Unity and Diversity — Free Biology HL Practice Questions

Influenza A virus is an enveloped virus with a segmented single-stranded RNA genome. The virus binds to sialic acid receptors on host respiratory epithelial cells via its haemagglutinin (HA) glycoprotein. After endocytosis, the low pH of the endosome triggers fusion of the viral envelope with the endosomal membrane, releasing the viral RNA segments into the cytoplasm. The viral RNA-dependent RNA polymerase (RdRp) replicates the viral genome and transcribes mRNA. New viral particles assemble at the host cell plasma membrane, where neuraminidase (NA) cleaves sialic acid to release progeny virions. A researcher treats infected cell cultures with oseltamivir, a competitive inhibitor of neuraminidase. After 24 hours, the number of viral particles released into the culture medium and the number remaining attached to the host cell surface are measured.

A laboratory culture of Escherichia coli is infected with bacteriophage T4. The phage injects its double-stranded DNA into the host cell, which then replicates the phage genome and synthesises phage proteins. New phage particles are assembled, and the host cell is lysed by phage-encoded lysozyme, releasing approximately 200 progeny phages per infected cell. This the lytic cycle. The culture initially contains $5.0 \times 10^{8}$ bacterial cells and is infected at a multiplicity of infection (MOI) of $0.1$ (1 phage per 10 bacterial cells).

A culture of $\text{CD4}^+$ T-cells was infected with HIV at time $= 0\,\text{h}$. The concentration of free virions in the culture medium was measured every $6\,\text{h}$ for $48\,\text{h}$. At $t = 12\,\text{h}$ the culture was divided into three treatment groups: - Group A: no drug (control) - Group B: treated with zidovudine (AZT), a nucleoside analogue that inhibits reverse transcriptase - Group C: treated with raltegravir, an inhibitor of viral integrase Viral concentrations (thousands of virions $\text{mL}^{-1}$) are shown below. Time (h) — 0 — 6 — 12 — 18 — 24 — 36 — 48 Group A — 0.0 — 0.5 — 2.0 — 8.0 — 32.0 — 512.0 — 8192.0 Group B — 0.0 — 0.5 — 2.0 — 2.1 — 2.2 — 2.4 — 2.6 Group C — 0.0 — 0.5 — 2.0 — 1.9 — 1.7 — 1.4 — 1.1

The Namib Desert beetle (Stenocara gracilipes) survives in one of Earth's driest environments by harvesting water from coastal fog. The beetle's wing surface has alternating hydrophobic (waxy) regions and hydrophilic bumps rich in hydroxyl ($-\text{OH}$) groups. When fog rolls in, water droplets condense on the hydrophilic bumps. The beetle tilts its body forward and water rolls down the wing into its mouth.

The vascular system of a giant sequoia (Sequoiadendron giganteum) transports water from roots to leaves over $80\,\text{m}$ high. Xylem vessels are composed of dead cells with heavily lignified walls. Transpiration from leaf surfaces creates a negative pressure (tension) that pulls water upward through the xylem.

Researchers investigate the role of water as a solvent in the cytoplasm of Escherichia coli using a theoretical diffusion model.

A phylogenetic tree was constructed using small subunit ribosomal RNA (SSU rRNA) sequences from three domains of life. The tree is rooted at a trifurcation point, with three branches of approximately equal length diverging simultaneously to Domain Bacteria (E. coli, 1542 nucleotides), Domain Archaea (Methanococcus, 1430 nucleotides), and Domain Eukarya (Homo sapiens, 1869 nucleotides). Within Domain Eukarya, the branch to Saccharomyces cerevisiae is longer than the branch to Homo sapiens.

A cladogram for five mammal species (platypus, kangaroo, bat, whale, and human) is constructed using three shared derived characters:
(1) mammary glands,
(2) placenta,
(3) echolocation. Presence is indicated by $1$, absence by $0$. Species — Mammary glands — Placenta — Echolocation Platypus — 1 — 0 Kangaroo — 1 — 0 Bat — 1 — 1 Whale — 1 — 0 Human — 1 — 0

The three-domain system (Bacteria, Archaea, Eukarya) is based on fundamental molecular and cellular differences. However, discoveries of organisms with mixed characteristics challenge the assumption of monophyly — a group containing a common ancestor and all its descendants — for each domain.

A simplified phylogenetic tree was constructed using rRNA gene sequences from three organisms: a methanogen (Methanococcus), a cyanobacterium (Anabaena), and a human (Homo sapiens). The tree indicates that the methanogen and human share a more recent common ancestor than either shares with the cyanobacterium. The methanogen's rRNA gene sequence is $85\%$ similar to the human sequence; the cyanobacterium's rRNA gene sequence is $60\%$ similar to the human sequence.

A phylogenetic tree of the three domains of life is constructed using rRNA sequence comparisons. Branch lengths are proportional to genetic distance, measured in substitutions per nucleotide site. The root is placed between Bacteria and the Archaea–Eukarya clade. - Branch length, Bacteria: $0.20$ substitutions per nucleotide site - Branch length, Archaea: $0.12$ substitutions per nucleotide site - Branch length, Eukarya: $0.05$ substitutions per nucleotide site

The three-domain classification system proposes that all life is divided into Bacteria, Archaea, and Eukarya.

The endosymbiotic theory proposes that mitochondria originated from free-living prokaryotes that were engulfed by a host cell.

Endosymbiotic theory proposes that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by a host eukaryotic cell.

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by a host cell. The following data were obtained from comparative genomic studies. Parameter — Value Genome size of free-living $\alpha$-proteobacterium (ancestral to mitochondria) — $\approx 4.0 \times 10^{6}\,\text{bp}$ Genome size of modern human mitochondrial DNA (mtDNA) — $\approx 1.66 \times 10^{4}\,\text{bp}$ Genome size of free-living cyanobacterium (ancestral to chloroplasts) — $\approx 3.5 \times 10^{6}\,\text{bp}$ Genome size of modern chloroplast DNA (cpDNA) in Arabidopsis thaliana — $\approx 1.54 \times 10^{5}\,\text{bp}$ Protein-coding genes in free-living $\alpha$-proteobacterium — $\approx 3{,}000$ Protein-coding genes in modern human mtDNA — $37$ Mitochondrial proteins in humans (nuclear-encoded) — $\approx 1{,}100$

The three-spined stickleback (Gasterosteus aculeatus) is a small fish found in both marine and freshwater environments in the Northern Hemisphere. After the last Ice Age (approximately 10,000 years ago), many marine stickleback populations became trapped in newly formed freshwater lakes and streams. These isolated populations have since diverged into distinct freshwater forms. Data from a study of two stickleback populations in British Columbia, Canada, are shown below: - Population A: Found in a large, deep lake (Loon Lake). Males build nests in open, sandy areas. Breeding occurs in June. Average number of lateral bony plates $= 32$. - Population B: Found in a small, shallow stream (Beaver Creek) that flows into Loon Lake. Males build nests under rocks in fast-flowing water. Breeding occurs in May. Average number of lateral bony plates $= 8$.

The apple maggot fly (Rhagoletis pomonella) originally infested only hawthorn (Crataegus) fruits in North America. Around 1850, a population began to infest introduced apple (Malus domestica) fruits. Today, the two host races are genetically distinct and show partial reproductive isolation. Mating occurs on the host fruit on which the female lays her eggs. In a laboratory experiment, female flies from the apple race were given a free choice of fruits on which to lay eggs. Results: - Apple fruits: $78$ eggs laid - Hawthorn fruits: $22$ eggs laid

The Grant family's long-term study of Darwin's finches (Geospiza spp.) on Daphne Major island in the Galápagos has provided direct evidence for natural selection in the wild. In 1977, a severe drought reduced seed availability. Small, soft seeds were rapidly depleted, leaving primarily large, hard seeds. Researchers measured beak depth in the medium ground finch (Geospiza fortis) population before and after the drought.

A researcher compares the efficiency of mitochondria from different tissues. Liver mitochondria consume $1.8 \times 10^{-16}\,\text{mol}$ of $\text{O}_2$ per second when respiring with excess substrate. Each liver mitochondrion contains 5000 copies of cytochrome c oxidase, the terminal enzyme of the electron transport chain.

A chloroplast in a leaf cell fixes $1.2 \times 10^{-16}\,\text{mol}$ of $\text{CO}_2$ per second during peak sunlight. The Calvin cycle requires 3 molecules of ATP and 2 molecules of NADPH to fix one molecule of $\text{CO}_2$.

A research team investigates a rare mitochondrial disorder in human fibroblasts. Cells from a patient show normal rates of glycolysis but significantly reduced overall ATP production. Electron microscopy reveals sparse, disorganised cristae and a less dense matrix compared with healthy cells.

A population of sea turtles nests on a beach undergoing tourism development. The bar chart shows the number of successful nests (eggs hatched) from 2015 to 2025.

A factory releases sulfur dioxide ($\text{SO}_2$) into the atmosphere, contributing to acid rain. The table shows the pH of rainwater collected at four sites at increasing distances from the factory. Site — Distance from factory / km — Rainwater pH A — 2 — 4.2 B — 5 — 4.5 C — 10 — 5.0 D — 20 — 5.6 Normal, unpolluted rainwater has a pH of approximately 5.6.

The bacterium E. coli can use either glucose or lactose as a carbon source. When both sugars are present, the lac operon is repressed until glucose is depleted. The graph below shows the relative intracellular lactose concentration over time after both sugars are added simultaneously at $t = 0$.

A researcher studies transcription of a gene in a eukaryotic cell. The gene is $1200$ base pairs long. RNA polymerase II moves along the template strand at a rate of $50$ nucleotides per second. - The gene contains $4$ introns, each $150$ base pairs long - Average mass of one ribonucleotide in mRNA: $5.0 \times 10^{-22}\,\text{g}$