bmc

Question 1

Explain what is meant by a gene

Answer 1

Sequence of nucleotides or length of DNA that code for a polypeptide chain

Question 2

Explain why blood taken from a person with an infectious disease may have a different
number of white blood cells compared with blood taken from a healthy person.

Answer 2

-more white blood cells because of immune response. Clonal expansion described, of, B- / T-, lymphocytes / cells; idea
of producing (large quantity of specific) antibody in context of immune response. Increased, production of
macrophages / phagocytes

Question 3

Suggest a reason why daughter cells are not identical immediately after cytokinesis

Answer 3

unequal sharing out of, cytoplasm / organelles / named organelles

Question 4

How to vaccination and treatments (oral rehydration) helps in preventing cholera

Answer 4

-vaccines stimulate (active) immunity to cholera. The immune system destroys pathogen before it can, because harm /
spread. Enough people effectively immunized to prevent spread (herd immunity). Rehydration therapy, decreases
recovery time / cures more quickly. Contaminated faces disposed of safely / decreased risk of spread via contaminated
faces. More able to practice good hygiene. Time spent in medical Centre acts as quarantine / infected people not in
general population. (So) decreases, density / proportion, of infected people to reduce risk of spread

Question 5

Describe the hydrogen bonding that occurs between the water molecules

Answer 5

hydrogen bond is a weak bond. Each oxygen forms two hydrogen bonds. Water is dipolar. Oxygen has, small / slight,
negative charge / δ–, and, hydrogen has, small / slight, positive charge / δ+

Question 6

Outline the importance of water as a solvent in plants

Answer 6

-1 dissolves / AW, ions / minerals / salts, and (named) polar molecules;
2 transports, solute(s) / named solutes / dissolved substance, in, xylem / phloem / xylem and phloem ;
3 storage of, solutes / named solutes, in vacuoles ;
4 metabolic / chemical / cellular, reactions occur in water ;
5 dissolves, carbon dioxide / oxygen, with ref to, respiration / photosynthesis;

Question 7

Explain the induced-fit hypothesis

Answer 7

1 active site is not (fully) complementary to substrate ;
2 active site, changes shape / moulds around, to fit the substrate ;
A conformational change for shape change
3 enzyme-substrate complex / ESC, forms ;
4 active site returns to original shape on release of product ;
5 AVP ;
e.g. change of shape (to give complementary fit) lowers activation energy / puts strain on bonds / AW

Question 8

Explain how malaria is transmitted.

Answer 8

1 vector is female Anopheles (mosquito) ;
2 mosquito / (female) Anopheles, takes blood from infected person ; I ‘bites’ alone
3 (vector / mosquito / Anopheles) inserts / AW, saliva / anticoagulant, with, pathogen / Plasmodium / parasite, into
(blood of) uninfected person.

Question 9

Discuss the factors that determine the global distribution of malaria

Answer 9

1 tropical / warm and humid / AW, climate / regions ; A in context of global warming (increases life span of Anopheles)
2 in areas where, Anopheles (mosquito) / vector, occurs ;
3 Anopheles mosquitoes only live in humid conditions ;
4 warm temperatures for, development / growth, of, parasite / Plasmodium (in vector / mosquito)
; 5 warm temperature for, development / growth, of mosquito larvae ;
6 mosquitoes require bodies of, still / AW, water for breeding ; A ponds, puddles, lakes, swamps
7 mosquitoes require places where there is sufficient rainfall ;
8 low altitude ; resistance
9 insecticide / repellent, resistance of mosquitoes ;
10 drug resistance of parasite ; prevention
11 in countries / areas, where, prevention / control measures, are not implemented by, governments / health
authorities ;
12 further detail e.g. any example of a, prevention / control measure that is not used or not implemented fully ;
immunity
13 immunity to malaria in human population (limits distribution) ; people
14 migration of infected people from areas (with high rates of malaria) ;
15 high rates of HIV infection

Question 10

Describe the differences between the blood arriving at the arterial end of the capillary
network and the tissue fluid surrounding the body cells

Answer 10

Question 11

Outline the changes that need to occur to form the functioning globular protein molecule

Answer 11

1 (formation of), secondary / 2° / second level, structure with, alpha-helix / α-helix / beta-pleated sheet(s) / β-pleated
sheet(s) ; β-conformation for β-pleated sheet
2 folding / coiling, to form tertiary / 3° / third level, structure ;
A ref. to 3D structure as alternative to, folding / coiling
A secondary structure, folds / coils, to form tertiary structure
R alpha helix / beta-pleated sheet, coils / folds
3 two of hydrogen bonds / ionic or electrostatic bonds / disulfide bonds or bridges / hydrophobic interactions ;
4 (formation of), quaternary / 4° / fourth level structure ;
A description e.g. polypeptides held together by, interactions / hydrogen bonds / ionic bonds / disulfide bonds
5 (globular protein) has hydrophilic, amino acids / R-groups / side chains, to, outside / AW ;
ora hydrophobic, amino acids / R-groups / side chains, towards centre / AW
6 example of other post-translational modification ;
e.g. removal, methionine / met
addition sugar group / glycosylation
addition phosphate group / phosphorylation
addition prosthetic group

Question 12

Suggest and explain how this vaccine will provide a person with active immunity

Answer 12

1 vaccine contains antigens ; in context of antigens of defective, T-lymphocytes and B-lymphocytes
(accept antibody of defective B-lymphocyte as AW antigen)
2 stimulates an immune response ;
3 detail ; e.g. macrophages and antigen presentation / AW
ref. to specific lymphocytes involved
clonal selection of lymphocytes
recognition by / activation of, lymphocytes
clonal expansion of lymphocytes
production of plasma cells in context of B-lymphocytes
production of memory cells
T-helper cells produce cytokine
4 antibody produced against (defective), T-lymphocytes / B-lymphocytes / antibody (that binds ACh receptors) ;
in context of primary or secondary response
R if context is related to the vaccine containing antibody
5 memory cells, remain / give long-lasting effect / AW ;
6 memory cells allow secondary immune response to (defective) lymphocytes (newly produced) ;

Question 13

Describe the differences in structure and function between RER and SER

Answer 13

1 RER has ribosomes R if context is within lumen or SER does not have ribosomes ;
2 RER, flattened sacs / cisternae, and SER tubular ; AW
3 RER continuous with, (external) nuclear membrane ; ora
4 ref. to RER, more regular / layered, arrangement or SER more irregular / disorganised, arrangement ;function
5 RER produces / transports, proteins / glycoproteins ; A polypeptides
6 SER produces, lipids / cholesterol / steroids ; A stores

Question 14

Discuss how the reasons listed in Fig. 2.1 contributed to the success of the eradication of
smallpox

Answer 14

1 same vaccine could be used (everywhere / for many years ) ; A no need to manufacture different vaccines in context
of ease of, administering/ planning/ production
2 (so) a person was protected for, a long time / life / each exposure (to virus) ; AW e.g. same virus so vaccinated
person did not become ill on repeated exposure A reduced / no, risk of vaccine becoming ineffective
A long-lasting immunity once only for mp2 or mp4
3 no, research / trials / development, required for new vaccine(s) ; in context of pre-mass productionlive and closely
related to
4 (closely related so) same antigens / (gives the desired) immune response / antibodies produced / memory cells
produced / memory cells remain ;
A long-lasting immunity once only for mp4 or mp2
5 live virus (replicates, so), gives a strong(er) immune response ;
A better immunity than inactive virus
6 no need for boosters ;
7 ref. to not smallpox virus, so fewer health and safety issues (at all stages of development and use) ; e.g. could not get
ill from smallpox
A not smallpox virus so more people willing to have vaccine

Question 15

Discuss the consequences of the development of antibiotic resistance in V. cholerae

Answer 15

inability to treat people with cholera using antibiotics ;
prolonged periods of illness ;
(so) increased risk of spread / AW ;
increased death rate from cholera ;
transfer of antibiotic resistance (alleles) to, other species of bacteria / other strains of V.cholerae ;
development of multiple resistance in other species of bacteria ;
inability to treat other bacterial diseases ;
increased death rate from other bacterial diseases ;
need to research alternative antibiotics

Question 16

Suggest why telomerase activity is important in stem cells.

Answer 16

allow DNA replication to continue ;
idea that stem cell to, self renew / keep dividing ;
idea that stem cells differentiate into (named) specialised cells ;
idea that mitosis produces cells for, cell replacement / tissue repair ;
(maintains telomeres, so) prevents the loss of genes

Question 17

Explain why air movement increases the rate of transpiration

Answer 17

humid air / water vapour, moves away from the leaves ;
water potential gradient is steeper ;
higher rate of evaporation into the air spaces in the leaf

Question 18

Explain why the amino acid sequence of the polypeptide cannot be used to deduce this
sequence of nucleotides in the gene that codes for the polyhedrin polypeptide

Answer 18

ref. to genetic code ; in correct context
amino acids coded for by more than one codon / two or more codons for (most) amino acids ;
A triplet (of, bases / nucleotides)
64 codons and 20 amino acids ;
third base in codon can be different / only first two bases in codon are same ;
AVP ; idea that amino acid sequence is only the result of exons / gives information only about introns
only sure of sequence for, two amino acids / met and trp
degenerate code / degeneracy of code R degenerative
ref. to control sequences
STOP codons (also) have more than one triplet (of bases)
STOP codons do not specify amino acids

Question 19

explain the advantage of calculating the initial rate of reaction in each experiment.

Answer 19

1 allows a (valid) comparison of the, activities / AW, of the enzymes ;
2 initial rate is the fastest rate ;
3 idea that if rate is calculated over a longer period of time than the linear part of the curve the rate will vary ;
4 not limited by substrate concentration / substrate is in excess ;
A ‘no limiting factors’ if substrate not mentioned
5 not influenced by (increase in concentration of) product

Question 20

Explain how molecules of cellulose are arranged into a microfibril.

Answer 20

1 straight (chains) / linear / unbranched ;
2 arranged in parallel ;
3 hydrogen bonds between (cellulose) molecules ;
4 any detail, e.g. between H of –OH groups and –O of –OH groups ;
A between -OH groups
5 AVP ;
e.g. cellulose molecules have, staggered / overlapping, ends

Question 21

Describe the function of elastic fibres in lung tissue

Answer 21

1 allow alveoli, to stretch and recoil (during inhalation and exhalation) or allow, lung (tissue) / named airway, to
expand and recoil ;
2 prevent alveoli, over-stretching / bursting ;
3 recoils to move air out of alveoli ;
A force / expel / push / AW
A alveolar duct

Question 22

Explain how water moves up the xylem vessels to the leaves in a continuous column

Answer 22

1 cohesion / hydrogen bonding, between water molecules ;
2 detail about H bonding ;e.g. oxygen has, small / δ negative charge, hydrogen has small / δ positive charge
water molecules are polar so attract each other
3 adhesion / hydrogen bonding, between water molecules and, cell wall / xylem wall / cellulose / hydrophilic regions
of lignin ;
4 adhesion / AW), supports the column of water ;
5 transpiration / evaporation (from leaves), pulls the column or ref. to tension from, above / leaves or
ref. to water potential gradient from roots to leaves ;

Question 23

State why it is necessary for chloride ions to enter the red blood cell as hydrogencarbonate
ions leave

Answer 23

idea of a negative ion (chloride) entering RBC replaces a negative ion (hydrogencarbonate) leaving / reducing charge
separation (across the membrane) / maintaining electrical neutrality of the cell ;

Question 24

Outline the steps in the production of monoclonal antibodies by the hybridoma method.

Answer 24

1 (named) small mammal, injected / AW, with antigen ;
2 immune response occurs (over several weeks) ; A immune response described
3 plasma cells / B-lymphocytes / B-cells / splenocytes, extracted from spleen ;
4 plasma cells / activated B-lymphocytes / activated B-cells, fused with, myeloma / tumour / cancer / AW, cells (to for
hybridomas) ;
5 AVP ; e.g. hybridoma cells separated (into wells) to produce clones screening / selection / AW, for hybridomas
producing desired, (monoclonal) antibodies / Mabs ref. to large scale production

Question 25

State reasons why triglycerides are described as hydrophobic

Answer 25

1 (hydrocarbon / fatty acid tails, are) non-polar / have no polar groups / not charged ;
A equal sharing of electrons in molecule
2 not soluble / insoluble, (in water) ;
3 cannot form hydrogen bonds with water

Question 26

Explain why triglycerides are not suitable as a component of cell surface membranes

Answer 26

1 no, hydrophilic / polar / phosphate, head / part, to, interact with / AW, water ;
2 cannot form hydrogen bonds with water ; I repel water
3 cannot form a bilayer ; in water triglycerides, form micelles / form spheres / ball shaped / form globules / do not
form thin films

Question 27

Describe what happens at stage 2, shown in Fig. 6.1, to shorten the length of the RNA
molecule

Answer 27

1 removal of introns (from primary transcript / RNA) ;
2 exons joined together ;
or
3 splicing ;
4 of, primary transcript / RNA

Question 28

Outline the role of cotransporter proteins in companion cells.

Answer 28

1 movement of sucrose with, protons / H+ ions, into companion cells ;
A correct ref. to (other named) assimilates
2 (sucrose moved) from, apoplast / cell wall / mesophyll cell ;
3 sucrose moves against its concentration gradient (into companion cell) ;
4 (needs cotransporter protein because) sucrose is polar so cannot pass through membrane ;
5 maintain concentration gradient for sucrose between companion cell and sieve tube (element) ;

Question 29

Outline other features of active transport

Answer 29

(movement of substances) against concentration gradient / from low(er) to high(er) concentration ;
(protein has) binding site(s) ;
specific ; A description e.g. shape complementary to shape of substance
can be in context of protein or binding site
ref. to (binding causes protein), conformational change / change of shape

Question 30

Suggest why glucose molecules need to be cotransported with Na+ when it enters the
cell through the membrane protein

Answer 30

1 glucose is moving against a (concentration) gradient ; AW e.g. low to high
2 movement powered by inward movement of Na+ ; AW
3 Na+ movement (into cell from gut lumen) down an electrochemical gradient or down a (Na+), concentration /
diffusion, gradient or by facilitated diffusion ; A diffusion if response includes ref. to the cotransport protein /
cotranporter e.g. idea that, cotransport needed so that all glucose will be removed from gut lumen ora
facilitated diffusion of glucose will only be to equilibrium / AW cotransporter has (specific) binding site for glucose
idea that there is no specific active uptake protein for glucose

Question 31

Outline the standard treatment that can be used for suspected cholera cases.

Answer 31

oral rehydration (salts / solution / therapy) ; A ORS / ORT A rehydration therapy if ref. to ‘drinking’ stated within
response. solution of / water with, glucose and salts ; A named example for salts e.g. sodium / potassium / citrate /
chloride. AVP ; e.g. antibiotics / doxycycline / tetracycline / furazolidone / choramphenicol / sulfaguanidine (standard
treatment in some countries) zinc treatment (standard treatment for children in some countries) ref. to isolation

Question 32

Outline the similarities and differences between the introns and the exons

Answer 32

Question 33

Suggest why mineral ions are found within phloem sieve tubes and state how they are
transported within phloem sieve tubes.

Answer 33

at (phloem) source, mineral ion entry (with water), from xylem / through plasmodesmata from companion cells ;
dissolved in water / as solutes in water / in solution / in sap solution ;
in context of entry from xylem or within phloem sap
(transport) as part of mass flow ; must be in context of phloem
flow down a pressure gradient

Question 34

Describe the xerophytic features of the waxy cuticle and the stoma shown in Fig. 3.1 and
explain how these features adapt the plant to a xerophytic mode of life

Answer 34

decreases / reduces, transpiration ; A lower transpiration rate accept in the correct context once only
counts towards max two max two for cuticle thick (waxy) cuticle ; increased waterproof layer ;
increased (diffusion) distance for water vapour / less water vapour lost ; in context of cuticular transpiration
max two for stoma sunken stoma ; A stoma in depression / AW
moist / humid, air collects in area near to external environment ; AW
decreases water potential gradient ; in context of between sub-stomatal air space or area above stoma and external
environment. reduces diffusion of water vapour (out) via the stoma

Question 35

Explain why only one strand of the DNA of each gene is involved in the production of the
subunits.(in transcription)

Answer 35

any three from:
(one strand only needed) to form mRNA / mRNA is single-stranded ;
(m)RNA is, used / needed, to produce, subunit (s) / polypeptide(s) ;
(only) one strand (of DNA) is the, template / transcribed, strand ;
idea that (complementary) copying / transcribing, other DNA strand would not result in, desired / AW, mRN
A / polypeptide

Question 36

-resolution of light microscope:

Answer 36

200 nm

Question 37

resolution of electron microscope:

Answer 37

0.5 nm

Question 38

advantages of using a light microscope over electron microscope

Answer 38

1) portable
2) easier to use; no technical training required
3) able to see natural colours
4) able to see living tissue
5) particular tissues/organelles can be stained for better visibility
6) no heavy metal staining

Question 39

-outline the function and structure of a cell surface membrane

Answer 39

1) 7 nm
2) selectively permeable membrane that allows for the exchange of certain substances
3) barrier between cytoplasm and external environment
4) cell recognition (surface antigens)
5) selecting substances that enter and leave the cell
6) cell signalling
7) cell-to-cell adhesion
8) formation of hydrogen bonds with water for stability

Question 40

outline the function and structure of the rough endoplasmic reticulum

Answer 40

1) series of flattened fluid-filled sacs
2) have cisternae (flattened membrane disk)
3) ribosomes are attached to it
4) proteins made by ribosomes enter it
5) they’re modified as they go through it
5) small sacs (vesicles) break off of the RER and join together to form the golgi body

Question 41

-outline the function and structure of centrioles (& centrosome)

Answer 41

1) 9 triplets of microtubules form a centriole
2) outside the nucleus of animal cells, 2 centrioles are present at right angles to each other in a
region called centrosome (also referred to as an MTOC)
3) assembles microtubules to produce the mitotic spindle

Question 42

-outline the function and structure of plasmodesmata

Answer 42

1) allows transport of water, sucrose, amino acids, ions, etc between cells
2) without crossing membranes
3) this is movement through the symplast
4) allows communication, signalling between cells

Question 43

-features of amylose

Answer 43

1) made by condensation reactions between α-glucose molecules
2) long unbranching chain of 1,4 linked glucose molecules
3) chains are curved and cool up into helical structures making the final molecule more compact

Question 44

features of amylopectin

Answer 44

1) also made of 1,4 linked α-glucose
2) chains are shorter than amylose and branch out to sides
3) branches are formed by 1-6 linkages

Question 45

Cellulose

Answer 45

1) polymer of β-glucose
2) in order to form a glycosidic bond with C atom 4 where OH group is below the ring, one glucose
molecule is rotated 180°
3) successive glucose molecules are linked 180° to each other therefore one oxygen is up and other
is down

Question 46

-Cellulose fibres

Answer 46

1) hydrogen bonds result in a strong molecule
2) cellulose molecules become tightly cross linked to form bundles called microfibrils
3) microfibrils are held together in bundles called fibres by hydrogen bonding
4) cellulose fibres have very high tensile strength - makes it possible for a cell to withstand high
turgopressures

Question 47

-secondary structure

Answer 47

Hydrogen bonding of peptide backbone causing amino acids to fold into a repeating pattern
* due to H bonding between oxygen of -CO- group of amino acid and the hydrogen of -NH- group
* corkscrew shape - alpha helix
* weaker H bonding results in β-pleated sheets

Question 48

Tertiary structure

Answer 48

Three dimensional folding of polypeptide chain due to side chain interactions
1) hydrogen bonds (form between strongly polar groups (NH2, CO, OH)
2) disulphide bonds (form between cysteine molecules, strongest)
3) ionic bonds (between ionised amine (NH3+) and carboxylic acid groups (COOH-). Broken by pH
changes.
4) hydrophobic interactions (between non polar R groups)

Question 49

Globular proteins

Answer 49

1) spherical/ball shaped eg Hb, myoglobin
2) polar hydrophilic r groups are on the outside which makes mixing + dissociating in water easier
3) usually soluble
6) precise shape - have roles in metabolic activities, specific in nature

Question 50

Collagen

Answer 50

1) structural protein
2) found in tendons, skin, cartilage, bone, teeth etc
3) consists of 3 polypeptide chains each in a helical shape
4) 3 polypeptides are wound together creating a triple helix
5) 3 strands are held together by H and console by bonds
6) every 3rd amino acid is glycine
7) each 3 stranded molecule interacts with other collagen molecules running parallel to it
8) covalent bonds form between R groups of amino acids forming fibrils
9) many fibrils lie alongside each other forming strong bonds called fibres
10) flexible but has tremendous tensile strength

Question 51

adv of immobilising enzymes

Answer 51

1. Enzyme is reused
2. Enzyme is easily recovered
3. Product isn’t contaminated with enzymes
4. Reduces product inhibition
5. Enzyme is more stable/less likely to denature
6. Longer shelf-life of enzyme

Question 52

-outline the difference between the induced fit mechanism and lock and key mechanism of enzyme
action [4]

Answer 52

induced fit
1. shape of substrates not fully complementary to shape of active site
2. active site flexible / moulds around substrates
3. provides better fit / fully complementary
lock & key
1. shape of substrates complementary to shape of active site
2. active site does not change shape
3. substrate fits into active site

Question 53

Structure of enzymes

Answer 53

• Enzymes are globular proteins (spherical shape) that catalyse metabolic
  reactions
• Function as biological catalysts
• Specific in nature
• Precise 3D shape with hydrophilic R groups on the outside ensuring they’re soluble

Question 54

Immobilising enzymes

Answer 54

• enzyme is mixed with a solution of sodium alginate
• droplets of this mixture are added to calcium
  chloride solution
• a reaction occurs forming jelly/beads
• enzyme is immobilised in the bead

Question 55

fluid mosaic model

Answer 55

‘Fluid’ refers to the movement of phospholipids while ‘mosaic’ refers to the scattered proteins (and
glycoproteins) in the phospholipid bilayer

Question 56

-Cholesterol

Answer 56

• Regulates the fluidity of membrane
• At low temperatures, cholesterol increases the fluidity of the membrane preventing it from being
  too rigid, this is because it prevents close packing of phospholipid tails
• At high temperatures, cholesterol decreases the fluidity of membrane and stabilises the cell

Question 57

glycolipids and glycoproteins

Answer 57

carbohydrate chains projecting out of lipids+proteins-
* Stabilises the membrane structure by forming hydrogen bonds with water molecules
surrounding the cell
* Glycocalyx - sugary cell coating formed by carbohydrate chains
* Act as receptor molecules: (Signalling receptors - recognise messenger molecules like
hormones and neurotransmitters; Endocytosis - bind to molecule to be engulfed by membrane)
* Act as cell markers/antigens allowing cell-cell recognition

Question 58

-cell signaling cascade

Answer 58

1. signal arrives at protein receptor in cell membrane
2. the receptor’s shape is complementary to the ligand
3. the signal brings about a change in the receptor’s shape
4. changing the shape of the receptor allows it to interact with the next component of the pathway
   so the message gets transmitted
5. binding triggers/stimulates reactions within the cell
6. cell signalling results in a response which may be intracellular or extracellular

Question 59

-G1 phase

Answer 59

cells increase in size and synthesize new proteins and organelles

Question 60

S phase

Answer 60

• synthesis of dna
• dna in the nucleus replicated so that each chromosome consists of two identical chromatids

Question 61

G2 phase

Answer 61

• cell continues to grow
• the new dna is checked and any errors are corrected
• preparations are made to start cell division eg sharp increase in the production of Tubulin is
  observed which is needed to make microtubules for mitotic spindle

Question 62

Prophase

Answer 62

• chromatin starts to condense
• nuclear envelope and nucleolus disappears
• centrosomes migrate to opposite poles of the cell and form piles of mitotic spindle
• asters and spindle fibres form

Question 63

Metaphase

Answer 63

centrosomes are at cell poles and organise production of spindle fibres
- chromosomes line up across equator of spindle

Question 64

Anaphase

Answer 64

Chromatids are pulled apart by the contraction of spindle fibres and move to opposite poles of the
cell

Question 65

Telophase

Answer 65

• nuclear envelope and nucleolus reforms
• nucleus divides

Question 66

Cytokinesis

Answer 66

Division of the cytoplasm during cell division

Question 67

Cellular changes that occur in development of cancer

Answer 67

1) mutation occurs in gene that controls cell division leading to formation of oncogene from
protooncogene
2) this causes uncontrolled division
3) coordination of cell cycle is lost (cell does not respond to or receive signals that tell it to stop
dividing)
4) loss of function and lack of specialisation occurs
5) a tumour is formed, an irregular mass of cells showing an abnormal change in shape

Question 68

Process of semi-conservative replication

Answer 68

1) double helix of DNA is unwound by helicase
2) primase synthesizes a short piece of RNA called primer which marks the starting point for
synthesis of new strand
3) DNA polymerase uses the primer and synthesizes a new strand in a 5’-3’ direction (which means
DNA is read in 3’-5’)
4) leading strand is made continuously
5) when using lagging strand as template, DNA polymerase adds nucleotides in short fragments
“Okazaki fragments” to overcome directionality problem
6) DNA ligase seals up fragments of DNA in both strands to form a continuous double strand helix
-direction in which new DNA strand is synthesized
DNA polymerase builds the new strand in 5’ to 3’ direction; this means it moves along the old
template strand in 3’ to 5’
process of transcription
1. RNA polymerase binds to the promoter signaling DNA to unwind so bases can be read from the
sense strand
2. RNA polymerase reads sense strand 3’-5’ and generates mRNA in 5’-3’
3. when RNA has reached the terminator sequence at the end of the gene, transcription stops;
enzyme detaches from gene and DNA winds again
4. last triplet transcribed to mRNA is DNA triplet coding for ‘STOP’ (ATT, ATC, ACT)

Question 69

-process of translation

Answer 69

1) mRNA leaves the nucleus and binds to the smaller ribosomal subunit
2) every 3 bases on mRNA (a codon) codes for a specific anticodon which is carried by a tRNA
molecule which is covalently linked to a particular amino acid
3) an initiator tRNA adheres to a start codon
4) tRNA that corresponds to next codon enters ribosome carrying an amino acid
5) peptide bonds form between adjacent amino acids (a ribosome can house 2 tRNA molecules at a
time)
6) polypeptide chain grows until STOP codon is reached

Question 70

-Explain why the hydrogen bonding between the two strands of DNA is important for it to carry out
its functions. (4)

Answer 70

1) important in contributing to 3-D structure of molecule
2) many hydrogen bonds so gives stability
3) hydrogen bonds more easily broken (than covalent bonds)
4) Hydrogen bonds weak so can be broken
5) (so strands can be separated) for DNA replication
6) hydrogen bonds only form between, specific bases so few mistakes
7) hydrogen bonds can easily re-form (without chemical reaction)

Question 71

cohesion-adhesion theory

Answer 71

1) cohesive forces between water molecules cause them to be attracted to each other
2) water that is transpired out pulls water molecules below causing a transpiration pull
3) as they ascend up the xylem, water molecules adhere to the lignified walls of xylem which
maintains a column of water

Question 72

transpiration

Answer 72

1) loss of water from the aerial parts of the plant
2) an inevitable consequence of gas exchange as stomata are open
3) also due to water potential between leaf and atmosphere

Question 73

root hair to xylem

Answer 73

1) water taken up by root hairs crosses the root cortex & enters xylem
2) water moving thorough cells walls; apoplastic pathway
3) water moving through plasmodesmata; symplastic pathway
4) water moves from one cell to another till it reaches xylem; movement is due to concentration
gradient due to concentrated sap vacuole of cells
5) apoplastic pathway is stopped at endodermis due to cells in it having a band of suberin forming
the casparian strip (an impenetrable barrier to water)
6) Suberin deposits increase with age of endodermal cells except for certain passage cells
THIS ARRANGEMENT:
- gives plants control over what mineral ions pass into xylem vessels (everything has to
cross cell membrane)
- May help with generation of root pressure
7) once across endodermis, water moves into xylem through pits in their walls

Question 74

root to stem and leaf in xylem

Answer 74

1) removal of water from xylem vessels in leaf reduces hydrostatic pressure in xylem
2) hydrostatic pressure at top of xylem becomes less than bottom
3) pressure difference causes water to move up the xylem

Question 75

sucrose loading into phloem [5]

Answer 75

1) sucrose is loaded into sieve tube elements by active transport
2) H+ ions are pumped out of companion cells creating an excess of H+ ions in the apoplastic
pathway outside the cell
3) H+ ions move back into the cell down their concentration gradient through a carrier protein for
both H+ and sucrose
4) sucrose molecules are carried against their concentration gradient by co-transporter molecules
5) sucrose can them move through plasmodesmata into the sieve tube elements

Question 76

source to sink [4]

Answer 76

1) when sucrose is loaded into a sieve tube element, the water potential decreases
2) water movies from surrounding tissue by osmosis
3) this increases the hydrostatic pressure
4) assimilates move down a pressure gradient to the sink

Question 77

Explain the ways in which the structure of an artery is adapted to its function

Answer 77

1) thick tunica media to withstand high blood pressure
2) endothelium/tunica intima are smooth so there’s little friction to blood flow
3) elastic fibres stretch to allow surges in blood flow and recoil to maintain blood pressure
4) smooth muscle contacts to regulate blood flow
5) collagen fibres avoid rupturing

Question 78

Structure of veins

Answer 78

1) large lumen
2) tunica externa is thickest in veins (mostly collagen fibres)
3) tunica media is very thin (contains some smooth muscle and fibres)

Question 79

tissue fluid

Answer 79

1) as blood flows through capillaries within tissues some plasma leaks out due to the pressure and
seeps into places between cells of tissues; this is tissue fluid
2) fluid tends to flow out of capillaries at arterial ends and into capillaries near venuous ends
3) if blood pressure is too high, too much fluid is forced out of capillaries causing accumulation of
fluid; oedema
6) it’s through tissue fluid that exchange between cells and blood occur

Question 80

outline the cardiac cycle

Answer 80

1) SAN (pacemaker) sends waves of excitation which stimulates atria to contact
2) non conducting tissue between atria and ventricles prevent them from contracting together
3) AVN delaying impulse allows atria to empty into ventricles/ contract first
4) AVN sends impulses down to bundle of his and along purkine fibres
5) this causes ventricles to contact from the base upwards
- brief break when no impulse is generated allowing blood to enter atria

Question 81

Bohr shift

Answer 81

1) presence of high partial pressures of carbon dioxide causing Hb to release oxygen is the Bohr
effect
2) carbon dioxide dissolved in cytoplasm of RBC forming carbonic acid, reaction is catalysed by
carbonic anhydrase
CO2 + H2O <—-> H2CO3
H2CO3 <—-> H+ + HCO3-
3) Hb has higher affinity for H+ ions than O2 therefore O2 is released

Question 82

Carbon dioxide transport

Answer 82

85% - HCO3- diffuse out of RBC into blood plasma and are carried in solution
5% - CO2 that hasn’t dissociated dissolves in blood plasma
10% - CO2 combines directly with anime groups of Hb forming carbaminohaemoglobin

Question 83

how carbon dioxide (CO2) and For hydrogen ions (H+) play a role in the unloading of oxygen from
haemoglobin [5]

Answer 83

1) diffusion of carbon dioxide
2) into red blood cell from correct source
3) carbonic acid formation followed by H+ production due to action of enzyme
4) carbonic anhydrase
5) haemoglobin has a higher affinity for hydrogen ions than oxygen; haemoglobin releases oxygen
more easily in acidic conditions
6) formation of haemoglobinic acid
7) higher partial pressures of CO2 linked to (oxy)haemoglobin releasing oxygen more readily; Bohr
shift
8) formation of carbamino-haemoglobin

Question 84

Explain the need for transport systems in multicellular plants and animals (3)

Answer 84

1) small surface area to volume ratio
2) long(er) distances (to reach cells / tissues)
3) diffusion (alone), too slow / insufficient / unable to satisfy needs

Question 85

Outline the role of Hb in the transport of CO2

Answer 85

1) Hb combines with carbon dioxide
2) carbon dioxide reacts with amine groups
3) to form carbaminohaemoglobin
4) each polypeptide can carry a molecule of CO2
5) CO2 remains bound to Hb until in a region of high pO2
6) carbonic acid dissociates to form H+ ions which bind to Hb forming haemoglobinic acid
7) hydrogencarbonate ions to plasma

Question 86

How alveoli are adapted for gas exchange

Answer 86

1) squamous epithelium is one cell thick
2) for short diffusion distance
3) collectively many so large surface area for gas exchange
4) surrounded by a network of capillaries
5) so RBC can come very close to air maintaining diffusion gradient

Question 87

elastic fibres in alveoli walls

Answer 87

1) stretch during inspiration
2) recoil during expiration
3) elasticity allows alveoli to expand according to volume of air breathed in

Question 88

Describe the role of macrophages in the lungs. [3

Answer 88

1. prevention of infections
2. prevents pathogens from entering rest of body/blood
3. carry out phagocytosis
4. engulf pathogens
5. macrophages are antigen presenting cells (APCs)

Question 89

Outline the function of cartilage in the human gas exchange system.

Answer 89

1. keep airways open
2. provides support
3. allow flexibility
4. rings allow widening during inspiration

Question 90

Outline the function of smooth muscle in the human gas exchange system.

Answer 90

1. contraction and relaxation
2. changes diameter of trachea, bronchus, bronchioles
3. controls air flow in bronchioles

Question 91

factors that led to the successful eradication of smallpox (5)

Answer 91

1) smallpox virus was stable/did not mutate
2) same vaccine was used for whole programme/vaccine did not need to be changed
3) vaccine was live/gave a strong immune response
4) one dose was enough to give life-long immunity/no boosters required
5) heat stable/freeze dried vaccine
6) suitable for hot countries/isolated areas/rural areas
7) few/no symptomless carriers
8) no animal reservoir/only in humans
9) infected people easy to identify

Question 92

advantage of calculating the number of cases per 100 000 population rather than stating the
number of cases alone (2)

Answer 92

1) number of cases per 100 000 shows, proportion of population affected
2) easier to visualise, the severity of the problem
3) more reliable for making comparisons between different countries

Question 93

how a person may become infected with TB (3)

Answer 93

1) Infected person coughs
2) aerosol droplet carrying the pathogen
3) inhaled by uninfected person
4) consumption of meat/milk of an organism carrying TB

Question 94

How infection of malaria occurs

Answer 94

1) female anopheles mosquito feed on human blood to obtain protein they need to develop eggs
2) if an infected person is bitten, mosquito takes up pathogen’s gametes with the blood meal
3) make and female gametes fuse in mosquito gut and develop into infective stages that move to
salivary glands
4) when mosquito feeds again, an anticoagulant is injected from salivary glands causing infective
stages to pass out as well
5) anticoagulant is injected to prevent blood from clotting
6) parasites enter bloodstream>liver cells>RBC

Question 95

Control of malaria

Answer 95

1) stocking ponds, irrigation and other permanent bodies of water with fish which feed on mosquito
larvae
2) introduction of dip stick to ensure fast diagnosis
3) sequencing entire genome of plasmodium that may lead to development of effective vaccines
4) drugs used in combination to reduce changes of drug resistance arising
5) spraying a preparation containing the bacteria which kills mosquito larvae but isn’t toxic to other
forms of life

Question 96

How HIV affects the body

Answer 96

1) genetic material of HIV is RNA
2) viral RNA is converted to DNA inside host cells to be incorporated into human chromosomes
3) infects and destroys t helper cells (control the immune system’s response to infection)
4) when no of t helper cells are low, body is unable to defend against infection allowing pathogens
to cause opportunistic infections
5) AIDs isn’t a disease; it’s a collection of opportunistic diseases associated with the
immunodeficiency caused by HIV infection

Question 97

treatment of HIV

Answer 97

1) drug therapy (eg zidovudine)
2) it binds to viral enzyme reverse transcriptase and blocks its action
3) this stops replication of viral genetic material and leads to increase in body’s lymphocytes
4) combination therapies are difficult to follow
5) people unable to keep up with regimen are susceptible to strains of HIV that have developed
resistance to drugs

Question 98

Explain why malaria is very difficult to control. [4]

Answer 98

1. no vaccine due to
2. Plasmodium is eukaryotic / antigens differ in different life stages / intracellular parasite / antigenic
   concealment
3. drug resistance in Plasmodium
4. insecticide resistance in Anopheles
5. cost of drugs
6. problems with funding research
7. lack of knowledge
8. infected people not identified
9. inaccessibility of some regions to healthcare

Question 99

phagocytosis

Answer 99

1) during an infection cells under attack respond by releasing histamines that attract neutrophils
2) this attraction towards a chemical stimulus is called chemotaxis
3) endocytosis
4) bacteria within a phagocytic vacuole
5) fusion of lysosomes and phagocytic vacuole
6) killing and digestion

Question 100

hybridoma method

Answer 100

1) antigen is injected into a mouse
2) spleen cells which produce lymphocytes which produce antibodies are removed
3) plasma cells from spleen are fused with cancer cells/myeloma cells forming hybridoma cells that
divide indefinitely
4) they divide by mitosis and produce antibodies

Question 101

using monoclonal antibodies in diagnosis

Answer 101

1) used to locate position of blood clots
2) locate cancer cells which have different cell surface proteins and therefore can be detected by
antibodies
3) used to identify exact strain of bacterium or virus causing an infection speeding up the treatment

Question 102

How vaccines provide long term immunity [5]

Answer 102

1) vaccine contains antigens that stimulates an immune response
2) macrophages take up virus by phagocytes and act as antigen presenting cells (APCs)
3) lymphocytes bind to these and undergo clonal selection
4) clonal expansion occurs by mitosis
5) memory cells are formed
6) booster is used to further stimulate memory cell formation

Question 103

process of cell-mediated immune-response

Answer 103

1) macrophage engulfs pathogen and becomes an APC
2) Th cell’s receptor (which is complementary to the antigen) binds to the antigen displayed by APC
3) Th cell releases interleukins
4) interleukins stimulate B-cells to differentiate into plasma cells and produce antibodies in the
humoral response

Question 104

process of humoral immune-response

Answer 104

1) clonal selection (process by which an antigen selectively binds to and activates only those
lymphocytes bearing receptors for the antigen)
- B cell with receptor for antigen binds to it and becomes a B-APC
- Th cell recognises B-APC and becomes activated, releasing interleukins to signal further actions
2) clonal expansion (rapid multiplication of B or T cell clones after activation by an antigen)
- B-APC divides and differentiates into plasma cells to make specific antibodies and also into
memory cells to prepare for secondary response

Question 105

Describe how the structure of an antibody molecule allows it to be specific for one antigen. [3]

Answer 105

1. antibody complementary shape to antigen
2. variable regions different for each antibody
3. specific antigen binding sites on antibody
4. sequence of amino acids varies between antibodies giving specificity
5. difference in primary structure leading to different tertiary structur