Continuity and Change — Free Biology SL Practice Questions

A student places a plant cell from a freshwater aquatic plant (Elodea canadensis) into a concentrated salt solution. The cell initially has a solute potential ($\Psi_s$) of $-0.8\,\text{MPa}$ and a pressure potential ($\Psi_p$) of $+0.3\,\text{MPa}$. The external salt solution has a water potential ($\Psi_w$) of $-1.4\,\text{MPa}$. After 10 minutes, the cell has undergone plasmolysis.

A student uses a pressure bomb to measure water potential in a branch of a maple tree (Acer saccharum). The branch is cut and placed in the pressure chamber with the cut end exposed. Pressure is applied until xylem sap just reappears at the cut surface. The pressure required is $0.8\,\text{MPa}$. In a pressure bomb, the applied pressure equals the magnitude of the xylem water potential, and the xylem water potential approximates the leaf cell water potential.

A plant physiologist investigates the water relations of the halophyte Salicornia europaea, which grows in coastal salt marshes. Cells in the stem accumulate high concentrations of sodium chloride (NaCl) and the organic solute glycine betaine to maintain turgor despite the high salinity of the surrounding soil water. A $5.0\,\text{cm}$ segment of S. europaea stem is placed in a sealed pressure chamber (Scholander bomb) containing soil solution. The stem segment is allowed to equilibrate fully with the soil solution before measurements are taken.

A proposed reforestation programme plants $500{,}000\,\text{ha}$ of tropical forest and $300{,}000\,\text{ha}$ of temperate forest. A mature tropical forest sequesters $2.5\,\text{t C ha}^{-1}\,\text{yr}^{-1}$ and a mature temperate forest sequesters $1.8\,\text{t C ha}^{-1}\,\text{yr}^{-1}$. The current global deforestation rate is $10{,}000\,\text{ha yr}^{-1}$. The carbon content of $\text{CO}_2$ by mass is $27.3\%$, and $1\,\text{t} = 1000\,\text{kg}$.

The Arctic sea ice minimum extent (the smallest area covered by ice each year) has decreased significantly since satellite records began in 1979. A study published in 2020 reported that the September 2019 minimum extent was $4.15$ million $\text{km}^2$, compared to an average of $6.85$ million $\text{km}^2$ for the period 1981–2010.

A carbon capture and storage (CCS) facility is proposed adjacent to a coal-fired power plant. The system captures $\text{CO}_2$ from flue gases and stores it in deep saline aquifers. Environmental impact assessments must consider both the energy penalty of the system and the biological consequences of potential $\text{CO}_2$ leakage.

A study investigated the reproductive strategies of two freshwater crustacean species, Daphnia pulex and Artemia franciscana. Both species can reproduce asexually (parthenogenesis) or sexually, depending on environmental cues. Population growth rates were measured in controlled laboratory cultures over 30 days: Species — Asexual rate $(\text{day}^{-1})$ — Sexual rate $(\text{day}^{-1})$ Daphnia pulex — $0.32$ — $0.08$ Artemia franciscana — $0.25$ — $0.12$ Genetic diversity was assed using gel electrophoresis of a polymorphic enzyme locus. The number of distinct alleles present in a population of 100 individuals after 30 days was: Species — Asexual reproduction — Sexual reproduction Daphnia pulex — 2 alleles — 8 alleles Artemia franciscana — 3 alleles — 6 alleles

The European white water lily (Nymphaea alba) is a flowering plant that reproduces sexually. Its life cycle involves alternation of generations, with both haploid ($n$) and diploid ($2n$) stages. The gametophyte generation is reduced and dependent on the sporophyte.

Hieracium pilosella (mouse-ear hawkweed) reproduces both asexually via apomixis (seed production without fertilisation) and sexually. A research team monitored two populations over three growing seasons. Population A (stable field — consistent rainfall and temperature): - Average seed set per plant: $450\,\text{seeds year}^{-1}$ - $95\%$ of seeds produced via apomixis; $5\%$ via sexual reproduction - Seedling survival rate: $72\%$ - $92\%$ of seedlings genetically identical to mother plant (microsatellite analysis) Population B (variable field — unpredictable rainfall, frequent pest outbreaks): - Average seed set per plant: $310\,\text{seeds year}^{-1}$ - $40\%$ of seeds produced via apomixis; $60\%$ via sexual reproduction - Seedling survival rate: $48\%$ - $38\%$ of seedlings genetically identical to mother plant (microsatellite analysis)

A geneticist is studying a family with a history of cystic fibrosis (CF), an autosomal recessive disorder caused by mutations in the CFTR gene. The pedigree below shows the inheritance of CF across three generations. - Individuals I-1 and I-2 are unaffected. - Individual II-1 is affected with CF and is the child of I-1 and I-2. - Individual II-2 is an unaffected child of I-1 and I-2. - Individual II-3 is unaffected; their brother, II-4, is affected with CF. - Individual III-1 is the unaffected child of II-2 and II-3. Use the allele symbols $F$ (dominant, normal) and $f$ (recessive, CF).

In pea plants, the allele for purple flower colour ($P$) is dominant over the allele for white flower colour ($p$). A farmer crosses a heterozygous purple-flowered plant with a white-flowered plant.

In a breeding experiment an agricultural research station, a true-breeding strain of pea plants with round, yellow seeds ($RRYY$) was crossed with a true-breeding strain with wrinkled, green seeds ($rryy$). The $F_1$ generation all had round, yellow seeds. The $F_1$ plants were then self-fertilized to produce an $F_2$ generation of 320 seeds. The following results were recorded: - Round, yellow seeds: 175 - Round, green seeds: 55 - Wrinkled, yellow seeds: 65 - Wrinkled, green seeds: 25

DNA replication is a highly coordinated process involving multiple enzymes. In a eukaryotic cell, replication begins at multiple origins of replication along the chromosome. The below (not to scale) shows a replication bubble with two replication forks moving in opposite directions away from a single origin.

A student analyses a of a replication fork in E. coli. The shows the following labelled components: DNA helicase, single-stranded binding proteins (SSBs), DNA primase, DNA polymerase III, and the leading and lagging strands.

The bacterium Escherichia coli has a single circular chromosome of $4.6 \times 10^{6}$ base pairs. Replication proceeds bidirectionally from a single origin of replication (oriC). Each replication fork adds nucleotides at $1000\,\text{nucleotides}\,\text{s}^{-1}$.

A research team investigates the effect of a new drug, GlucoStat, on blood glucose regulation in healthy laboratory rats. Rats are divided into a control group (injected with saline) and an experimental group (injected with GlucoStat). After 30 minutes, both groups receive an intravenous injection of glucose solution ($0.5\,\text{g kg}^{-1}$ body mass). Blood glucose concentration is measured every 15 minutes for 120 minutes. - Baseline blood glucose (both groups): $4.8\,\text{mmol L}^{-1}$ - Control group: peak at 15 min = $10.2\,\text{mmol L}^{-1}$; returns to baseline by 90 min - GlucoStat group: peak at 15 min = $9.1\,\text{mmol L}^{-1}$; returns to baseline by 60 min

A researcher investigates insulin action using cultured human skeletal muscle cells (myocytes). Cells are incubated in medium containing $20\,\text{mmol}\,\text{L}^{-1}$ glucose. One group is treated with $100\,\text{nM}$ insulin; a control group receives no insulin. After 30 minutes, GLUT4 glucose transporter proteins are counted. Group — GLUT4 on cell surface membrane — GLUT4 internal vesicles Control — 200 per cell — 4800 per cell Insulin-treated — 4800 per cell — 200 per cell Total GLUT4 per cell (surface + internal) = 5000 in both groups.

A clinical study investigated blood glucose regulation in two groups of healthy adults ($n = 15$ each) following an oral glucose tolerance test (OGTT). Participants fasted for 12 hours, then consumed $75\,\text{g}$ of glucose dissolved in water. Blood samples were taken every 30 minutes for 180 minutes. Mean blood glucose concentration ($\text{mmol}\,\text{L}^{-1}$): Time (min) — Group A (control) — Group B (soluble fibre) 0 — 4.8 — 4.7 30 — 8.2 — 6.9 60 — 7.5 — 6.1 90 — 6.0 — 5.2 120 — 5.1 — 4.9 180 — 4.9 — 4.8 Group B consumed $10\,\text{g}$ of soluble fibre 30 minutes before the glucose drink.

A study was conducted on the expression of the $\beta$-globin gene in human erythroblast precursor cells. Researchers isolated mRNA from these cells and used it as a template for complementary DNA (cDNA) synthesis. The cDNA was then sequenced. The following data were obtained from the coding strand of the $\beta$-globin gene and the corresponding mature mRNA transcript. - Length of $\beta$-globin gene (coding strand): $1600\,\text{bp}$ - Length of $\beta$-globin mature mRNA (after processing): $626\,\text{nt}$ - Number of amino acids in the final $\beta$-globin polypeptide: $146$ - The gene contains three exons and two introns.

The below shows a ribosome during translation in a prokaryotic cell. The mRNA sequence is $5'\text{-AUGGCUAAC-}3'$. A large and small ribosomal subunit are shown with mRNA threaded between them. Three tRNA binding sites are visible: the E site (empty), the P site (occupied by a tRNA with anticodon $3'\text{-UAC-}5'$ carrying methionine and a growing peptide), and the A site (approached by a tRNA carrying alanine). The codons are $5'\text{-AUG-}3'$, $5'\text{-GCU-}3'$, and $5'\text{-AAC-}3'$.

In an investigation of translation efficiency, researchers used an in vitro translation system containing ribosomes, tRNAs, amino acids, ATP, and GTP. They added a synthetic mRNA with the sequence $5'\text{-AUG GUA CCC UAA GCA UGG-}3'$.

The peppered moth (Biston betularia) is a classic example of natural selection. Before industrialisation, most peppered moths in England had light, speckled colouration (typica morph); a dark melanic morph (carbonaria morph) was very rare. During the Industrial Revolution, soot from factories darkened tree trunks and killed light-coloured lichens. Bird predation experiments showed that on soot-darkened trees, typica moths were eaten more frequently than carbonaria moths. - Frequency of carbonaria morph in Manchester in 1848: $2\%$ - Frequency of carbonaria morph in Manchester in 1895: $98\%$ - Generation time of peppered moth: $1$ year

A population of cheetahs (Acinonyx jubatus) lives in a nature reserve in Namibia. A researcher measures the running speed of $200$ adult cheetahs. The fastest $10\%$ of cheetahs (those running over $110\,\text{km\,h}^{-1}$) produce, on average, $4$ surviving cubs per year. The slowest $10\%$ (those running under $85\,\text{km\,h}^{-1}$) produce, on average, $1$ surviving cub per year. The remaining $80\%$ produce, on average, $2$ surviving cubs per year. Running speed is a heritable, polygenic trait.

The peppered moth (Biston betularia) is a classic example of natural selection driven by industrial pollution. In 19th-century England, two phenotypes existed: light-coloured (typica) and dark-coloured (carbonaria). Before industrialisation, typica moths were camouflaged against lichen-covered bark. After pollution darkened tree trunks with soot, carbonaria increased in frequency. Observed frequencies: Period — carbonaria frequency — typica frequency Pre-industrial (1848) — $2\%$ — $98\%$ Post-industrial (1895) — $95\%$ — $5\%$ Relative fitness values ($w$): Phenotype — Polluted forest — Unpolluted forest carbonaria — $0.95$ — $0.40$ typica — $0.50$ — $0.90$ Generation time of Biston betularia: 1 year.

A researcher studies the effect of sustainable agricultural practices on soil biodiversity in a grassland ecosystem. She compares two adjacent fields over 5 years: Field A is managed with conventional tillage and synthetic pesticides; Field B uses no-till farming, cover cropping, and integrated pest management (IPM). The Shannon-Wiener diversity index ($H$) for soil invertebrates is measured each year. A higher $H$ value indicates greater biodiversity. Year — Field A ($H$) — Field B ($H$) 1 — $1.2$ — $1.8$ 3 — $0.9$ — $2.3$ 5 — $0.7$ — $2.5$

A field study was conducted in a temperate deciduous forest to evaluate the sustainability of harvesting wood for biofuel. Two $1\,\text{ha}$ plots were established: Plot A (unharvested control) and Plot B (selective logging of $30\%$ of tree biomass every 5 years). Data collected after 20 years are shown below. Parameter — Plot A — Plot B Total woody biomass — $450\,\text{t}$ — $320\,\text{t}$ Simpson's Diversity Index ($D$) — $0.87$ — $0.72$ Soil organic carbon — $120\,\text{t}$ — $95\,\text{t}$ Annual NPP — $12.5\,\text{t}\,\text{ha}^{-1}\,\text{yr}^{-1}$ — $10.8\,\text{t}\,\text{ha}^{-1}\,\text{yr}^{-1}$ - Harvested wood from Plot B over 20 years: $210\,\text{t}$ - Energy content of dry wood: $18.5\,\text{MJ}\,\text{kg}^{-1}$ $$D = 1 - \frac{\sum n_i(n_i - 1)}{N(N-1)} \qquad \text{Percentage change} = \frac{\text{new} - \text{original}}{\text{original}} \times 100\%$$

A coastal community is evaluating the installation of a tidal barrage for renewable electricity generation. The barrage would enclose a $5.0\,\text{km}^2$ estuary. Environmental impact assessments collected the following baseline

A karyotype analysis of a human patient reveals a chromosomal mutation. The patient has 46 chromosomes, but one copy of chromosome 7 has a segment that is inverted relative to the normal sequence. A different patient has 45 chromosomes formed by the fusion of two acrocentric chromosomes at their centromeres.

A researcher uses the CRISPR-Cas9 system to disrupt a gene in human cells grown in culture. She designs a single guide RNA (sgRNA) that targets a sequence near the start of the gene. After editing, she uses PCR with primers flanking the target site to amplify the region. The PCR products are separated by gel electrophoresis. Cells that were not exposed to CRISPR-Cas9 produce a single band at $500\,\text{bp}$. Cells exposed to CRISPR-Cas9 produce two bands: one at $500\,\text{bp}$ and one at $320\,\text{bp}$.

A research team studies a gene in the bacterium E. coli that codes for an enzyme involved in lactose metabolism. The following segment of the coding strand (non-template strand) of the wild-type gene is shown: $$5'\text{-ATG GGC TAC CCT GAA TTC-}3'$$ The wild-type peptide produced from this segment is: Met–Gly–Tyr–Pro–Glu–Phe. After exposure to a chemical mutagen, two mutant strains are isolated, both unable to metabolise lactose. - Mutant 1: a single guanine (G) nucleotide is inserted between the 4th and 5th nucleotides of the coding strand shown above. - Mutant 2: a single base substitution changes the cytosine (C) at position 10 of the coding strand to adenine (A). Use the genetic code in the IB Data Booklet where required.

A single diploid cell ($2n = 4$) undergoes nuclear division as shown below. Cell 1: Four chromosomes (two long, two short), each consisting of two sister chromatids, arranged as two homologous pairs. Cell 2: Homologous pairs have separated; one long and one short chromosome move to opposite poles. Cell 3: Two cells, each containing two chromosomes (one long, one short), each still consisting of two sister chromatids. Cell 4: Four cells, each containing two chromosomes (one long, one short) as single chromatids.

A diploid cell ($2n = 4$) undergoes meiosis. The shows the cell at metaphase I, with two homologous pairs of chromosomes arranged at the equator of the cell. Chiasmata are visible between homologous chromosomes.

A research team investigates the effects of a novel compound, Karyostatin, on cell division in the root meristem of Allium cepa. Root tip cells are treated with Karyostatin for 24 hours, then fixed, stained, and observed under a light microscope. Cells are classified by phase. Results are shown below. Phase — Control (500 cells) — Karyostatin-treated (500 cells) Interphase — 120 — 185 Prophase — 240 — 100 Metaphase — 60 — 200 Anaphase — 50 — 10 Telophase — 30 — 5