Mark Scheme (Results)

Summer 2009

GCE

GCE SNAB Biology (6136)

Edexcel Limited. Registered in England and Wales No. 4496 50 7 Registered Office: One90 High Holborn, London WC1V 7BH

Unit SN6 (6136/01) - Synoptic paper

Question Answer Mark Number 1(a)

max(3)

1. (they published) the theory of evolution by natural selection / eq ;

2. theory (appeared to) contradict prevailing {creationist / Genesis / biblical / religious / eq} point of view / eq ;

3. especially idea that humans evolved from {primates / apes / monkeys / simpler organisms / eq} ;

4. (theory required) Earth to be much older than thought by most people at the time / eq ;

5. theory caused heated debate / eq ;

6. lack of knowledge of {genes / inheritance / eq} (at the time) ;

Question Answer Mark Number 1(b)

max(4)

1. if a CREB gene does not work then (long-term) memories cannot form / eq ;

2. (this is evidence) for role of nature in ability to form memories / eq ;

3. {the act of learning / environmental factors / stimulus / eq} turns on the genes / if you do not learn anything genes are not activated / eq ;

4. (this is evidence) for the role of {nurture / environment} in ability to form memories / eq ;

5. CREB genes needed to form new synapses ;

6. synapses store memories in the brain / eq ;

Question Answer Mark Number 1(c)

Markchang

max(4)

as a pair i.e. change & effect – maximum 2 es

1. habitat destruction by humans / deforestation / eq ;

2. effect on human population e.g. less water available ;

3. climate change / global warming / eq ; 4. effect on human population e.g. flooding ;

5. overfishing / overstocking / overhunting / eq ;

6. effect on human population e.g. loss of food supply ;

7. named pollutant e.g. heavy metal ; 8. effect on human population e.g.

contaminated water ;

9. overcrowding / eq ; 10. e.g. more disease / lack of clean water

supplies / eq ;

Question Answer Mark Number 1(d)

Reas reement about If th

If th

max(2)

on must be linked to agreement or disagStephen Mobley’s defence.

ey disagree with his defence:

1. {environment / nurture} also effects {behaviour /personality / genes} /eq ;

2. knowledge of {instinct /criminal tendency /eq} can make it possible to {overcome /avoid /override} that instinct / eq ;

3. we still have choices / free will despite influence of genes / eq ;

ey agree with the defence:

1. his inherited genes from his criminal ancestors caused him to commit murder ;

2. genes determine his personality / nature causes his instinct /eq ;

3. genes prevent ‘free will’ / eq ;

Question Answer Mark Number 1(e)

max(2)

1. if {young rats / pups} are groomed by their mothers, {their DNA is not methylated /a methyl group is not added to their DNA} ;

2. a receptor gene (in the hippocampus) responsible for mediation of stress not affected / eq ;

3. groomed rats produce lower levels of stress hormones ;

Question Answer Mark Number 1(f)

max(2)

1. twin studies help to quantify the relative contribution of genes and the environment (in determining a characteristic) /eq ;

2. {identical twins /twins with the same genes} have 80% correlation (in body mass) AND {fraternal twins/non-identical twins} have {43% / less} correlation (in body mass) ;

3. (therefore) both {genes / nature} and {the environment / nurture} have an influence on body mass ;

4. if identical twins show any differences it must be due to the environment / eq ;

Question Answer Mark Number 1(g)

max(3)

1. gene {expression / transcription /induction} is influenced by other {genes / proteins made by other genes} ;

2. transcription factors are products of other genes / eq ;

3. correct reference to {promoters / transcription initiation complexes} ;

4. characteristics can be determined by {several different genes / environmental factors / polygenic /multifactoral} ;

5. correct reference to epistasis ;

Question Answer Mark Number 2(a)

max(4)

(i) 1. natural selection / survival of the fittest / eq ;

2. different selective pressures in each place / evolved according to local environmental conditions / eq ;

3. (random) mutations (in each place) / founder effect ;

4. reproductive isolation / different species can not inbreed / eq ;

5. geographical isolation / (sea is a) barrier to gene flow / eq ;

6. (sea is a) barrier to exchange of pollen / seeds ;

7. reference to {allopatric/ eq} speciation ;

8. genetic drift / eq ;

Question Answer Mark Number 2(a)

succ

(1) (ii) ession / colonisation ;

Question Answer Mark Number 2(a)

redu

(1) (iii) ced competition (from other plants) / eq ;

Question Number

Answer Mark

2(a)(iv)

limited genetic diversity / small gene pool / inbreeding / less

(1)

frequent mutations / eq ;

Question Number

Answer Mark

2(b)

1. {characteristic of / phenotype of / round / purple

2. because these characters are the same in both

3. {stem length / spacing of leaves} is

inerals}} /

4. because these characters are different in both

5. {stem length / spacing of leaves} is caused by {soil

max(3)

/ hairy} leaves are determined by genes / eq ;

soils / eq ;

{environmental / affected by {soil / meq ;

soils / eq ;

/ edaphic} conditions rather than climate / eq ;

Question Number

Answer Mark

2(c)

1. numbers (of Edmondston’s Chickweed) have been

2. the number of growing days has been increasing /

3. growing days data very variable (trend difficult to

4. possible increase in rainfall / pattern {not a very

5. (according to Figure 5) the numbers (of

variable

6. so {difficult / too early} to establish if there is a

7. (according to Figure 6) plant data only comes

of

8. no direct measurement of temperature / eq ;

9. (according to Figure 5) no data was collected

10. no statistical evidence / eq ;

11. reference to other factors may affect growth ;

12. credit specific reference to data illustrating a

max(4)

declining since 1995 / the decline was steep 1996 to 2000 ;

eq ;

recognise) / not a long enough series of data to establish long term trends / might be only short term patterns / eq ;

clear one / inconclusive} ;

Edmondston’s Chickweed) have been very(1978-2006 / over past three decade) ;

long-term trend / eq ;

from one quadrat – may not be representative the whole site ;

between 1996 and 2006 ;

point or trend e.g. maximum growth does not correspond with {maximum rainfall / maximumgrowing days} ;

Question Number

Answer Mark

2(d)

1. allows observations on exactly the same area /

2. allows comparisons year on year / eq ;

(2)

eq ;

Question Number

Answer Mark

2(e)

1. GPP = photosynthesis only / eq ;

2. NPP = photosynthesis – respiration / eq ;

3. temperature increases {metabolism / chemical

4. reference to enzymes (being effected by

5. (temperature has) greater effect on respiration /

6. photosynthesis limited by another factor (other

max(4)

reactions / respiration} / reference to kinetic effect ;

temperature) ;

respiration going up faster than photosynthesis / eq ;

than temperature) / eq ;

3. While our biological understanding and medical technology may have advanced

ctised

Blood – The fluid of life’: Write an essay about the importance of blood and circulatory

Breadth: maximum of 6 marks

te if e

le. If

ly

to

cript

Depth: maximum of 8 marks andidates

e an idea

m of 4 B marks per

he script as

considerably over the last 200 years, the art of balancing bodily fluids has been prafor centuries. ‘systems.

These marks are to be awarded to the candidathey successfully introduce thgeneral area of Biology relevant to the essay tita relevant B point is awarded then the corresponding A pointshould also be awarded. Key ideas to look for are in bold type – the candidate need onshow evidence that he or she realises that key idea is appropriate in the essay gain a breadth (A) mark. Indicate A1 to A10 on the sas appropriate

These marks are awarded to cfor demonstrating an understanding of relevant A level biological detail expanding on the areas of biologyintroduced in A. This list is not exhaustive but is designed to givof the type of response worthy of credit for a (B) mark. Allow a maximucorresponding A mark. Indicate B1a to B10a on tappropriate

1 iscussion/explanation of

1a

1b

1c

escription of mass transport

iscussion of limitations of diffusion to

escription of what is meant by blood

A Dwhy many animals/humanshave a heart and circulation

B B B

D Dprovide an organisms needs Dpressure and how it is involved in masstransport

2

iscussion of the role of

2a

2b

2c

2d

tructure of arteries and veins compared

tructure of capillaries related to their

iscussion of the advantages of a closed

etailed description provided of the es

A Dblood vessels

B B B B

Sand related to their functions Sfunction in exchange systems Dcirculatory system compared to an open circulatory system Ddifferent layers of tissue in the arteriand veins related to their functions e.g. tunica media, collagen and elastin fibres etc.

A3

Discussion of the structure and function of the heart

B3a B3b B3c B3d

Discussion of the double circulatory system and why it is an advantage to pump the blood twice for each complete circuit of the body Good description of the structure of the heart including 4 chambers, valves, etc – may be included in a diagram Good description of the cardiac cycle including diastole, atrial systole, ventricular systole Description of the control of the cardiac cycle including {SAN/eq} and AVN as a minimum

A4

Discussion of the role of the blood in transport around the body

B4a B4b B4c B4d B4e

At least 3 named examples of materials carried around the body via the blood Role of blood in the endocrine system explained with at least one example Good description of at least one exchange network together with the molecules exchanged and why e.g. oxygen and glucose going into tissue fluid for providing cells the things needed for aerobic respiration, while removing carbon dioxide for excretion in the lungs Description of how carbon dioxide is carried in the blood including reference to hydrogen carbonate Correct reference to the Bohr shift and how it helps the release of oxygen from haemoglobin in actively respiring tissues

A5

Description of the cells associated with blood

B5a B5b B5c B5d

Reference to red blood cells, white blood cells and platelets Description of how oxygen is carried in red blood cells involving haemoglobin Description of the role of platelets in blood clotting Discussion of the role of the bone marrow / stem cells in the production of new blood cells

B5e

Discussion of the role of erythropoietin in the stimulation of the production of more red blood cells

A6

Description of the role of blood in homeostasis

B6a B6b B6c B6d B6e

Explanation of the role of negative feedback in maintaining systems within narrow limits Description of a control mechanism for CO2 levels Description of the role of blood in the distribution of heat and thermoregulation, especially vasoconstriction / vasodilation Osmoregulation /oedema related to role of blood / blood pressure / damage to blood vessels Reference to receptors in circulatory system e.g. chemoreceptors

A7

Discussion of the role of the blood system in defence and immunity

B7a B7b B7c B7d B7e B7f B7g

Description of the blood clotting process including at least 2 of thromboplastin, prothrombin and fibrinogen Discussion of antibodies and how they can affect antigens Description of how plasma cells are produced and stimulated into producing antibodies – include antigen activation and role of cytokines Description of role of killer / cytotoxic T cells Discussion of how immunity may be acquired including the role of memory cells Description of role of phagocytic blood cells such as macrophages in the non specific immune system / antigen presentation / describe the role of blood cells in the inflammatory response Reference to role of T helper cells and cytokines

A8

Discussion about conditions that can affect the health of the blood or circulatory system.

B8a B8b B8c B8d B8e B8f B8g B8h B8i B8j

Clear description of atherosclerosis / blood clots leading to stroke / myocardial infection / eq Discussion of the potentially damaging effects of high blood pressures and conditions that can lead to high blood pressure e.g. obesity / stress Discussion of inheritance of risk such as hypercholesterolemia. Discussion of effect of diet / drugs on balance / risk of atherosclerosis etc with at least one clear named and explained example Discussion about leukaemia and its effects Discuss the importance of balancing LDL and HDLs Haemophilia Auto-immune responses Description of the effect of blood loss e.g. shock Sickle cell anaemia

A9

Discussion about medical intervention and the heart/blood/circulatory system.

B9a B9b B9c B9d B9e B9f

Discussion of at least one named drug and its effect on the blood e.g. warfarin, aspirin, etc. Discussion of surgery e.g. balloon angioplasty, heart transplants, valve replacements etc. Discussion of xenotransplantation as a potential alternative to human donor organs due to shortages of suitable donors Discussion about lifestyle factors that can be changed to reduce risk of CVD (do not credit exercise here) Reference to blood donors saving lives. Blood used to test / research drugs / eq

A10

Discussion about exercise and the effects it can have on the circulatory system

B10a

Discuss the benefits of exercise on the cardiovascular system such as strengthening the cardiac muscle, stimulating red blood cell formation, reducing cholesterol levels etc.

Unpacking the question:

C1 Response shows a clear understanding that a circulatory system enables animals to grow bigger and maintain a higher metabolic rate. C2 Good description conveying a clear message that a healthy circulatory system is needed for the health of the entire organism. C3 Descriptions of the effects of the sympathetic and parasympathetic nervous systems on the circulatory system. C4 Clear description of tissue fluid formation and how tissue fluid is returned to the circulatory system. C5 Outline description of the lymphatic system and its roles in association with the circulatory system. C6 Response includes a clear comparison of the systemic and pulmonary circulatory systems together with their roles. C7 Response shows a clear understanding that control of the circulatory system enables the body to respond to different demands placed upon it. E.g. in response to adrenaline increased blood flow to muscles and brain, but decreased flow to digestive system enables improved response to danger. C8 Comparison between blood flow in veins and arteries, including how movement of blood is achieved in veins. C9 Clear description about how cardiac muscle is unique and different to skeletal and smooth muscle, e.g. myogenic and does not fatigue. C10 Importance of the coronary circulatory system to supply the cardiac muscle with what it needs.

D: Coherence, clarity and expression of the answer: maximum of 4 marks This strand will award students for style of their answer and is quite distinct from mentioning the big ideas (C). It isn’t what candidates say but how they say it. 4 marks: A truly synoptic essay which links together information from different parts of the specification in a coherent and logical style (introduction, conclusion, good use of paragraphs and well illustrated by examples). Good spelling, punctuation, grammar and sound use of technical terminology. 3 marks: Good logical structure with good spelling, punctuation, grammar and sound use of technical terminology, but tends to be a collection of information which, although relevant, tends to be disjointed and only partly attempts to synthesise information. 2 marks: A reasonably coherent account that includes satisfactory spelling, punctuation and grammar, which tends to be disjointed. A collection of information with little or no attempt to link ideas together. 1 mark: Some relevant information is presented in an intelligible way using correctly formulated simple sentences. 0 marks: The use of English is not adequate to convey scientific information beyond naming a list of examples. A candidate who has scored some marks (particularly in strand A) for mentioning some relevant points may nevertheless fail to score marks in strand D if he or she fails to form simple sentences. Note that the maximum total mark which can be awarded is 20

4. In the absence of enzymes, chemical reactions in organisms would be too slow to sustain life. ‘Enzymes – The miracle molecules’: Write an essay about the structure and functions of enzymes.

Breadth: maximum of 6 marks These marks are to be awarded to the candidate if they successfully introduce the general area of Biology relevant to the essay title. If a relevant B point is awarded then the corresponding A point should also be awarded. Key ideas to look for are in bold type – the candidate need only show evidence that he or she realises that key idea is appropriate in the essay to gain a breadth (A) mark. Indicate A1 to A11 on the script as appropriate

Depth: maximum of 8 marks These marks are awarded to candidates for demonstrating an understanding of relevant A level biological detail expanding on the areas of biology introduced in A. This list is not exhaustive but is designed to give an idea of the type of response worthy of credit for a (B) mark. Allow a maximum of 4 B marks per corresponding A mark. Indicate B1a to B11 d on the script as appropriate

A1

Description of enzymes as catalysts

B1a B1b B1c B1d B1e

Lower activation energy Provide alternative reaction pathway (via enzyme/substrate complex) Speed up reactions without being used up themselves The presence of an enzyme does not alter the nature or properties of the end products of the reaction Enzymes are very efficient: a very small amount of enzymes brings about the change of a large amount of substrate.

A2

Enzymes are proteins

B2a B2b B2c B2d

Globular proteins Hydrophilic {R groups/eq} on outside of the protein and inside active site enable enzymes to work in aqueous solutions Most enzymes are far larger than the substrates they act on Good description of levels of protein structure and shape provided

A3

Mechanism of enzyme action

B3a B3b B3c B3d

Enzymes are specific to their substrate due to the shape of their active site Correct description of lock and key hypothesis of enzyme action Correct description of induced fit hypothesis of enzyme action Evidence for enzyme action has come from X ray crystallography and/or computer assisted modelling

A4

Factors affecting the rate of enzyme reactions

B4a B4b B4c B4d B4e B4f B4g B4h B4i B4j B4k

pH and temperature Substrate concentration and enzyme concentration Rate of reaction is measured by the amount of substrate changed, or amount or product formed, during a period of time Description of temperature coefficient Q10

Temperature affects kinetic energy of molecules and shape of the protein / can denature the enzyme Explanation that changes in pH alter the ionic charge of acidic and basic groups of amino acids and therefore effect ionic bonding in the enzyme Small changes in pH may produce reversible changes in the shape of the active site, larger changes will produce permanent denaturation Enzyme/substrate concentrations (and kinetic energy) effect number of collisions and therefore number of enzyme/substrate complexes formed Enzyme inhibitors mentioned Good description of competitive or non competitive inhibitors provided Reference to role of coenzymes / cofactors

A5

Role of enzymes in respiration/photosynthesis

B5a B5b B5c B5d B5e B5f

Role of coenzymes described for photosynthesis or respiration e.g. NAD, NADP, Coenzyme A Most enzymes associated with respiration and photosynthesis are found packaged into a specialised organelle to increase their concentration Matrix of mitochondria / stroma of chloroplast Calvin cycle / glycolysis / Krebs cycle / link reaction enzyme controlled Example of specific action of a named enzyme within photosynthesis or respiration e.g. ribulose bisphosphate carboxylase (Rubisco), alcohol dehydrogenase, glucose isomerase, decarboxylase enzymes, dehydrogenase enzymes Reference to ATPase / chemiosmosis

A6

Role of enzymes in replication/transcription/cell cycle

B6a B6b B6c B6d B6e B6f B6g B6h

Helicase in DNA replication DNA polymerase needed to synthesise new DNA strand in replication DNA ligase used to fill in gaps on one of the newly synthesised DNA strands RNA polymerase needed to make RNA in transcription Enzymes needed to load tRNA molecules with amino acids Enzymatic action of ribosomes forms peptide bonds between amino acids in translation Active CDK stimulates mitosis in the cell cycle Reverse transcriptase / integrase in retrovirus replication

A7

Role of enzymes in anabolism/catabolism of other biological molecules e.g. digestion

B7a B7b B7c B7d B7e B7f B7g B7h B7i

Role of enzymes in the digestive system described Specific examples of at least 2 digestive enzymes mentioned Role of enzymes in forming bonds by condensation reactions to build up biological molecules e.g. starch/ cellulose/ glycogen/ proteins, etc. Describe digestion of food as an example of a hydrolysis reaction. Role of enzymes in germinating seeds described Role of enzymes in decomposition Role in autolysis of cells Role of ATPase in hydrolysis of ATP in muscle contraction Role of enzymes in acrosome reaction

A8

Role of enzymes in gene technology

B8a B8b B8c B8d B8e B8f

DNA polymerase needed for PCR Restriction enzymes used to cut DNA Ligase used to join strands of DNA together e.g. making recombinant DNA for genetic modification, gene therapy, etc. Cellulases used to make plant protoplasts for genetic modification Reverse transciptase Integrase

A9

Other Industrial applications of enzymes

B9a B9b B9c

Biological washing powders/stain removers Used in brewing e.g. malting/mashing process Predigest baby foods e.g. trypsin

B9d B9e B9f B9g B9h B9i B9j B9k B9l B9m B9n

(B galactosidase/lactase) used to digest lactose in milk for lactose intolerance (Cellulases/pectinases) to produce fruit juice (Rennin) to make cheese Meat tenderisers (Amylases) used to make syrups from starch (Glucose isomerase) converts glucose into fructose for high sweetness but low calorific values Used in paper industry Cellulases used to break down cellulose into sugars for the biofuel industry. Proteases used in contact lens cleaners to remove proteins and prevent infections (Proteases) used to reclaim silver from photographic film Reference to immobilisation of enzymes

A10

Role of enzymes in controlling synapses

B10a B10b B10c B10d

Enzymes break down neurotransmitters at synapses (to prevent continuous activation of the synapse) Drugs may affect synapses by blocking/inhibiting the action of the enzymes that breakdown the neurotransmitter Example e.g. acetylcholinesterase Reference to condition caused by an imbalance of neurotransmitter if linked to enzymes activity e.g. Parkinson’s disease

A11

Enzymes associated with the immune system

B11a B11b B11c B11d

Lysozyme to digest cell walls in bacteria in tears, etc. Role of enzymes in blood clotting e.g. thromboplastin, thrombin Proteases released by {killer / cytotoxic} T cells to destroy infected cells Hydrolytic enzymes used by phagocytic white blood cells to destroy pathogens etc

Unpacking the question:

C1 Reactions can take place at a high rate at a temperature suitable for living organisms due to action of enzymes.

C2 Enzymes often work together in metabolic pathways, where the reactions proceed in a controlled manner due to the specific nature of the enzymes.

C3 Mutations in genes can alter the structure of the protein made, effecting {the tertiary structre/ 3D shape} and therefore may change the active site and function of an enzyme.

C4 Description of lac operon to convey the control of enzyme production by the substrate it acts on.

C5 Coenzymes act as cofactors but do not remain attached to the enzyme between reactions e.g. NAD, coenzyme A

C6 Some enzyme pathways are regulated by the products they produce – end- product inhibition or negative feedback inhibition.

C7 Homeostasis is vital in the body to maintain optimum conditions for enzyme activity.

C8 Enzymes are adapted to have optimums corresponding to the position of the enzymes activity e.g. pepsin optimum pH ~2.

C9 Bacteria thrive in a wide range of environments and therefore their enzymes may work in environmental extremes as well, making them useful for industrial applications.

D: Coherence, clarity and expression of the answer: maximum of 4 marks This strand will award students for style of their answer and is quite distinct from mentioning the big ideas (C). It isn’t what candidates say but how they say it. 4 marks: A truly synoptic essay which links together information from different parts of the specification in a coherent and logical style (introduction. Conclusion, good use of paragraphs and well illustrated by examples). Good spelling, punctuation, grammar and sound use of technical terminology. 3 marks: Good logical structure with good spelling, punctuation, grammar and sound use of technical terminology, but tends to be a collection of information which, although relevant, tends to be disjointed and only partly attempts to synthesise information. 2 marks: A reasonably coherent account that includes satisfactory spelling, punctuation and grammar, which tends to be disjointed. A collection of information with little or no attempt to link ideas together. 1 mark: Some relevant information is presented in an intelligible way using correctly formulated simple sentences. 0 marks: The use of English is not adequate to convey scientific information beyond naming a list of examples. A candidate who has scored some marks (particularly in strand A) for mentioning some relevant points may nevertheless fail to score marks in strand D if he or she fails to form simple sentences. Note that the maximum total mark which can be awarded is 20