malate and passed to the bundle sheath cells – hence maintaining a high concentration of carbon dioxide – for which CO2 is fixated with
RuBP in the light independent reaction; photorespiration is avoided

Question 79

Answer 79

Question 80

Answer 80

Question 81

The high surface area of the

Answer 81

thylakoid membrane and the large size and number of grana
results to the high absorption of light, hence high photophosphorylation and more
chemiosmosis; the large thylakoid space helps increase the proton gradient hence more
ATP and reduced NADP produced, thus high rate of light independent reaction

Question 82

Homeostasis

Answer 82

to maintain a constant/stable internal environment in the body

Question 83

The importance of homeostasis in a mammal:

Answer 83

 To maintain a constant internal environment of blood and tissue fluids within
narrow limits / set point, effects:
 Low temperature, consequence: slowed metabolism / enzymes less active
 High temperature, consequence: enzymes denatured
 Low water potential, consequence: water leaving cells / cells shrink
 High water potential, consequence: water enters cells / cells burst
 Low blood glucose, consequence: effect on respiration
 High blood glucose, consequence: water leaving cells / cells shrink
 Control of pH, consequence: enzymes become less active

Question 84

Control mechanisms use a negative feedback loop involving:

Answer 84

 Receptor (sensor) detects changes in both internal and external stimuli (any
change in a physiological factor being regulated) away from the set-point; nerve
impulse sent to a central control or hormone released, which then reaches the
effectors (muscles and glands) / target organs; effector performs corrective action,
hence factor returns to set-point

Question 85

Continuous monitoring of the factor by receptors produces a

Answer 85

steady stream of information
to the control centre that makes continuous adjustments to the output, hence the factor
fluctuates around a particular set point

Question 86

Negative feedback:

Answer 86

the mechanism to keep changes in the factor within narrow limits, by
increasing or decreasing accordingly during a change in the factor

Question 87

Two coordination systems in mammals:

Answer 87

 Nervous system, by electrical impulses transmitted along neurones
 Endocrine system, by hormones (chemical messengers) travel in the blood

Question 88

Answer 88

Question 89

Thermoregulation

Answer 89

is the control of body temperature involving both coordination systems,
controlled by the hypothalamus – receives constant input of sensory information about temperature of the blood (by the themorecepter cells monitoring the core temperature)
and the surroundings (skin receptors)

Question 90

If there is a decrease in temperature,

Answer 90

ypothalamus sends impulses that activate several
physiological responses which decreases the loss of heat from the body and increases heat
production:

Question 91

physiological responses which decreases the loss of heat from the body and increases heat
production:

Answer 91

 Vasoconstriction – contraction of the muscles in the walls of the arterioles in skin
surface, narrowing the lumens, reducing the supply of blood, hence less heat lost
from the blood
 Shivering – involuntary contraction of the skeletal muscles generate heat, absorbed
by the blood
 Raising body hairs – contraction of the muscles attached to the hairs, increasing the
depth of fur and the layer of insulation, trapping air close to the skin
 Decrease in sweat production – reduces heat loss by evaporation from skin surface
 Increase secretion of adrenaline – increases the rate of heat production in the liver

Question 92

A decrease in temperature gradually (e.g. winter),

Answer 92

the hypothalamus releases a hormone
which activates the anterior pituitary gland to release thyroid stimulating hormone (TSH)

Question 93

what does TSH do

Answer 93

 TSH stimulates the thyroid gland to secrete thyroxine hormone into the blood,
increases the metabolic rate, increases the heat production
 When temperature starts to increase again, the hypothalamus responds by
reducing the release of TSH by the anterior pituitary gland, hence less thyroxine
released from the thyroid gland

Question 94

If there is a increase in temperature

Answer 94

hypothalamus increases the loss of heat from the
body and reduces heat production:

Question 95

hypothalamus increases the loss of heat from the
body and reduces heat production:

Answer 95

 Vasodilation – relaxation of the arterioles in skin, hence it widens, more blood flows
to the capillaries, heat energy lost
 Increasing sweat production – sweat glands increase production of sweat which
evaporates on the surface of the skin, removing heat from the body
 Lowering body hairs – relaxation of the muscles attached to the hairs, hence they
lie flat, reducing the depth of fur and layer of insulation

Question 96

Excretion:

Answer 96

the removal of unwanted products (e.g. ammonia – toxic) of metabolism

Question 97

 Urea is produced in the

Answer 97

liver from excess amino acids, transported to the kidney in
solution in the blood plasma through diffusion from liver cells, which will then be
removed from the blood, dissolved in water and excreted as urine

Question 98

The formation of urea from excess amino acids by liver cells:

Answer 98

 Deamination / removal of amine group and ammonia (NH3) formed, which is then
combined with carbon dioxide forming the urea cycle
 Ammonia is a soluble and toxic compound, hence needed to be converted into urea
(main nitrogenous excretory product) – less soluble and less toxic

Question 99

Structure of kidney:

Answer 99

 Each kidney receives blood from a renal artery; return blood via a renal vein
 Narrow tube – ureter – carries urine from kidney to bladder
 Urethra – single tube – carries urine to the outside of the body

Question 100

Answer 100

Question 101

A longitudinal section through a kidney (Fig 14.6) shows its main areas

Answer 101

 Capsule covering the whole kidney
 Cortex lying beneath the capsule
 Medulla – central area of kidney
 Pelvis – where ureter joins

Question 102

Answer 102

Question 103

A kidney is made up of thousands of

Answer 103

tiny tubes called nephrons and many blood
vessels

Question 104

nephron structure

Answer 104

One end of the nephron forms a cup-shaped structure called Bowman’s capsule,
surrounding a tight network of capillaries called a glomerulus (both located on the
cortex)
 The tube then forms a twisted region called the proximal convoluted tubule
 Which then runs down towards the centre of the kidney (medulla) forming the loop
of Henle
 It then runs back upwards into the cortex forming another twisted region called
distal convoluted tubule
 Before finally joining a collecting duct that leads down through the medulla and
into the pelvis of the kidney
 Each glomerulus is supplied with blood from a branch of renal artery called an
afferent arteriole
 The capillaries of the glomerulus rejoins to form an efferent arteriole, which leads
off to form a network of capillaries running closely alongside the rest of the
nephron, where it then flows into a branch of the renal vein

Question 105

Answer 105

Question 106

The kidney makes urine in a two-stage process:

Answer 106

 Ultrafiltration – filtering of small molecules including urea into the Bownman’s
capsule from the blood
 Selective reabsorption – taking back useful molecules from the fluid in the nephron
as it flows along

Question 107

Water potential:

Answer 107

tendency of water molecules to move from one region to another

Question 108

Ultrafiltration

Answer 108

The blood in the glomerular capillaries is separated from the lumen of the
Bowman’s capsule by two cell layers (endothelium of the capillary and epithelial
cells (podocytes – having finger-like projections with gaps in between them) making
the inner lining of the Bowman’s capsule) and a basement membrane  The diameter of lumen of the afferent arteriole is wider than efferent arteriole,
which leads to high blood pressure (hydrostatic pressure) and low pressure in the
Bowman’s capsule, hence plasma/fluid passes through the fenestrations between
the endothelial cells of the capillaries; however, red and white blood cells / large
proteins (plasma proteins) / molecules greater than 68 000(MM), cannot pass
through due to the basement membrane which acts as a selective barrier; filtrate
through the basement membrane can freely pass through the podocytes due to its
fenestrations and forced into the Bowman’s capsule (renal capsule)

Question 109

Reabsorption in the proximal convoluted tubule

Answer 109

 Process called selective reabsorption (above 180 mg, no further absorption, as
carriers in the PCT are saturated)
 Lining of the proximal convoluted tubule is made of a single layer of cuboidal
epithelial cells

Question 110

single layer of cuboidal
epithelial cells which are adapted to their function of reabsorption by having:

Answer 110

 Microvilli to increase the surface area of the inner surface facing the lumen
to increase absorption of Na+ / glucose / amino acids
 Tight junctions to hold adjacent cells together so that fluid cannot pass
between the cells (all reabsorbed substances must go through the cells)
 Many mitochondria to provide ATP for sodium-potassium (Na+-K+) pump
proteins in the outer membranes of the cells
 Many co-transporter proteins in the membrane facing the lumen
 Folded basal membrane to increase surface area to increase sodium
potassium pumps to move Na+ into the blood
 More ER for increase in protein synthesis

Question 111

 The folded basal membranes of the cells lining the proximal convoluted tubule are
those nearest the blood capillaries; s

Answer 111

odium–potassium pumps in these membranes
move sodium ions out of the cells; the sodium ions are carried away in the blood,
lowering the concentration of sodium ions inside the cell, so that they passively
diffuse into it, down their concentration gradient, from the fluid in the lumen of the
tubule; however, sodium ions do not diffuse freely through the membrane – only
enter through special co-transporter proteins in the membrane, each of which
transports something else, such as a glucose molecule or an amino acid, at the
same time as the sodium ion (the passive movement of sodium ions into the cell
down their concentration gradient provides the energy to move glucose molecules,
against a concentration gradient – indirect or secondary active transport, since the
energy (as ATP) is used in the pumping of sodium

Question 112

All of the glucose, amino acids, vitamins and many Na+ and Cl- ions, some urea and
most water are

Answer 112

reabsorbed from the glomerular filtrate into the blood, which
increases the water potential in the filtrate, hence water moves down this gradient
through the cells into the blood

Question 113

Answer 113

Question 114

Answer 114

Question 115

 Reabsorption in the loop of Henle and collecting duct

Answer 115

absorption in the loop of Henle and collecting duct
 Descending limb is permeable to water, ascending limb does not
 In the ascending limb, active transport of Na+ and Cl- ions out of loop into the tissue
fluid, which decreases the water potential in the tissue fluid and increases the
water potential of the ascending limb’s water potential
 Descending limb permeable to both water and Na+ and Cl- ions, hence as fluid
moves down the loop, water from filtrate moves down a water potential gradient
into the tissue fluid by osmosis, while Na+ and Cl- ions diffuse into the loop, down
their potential gradient, thus the fluid becomes more concentrated towards the
bottom of the loop; the longer the loop, the more concentrated the fluid can
become
 Concentrated fluid flows up the ascending limb where Na+ and Cl- ions diffuse out in
the lower part of ascending limb; and active transported out on the upper part of
ascending limb Counter-current multiplier mechanism used – fluid flowing in vertically opposite
directions to maximise the concentration built up of solutes both inside and outside
the tube at the bottom of the loop

Question 116

Answer 116

Question 117

As fluid flows up the ascending limb of the loop of Henle,

Answer 117

it loses sodium and
chloride ions as it goes, so becoming more dilute and having a higher water
potential; cells of the ascending limb of the loop of Henle and the cells lining the
collecting ducts are permeable to urea which diffuses into the tissue fluid, hence
urea is also concentrated in the tissue fluid in the medulla, so water can move out
of the collecting duct by osmosis, due to the tissue fluid’s high solute concentration
and low water potential

Question 118

Reabsorption in the distal convoluted tubule and collecting duct

Answer 118

 Na+ ions are actively pumped from the fluid in the tubule into the tissue fluid, into
the blood
 K+ ions are actively transported into the tubule, where the rate of transfer of the
two ions are variable, helps regulate the concentration of these ions in the blood

Question 119

Osmoregulation:

Answer 119

: the control of the water potential of body fluids

Question 120

Roles of hypothalamus, posterior pituitary, collecting ducts and ADH in osmoregulation:

Answer 120

 Hypothalamus detects changes in water potential of the blood, as osmoreceptors
(in the hypothalamus) shrink when there is a low water potential (ADH produced in
hypothalamus), and released into the blood via the posterior pituitary gland
 Nerve impulses are sent from the hypothalamus to posterior pituitary gland
 ADH bind to receptor proteins on the collecting duct cell surface membranes and
affects the collecting duct by activating series of enzyme controlled reactions,
activating vesicles containing aquaporins in their membranes to move to cell
surface membrane on lumen side; fuses with the cell surface membrane, hence
increases water permeability of collecting duct cells, causing more water
reabsorption / more concentrated urine, as water moves through the aquaporins,
out of the tubule into the tissue fluid down water potential gradient

Question 121

Answer 121

Question 122

Answer 122

Question 123

Homeostatic control of blood glucose concentration is carried out by

Answer 123

two hormones
secreted by endocrine tissue – consisting of groups of cells known as islets of Langerhans
(containing cells secreting glucagon; cells secreting insulin) in the pancreas

Question 124

Question 125

Insulin is

Answer 125

a signalling molecule which binds to a receptor in the cell surface membrane and
affects the cell indirectly through the mediation of intracellular messengers

Question 126

signalling molecule which binds to a receptor in the cell surface membrane and
affects the cell indirectly through the mediation of intracellular messengers, following
these steps

Answer 126

 Increase in blood glucose concentration detected by β cells in the islets of
Langerhans, hence more insulin secreted into the blood
 Resulting to increase in glucose absorption in the liver by phosphorylating glucose –
traps glucose inside cells as phosphorylated glucose cannot pass through the
transporters in the cell surface membrane; increases the permeability of glucose in
muscle / fat cells by addition of GLUT 4 proteins to cell surface membranes of these
cells; increases the rate of respiration of glucose; conversion of glucose to glycogen;
inhibits secretion of glucagon; process called negative feedback

Question 127

 The action of glucagon on liver cells in the regulation of blood glucose concentration:

Answer 127

 Decrease in blood glucose concentration detected by cells and responds by
secreting glucagon
 Glucagon binds to receptors in cell surface membrane of liver cell
 Receptor changes conformation and G-protein activated
 Adenylate cyclase activated causing ATP to be converted to cyclic AMP which is a
second messenger
 Cyclic AMP activates kinase protein which activate enzymes through
phosphorylation resulting to enzyme cascade
 Glycogen phosphorylase activated catalysing the breakdown of glycogen to glucose
 Glucose diffuses out of liver cell through GLUT2 transporter proteins into the blood
 Gluconeogenesis – glucose made from amino acids and lipids
 Increase in blood glucose concentration

Question 128

The main stages of cell signalling in the control of blood glucose concentration by
adrenaline:

Answer 128

 Adrenaline binds to receptors in the cell surface membrane
 Receptor changes conformation and G proteins activated
 Adenylyl cyclase activated resulting to ATP converted to cyclic AMP which is a
second messenger
 Cyclic AMP activates kinase protein which activates enzymes through
phosphorylation resulting to enzyme cascade
 Glycogen phosphorylase activated catalysing the breakdown of glycogen to glucose
 Glucose diffuses out of liver cell through GLUT2 transporter proteins into the blood
 Increase in blood glucose concentration

Question 129

Diabetes mellitus are of two forms:

Answer 129

 Type 1 diabetes: insulin-dependent diabetes, where pancreas is incapable of
secreting sufficient insulin, early onset
 Type 2 diabetes: non-insulin-dependent diabetes, where pancreas does secrete
insulin, but liver and muscle cells do not respond properly to it, late onset –
associated with diet and obesity

Question 130

The symptoms of diabetes mellitus include the tendency to drink a lot of water and a loss
of body mass because:

Answer 130

 High blood glucose concentration causes decrease in water potential of the blood,
which is detected by osmoreceptors resulting to the feelings of thirst
 Less glucose converted to glycogen, as glucose lost in urine (above the renal
threshold), hence glucose is not taken up by cells, hence fats are metabolised,
resulting to build up in ketones which decreases the blood pH causing come

Question 131

Dip stick can be used to measure glucose concentration by:

Answer 131

 Immobilised glucose oxidase enzyme stuck onto pad at the end of the stick
 Dip stick lowered into urine and if it contains glucose, glucose oxidase oxidises
glucose into gluconic acid (gluconolactone) and hydrogen peroxide
 Peroxide reacts with chromogen (using peroxidase enzyme) on pad to form a brown
compound, due to the oxygen produced resulting to the oxidation of chromogen by
oxygen, which produces a range of colour
 Darkness of colour / range of colours is matched against a colour chart and is
proportional to concentration of glucose (the darker the colour, the more glucose
present)
 Does not give the current blood glucose concentration (only that it is higher than
the renal threshold
 Important to keep a fixed time in observing colour changes

Question 132

Biosensor can be used to measure glucose concentration by:

Answer 132

 The pad contains glucose oxidase enzyme reacts with glucose in the blood to
produce gluconolactone and oxygen
 Oxygen is detected and an electric current is generated which is detected by an
electrode, amplified and gives numerical value of blood glucose concentration
 The greater the current, the greater the reading from the biosensor, the greater the
glucose present

Question 133

Advantages of biosensor over dip stick:

Answer 133

 Gives the actual reading of blood glucose concentration
 Re-usable
 Quantitative, hence more precise reading

Question 134

Stomata have daily rhythms of

Answer 134

opening and closing (opens during the day to maintain the
inward diffusion of carbon dioxide and the outward diffusion of oxygen and water vapour
in transpiration; the closure of stomata at night when photosynthesis does not occur to
reduce rates of transpiration and conserve water) and also respond to changes in
environmental conditions to allow diffusion of carbon dioxide and regulate water loss by
transpiration

Question 135

stomata open in response to

Answer 135

 Increase in light intensity to gain CO2 for photosynthesis, allowing oxygen to
diffuse out
 Allows transpiration to occur for which brings water / mineral ions in for
photosynthesis

Question 136

stomata close in response to

Answer 136

 Decrease in light intensity as CO2 is not required (no photosynthesis)
 Low humidity, high temperature, high wind speed and water stress
 To prevent water loss by transpiration (maintains cell turgidity)

Question 137

Guard cells open when they gain

Answer 137

water to become turgid and close when they lose water
to become flaccid, by osmosis

Question 138

Mechanism by which guard cells open stomata:

Answer 138

 Proton pumps in cell surface membranes of guard cells actively pump H+ out of the
cells, which causes a lower H+ concentration inside the cell, hence inside of cell is
more negatively charged than the outside
 K+ channels open to move K+ into the cell by facilitated diffusion down an
electrochemical gradient
 Water potential of cell decreases, due to increase in solute potential, hence water
moves into the cell by osmosis down a water potential gradient through the
aquaporins in the membrane
 Volume of the guard cells increases becoming turgid opening the stoma
 Unequal thickness of the cell wall of the guard cells (thicker wall adjacent to the
pore

Question 139

Answer 139

Question 140

Stomata close when

Answer 140

proton pumps in cell surface membranes of guard cells stop and K+
ions diffuses out of the guard cells through K+ channels to enter the neighbouring cells,
creating a water potential gradient in the opposite direction, hence water leaves the guard
cells so it becomes flaccid and stoma closes, reducing the CO2 uptake for photosynthesis
and reduces rate of transpiration; in conditions of water stress, abscisic acid (ABA)
hormone stimulates stomatal closure

Question 141

The role of abscisic acid in the closure of stomata:

Answer 141

 Plant secretes abscisic acid during times of water stress
 Abscisic acid is a stress hormone which binds to receptors on the cell surface
membranes of guard cells, and inhibits proton pump (H+ not pumped out of cell)
 High H+ concentration inside cell, resulting to change in charge, stimulating Ca2+
influx into the cytoplasm which acts as second messenger, and encourages K+ efflux
(K+ channels open)
 Water potential of the cell increases, hence water moves out of cell by osmosis
 Volume of guard cells decreases, becoming flaccid
 The response is very fast

Question 142

The mammalian nervous system is made up of

Answer 142

central nervous system (brain and spinal
cord) and peripheral nervous system (cranial and spinal nerves)

Question 143

Communication using nerve action potentials

Answer 143

(brief changes in the distribution of electrical
change across the cell surface membrane, caused by rapid movement of Na+ and K+ ions) /
impulses which travel along nerve cells (neurones) at very high speeds to their target cells

Question 144

Compare the nervous and endocrine systems as communication systems that co-ordinate
responses to changes in the internal and external environment in mammals:
differences

Answer 144

Question 145

similarities Compare the nervous and endocrine systems as communication systems that co-ordinate
responses to changes in the internal and external environment in mammals

Answer 145

Question 146

 Motor neurone:

Answer 146

 Dendrites lead to cell body containing nucleus, many mitochondria and RER
 Has one long axon with synaptic knobs at the end furthest from cell body
 For some of its length, covered in myelin sheath made by specialised cells: Schwann
cells
 Has small uncovered areas of axon between Schwann cells: nodes of Ranvier

Question 147

 Sensory neurone:

Answer 147

 Dendrites at the ends of the long axon with synaptic knobs, with the cell body
containing nucleus, many mitochondria and RER near the source of stimuli or
ganglion
 For some of its length, covered in myelin sheath made by specialised cells: Schwann
cells

Question 148

Relay neurone found in the

Answer 148

central nervous system

Question 149

Answer 149

Question 150

The functions of sensory, relay and motor neurones in a reflex arc:

Answer 150

 Sensory neurone – receives impulses from receptor
 relay neurone – passes impulses on to motor neurone
 motor neurone – sends impulses to the effector

Question 151

The transmission of action potential in a myelinated neurone and its initiation from a
resting potential

Answer 151

 Initially at a resting potential (p.d. between -60 mV to -70mV), as inside is slightly
more negative than the outside, which is produced and maintained by sodium
potassium pumps in the cell surface membrane (3 Na+ removed for every 2 K+
brought in across the potential gradient by active transport)
 Na+ channels open and Na+ enters the cell down their electrochemical gradient,
where the p.d. becomes positive (+40 mV) due to depolarisation
 Na+ channels close and K+ channels open, hence K+ moves out of cell down their
concentration gradient, hence p.d., becomes negative due to repolarisation
 Local circuits occur
 Myelin sheath acts as an insulator hence prevents movement of ions, hence action
potentials / depolarisation occurs only at nodes of Ranvier, process called saltatory
conduction (action potential jumps from node to node)
 Process is a one-way transmission due to the refractory period
 If the p.d. generated does not reach the threshold potential (between – 60 mV and –70 mV), action potential does not occur

Question 152

Answer 152

Question 153

Answer 153

Question 154

The length of the refractory period determines the

Answer 154

maximum frequency at which impulses
are transmitted along neurones, as the transferred region will still be recovering from the
action potential it just had (sodium ion voltage-gated channels are ‘shut tight’ and cannot
be stimulated to open, however great the stimulus)

Question 155

Answer 155

Question 156

Answer 156

Question 157

The difference in strength of stimulus causes the

Answer 157

different frequency of action potentials,
where a strong stimulus produces a rapid succession of action potentials, each one
following along the axon just behind its predecessor; a weak stimulus results in fewer
action potentials per second; a strong stimulus stimulate more neurones than a weak
stimulus
 The brain can therefore interpret the frequency of action potentials arriving along the axon
of a sensory neurone, and the number of neurones carrying action potentials, to get
information about the strength of the stimulus being detected. The nature of the stimulus,
whether it is light, heat, touch or so on, is deduced from the position of the sensory
neurone bringing the information. If the neurone is from the retina of the eye, then the
brain will interpret the information as meaning ‘light’

Question 158

Sensory receptor cell

Answer 158

a cell that responds and converts a stimulus into an electrical
impulse by initiating an action potential (e.g. chemoreceptor)

Question 159

The roles of sensory receptor cells in detecting stimuli and stimulating the transmission of
nerve impulses in sensory neurones (e.g. chemoreceptor cell found in human taste buds):

Answer 159

 Chemicals act as a stimulus
 Each chemoreceptors contain a specific receptor protein which detects a particular
chemical
 Sodium ions diffuse into cell via Microvilli, causing the membrane to depolarise,
which increases the positive charge inside the cell known as receptor potential
 Sufficient stimulation (above the threshold potential (below will only cause a local
depolarisation of the receptor cell)) of Na+, stimulates opening of calcium ion
channels, hence calcium ions enter cell, causing movement of vesicles containing
neurotransmitter which is released exocytosis
 Neurotransmitter stimulates action potential in sensory neurone
 Chemoreceptors are transducers as they convert energy in one form into emergy in
an electrical impulse in a neurone
 All or nothing law where neurones either transmit impulses from one end to
another or they do not
 Chemoreceptor cells found in the papilla of the tongue

Question 160

Answer 160

Question 161

The gap between two neurones is called

Answer 161

synaptic cleft; the parts of the two neurones near
the cleft, plus the cleft itself, make up a synapse

Question 162

An action potential occurs at the

Answer 162

cell surface membrane of the presynaptic neurone
 Action potential causes the release of molecules of transmitter substance into the cleft
 The molecules of neurotransmitter diffuse across the synaptic clef and bind temporarily on
the postsynaptic neurone
 The postsynaptic neurone responds to all the impulses arriving at any one time by
depolarising; sends impulses if the overall depolarisation is above its threshold
 Synapses that uses acetylcholine as its neurotransmitter is known as cholinergic synapses

Question 163

 How a cholinergic synapse functions:

Answer 163

 Action potential reaches presynaptic membrane causing depolarisation, and Ca2+
channels to open in the presynaptic membrane
 An influx of Ca2+ ions (facilitated diffused down its electrochemical gradient),
stimulates vesicles containing ACh to move towards the presynaptic membrane and
fuse with it
 ACh released into synaptic cleft by exocytosis, and diffuses across the cleft
 ACh binds to receptor proteins on the postsynaptic membrane; proteins change
shape causing Na+ channels to open
 Na+ facilitated diffuse into postsynaptic neurone causing depolarisation of the
postsynaptic membrane creating an action potential, if the potential difference is
above the threshold for that neurone
 Acetylcholinesterase catalyses the hydrolysis of each ACh molecule into acetate and
choline to prevent permanent depolarisation of the postsynaptic neurone; choline
is taken back into the presynaptic neurone to be combined with acetyl CoA to form
ACh once more and transported into the presynaptic vesicles

Question 164

 Role of calcium ions in synaptic transmission:

Answer 164

 Enter the presynaptic neurone, and causes vesicles containing acetylcholine (ACh)
to fuse with the presynaptic membrane, hence releasing ACh into the synaptic cleft
(exocytosis)

Question 165

Answer 165

Question 166

Some insecticides have a similar structure to acetylcholine, hence affects the functioning of
acethycholinesterase by:

Answer 166

 Acting as a competitive inhibitor – complementary to the active site – hence binds
with the active site, this ACh is not broken down

Question 167

Answer 167

 A: Na+ cannot enter post-synaptic neurone hence no depolarisation / action potential in
the post-synaptic neurone
 B: Ca2+ cannot enter pre-synaptic neurone hence vesicles cannot move towards the pre
synaptic membrane
 C: ACh cannot be released into synaptic cleft
 D: ACh is not broken down hence continuous depolarisation / action potential, of post
synaptic neurone

Question 168

Ways in which the toxin (acts at cholinergic synapses) in cobra venom may cause muscle
paralysis:

Answer 168

 Binds to receptors on postsynaptic membrane, hence stops ACh from binding,
inhibiting depolarisation, hence no action potentials / Na+ ion channels stay shut
 Stimulates ACh receptors causing continuous depolarisation / opens Na+ ion
channels
 Stops the release of ACh from presynaptic neurone
 Inhibits acetyl cholinesterase

Question 169

The roles of synapses in the nervous system

Answer 169

 Ensures one-way transmission as vesicles are only found in the presynaptic
neurone, hence ACh will only be released from presynaptic neurone
 Involved in memory and learning, where an increase in the number of synapses
allows more interconnection of nerve pathways for memory, allowing a wider range
of response
 Allows interconnection of nerve pathways

Question 170

 The roles of synapses in the nervous system

Answer 170

 Ensures one-way transmission as vesicles are only found in the presynaptic
neurone, hence ACh will only be released from presynaptic neurone
 Involved in memory and learning, where an increase in the number of synapses
allows more interconnection of nerve pathways for memory, allowing a wider range
of response
 Allows interconnection of nerve pathways

Question 171

Striated muscle tissue makes up the

Answer 171

many muscles in the body that are attached to the
skeleton; only contracts when it is stimulated to do so by impulses that arrive via motor
neurones; muscle tissue like this is described as being neurogenic, whereas the cardiac
muscle in the heart is myogenic – it contracts and relaxes automatically, with no need for
impulses arriving from neurones

Question 172

Answer 172

Question 173

Multinucleate muscle fibres are called

Answer 173

synctium instead of cell; sacrolemma instead of cell
surface membrane; sarcoplasm instead of cytoplasm; sarcoplasmic reticulum (SR) instead
of endoplasmic reticulum; the deep infolding of the cell surface membrane into the interior
of the muscle fibre is called transverse system tubules or T-tubules, which runs close to
the sarcoplasmic reticulum

Question 174

Membranes of the sarcoplasmic reticulum have

Answer 174

huge numbers of protein pumps that
transport calcium ions into the cisternae of the SR

Question 175

The sarcoplasm contains a large number of

Answer 175

mitochondria, packed tightly between the
myofibrils to carry out aerobic respiration, generating the ATP that is required for muscle
contraction

Question 176

Each myofibrils are made up of

Answer 176

filaments (made from protein), where parallel groups of
thick filaments (made up of myosin) lie between groups of thin ones (made up of actin),
creating the stripes or striations

Question 177

The darkest parts of the A band are produced by the

Answer 177

overlap of thick and thin filaments,
while the lighter area within the A band, known as the H band, represents the parts where
only the thick filaments are present

Question 178

The Z line provides an attachment for

Answer 178

the actin filaments, while the M line does the same
for the myosin filaments; the part of a myofibril between two Z lines is called a sarcomere

Question 179

Answer 179

Question 180

Answer 180

Question 181

The sliding filament model of muscular contraction (shortening of I-band):

Answer 181

 When sarcoplasmic reticulum (SR) depolarised
 Calcium ion channels open
 Tropomyosin covers / uncovers the myosin binding sites on actin; when calcium
ions diffuse down a potential gradient through open channels from the SR to bind
to troponin hence changes shape (so tropomyosin and troponin move away),
allowing myosin to bind to actin, forming cross-bridges
 ATP hydrolysis (ATP → ADP + Pi) causes myosin head (acts as ATPase) to tilt
 ADP and Pi detach and myosin head swings back / returns to previous position
 Actin is moved and power stroke occurs
 New ATP binds
 Myosin head detaches from actin and the cross-bridges break

Question 182

No ATP in striated muscle results to:

Answer 182

 No pumping of calcium ions into SR
 No detachment of myosin heads:
 Hence no hydrolysis of ATP
 Hence no cross bridge formation
 Hence no power stroke / pulling of actin
 Hence no recovery stroke / myosin head does not return to original position

Question 183

Stimulating contraction in striated muscle:

Answer 183

 Action potential arrives resulting to the opening of Ca2+ channels in the presynaptic
membrane, hence Ca2+ enters into the presynaptic neurone
 Releasing vesicles containing acetylcholine to move towards and fuse to the
presynaptic membrane
 Acetylcholine released by exocytosis and diffuses across the cleft; binds to
receptors on the sacrolemma
 Na+ channels open, hence Na+ ions enter to depolarise the sacrolemma and
generate action potential
 Impulses then pass along the T-tubules towards the centre of the muscle fibre
 Causing Ca2+ ions to bind with the troponin molecules
 When there is no longer stimulation from the motor neurone, no impulses
conducted along the T-tubules, hence the Ca2+ channels close and calcium pumps
move Ca2+ back into stores in the SR; as Ca2+ leaves the binding sites on troponin,
tropomyosin moves back to cover the myosin-binding sites, hence no more cross
bridges

Question 184

Muscles also have another source of ATO produced from

Answer 184

creatine phosphate, keeping
their stores in their sarcoplasm as their immediate source of energy

Question 185

Answer 185

Question 186

Answer 186

Question 187

Answer 187

Question 188

Venus fly trap:

Answer 188

 Each trap consists of a pair of modified leaves joined by a midrib of hinge cells
 The modified leaves have touch-sensitive hairs; if two hairs are touched within 20
seconds, or the same hair is touched twice in rapid succession, the trap closes
 The surface of the lobes has many glands that secrete enzymes for the digestion of
trapped insects
 The touch of insects on the sensory hairs activates calcium ion channels in cells at
the base of the hair to open, causing an influx in Ca2+ to generate action potential
 Auxin increase triggered in hinge cells
 H+ ions pumped into the cell walls and calcium pectate ‘glue’ in cell wall dissolved
 Ca2+ ions enter hinge cell
 Water follows by osmosis down its water potential gradient
 Hinge cells expand
 Trap lobes, flip from convex to concave (change in elastic tension)

Question 189

Two types of plant growth regulators (hormones):

Answer 189

Auxins: influence many aspects of growth including elongation growth which
determines the overall length of roots and shoots; synthesised in the growing tips
(meristems) of shoots and roots, where cells are dividing; actively transported from
cell to cell
 Gibberellins: involved in seed germination and controlling stem elongation

Question 190

Role of auxin in cell elongation in plants:

Answer 190

 Acid-growth hypothesis
 Auxin stimulates proton pumps in the cell surface membrane
 H+ pumped into the cell wall by active transport, hence the pH of cell wall
decreases, so pH-dependent enzymes activated (expansins) and loosen the bonds
between cellulose microfibrils
 Cell wall ‘loosens’ / becomes more elastic / able to stretch, hence more water
enters the cell and turgor pressure increases, resulting to the expansion of the cell
wall

Question 191

Answer 191

Question 192

The role of gibberellin in the germination of wheat or barley:

Answer 192

 Seed is initially dormant
 When the seed absorbs water, embryo produces gibberellin
 Gibberellin diffuses onto the aleurone layer and stimulate the cells to synthesise
amylase, by the affecting the gene transcription of mRNA for amylase (breakdown
of DELLA proteins which inhibit germination)
 Amylase hydrolyses starch in the endosperm to soluble maltose molecules, which
are converted to glucose and transported to the embryo for respiration (ATP for
growth)
 Amylase hydrolyses starch in the endosperm to soluble maltose molecules, which
are converted to glucose and transported to the embryo for respiration (ATP for
growth)

Question 193

 The height of some plants is partly controlled by their genes, e.g

Answer 193

tallness in pea plants is
affected by a gene with two alleles; if the dominant allele, Le, is present, the plants can
grow tall, but plants homozygous for the recessive allele, le, always remain short
 The dominant allele of this gene regulates the synthesis of the last enzyme in a
pathway that produces an active form of gibberellin, GA1
 Active gibberellin stimulates cell division and cell elongation in the stem, so causing
the plant to grow tall
 A substitution mutation in this gene gives rise to a change from alanine to
threonine in the primary structure of the enzyme near its active site, producing a
non-functional enzyme; this mutation has given rise to the recessive allele, le;
hence homozygous plants, lele, are genetically dwarf as they do not have the active
form of gibberellin

Question 194

Homologous chromosomes

Answer 194

are a pair of chromosomes in a diploid cell that have the same
structure as each other, with the same genes (but not necessarily the same alleles of those
genes) at the same loci, and that pair together to form a bivalent during the first division of
meiosis

Question 195

There are 22

Answer 195

matching chromosomes in humans (homologous chromosomes) – autosomes – and a non-matching pair labelled X and Y (sex chromosomes); two sets of 23
chromosomes – one set of 23 from the father and one set of 23 from the mothe

Question 196

A gene

Answer 196

is a length of DNA that codes for a particular protein or polypeptide

Question 197

An allele

Answer 197

is a particular variety of a gene

Question 198

A locus

Answer 198

is the position at which a particular gene is found on a particular chromosome; the
same gene is always found at the same locus

Question 199

Diploid cell

Answer 199

one that possesses two complete sets of chromosomes; the abbreviation for
diploid is 2n

Question 200

Haploid cell:

Answer 200

: one that possesses one complete set of chromosomes; the abbreviation for
haploid is n

Question 201

Without halving the number of chromosomes into

Answer 201

haploid gametes (meiosis – reduction
division), it would double every generation

Question 202

Answer 202

Question 203

Answer 203

Question 204

Answer 204

Question 205

Answer 205

Question 206

Answer 206

Question 207

Answer 207

Question 208

Answer 208

Question 209

Answer 209

Question 210

Answer 210

Question 211

Answer 211

Question 212

Answer 212

Question 213

Answer 213

Question 214

Answer 214

Question 215

Answer 215

Question 216

Answer 216

Question 217

Answer 217

Question 218

Answer 218

Question 219

Answer 219

Question 220

Answer 220

Question 221

The formation of male gametes:

Answer 221

: spermatogenesis (testes)

Question 222

The formation of female gametes:

Answer 222

: oogenesis (ovaries)

Question 223

Sperm production takes place inside

Answer 223

tubules in the testes. Here, diploid cells divide
by mitosis to produce numerous diploid spermatogonia, which grow to form
diploid primary spermatocytes. The first division of meiosis then takes place,
forming two haploid secondary spermatocytes. The second division of meiosis then
produces haploid spermatids, which mature into spermatozoa.

Question 224

Ovum production takes place inside the

Answer 224

ovaries, where diploid cells divide by
mitosis to produce many oogania which begins to divide by meiosis but stops at
prophase I, primary oocytes are formed. During puberty, some of the primary
oocytes proceed from prophase I to the end of the first meiotic division forming
two haploid cells (secondary oocyte – gets most of the cytoplasm – and polar body – has no role in reproduction)

Question 225

Answer 225

Question 226

The male gametes are nuclei inside

Answer 226

pollen grains, which are made in the anthers of a flower

Question 227

The female gametes are nuclei inside

Answer 227

the embryo sacs, which are made in the
ovules inside the ovaries of a flower

Question 228

Inside the anthers,

Answer 228

pollen mother cells divide by meiosis to form four haploid cells,
which nuclei divide by mitosis to form two haploid nuclei in each cell; matures into
pollen grains; one of the nuclei is the male gamete nucleus which can fuse with a
female nucleus to produce a diploid
zygote which grows into an embryo
plant

Question 229

Inside each ovule,

Answer 229

a large, diploid spore mother cell develops, which divides by
meiosis to produce four haploid cells; all but one of these degenerates, which then
develops into an embryo sac, which grows larger and its haploid nucleus divides by
mitosis three times, forming eight haploid nucleus (one of these will become the
female gamete

Question 230

Note that in plants, unlike animals,

Answer 230

e gametes are not formed directly by meiosis.
Instead, meiosis is used in the production of pollen grains and the embryo sac and
the gametes are then formed inside these structures by mitotic divisions

Question 231

During prophase I of meiosis,

Answer 231

as the two homologous chromosomes lie side by side, their
chromatids form links called chiasmata (singular: chiasma) with each other. When they
move apart, a piece of chromatid from one chromosome may swap places with a piece
from the other – crossing over – resulting in each chromosome having different
combinations of alleles as it did before

Question 232

Answer 232

Question 233

Independent assortment.

Answer 233

At metaphase of meiosis I,
the pairs of homologous chromosomes line up on the
equator independently of each other. For two pairs of
chromosomes, there are two possible orientations; at
the end of meiosis II, each orientation gives two types
of gamete. There are therefore four types of gamete
altogethe

Question 234

A genotype is

Answer 234

the alleles possessed by an organism

Question 235

Homozygous means

Answer 235

having two identical alleles of a gene (e.g. HbAHbA)

Question 236

Heterozygous means

Answer 236

having two different alleles of a gene (e.g. HbAHbS)

Question 237

Genotype affects phenotype:

Answer 237

 HbSHbS: coding for the production of the sickle cell -globin polypeptide, sickle cell
anaemia
 HbAHbA: coding for the normal -globin polypeptide
 HbAHbS: Half of the person’s Hb will be normal, and half will be sickle cell Hb - sickle
cell trait – can be referred to as ‘carriers’ – they have enough normal haemoglobin
to carry enough oxygen, and so will have no problems at all and immune to malaria

Question 238

An organism’s phenotype is its

Answer 238

characteristics, often resulting from an interaction between
its genotype and its environment

Question 239

During every fertilisation,

Answer 239

either an HbA sperm or an HbS sperm may fertilize either an HbA
egg or an HbS egg. The possible results can be shown like this:

Question 240

Codominant alleles both have

Answer 240

an
effect on the phenotype of a
heterozygous organism

Question 241

A dominant allele is

Answer 241

one whose
effect on the phenotype of a
heterozygote is identical to its effect in a homozygote

Question 242

A recessive allele is

Answer 242

one that is only expressed when no dominant allele is present
 Dominant and recessive example (tomato plants):

Question 243

The F1 generation

Answer 243

is the offspring resulting from a cross between an organism with a
homozygous dominant genotype, and one with a homozygous recessive genotyp

Question 244

The F2 generation

Answer 244

is the offspring resulting from a cross between two F1 (heterozygous(
organisms

Question 245

A test cross is a

Answer 245

genetic cross in which an organism showing a characteristic caused by a
dominant allele is crossed with an organism that is homozygous recessive; the phenotypes
of the offspring can be a guide to whether the first organism is homozygous or
heterozygous:
 E.g. a purple stem tomato plant might have the genotype AA or Aa; to find out its
genotype, it could be crossed with a green-stemmed tomato plant aa

Question 246

Answer 246

Question 247

XX:

Answer 247

female

Question 248

XY

Answer 248

malewhere Y chromosome
is much shorter than the X

Question 249

Sex linkage.

Answer 249

E.g. haemophilia (sex-linked gene), in which the blood fails to clot properly
due to the recessive allele h, resulting to the lack of factor VIII; where dominant allele, H,
produces normal factor VIII

Question 250

The gene for haemophilia is on the

Answer 250

X chromosome, and not on the autosome, hence affects
the way that is inherited, e.g. a man does not have haemophilia while the woman is a
carrier (0.25 probability: normal girl, boy, carrier girl and boy with haemophilia):

Question 251

Dihybrid crosses

Answer 251

(inheritance of two genes at once

Question 252

dihybrid cross between
a heterozygous organism and a
homozygous recessive organism where the
alleles show complete dominance

Answer 252

1 : 1 : 1 : 1 ratio

Question 253

dihybrid cross between two heterozygous
organisms where the two alleles show
complete dominance and where the genes
are on different chromosomes

Answer 253

9 : 3 : 3 : 1 ratio

Question 254

Linkage

Answer 254

is the presence of two genes on the same chromosome, so that they tend to be
inherited together and do not assort independently (e.g. the fruit fly, Drosophila. The gene
for body colour and the gene for antennal shape are close together on the same
chromosome and so are linked)

Question 255

Recombinant (offspring caused by crossing over):

Answer 255

Cross over value is the percentage of offspring that belong to the recombinant class

Question 256

Chi-squared (χ2) test

Answer 256

allows comparison between observed results and expected results,
and decide whether there is a significant difference between them (e.g. two heterozygous
tomato plants – 144 offspring):

Question 257

Gene mutation:

Answer 257

a change in the structure of a DNA molecule, producing a different allele
of a gene

Question 258

Mutagen

Answer 258

a substance that increases the chances of mutation occurring

Question 259

Three different ways in which the sequence of bases in a gene may be altered (gene
mutations):

Answer 259

 Base substitution, where one base takes the place of another, e.g. CCT GAG GAG
into CCT GTG GAG
 Base addition, where one or more extra bases are added to the sequence, e.g. CCT
GAG GAG into CCA TGA GGA G
 Base deletion, where one or more bases are lost from the sequence, e.g. CCT GAG
GAG into CCG AGG AG

Question 260

Base deletion and addition have significant effect on

Answer 260

the structure, therefore function of
the polypeptide that the allele codes for, and causes frame shifts

Question 261

Base substitution may not have

Answer 261

any apparent effect, called silent mutation

Question 262

Sickle cell anaemia:

Answer 262

Question 263

Huntington’s disease:

Answer 263

Question 264

Albinism:

Answer 264

Question 265

Genetic variation is caused by:

Answer 265

 Independent assortment of chromosomes, and therefore alleles, during meiosis
 Crossing over between chromatids of homologous chromosomes during meiosis
 Random mating between organisms within a species
 Random fertilisation of gametes
 Mutation (excluding somatic cells – apart of cells in the reproductive organs

Question 266

Genetic variation provides the

Answer 266

raw material on which natural selection can act, where
some individual have features that give them an advantage over other members in a
population

Question 267

Phenotypic variation is also caused by

Answer 267

the environment, e.g. some organisms might be
larger than others due to better access of quality food while they were growing, however
these variations will not be passed onto the offsprin

Question 268

Qualitative differences fall into

Answer 268

learly distinguishable categories, with no intermediates –
e.g. four possible ABO blood groups: A, B, AB or O. This is discontinuous variatio

Question 269

Quantitative differences where there is a

Answer 269

range of heights between two extremes (Figure
17.2). This is continuous variation

Question 270

Answer 270

Question 271

Both qualitative and quantitative differences in phenotype may be inherited; may involve
several different genes; differences between them are:

Answer 271

 In discontinuous (qualitative) variation:
 Different alleles at a single gene locus have large effects on the phenotype
 Different genes have quite different effects on the phenotype
 In continuous (quantitative) variation (e.g. organism’s height):
 Different alleles at a single gene locus have small effects on the phenotype
 Different genes have the same, often additive, effect on the phenotype
 A large number of genes may have a combined effect on a particular
phenotypic trait; these genes are known as
polygenes

Question 272

Environmental effects may allow the full genetic potential height to be reached or may
stunt it in some way:

Answer 272

 One individual might have less food, or less nutritious food, than another with the
same genetic contribution
 A plant may be in a lower light intensity or in soil with fewer nutrients than another
with the same genetic potential height.
 Himalayan colouring of rabbits and of Siamese and Burmese cats, colouring is
caused by an allele which allows the formation of the dark pigment only at low
temperature, hence the extremities are the coldest parts of the animals

Question 273

The t-test is used to

Answer 273

assess whether or not the means of two sets of data with roughly
normal distributions, are significantly different from one another (p. 500)

Question 274

Various environmental factors come into play to keep down a population’s number

Answer 274

 Biotic – caused by other living organisms such as through predation, competition
for food, or infection by pathogens
 Abiotic – caused by non-living components of the environment such as water
supply or nutrient levels in the soil

Question 275

Once the population increases,

Answer 275

the pressure of the environmental factors will be
sufficiently great, then the population size will decrease, only when they have fallen
considerably will the numbers be able to grow again; over a period of time, the population
will oscillate about a mean level

Question 276

Natural selection occurs as

Answer 276

populations have the capacity to produce many offspring that
compete for resources; in the ‘struggle for existence’ only the individuals that are best
adapted survive to breed and pass on their alleles to the next generation (selection
pressure)

Question 277

Fitness

Answer 277

is the capacity of an organism to survive and transmit its genotype to its offspring

Question 278

Selection pressure

Answer 278

an environmental factor that gives greater chances of survival and
reproduction on some individuals than on others in a population (e.g. predation and
camouflage)

Question 279

Environmental factors can act as:

Answer 279

Question 280

Changes in allele frequency creates the

Answer 280

basis of evolution

Question 281

Antibiotic resistance:

Answer 281

 There may be one or more individual bacteria with an allele giving resistance to
penicillin – such as Staphylococcus – by producing penicillinase, which inactivates
penicillin. These bacteria can survive and reproduce, while others will die (selection
pressure)

Question 282

Industrial melanism

Answer 282

 Black forms of moth with allele, C, increases in areas near industrial cities; whereas
speckled forms of moth with allele, c, stays constant in non-industrial areas
(predation selection pressure)
 These mutations are not caused by pollution (changes in environmental factors only
affect the likelihood of an allele surviving in a population; not affecting the
likelihood of such an allele arising by mutation

Question 283

Sickle cell anaemia:

Answer 283

 Places where sickle cell allele is most common are parts of the world where malaria
(caused by protoctist parasite, Plasmodium, where it enters the RBC and multiply) is
found (selection pressure occurs hence selective advantage occurs)
 There are two strong selection pressures acting on these two allele:
 Selection against people who are homozygous for the sickle cell allele,
HbSHbS, is very strong, because they become seriously anaemic
 Selection against people, who are homozygous HbAHbA is also very strong,
as they are more likely to die from malaria
 Heterozygous people with malaria only have about one-third the number of
Plasmodium in their blood as do HbAHbA homozygotes

Question 284

Genetic drift

Answer 284

s a change in allele frequency that occurs by chance, as only some of the
organisms of each generation reproduce (e.g. when a small number of individuals are
separated (isolated) from the rest of a large population, resulting to different allele
frequencies; further genetic drift will alter the allele frequencies even more and evolution
will cause significant difference with the parent population) – founder effect

Question 285

Hardy-Weinberg principle

Answer 285

allow the proportions of each of the genotypes in a large,
randomly mating population to be calculated (the frequency of a genotype is its proportion
to the total population; p represents the frequency of the dominant allele & q represents
the frequency of the recessive allele):
 E.g. two alleles of a single gene, A/a, thus three genotypes will be in the population

Question 286

Hardy-Weinberg calculations do not apply when the population is small or when
there is:

Answer 286

 Significant selective pressure against one of the genotypes
 Migration of individuals carrying one of hte two alleles into, or out of, the
population
 Non-random mating

Question 287

Selective breeding of dairy cattle:

Answer 287

 Artificial selection: When humans purposely apply selection pressures to
populations
 Desired features include docility (making the animal easier to control), fast growth
rates and high milk yields have been achieved by selective breeding
 Individuals showing one or more of these desired features are chosen for breeding
 Some of the alleles granting these features are passed on to the individuals’
offspring
 Over many generations, alleles granting the desired characteristics increase in
frequency, while those conferring characteristics not desired by the breeder
decrease in frequency
 Background genes (the alleles of genes that adapt to its particular environment)
results in offspring obtaining the same adaptations, however will not be well
adapted to a new environment (even though it will show selected traits)

Question 288

Crop improvement:

Answer 288

 Gene technology is used to alter or add genes into a species in order to change it
characteristics
 Selective breeding:
 Produced many different varieties of wheat and rice – most is grown to
produce grains rich in gluten
 Resistance towards various diseases (wheat and rice)
 Shorter stems, for easy harvest hence higher yields (less energy used to
grow tall, more to growing of seeds) – wheat and rice
 Most of the dwarf varieties of wheat carry mutant alleles of two reduced height
(Rht) genes, which code for DELLA proteins to reduce the effect of gibberellins ongrowth. The mutant alleles cause dwarfism by producing more of, or more active
forms of, these transcription inhibitors
 A mutant allele of a different gene, called ‘Tom Thumb’, has its dwarfing effect
because the plant cells do not have receptors for gibberellins and so cannot
respond to the hormone
 Interbreeding and hybridisation of maize for uniformity and heterozygosity:
 If maize plants are inbred (crossed with other plants with genotypes like
their own), the plants in each generation become progressively smaller and
weaker – inbreeding depression – due to the less vigorous homozygous
plants compared to heterozygous
 Homozygous plants obtained from companies, then crossing them,
producing F1 plants that all have the same genotype which have
characteristics such as high yields, resistance to more pests and diseases,
and good growth in nutrient-poor soils or where water is in short supply

Question 289

Speciation

Answer 289

: The production of new species

Question 290

A species

Answer 290

is a group of organisms with similar morphological, physiological, biochemical
and behavioural features, which can breed together naturally to produce fertile offspring,
and are reproductively isolated from other species

Question 291

Reproductive isolation:

Answer 291

The inability of two groups of organisms of the same species to
breed with one another, e.g. because of geographical separation or because of behavioural
differences

Question 292

Reproductive isolation can take very different forms:

Answer 292

pre zygotic and postzygotic

Question 293

Prezygotic (before a zygote is formed) isolating mechanisms include:

Answer 293

 Individuals not recognising one another as potential mates or not responding to
mating behaviour
 Animals being physically unable to mate
 Incompatibility of pollen and stigma in plants
 Inability of a male gamete to fuse with a female gamete

Question 294

Postzygotic isolating mechanisms include:

Answer 294

 Failure of cell division in the zygote
 Non-viable offspring (offspring that soon die)
 Viable, but sterile offspring

Question 295

 Allopatric speciation (geographical isolation/separation):

Answer 295

 Requires a barrier to arise between two populations of the same species,
preventing them from mixing (e.g. a stretch of water, deforestation)
 The selection pressures on these two places might be very different, resulting in
different alleles being selected for
 Over time, the two population can no longer interbreed, hence a new species had
evolved

Question 296

Sympatric speciation (ecological and behavioural separation):

Answer 296

 Two groups of individuals living in the same area may become unable to breed
together (e.g. one group develops courtship behaviours that no longer match with
the other groups, because they love in different habitats in the same area
(ecological separation))

Question 297

Molecular evidence that reveals similarities between closely related organisms with
reference to mitochondrial DNA and protein sequence data:

Answer 297

 Mitochondria contain a single DNA molecule that is passed on down the female
line; analysis of mitochondrial DNA (mtDNA) can be used to determine how closely related two different species are; the more similar the sequence of bases in the
DNA, the more closely related they are considered to be
 Amino acid sequences in proteins can be used (the protein cytochrome c, involved
in the electron transport chain, is found in a very wide range of different organisms,
suggesting that they all evolved from a common ancestor, where differences in the
amino acid sequences in cytochrome c suggest how closely related particular
species are

Question 298

Extinctions tend to be caused by:

Answer 298

 Climate change, for example, global warming can result in some species inability to
find adapted habitats
 Competition, for example, a newly evolved species or an alien species, may out
compete a resident species
 Habitat loss, for example, large deforestations
 Direct killing by humans

Question 299

Structural genes:

Answer 299

 Genes that code for proteins required by a cell

Question 300

Regulatory genes:

Answer 300

 Genes that code for proteins that regulate the expression of other genes

Question 301

Difference between repressible and inducible enzymes

Answer 301

 The synthesis of a repressible enzyme can be prevented by binding a repressor
protein to a specific site, called an operator, on bacterium’s DNA
 The synthesis of an inducible enzyme occurs only when its substrate is present
(transcription of a gene occurs as a result of the inducer (enzyme’s substrate)
interacting with the protein produced by the regulatory gene)

Question 302

 The lac operon consists of a cluster of three structural genes and a length of DNA
including operator and promoter regions, the three structural genes are

Answer 302

 lacZ, coding for -galactosidase
 lacY, coding for permease (allows lactose to enter the cell)
 lacA, coding for transacetylase

Question 303

When there is no lactose in the medium in which the bacterium is growing:

Answer 303

 The regulatory gene codes for a protein called a repressor
 The repressor binds to the operator region, close to gene lacZ
 In the presence of bound repressor at the operator, RNA polymerase cannot
bind to DNA at the promoter region
 No transcription of the three structural genes take place

Question 304

The repressor protein is

Answer 304

allosteric (two binding sites), hence when lactose binds to
its site, the shape of the protein changes so that the DNA-binding site is closed

Question 305

When lactose is present in the medium in which the bacterium is growing:

Answer 305

 Lactose is taken up by the bacterium
 Lactose binds to the repressor protein, distorting its shape and preventing it
from binding to DNA at the operator site
 Transcription is no longer inhibited and messenger RNA is produced from
the three structural genes
 -galactosidase is an inducible enzyme

Question 306

Transcription of a gene is controlled by transcription factors

Answer 306

proteins that bind to
a specific DNA sequence and control the flow of information from DNA to RNA by
controlling the formation of mRNA, role is to make sure that genes are expressed in
the correct cell at the correct time and to the correct extend

Question 307

effects of mrna

Answer 307

 Necessary for transcription to occur, form part of the protein complex that
binds to the promoter region of the gene concerned
 Activate appropriate genes in sequence
 Responsible for the determination of sex in mammals
 Allow responses to environmental stimuli, e.g. switching on the correct
genes to respond to high environmental temperatures
 Hormones have their effect through transcription factors

Question 308

Gibberellin controls seed germination in plants by

Answer 308

increasing the transcription of
mRNA coding for amylase, done by breaking down of DELLA proteins (inhibits the
binding of a transcription factor, such as PIF to a gene promoter), causing PIF to
bind to its target promoter resulting to an increase in amylase production

Question 309

Answer 309