Biology HL Paper 2 Long response questions

Explain how the properties of water that are essential to living things arise from the dipolar nature of water.

1. oxygen in water is slightly negatively charged and hydrogens are slightly positive

2. hydrogen bonding due to dipolar nature

3. Water molecules are cohesive due to hydrogen bonding

4. cohesion useful in xylem transport

5. hydrogen bonds with other structures giving adhesive properties

6. adhesion of water cellulose in cell walls

7. high boiling point due to cohesion/hydrogen bonding

8. water is liquid rather than a gas over the global temperature range

9. high latent heat of vaporisation as energy needed to rbak hydrogen bonds

10. Use of sweat for cooling the body

11. high specific heat capacity as hydrogen bonds must be broken to warm water up

12. water is a thermally stable habitat

Describe the adaptions of plants in deserts for water conservation

1. Thick waxy cuticle to reduce cuticular transportation

2. few/small stomata

3. stomata that open at night when it is cooler

4. Leaf surface are small reduced

5. water storage tissue in leaves/stems/roots

6 Deep extensive roots

Outline the use of models to investigate the transport of water in xylem

1. Models allow for one factor/aspect to be studied independently

2. Glass capilalry tubes to model adhesion between water and xylem vessel walls

3. Porous pot to model flow in a xylem vessel due to transportation for the lead

4. Blotting paper

Explain the exchange of materials between the mother and the foetus in the uterus

Outline how William harvey changed the understanding of blood flow around the human body

Describe how in microscope images, blood vessels can be identified as arteries, capillaries and veins

Explain how new species can emerge by:

1. directional selection

2. disruptive selection

3. Polyploidy

Outline the advantages to scientists of the binomial system for naming species

Describe the use of the dichotomous keys for the identification of species

Explain how the properties of water that are essential to all living things are attributed to their dipolar nature

Hydrogen bonding due to dipolar nature

Water molecules are cohesive due to hydrogen bonding

Cohesion useful in xylem transport

Adhesion of water to cellulose in cell walls

High boiling point due to hydrogen bonding

Water is liquid at high temperatures rather than gas

High latent heat of vaporisation

High specific heat

Thermally stable habitat

Used in sweat as body coolant

Describe the adaptations of plants in deserts for water conversation [4]

Thick waxy cuticle to reduce cuticular transpiration

Few/small stomata

Stomata that open at night when it is cooler

Leaf surface reduced

Water storage tissue in leaves

Deep or more extensive roots

Outline the use of models to investigate the transport of water in xylem [3]

Models allow one aspect to be studied independently

Glass capillary tubes to model adhesion between xylem and water

Pourous pot to model flow in a xylem vessel due to transpiration from the leaf

Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. [5]

Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands. [5]

four nucleotides shown in diagram with one nucleotide clearly labelled; base, phosphate and deoxyribose (shown as pentagon) connected between the correct carbons and labelled at least once; backbone labelled as covalent bond between nucleotides correctly shown as 3′ to 5′ bond; two base pairs linked by hydrogen bonds drawn as dotted lines and labelled; two H bonds between A and T and three H bonds between C and G; adenine to thymine and cytosine to guanine; do not accept initials of bases antiparallel orientation shown;

Explain the methods and aims of DNA profiling. [8]

DNA sample obtained; from hair/blood/semen/human tissue; DNA amplified / quantities of DNA increased by PCR/polymerase chain reaction; satellite DNA/highly repetitive sequences are used/amplified; DNA cut into fragments; using restriction enzymes/restriction endonucleases; gel electrophoresis is used to separate DNA fragments; using electric field / fragments separated by size; number of repeats varies between individuals / pattern of bands is unique to the individual/unlikely to be shared; Award [5 max] for methods.

forensic use / crime scene investigation; example of forensic use e.g. DNA obtained from the crime scene/victim compared to DNA of suspect / other example of forensic use; paternity testing use e.g. DNA obtained from parents in paternity cases; biological father if one half of all bands in the child are found in the father; genetic screening; presence of particular bands correlates with probability of certain phenotype / allele; other example; brief description of other example; Award [4 max] for aims.

Distinguish between RNA and DNA. [3]

DNA is double-stranded while RNA is single-stranded; DNA contains deoxyribose while RNA contains ribose; the base thymine found in DNA is replaced by uracil in RNA; one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);

Explain the process of DNA replication. [8]

occurs during (S phase of ) interphase/in preparation for mitosis/ cell division; DNA replication is semi-conservative; unwinding of double helix / separation of strands by helicase (at replication origin); hydrogen bonds between two strands are broken; each strand of parent DNA used as template for synthesis; synthesis continuous on leading strand but not continuous on lagging strand; leading to formation of Okazaki fragments (on lagging strand); synthesis occurs in 5´ → 3´ direction; RNA primer synthesized on parent DNA using RNA primase; DNA polymerase III adds the nucleotides (to the 3´ end) added according to complementary base pairing; adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.) DNA polymerase I removes the RNA primers and replaces them with DNA; DNA ligase joins Okazaki fragments; as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form bond; Accept any of the points above shown on an annotated diagram.

Outline how enzymes catalyse reactions. [7]

They increase rate of reaction; remains unused/unchanged at the end of the reaction ; lower activation energy; activation energy is energy needed to overcome energy barrier that prevents reaction; annotated graph showing reaction with and without enzyme; substrate joins with enzyme at active site; to form enzyme substrate complex; active site specific for a particular substrate; enzyme bidning with substrate brings reactants closer together to facilitate chemical reactions; induced fit model, when enzyme substrate forms; making substrate more reactive

Distinguish between RNA and DNA. [3]

DNA is double-stranded while RNA is single-stranded; DNA contains deoxyribose while RNA contains ribose; the base thymine found in DNA is replaced by uracil in RNA; one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);

Explain the process of DNA replication. [8

occurs during (S phase of ) interphase/in preparation for mitosis/ cell division; DNA replication is semi-conservative; unwinding of double helix / separation of strands by helicase (at replication origin); hydrogen bonds between two strands are broken; each strand of parent DNA used as template for synthesis; synthesis continuous on leading strand but not continuous on lagging strand; leading to formation of Okazaki fragments (on lagging strand); synthesis occurs in 5 ́ → 3 ́ direction; RNA primer synthesized on parent DNA using RNA primase; DNA polymerase III adds the nucleotides (to the 3 ́ end) added according to complementary base pairing; adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.) DNA polymerase I removes the RNA primers and replaces them with DNA; DNA ligase joins Okazaki fragments; as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form bond; Accept any of the points above shown on an annotated diagram.

Outline how enzymes catalyse reactions. [7]

they increase rate of (chemical) reaction; remains unused/unchanged at the end of the reaction; lower activation energy; activation energy is energy needed to overcome energy barrier that prevents reaction; annotated graph showing reaction with and without enzyme; substrate joins with enzyme at active site; to form enzyme-substrate complex; active site/enzyme (usually) specific for a particular substrate; enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron transfer); induced fit model / change in enzyme conformation (when enzyme-substrate/ES complex forms); making the substrate more reactive;

Draw a labelled diagram showing the ultra-structure of a liver cell. [4]

(plasma) membrane—single line surrounding cytoplasm; nucleus — with a double membrane and pore(s) shown; mitochondria(ion) — with a double membrane, the inner one folded into internal projections, shown no larger than half the nucleus; rough endoplasmic reticulum multi folded membrane with dots/ small circles on surface; Golgi apparatus—shown as a series of enclosed sacs with evidence of vesicle formation; ribosomes — dots/small circles in cytoplasm/ribosomes on rER; lysosome;

Explain prokaryotic DNA replication. [8]

DNA replication is semi-conservative / each strand of DNA acts as template;

(DNA) helicase separates two strands/forms a replication fork; new strand built / nucleotides added in a 5 ́ to 3 ́ direction; (deoxy)nucleoside triphosphates hydrolysed to provide energy for nucleotide formation/base pairing; on one strand DNA polymerase III builds continuous strand; on other strand short chains of DNA/Okazaki fragments are formed; each short chain starts with RNA primer; added by RNA primase; then remainder of chain of DNA built by DNA polymerase III; DNA polymerase I removes RNA primer and replaces it by DNA; DNA ligase joins DNA fragments together forming complete strand; replication only occurs at a single replication fork; Award credit for any of the above points clearly drawn and accurately labelled.

State four functions of proteins, giving a named example of each. [4]

structure — collagen; transport—transthyretin / hemoglobin; enzyme/catalyst — lysozyme; movement — actin / tubulin; hormones — insulin; antibodies — immunoglobulin; storage —albumin; Accept any other valid function of proteins with a named example. For example, sodium potassium pump, but do not accept simply "in membranes" without a clear function. To award [4 max], responses need a function of protein and a named example. Only accept the first four answers.

Outline the structure of ribosomes. [6]

made of protein; made of rRNA; large subunit and small subunit; three tRNA binding sites; Aminacyl/A, Peptidyl/P and Exit/E; mRNA binding site (on small subunit); 70S in prokaryotes / 80S in eukaryotes; can be free / bound to RER (in eukaryotes);

Explain the process of transcription leading to the formation of mRNA. [8]

RNA polymerase; (polymerase number is not required) binds to a promoter on the DNA; unwinding the DNA strands; binding nucleoside triphosphates; to the antisense strand of DNA; as it moves along in a 5′→3′ direction; using complementary pairing/A-U and C-G; losing two phosphates to gain the required energy; until a terminator signal is reached (in prokaryotes); RNA detaches from the template and DNA rewinds; RNA polymerase detaches from the DNA; many RNA polymerases can follow each other; introns have to be removed in eukaryotes to form mature mRNA;

(Plus up to [2] for quality)

Living organisms at every trophic level are part of the carbon cycle. Draw a labelled diagram of the carbon cycle to show the processes involved. [9]

Award [1] for each of the following shown on a diagram of the carbon cycle. Award [5 max] for points not shown on a diagram.

The following show carbon which is static within the cycle at this point in time. carbon dioxide in air/water; (sugars/carbon compounds in) plants/producers;

(carbon compounds in) animals/consumers; (carbon trapped in) coal/oil/gas/fossil fuels;

The following should show arrows in direction of carbon flow.

carbon dioxide absorbed by plants/producers and used in photosynthesis; carbon dioxide released by (cell) respiration in plants/producers; plants/producers eaten by animals/primary consumers/herbivores; primary consumers eaten by secondary consumers; carbon dioxide released by (cell) respiration in animals/consumers; plants/animals die and are decomposed by (saprotrophic) bacteria/fungi; carbon dioxide released by combustion of coal/oil/gas/fossil fuels; carbon dioxide released by (cell) respiration in bacteria/fungi/ decomposers; forest fires/combustion releases carbon dioxide from trees/plants; carbon dioxide emitted by volcanoes;

Explain, using an example of a food chain, how trophic levels can be deduced. [4]

diagram of food chain showing at least three organisms and two linkages with arrows showing direction of energy flow; trophic level is a step/position in the movement/flow of energy through an ecosystem; (in a field situation) observe which organisms eat each other; producer/name from example (first trophic level) does not eat other organisms/captures energy through photosynthesis; primary consumer/name from example (second trophic level) feeds on producers; secondary consumer/name from example (third trophic level) feeds on primary consumers; Since the command term is explain, the answer must be explicit to gain marking points d-f. Named examples for producer and consumers in diagram or explained example must represent a coherent food chain. Reject chains using general names such as fish or tree or grass. But, accept sardine or oak.

Explain methods that can be used to measure the rate of photosynthesis. [5]

measure production of oxygen; because oxygen is a by-product of photosynthesis; example of technique for measuring oxygen production (count bubbles/use sensors/other); measure uptake of carbon dioxide; because carbon dioxide is used during photosynthesis; example of technique for measuring carbon dioxide production (sensor, aquatic pH shift); measure biomass of (batches of) plants; increase in biomass gives (indirect) measure of rate of photosynthesis; Since the command term is explain, reasons must be given to receive full marks.

Outline condensation and hydrolysis reactions using a different example for each. [5]

condensation reactions involve joining subunits/molecules/monomers; with the release of water; hydrolysis reactions involve splitting molecules into subunits/ molecules/monomers;

with the addition of water; example of condensation reaction; (e.g. amino acid + amino acid yields dipeptide + water) example of hydrolysis reaction; (e.g. disaccharide + water yields two monosaccharides) Examples can be shown in words or chemical form.

Outline the effect of temperature and substrate concentration on the activity of enzymes. [4]

enzymes most active at one temperature/optimum temperature; any deviation from that temperature lowers the enzyme activity; denaturing/change in active site/no activity at higher temperatures / inactivated at (very) low temperatures; increasing the substrate concentration increases the enzyme activity/more enzyme-substrate complex formed/more collisions between enzyme and substrate; eventually no increase in enzyme activity with increased substrate concentration / plateau when enzymes are working to the maximum/when all active sites occupied/saturated; Accept answers shown graphically.

Explain methods by which the rate of photosynthesis can be measured, including conditions that affect the rate. [9]

measuring oxygen release; measuring volume / counting rising oxygen bubbles / counting rising disks; measuring carbon dioxide intake/uptake; CO2 can be measured by change in pH / increase in pH shows an increase in CO2 fixation; increase in biomass would be an indirect measure of photosynthesis / measure of net photosynthesis; measure starch production / dry organic mass; increasing temperature would increase the rate of photosynthesis; provided the temperature did not go above optimum temperature of enzymes; increasing carbon dioxide concentration would increase the rate of photosynthesis; higher light intensity would increase the rate of photosynthesis; light of different wavelengths / blue and red light can affect photosynthesis / green wavelength usually not absorbed; example of a detailed experiment; (e.g. drawing or explanation of waterweed under funnel and tube collecting bubbles of oxygen)

All organisms in an ecosystem are involved in the carbon cycle. Outline the roles of living organisms in the carbon cycle. [8]

plants/producers fix carbon (dioxide)/use carbon (dioxide) in photosynthesis; sugars/carbon compounds (produced) in plants/producers from photosynthesis;

(carbon compounds in) plants/producers eaten by animals/primary consumers/herbivores; (carbon compounds in) primary consumers eaten by secondary consumers/ passed along food chain;

carbon compounds/sugars/organic molecules digested and absorbed by consumers; carbon dioxide released by cell respiration (in plants/animals/ consumers);

plants/animals die and are decomposed by (saprotrophic) bacteria/fungi; carbon dioxide released by cell respiration in bacteria/fungi/ decomposers;

enzymes released to digest/hydrolyse carbon compounds in organic matter; forest fires/combustion releases carbon dioxide; humans burn fossil fuels adding carbon dioxide to the atmosphere; Award any of the above points if clearly drawn in an annotated diagram.

Explain how triose phosphate is produced and used in the chloroplasts of a plant. [5]

ribulose bisphosphate/RuBP and carbon dioxide react together; (this is) carbon fixation/part of light-independent reactions; catalysed by RuBP carboxylase/Rubisco; glycerate 3-phosphate/GP produced; glycerate 3-phosphate/GP reduced/converted to triose phosphate/TP; using NADPH/(NADPH+H+) and ATP; from the light-dependent reactions; some triose phosphate used to regenerate RuBP; some triose phosphate used to synthesize glucose (phosphate)/starch;

Explain the conditions that are needed to allow a seed to germinate. [5]

water needed to rehydrate the seed; gibberellin released / active after water absorbed; gibberellin needed to produce amylase; water needed to allow substances inside the seedling to be transported; oxygen needed for (aerobic) cell respiration; warmth needed to speed up metabolism/enzyme activity; warmth indicates that it is a favourable season for germination/spring; some seeds need a cold period to stimulate germination; some seeds need fire to stimulate germination; some seeds need to pass through an animal (gut) to stimulate germination;

State four elements that are needed by living organisms, other than carbon, hydrogen and oxygen, giving one role of each. [4]

nitrogen — structure of organic molecules/proteins/nucleotides; sulphur — amino acid / protein structure; phosphorus — nucleic acids / energy carriers; calcium — bone structure / trigger exocytosis (e.g. neurotransmitters); iron — prosthetic groups / hemoglobin; sodium —membrane potential; Accept other valid roles for those elements already listed. Accept other valid examples of elements with their roles. To award [4 max], responses need an appropriate role for each element stated.

Outline how light energy is used and how organic molecules are made in photosynthesis. [6

chlorophyll is the (main) photosynthetic pigment; absorbs (mainly) red and blue light; green light is reflected; light energy absorbed is converted into chemical energy; ATP produced; water split; to form oxygen and hydrogen; ATP and hydrogen used to fix carbon dioxide to make organic molecules;

Explain the significance of complementary base pairing for replication, transcription and translation. [8]

A-T and C-G in DNA; A-U and C-G in RNA; complementary base pairing in replication ensures identical nucleotide sequence of new complementary strands; semi-conservative replication; transcription produces RNA sequence complementary to the DNA sequence (of the gene); triplets of nucleotides on mRNA are codons; translation converts mRNA sequence of information into a specific amino acid chain (polypeptide); (each class of) tRNA carries a specific triplet of (three) bases called an anticodon; anticodons bind to codons by complementary base pairing; (each class of) tRNA with specific complementary anticodons carry specific amino acids; sequence of mRNA codons translates into specific amino acid sequence; enables conservation of information transfer from DNA to RNA to polypeptide;

Draw a labelled diagram of the structure of a chloroplast as seen with an electron microscope. [4]

Award [1] for each of the following clearly drawn and correctly labelled. Label lines must be unambiguous in terms of what they are indicating. double/inner and outer membrane/envelope—shown as two concentric continuous lines close together; granum/grana —shown as a stack of several disc-shaped subunits; (intergranal) lamella — shown continuous with thylakoid membrane; thylakoid — one of the flattened sacs; stroma; (70S) ribosomes/(circular) DNA / lipid globules / starch granules / thylakoid space;

Describe how water is carried by the transpiration stream. [7]

transpiration is water loss (from plant) by evaporation; flow of water through xylem from roots to leaves is the transpiration stream; evaporation from spongy mesophyll cells; replaced by osmosis from the xylem; (diffusion of water vapour) through stomata; water lost replaced from xylem / clear diagram showing movement of water from xylem through cell(s) (walls) to air space; water pulled out of xylem creates suction/low pressure/tension; transpiration pull results; water molecules stick together/are cohesive; due to hydrogen bonding/polarity of water molecules; xylem vessels are thin (hollow) tubes; adhesion between water and xylem due to polarity of water molecules; creates continuous column/transpiration stream;