Biology - Past Paper questions

PAPER 2 (5/2/25)

PAPER 2 (5/2/25)

2b) Compare and contrast mitochondria and chloroplasts in terms of the substrates they use and the products they produce [2]

• ATP produced by both

• oxygen produced by chloroplasts and used by mitochondria

• carbon dioxide produced by mitochondria and used by chloroplasts

• carbon/organic compounds built up in chloroplasts (anabolism)

• carbon/organic compounds broken down in mitochondria (catabolism)

2c) Outline how the compounds produced by chloroplasts are distributed throughout the plant [3]

• compounds distributed using the phloem

• loading into sieve tubes by active transport/by companion cells

• entry of water to phloem by osmosis (due to high solute concentration)

• causes high/hydrostatic pressure

• flow from high pressure to lower pressure down the concentration gradient from source to sink

3a) Describe how the exchange of gases is brought about between the alveoli and the blood [3]

• concentration of oxygen in the blood is lower than the concentration of oxygen in the air (blood arriving at the alveoli is deoxygenated)

• oxygen moves into the blood by diffusion

• oxygen moves down teh concentration gradient from the alveoli into the blood

• During inhalation the concentration of oxygen in the alveoli increases establishing a concentration gradient

• flow of blood in capillaries maintains concentration gradient

3b) Describe two features common to the exchange surfaces of alveoli and gills

• rich capillary supply

• large surface area

• epithelium is one cell thick

• moist surface

4b) Down syndrome is a genetic condition in which the cells of an individual contain three copies of chromosome 21. Explain how non-dijunction leads to trisonmy 21

• sex cell contains one copy of chromosome 21

• non-disjunction is when chromosomes do not separate during meiosis

• for down-syndrome to occur the sex cells contain 2x copies of chromosome 21

• chromosmes fail to separate in the first/second meiotic division

• this sex cell with 2 copies, fertilizes normal sex cell with one copy of chromo 21

• non-disjunction of chromosome 21 is more frequent in females and increases with age

5a) identify the recombinant phenotypes amongst the offspring

• brown body, vestigial wings and black body, normal wings

• unlinked genes

5b) Explain whether these results fit the predicted Mendelian ratios for this cross

• No - Mendelian ratios are 1:1:1:1 wheras offspring phenotype ratios are 3:1:1:3

• suggests linkage between genes

5c) Outline the statistical test that could be used to provide more evidence of the type of inheritance

• chi-squared statistical test

• compares expected ratios with real ratios

• assume no sleective advantage to any of the genotypes

• if tabulated value is exceeded then there is a significant difference

6a) Plasma cells (differentiated B cells) secrete antibodies against specific antigens. Outline how plasma cells become activated

• macrophages/phagocytes engulf the pathogen and display the antigens

• antigens bind to the T-helper cells

• antigen binds to antibodies in membrane of the B cells

• Activated T cells activate B cells

• activated B cells divide to produce a clone of cells

• active plasma cells develop from clone of cells

6b) A hybridoma is a cell produced by the fusion of a plasma cell with a tumour cell. Explain the advantages of using hybridoma cells in the production of monoclonal antibodies

• endless cell divisions/unregulated mitosis

• monoclonal antibodies are produced at a fast rate allowing the harvesting of antibodies

• all cells in clone produce same type of antibody

• large amount of chosen antibody can be produced

6c) State one use of monoclonal antibodies

• pregancy testing - detection of hCG

• producing antibodies for treating disease e.g. athritis or psoriasis - gives passive immunity

• blood typing

• testing urine for drugs

7a) Describe the genetic and hormonal control of male sexual characteristics in a human

• male characteristics are determined genetically by XY chromosomes

• gene on Y chromosome promotes development of the testes

• testes secrete testosterone

• testosterone stimulates sperm production

• testosterone stimulates the development of male genitals/sexual characteristics in fetus

• secondary sexual characteristics are determined by hormones e.g. testosterone

7b) Outline how the hormone auxin controls phototropism in plant shoots

• auxin causes shoot to grow/bend towards the light

• auxin moves from the light side to the shadier side

• auxin is moved by auxin efflux pumps

• auxin promotes cell elongation

• more growth on shady side of stem due to auxin concentration gradient

• auxin binds to auxin receptors in target cells

• auxin promotes growth gene expression

7c) Toxins often act as inhibitors. Compare and contrast competitive and non-competitive enzyme inhibition

similarities:

• slow down enzyme activity

• interact with R chains of amino acids at the surface of the enzyme molecule

differences:

• C inhibitor binds to active site, NC inhibitor binds to allosteric site

• C - substrate cannot bind due to blocakge of active site/NC - substrate cannot bind due to change in shape of active site

• C - inhibition is reversible/ NC - inhibition is permanent

• C - inhibitor has similar shape to substrate

• C - can be reversed by increasing substrate concentration / NC - adding substrate does not reuce inhibition

8b) Compare and contrast nervous and hormonal communication

Similarities

• communication between cells/parts of the body

• both cause response in target cells

• both use chemicals that bind to receptors

• both can stimulate/inhibit processes

• both can work over long distances

• both under overall control of brain/CNS

• both use feedback mechanisms

Differences

• chemical vs electrical

• transported in blood vs neurons

• slower vs faster

• carried throughout body vs carried to specific cell

• wide range of tissues affected vs only muscles/glands receive signals

• long term vs short duration

8c) Describe barriers that exist to hybridisation between species

• courtship behavior is complex and unique to species

• rejection if characteristic behaviour not exhibited

• species spefic egg feritilisation

• sterility of hybrids

• differences in chromosme number

PAPER 1 - MAY 2024 HL

PAPER 1 - MAY 2024 HL

4. What explains the movement of glucose molecules down a concentration gradient across the cell surface membrane?

They move through hydrophilic channels because they are polar

9. The graph shows the effect of increasing light intensity on teh rate of photosynthesis during an experiment carried out at optimum temperature and normal atmospheric CO₂ concentration. Which factor could be limiting photosynthesis at point X?

carbon dioxide concentration

11. The Human Genome Project complete the sequencing of the human genome by the year 2003. Which could have been a source of the entire genome in humans?

The nucleus and mitochondria of a skin cell

13. Nonsyndromic Hearling Loss and Deafness is an inherited cause of deafness in humans. The pedigree chart shows this inheritance in a family. Where is the allele found in family members with this condition?

on a pair of autosomes

14. What can be found in each band of a DNA profile?

Dna fragments with the same number of base pairs

15. What do teh letters P, Q and R represent in an ecosystem?

P = Light

Q = heat

R = inorganic nutrients

32. how does lignin contribute to the transport of water up the stem? (xylem)

They allow xylem vessels to withstand low pressure

33. Which structures are sources and sinks? (potato)

source = tuber

sink = developing stems

NOV 2022 P2

NOV 2022 P2

2a) Identify the stage of meiosis where exchange of genetic material occurs

prophase 1

2b) Explain the reasons why the results do not agree with expected Mendelian rations in a dihybrid cross

a) gene linkage/genes located on the same chromosome

b) independent assortment does not occur

c) no recombination unless there is crossing over

3b) Explain how increased levels of atmospheric carbon dioxide contribute to global warming

• carbon dioxide absorbs long wave/infra-red radiation

• more heat trapped in/less heat escapes atmosphere with more carbon dioxide

• short wave/UV radiation from the sun passes through the atmosphere and reaches earth’s surface

• radiation from the sun/sunlight warms the surface of the Earth

• long wavelength/infrared radiated from the warmed Earth’s surface

4b) Outline how nucleosomes affect the transcription of DNA

• can promote and inhibit the transcription of genes

• nucleosomes can prevent transcription by condensation/supercoiling of DNA

• nucleosomes can allow or prevent the binding of RNA polymerase or transcription factors

• acetylation/methylation of nucleosomes/histones can respectively promote/inhibit transcription

• movement of histones/nucleosomes along DNA can affect which genes are transcribed

4c)i) Which enzyme copies the DNA sequence

RNA polymerase

4d) Explain the role of lactose in the expression of the gene for lactase production

a) lactose binds to repressor protein

b) repressor protein with lactose bound cannot bind to the promoter

c) RNA polymerase binds to the promoter and transcribes gene

d) lactase producedif lactose is present/lactase production inhibited if lactose is absent