Cambridge Pre-U Biology (Principal Subject) Flashcards

24 sample cards from the Biology (Principal Subject) deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. How does meiosis differ from mitosis in outcome and chromosome number?

   Meiosis involves two divisions producing four genetically different haploid gametes (chromosome number halved, $2n \to n$), with genetic variation from crossing over and independent assortment; mitosis gives two identical diploid cells.

2. In a monohybrid cross between two heterozygotes ($Tt \times Tt$), state the genotypic and phenotypic ratios.

   Genotypic ratio $1\,TT : 2\,Tt : 1\,tt$; phenotypic ratio $3$ dominant : $1$ recessive.

3. What phenotypic ratio is expected in the F2 of a dihybrid cross between two heterozygotes ($RrYy \times RrYy$)?

   $9:3:3:1$ (9 both dominant : 3 one dominant : 3 other dominant : 1 both recessive), assuming the genes assort independently.

4. State the Hardy-Weinberg equations and what the terms represent.

   $$p + q = 1 \qquad p^{2} + 2pq + q^{2} = 1$$ where $p$ and $q$ are allele frequencies; $p^{2}$ = homozygous dominant, $2pq$ = heterozygous, $q^{2}$ = homozygous recessive frequencies.

5. Distinguish natural selection from genetic drift.

   Natural selection is the non-random differential survival and reproduction of organisms based on advantageous alleles. Genetic drift is a random change in allele frequencies due to chance, most significant in small populations.

6. Differentiate directional, stabilising, and disruptive selection.

   Directional: favours one extreme, shifting the mean. Stabilising: favours the intermediate phenotype, reducing variation. Disruptive: favours both extremes against the mean, potentially splitting the population.

7. Define a species and outline allopatric speciation.

   A species is a group of organisms that can interbreed to produce fertile offspring. Allopatric speciation: a geographical barrier isolates populations, which then diverge under different selection pressures/drift until they can no longer interbreed.

8. How is recombinant DNA produced and inserted into a bacterial plasmid?

   A restriction endonuclease cuts both the gene and the plasmid, leaving complementary sticky ends; the gene and plasmid are joined by DNA ligase (forming phosphodiester bonds), and the recombinant plasmid is taken up by a host bacterium as a vector.

9. Describe the three repeating temperature steps of one PCR cycle.

   Denaturation at about $95\,^{\circ}\text{C}$ separates the strands; annealing at about $55\,^{\circ}\text{C}$ lets primers bind; extension at about $72\,^{\circ}\text{C}$ lets Taq polymerase synthesise new strands. The cycle roughly doubles DNA each time.

10. State the overall balanced equation for photosynthesis.

    $$\ce{6CO2 + 6H2O ->[\text{light}][\text{chlorophyll}] C6H12O6 + 6O2}$$

11. Where do the light-dependent and light-independent reactions occur, and what does each produce?

    Light-dependent reactions occur on the thylakoid membranes, producing ATP, reduced NADP and $\ce{O2}$ (from photolysis of water). The light-independent reactions (Calvin cycle) occur in the stroma, fixing $\ce{CO2}$ into carbohydrate using ATP and reduced NADP.

12. Outline the Calvin cycle.

    $\ce{CO2}$ combines with ribulose bisphosphate (RuBP, 5C) catalysed by rubisco to form two molecules of glycerate-3-phosphate (GP, 3C); GP is reduced to triose phosphate (TP) using ATP and reduced NADP; some TP forms glucose, and the rest regenerates RuBP.

13. State the overall balanced equation for aerobic respiration.

    $$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}$$ releasing energy used to make ATP.

14. Name the four stages of aerobic respiration and their locations.

    Glycolysis (cytoplasm), the link reaction (mitochondrial matrix), the Krebs cycle (matrix), and oxidative phosphorylation/electron transport chain (inner mitochondrial membrane/cristae).

15. What is the net ATP yield of glycolysis and the final electron acceptor in aerobic respiration?

    Glycolysis yields a net of $2$ ATP (and $2$ reduced NAD) per glucose. The final electron acceptor of the electron transport chain is oxygen, which combines with electrons and $\ce{H+}$ to form water.

16. Compare the products of lactate (animal) and ethanol (yeast) anaerobic respiration.

    In animals, pyruvate is reduced to lactate (regenerating NAD): $\ce{C3H4O3 + 2H -> C3H6O3}$. In yeast/plants, pyruvate is decarboxylated and reduced to ethanol and $\ce{CO2}$: $\ce{C6H12O6 -> 2C2H5OH + 2CO2}$.

17. Define habitat, population, community, and ecosystem.

    Habitat: the place an organism lives. Population: all individuals of one species in an area. Community: all populations of different species in an area. Ecosystem: the community plus the abiotic (non-living) environment, interacting as a unit.

18. Why does biomass and energy decrease along a food chain, and roughly by how much per trophic level?

    Energy is lost at each level through respiration (heat), excretion and unineaten/undigested material, so only about $10\%$ of energy is transferred to the next trophic level; this limits chains to about $4$–$5$ links.

19. Outline the role of microorganisms in the nitrogen cycle.

    Nitrogen-fixing bacteria convert $\ce{N2}$ to ammonia; nitrifying bacteria oxidise ammonia to nitrite then nitrate ($\ce{NH3 -> NO2^- -> NO3^-}$); denitrifying bacteria reduce nitrate back to $\ce{N2}$; decomposers (ammonifying bacteria) release ammonia from dead organic matter.

20. Explain how eutrophication leads to the death of aquatic organisms.

    Excess nitrate/phosphate (e.g. from fertiliser run-off) causes an algal bloom; algae block light, killing plants; decomposing dead matter is broken down by aerobic bacteria whose multiplication depletes dissolved oxygen, suffocating fish and other aerobic organisms.

21. How does the greenhouse effect contribute to global warming?

    Greenhouse gases (e.g. $\ce{CO2}$, $\ce{CH4}$) absorb and re-emit long-wave infrared radiation emitted by the Earth, trapping heat in the atmosphere; rising concentrations from burning fossil fuels enhance this, raising mean global temperature.

22. Distinguish in-situ from ex-situ conservation with examples.

    In-situ conservation protects species in their natural habitat (e.g. nature reserves, national parks). Ex-situ conservation protects species outside their habitat (e.g. zoos, botanic gardens, seed banks, captive breeding programmes).

23. Define biodiversity and name the levels at which it is measured.

    Biodiversity is the variety of living organisms in an area. It is measured at three levels: genetic diversity (variety of alleles within a species), species diversity (number and relative abundance of species), and ecosystem/habitat diversity.

24. State Simpson's Index of Diversity formula and what a higher value indicates.

    $$D = 1 - \sum \left( \frac{n}{N} \right)^{2}$$ where $n$ is the number of individuals of each species and $N$ the total; a value closer to $1$ indicates higher biodiversity and a more stable ecosystem.

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