Interaction and Interdependence — Free Biology HL Practice Questions

Auxin (indole-3-acetic acid, IAA) is a plant hormone that regulates phototropism. In a classic experiment, coleoptile tips of oat (Avena sativa) were removed and placed on agar blocks. The blocks were then placed asymmetrically on decapitated coleoptiles, and the angle of curvature was measured.

Cytokinins are plant hormones that promote cell division and delay senescence. They are produced primarily in roots and transported to shoots. A research team investigated the effect of the synthetic cytokinin 6-benzylaminopurine (BAP) on chlorophyll retention in detached leaves of Phaseolus vulgaris (bean). - Leaf discs ($1\,\text{cm}$ diameter) were floated on BAP solutions of varying concentration. - After 5 days in darkness, chlorophyll content was measured spectrophotometrically at $663\,\text{nm}$. Treatment — Absorbance at $663\,\text{nm}$ Control (distilled water) — $0.12$ $10\,\mu\text{M}$ BAP — $0.35$ $100\,\mu\text{M}$ BAP — $0.48$

Endocrine disruptors are environmental chemicals that interfere with hormone signalling pathways. Bisphenol A (BPA) is a monomer used in polycarbonate plastics that leaches from plastic waste into aquatic ecosystems. BPA is structurally similar to oestradiol, a lipid-soluble steroid hormone that crosses the plasma membrane and binds to an intracellular receptor; the resulting hormone–receptor complex acts a transcription factor. Researchers measured BPA concentration in water and in the tissues of organisms at successive trophic levels in a contaminated lake. Sample — BPA concentration Lake water — $2.5\ \mu\text{g}\,\text{L}^{-1}$ Algal tissue — $15.3\ \mu\text{g}\,\text{kg}^{-1}$ Zooplankton tissue — $48.7\ \mu\text{g}\,\text{kg}^{-1}$ Small fish tissue — $162.4\ \mu\text{g}\,\text{kg}^{-1}$ Additional information: - BPA has a lower binding affinity for oestradiol receptors than oestradiol itself. - BPA is more resistant to metabolic breakdown in animal tissues than oestradiol.

The below shows a simplified overview of aerobic respiration in a eukaryotic cell. Use the following ATP yields for parts (b) and (c): glycolysis $= 2\,\text{ATP}$; link reaction $= 0\,\text{ATP}$; Krebs cycle $= 2\,\text{ATP}$; oxidative phosphorylation $= 28\,\text{ATP}$.

A researcher investigates aerobic cell respiration using isolated rat liver mitochondria. The mitochondria are supplied with pyruvate, ADP, and inorganic phosphate ($\text{P}_\text{i}$). Oxygen consumption is measured over 10 minutes; $0.5\,\mu\text{mol}$ of $\text{O}_2$ is consumed. Assume: complete oxidation of one pyruvate via the link reaction and Krebs cycle yields $4\,\text{NADH}$, $1\,\text{FADH}_2$, and $1\,\text{ATP}$ (substrate-level phosphorylation). Each NADH yields $2.5\,\text{ATP}$; each FADH₂ yields $1.5\,\text{ATP}$. Each $\text{O}_2$ molecule accepts $4$ electrons; each NADH or FADH₂ donates $2$ electrons to the electron transport chain.

A researcher studies the effectiveness of two experimental vaccines against a bacterial pathogen that produces a potent exotoxin. The vaccines are tested in three groups of ten mice each: - Group A: vaccinated with a chemically inactivated exotoxin (toxoid) at week 0 and week 4. - Group B: injected with a saline placebo at week 0 and week 4. - Group C: injected with a single dose of a live attenuated bacterial vaccine at week 0. At week 8, all mice are challenged with a lethal dose of the live pathogen. Results are shown below. Group — Survival — Time to symptom onset — Outcome A — 10/10 — 48 hours — Resolved by 72 hours B — 0/10 — 12 hours — Death by 24 hours C — 9/10 — 36 hours — Resolved by 60 hours; one mouse died at 30 hours

Severe combined immunodeficiency (SCID) is a genetic disorder in which both B-lymphocytes and T-lymphocytes are absent or non-functional. A patient with SCID received a bone marrow transplant from a healthy sibling. The table below shows lymphocyte counts in the patient's blood over 12 months following the transplant, alongside normal reference ranges. Time (months) — T-cells $(\text{cells}\,\mu\text{L}^{-1})$ — B-cells $(\text{cells}\,\mu\text{L}^{-1})$ 0 — 0 6 — 800 — 200 1200 — 600 Normal range — 800–2000 — 100–500

A researcher studying the nervous system of the squid (Loligo pealeii) measures the diameter of a giant axon and finds it to be $1.0\,\text{mm}$. In humans, a typical myelinated motor neuron has an axon diameter of $10\,\mu\text{m}$ but conducts action potentials at $120\,\text{m s}^{-1}$, compared to $25\,\text{m s}^{-1}$ in the squid giant axon.

A neurobiologist examines a cross-section of a peripheral nerve under an electron microscope. She observes that some axons are surrounded by multiple layers of Schwann cell membrane forming a myelin sheath, while others have only a single Schwann cell layer without myelin.

A coastal marine ecosystem contains the following organisms: - Phytoplankton (primary producers) - Zooplankton and mussels (primary consumers) - Small fish and crabs (secondary consumers; crabs eat mussels and zooplankton; small fish eat zooplankton) - Seabirds and large fish (tertiary consumers; both eat small fish and crabs) The total energy stored in phytoplankton biomass is $50{,}000\,\text{kJ m}^{-2}$. Ecological efficiencies are: phytoplankton → primary consumers $= 10\%$; primary consumers → secondary consumers $= 15\%$; secondary consumers → tertiary consumers $= 8\%$. Assume primary consumers collectively consume all available phytoplankton energy.

In a coastal salt marsh ecosystem, the water cycle and nitrogen cycle interact closely. During a spring tide, seawater floods the marsh, depositing dissolved inorganic nitrogen (mainly $\text{NO}_3^-$ and $\text{NH}_4^+$) at a concentration of $0.8\,\text{mg L}^{-1}$. The total volume of seawater flooding the marsh is $5000\,\text{m}^3$. The marsh plants, primarily Spartina alterniflora, take up this nitrogen for growth. After the tide recedes, evaporation and plant transpiration remove water from the marsh soil.

A pharmaceutical company is testing drug Y as a potential treatment for hypertension. Drug Y inhibits angiotensin-converting enzyme (ACE), which converts angiotensin I to angiotensin II. The kinetics of ACE were studied in vitro at fixed enzyme concentration. $$\frac{1}{v} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$$ $[\text{Angiotensin I}]\ (\text{nM})$ — $v\ (\text{nmol min}^{-1})$ — no drug — $v\ (\text{nmol min}^{-1})$ — with $10\ \text{nM}$ drug Y $2$ — $0.80$ — $0.50$ $5$ — $1.50$ — $1.00$ $10$ — $2.20$ — $1.70$ $20$ — $2.80$ — $2.50$ $50$ — $3.30$ — $3.20$

The shows a metabolic pathway in Escherichia coli that synthesises product Z from substrate X via intermediate Y.

A population of Daphnia (water fleas) in a small pond grows according to a logistic model. The carrying capacity of the pond is $K = 800$ individuals. The graph below shows population size ($N$) against time ($t$) in weeks.

An ecologist studies a population of algae (Chlorella vulgaris) in a laboratory culture flask. The population grows according to the logistic growth model. The carrying capacity of the flask is $5.0 \times 10^{6}$ cells $\text{mL}^{-1}$. The graph below shows the population size over time.

A clinical study investigates the role of the hypothalamus–pituitary–adrenal (HPA) axis integrating nervous and endocrine responses to acute psychological stress. Twenty healthy volunteers perform a 10-minute public speaking task. Blood samples are taken before the task (baseline), immediately after (0 min), and at 30 and 60 minutes after the task. Time point — Plasma cortisol ($\mu\text{g/dL}$) — Plasma ACTH (pg/mL) — Heart rate (bpm) Baseline — 10 — 20 — 70 0 min — 25 — 60 — 110 30 min — 35 — 45 — 80 60 min — 15 — 22 — 72 Additional information: - HPA axis pathway: stress $\rightarrow$ hypothalamus releases CRH $\rightarrow$ anterior pituitary releases ACTH $\rightarrow$ adrenal cortex releases cortisol; cortisol exerts negative feedback on the hypothalamus and anterior pituitary. - The sympathetic nervous system is activated during stress, releasing noradrenaline from nerve terminals and adrenaline from the adrenal medulla.

A student investigates the effect of exercise on heart rate. She measures her resting heart rate, then runs on a treadmill for 5 minutes at a constant speed, recording her heart rate every minute. Time — Heart rate / bpm Rest (0 min) — 72 1 min — 98 2 min — 115 3 min — 128 4 min — 135 5 min — 138