2023 IB DP Biology Practice Paper with Answers

During exercise, increased cell respiration produces more carbon dioxide, which reacts with water to form carbonic acid and decreases blood pH. This decrease in pH is detected by chemoreceptors in the medulla oblongata, which then sends sympathetic nerve impulses to the sinoatrial (SA) node, causing it to increase the rate of depolarization and therefore increase heart rate.

Award 1 mark for the correct option A. Reject B, C, and D as they inaccurately describe the receptors, pH changes, autonomic pathways, or node actions.

Dehydration increases blood solute concentration, which is detected by osmoreceptors in the hypothalamus. The posterior pituitary gland is then stimulated to secrete ADH (antidiuretic hormone). ADH travels to the kidneys and increases the number of aquaporins in the membrane of the collecting duct cells, enhancing water reabsorption.

Award 1 mark for the correct option C. Reject other options because they name incorrect detection sites, hormone release sites, or aquaporin responses.

The greenhouse effect begins with short-wave solar radiation passing through the atmosphere to heat the Earth's surface. The Earth's surface then re-radiates this energy as long-wave infrared radiation. Greenhouse gases in the troposphere absorb this outgoing long-wave radiation and re-emit it in all directions, trapping heat in the lower atmosphere.

Award 1 mark for the correct option B. Reject options suggesting greenhouse gases directly absorb short-wave solar radiation (C) or that ozone trapping heat in the lower atmosphere is the main driver (A).

Dissolved carbon dioxide forms carbonic acid, which dissociates into hydrogen ions and bicarbonate. The increase in hydrogen ions lowers the ocean's pH. These hydrogen ions also react with carbonate ions to form more bicarbonate, thereby reducing the concentration of free carbonate ions available for calcifying organisms (like corals and molluscs) to build their calcium carbonate shells.

Award 1 mark for the correct option A. Reject B, C, and D as they incorrectly describe the chemical reactions, the direction of pH change, or the ions involved.

Type I pneumocytes are extremely thin, flattened cells that minimize the diffusion distance for gas exchange. Type II pneumocytes are cuboidal cells that secrete pulmonary surfactant, which reduces surface tension and prevents the alveoli from collapsing.

Award 1 mark for the correct option A. Reject B, C, and D because they swap functions, describe active transport of gases, or incorrectly state that surfactant increases surface tension.

During quiet, restful exhalation, the process is passive. The diaphragm relaxes and domes upwards, while the external intercostal muscles relax, lowering the ribcage. This decreases the thoracic volume, which increases the air pressure inside the lungs above atmospheric pressure, causing air to leave the lungs.

Award 1 mark for the correct option C. Reject other options because active contraction of internal intercostals only occurs during forced exhalation, and inhalation requires contraction of the diaphragm and external intercostals.

Antibiotic resistance is caused by pre-existing mutations within a gene pool before the antibiotic is introduced. The presence of the antibiotic acts as a selective pressure, killing susceptible cells while allowing resistant cells to survive and reproduce. This shifts the population frequency towards the resistance allele.

Award 1 mark for the correct option B. Reject Lamarckian explanations (A) where the antibiotic causes the mutation, active adaptations (C), or changes in binary fission rate as an active adaptation (D).

The scarcity of small seeds created a selective pressure where finches with deeper, larger beaks were better able to feed on the remaining large seeds and survive to reproduce. This led to directional selection, shifting the mean beak size of the population to be larger.

Award 1 mark for the correct option D. Reject A (Lamarckian inheritance), B (not disruptive), and C (stabilizing selection keeps the mean the same).

During strenuous exercise, high levels of carbon dioxide are produced, which react with water to form carbonic acid, lowering blood pH. This decrease in pH is detected by chemoreceptors (both central in the medulla and peripheral in the carotid and aortic bodies). These chemoreceptors send nerve impulses to the cardiovascular center of the medulla oblongata. The cardiovascular center coordinates a response, sending sympathetic nervous signals down the cardiac nerve to the sinoatrial (SA) node of the heart, causing it to increase the heart rate and thus speed up blood flow to remove carbon dioxide via the lungs.

Award 1 mark for the correct option (C). Correct option identifies that chemoreceptors detect the pH drop, send impulses to the medulla oblongata, which then uses sympathetic stimulation of the SA node to increase heart rate.

When core body temperature falls, the hypothalamus initiates responses to conserve and generate heat. Shivering is controlled by the somatic nervous system, triggering rapid skeletal muscle contraction to release heat via cellular respiration. Simultaneously, the sympathetic nervous system stimulates the constriction of arterioles supplying skin capillary loops (vasoconstriction), reducing blood flow to the skin surface to minimize heat loss.

Award 1 mark for the correct option (B). Option B correctly integrates somatic control of shivering and sympathetic control of cutaneous vasoconstriction.

Solar radiation mostly consists of short-wavelength radiation (such as UV and visible light) which passes through the atmosphere and is absorbed by the Earth's surface. The Earth then re-emits this energy as longer-wavelength, lower-energy infrared radiation (heat). Greenhouse gases in the atmosphere absorb this long-wavelength infrared radiation and re-radiate it in all directions, including back down towards the Earth, trapping heat within the atmosphere.

Award 1 mark for the correct option (B). This correctly describes the absorption of outgoing long-wavelength radiation and subsequent re-radiation.

A positive feedback loop is a process where the output of a system amplifies or accelerates the initial change. Ice has a high albedo (reflectivity) and reflects a large proportion of solar radiation. As temperatures rise, polar ice caps melt, exposing water or land with a much lower albedo. This darker surface absorbs more solar radiation, which further heats the Earth and leads to even more ice melting.

Award 1 mark for the correct option (B). Positive feedback loops amplify change, and the albedo effect of melting ice is a primary positive feedback mechanism.

Type I pneumocytes are extremely thin, flattened alveolar cells that make up the vast majority of the alveolar wall, minimizing the diffusion distance to facilitate rapid gas exchange of oxygen and carbon dioxide. Type II pneumocytes are larger, rounded, secretory cells that produce and secrete pulmonary surfactant, which reduces surface tension within the alveoli to prevent them from collapsing during expiration.

Award 1 mark for the correct option (A). Option A correctly identifies the structure and function of Type I pneumocytes.

Inspiration is an active process requiring muscle contraction. The diaphragm contracts and flattens (moving downwards). The external intercostal muscles contract (while the internal intercostal muscles relax), which pulls the ribcage upwards and outwards. This dual action increases the volume of the thoracic cavity, lowering the pressure inside the lungs below atmospheric pressure, causing air to flow in.

Award 1 mark for the correct option (B). B is the only option that correctly describes the diaphragm moving down, ribcage moving up and out, and the correct antagonistic action of the intercostal muscles during inspiration.

Natural selection operates on pre-existing genetic variation. Within a large bacterial population, some individual cells may carry rare mutations that grant them resistance to an antibiotic before they ever encounter it. When the antibiotic is introduced, it acts as a strong selective pressure, killing the susceptible (non-resistant) bacteria. The resistant individuals survive, reproduce by binary fission, and pass the resistance genes to their offspring, rapidly increasing the frequency of the resistant phenotype in the population.

Award 1 mark for the correct option (B). Correctly identifies that natural selection selects from pre-existing variations and does not cause mutations directly.

The drought acts as a selective pressure. Finches with deeper, stronger beaks are physically capable of cracking and eating the large, hard seeds, allowing them to survive. Finches with shallower beaks cannot access this food source and are more likely to die of starvation. The surviving deep-beaked finches reproduce more successfully and pass the alleles for deep beaks to their offspring. Over several generations, the average beak depth in the population increases (directional selection).

Award 1 mark for correct option (C). C accurately describes natural selection resulting in directional change over generations without suggesting Lamarckian inheritance.

During exercise, increased carbon dioxide production leads to more carbonic acid in the blood, which dissociates into hydrogen ions and bicarbonate, lowering blood pH. Chemoreceptors located in the medulla oblongata, carotid arteries, and aorta detect this drop in pH. They send sensory nerve impulses to the ventilation and cardiovascular centres in the medulla oblongata. In response, the cardiovascular centre sends sympathetic nerve impulses to the sinoatrial node to increase heart rate, while the ventilation centre increases breathing rate and depth, helping to expel CO2 and restore normal blood pH.

Award [1] for the correct option (A). Reject all other options.

After eating a meal, glucose is absorbed from the small intestine into the capillaries of the villi, which drain into the hepatic portal vein. Thus, glucose levels are extremely high in the hepatic portal vein. As this blood passes through the liver sinusoid capillaries, the high glucose concentration triggers the secretion of insulin from pancreatic beta cells. Insulin stimulates hepatocytes to absorb excess glucose and convert it into glycogen (glycogenesis). Consequently, the blood leaving the liver via the hepatic vein has a lower, regulated glucose concentration. This prevents hyperglycemia in the general systemic circulation.

Award [1] for the correct option (A). Reject all other options.

A positive feedback loop is a process where the output of a system amplifies or accelerates the initial change. In this scenario, the warming temperatures (initial change) cause permafrost to melt, releasing trapped greenhouse gases like methane (the output). Methane absorbs heat, further accelerating global warming (amplification). The other options describe negative feedback loops, which counteract or buffer the initial change.

Award [1] for the correct option (B). Reject all other options.

As global temperatures rise, the climate zones typical of specific altitudes shift upward. Montane species must migrate up the mountain slope to remain within their tolerable temperature range. However, as they move higher, the total land area of the mountain peak decreases (range contraction). If temperatures continue to rise, species already living at the highest altitudes have no higher ground to migrate to, putting them at extreme risk of extinction.

Award [1] for the correct option (C). Reject all other options.

Inhalation (inspiration) requires air to flow into the lungs, which happens when the pressure inside the lungs is lower than atmospheric pressure. This pressure drop is achieved by increasing the volume of the thoracic cavity. The volume increases when the external intercostal muscles contract (moving the ribcage up and out) while the internal intercostal muscles relax, and the diaphragm contracts and flattens (moves downwards). This antagonistic action increases thoracic volume, decreases pressure, and draws air in.

Award [1] for the correct option (A). Reject all other options.

Type II pneumocytes are cuboidal cells that secrete a fluid containing a surfactant. This surfactant forms a monolayer over the internal moist surface of the alveolus, reducing the surface tension of the water layer. This prevents the alveoli from collapsing during expiration and ensures they can re-expand easily during inspiration. Type I pneumocytes, on the other hand, are thin, flat cells that minimize diffusion distance.

Award [1] for the correct option (C). Reject all other options.

Natural selection operates on pre-existing genetic variation. The pesticide acts as a selective pressure. In the presence of the pesticide, individuals with the pre-existing resistance mutation have a selective advantage; they survive and reproduce at a higher rate than non-resistant individuals. Over generations, they pass this heritable resistance allele to their offspring, causing the allele frequency of the resistance gene to increase in the population's gene pool. Mutations are random and are not actively caused or directed by the environmental challenge.

Award [1] for the correct option (C). Reject all other options.

Natural selection relies on variation within a population. In a bacterial population, some individuals may possess resistance alleles due to random mutation or horizontal gene transfer. When an antibiotic is introduced, it acts as a strong selective agent. It kills the susceptible bacteria, leaving the resistant ones alive. These survivors reproduce, passing on their resistance genes, which increases the proportion of resistant bacteria in the population over time. Environmental pressures do not induce targeted mutations.

Award [1] for the correct option (B). Reject all other options.

During exercise, cellular respiration increases and produces more carbon dioxide, which reacts with water in blood plasma to form carbonic acid, lowering blood pH. Chemoreceptors in the medulla, carotid bodies, and aortic bodies detect this decrease in pH. They send nerve impulses to the cardiovascular and respiratory control centers in the medulla oblongata. The medulla oblongata then increases sympathetic stimulation to the sinoatrial node of the heart (increasing heart rate) and sends more frequent impulses via the phrenic and intercostal nerves to increase the ventilation rate.

Award 1 mark for option B. Option A is incorrect because baroreceptors detect blood pressure, not pH, and the parasympathetic system would decrease heart rate. Option C is incorrect because thermoreceptors detect temperature, not oxygen, and the cerebellum does not directly control ventilation rate. Option D is incorrect because lactate decreases pH, not increases it, and adrenaline is released by the adrenal glands, not the pituitary.

Adrenaline acts on the circulatory system to redirect blood flow to active tissues, causing vasodilation in arterioles supplying skeletal muscles and vasoconstriction in non-essential organs like the gut. Metabolically, adrenaline stimulates the liver to break down stored glycogen into glucose (glycogenolysis), increasing blood glucose levels to support rapid cellular respiration.

Award 1 mark for option B. Option A is incorrect because adrenaline causes vasodilation (not vasoconstriction) in skeletal muscles, and glycogenolysis (not glycogenesis) in the liver. Option C is incorrect because blood flow to the brain is preserved, and glucagon secretion is not inhibited. Option D is incorrect because adrenaline causes vasoconstriction in the digestive system and does not stimulate fat synthesis (lipogenesis).

Arctic sea ice has a high albedo, reflecting most of the incoming solar radiation back into space. When temperature increases cause this ice to melt, darker ocean water is exposed. This water has a lower albedo and absorbs significantly more solar radiation, which warms the ocean and increases local atmospheric temperatures, leading to even more ice melt. This self-reinforcing cycle is a positive feedback loop.

Award 1 mark for option B. Option A is incorrect because increasing volume does not create a positive feedback loop that accelerates warming in this manner. Option C is incorrect because while cold water dissolves more CO2, the primary feedback loop of ice melt is related to albedo. Option D is incorrect because oxygen does not react with methane in this way to drive greenhouse warming.

Methane is a potent greenhouse gas because its molecules are much more effective at absorbing and trapping longwave infrared radiation emitted from the Earth's surface compared to carbon dioxide. Even though methane is present in lower concentrations and has a shorter atmospheric lifetime than carbon dioxide, its high global warming potential (GWP) makes it a critical factor in climate change.

Award 1 mark for option C. Option A is incorrect because greenhouse gases absorb longwave infrared radiation, not shortwave solar radiation. Option B is incorrect because methane has a shorter atmospheric lifetime than carbon dioxide (about 12 years compared to centuries for CO2). Option D is incorrect because methane is not directly converted into ozone as the primary driver of global warming.

According to Fick's law, the rate of diffusion is inversely proportional to the diffusion distance. Type I pneumocytes are extremely thin, flattened alveolar cells that form a single-layered epithelium. This extremely thin barrier, combined with the thin capillary endothelial wall, minimizes the distance that oxygen and carbon dioxide must diffuse, significantly increasing the rate of gas exchange.

Award 1 mark for option B. Option A is incorrect because the thin fluid layer actually facilitates the dissolution and diffusion of gases, though surfactant is needed to prevent collapse. Option C is incorrect because bronchiolar branching increases surface area, not directly the partial pressure gradient of CO2. Option D is incorrect because surfactant decreases surface tension and does not increase membrane thickness (which would reduce diffusion rate).

Natural selection acts on pre-existing genetic variation within a population. In this scenario, some bacteria already possess alleles that confer tolerance or resistance to the heavy metal due to random mutations that occurred before exposure. When the heavy metal pollutant is introduced, it acts as a selective pressure, killing non-resistant individuals. The resistant individuals survive, reproduce, and pass on their resistance alleles, leading to an increase in the frequency of these alleles in the population over time.

Award 1 mark for option C. Option A represents a Lamarckian view of acquired inheritance, which is incorrect. Option B is incorrect because selection acts on pre-existing variation and does not 'force' uniform beneficial mutations. Option D is incorrect because it describes the opposite of selection for resistance.

(a) The medulla oblongata is the primary integration center in the brain for heart and ventilation rates.

(b) During exercise, increased cellular respiration in muscles produces more \(\text{CO}_2\). This \(\text{CO}_2\) diffuses into the blood plasma where it reacts with water to form carbonic acid (\(\text{H}_2\text{CO}_3\)), which dissociates into hydrogen ions (\(\text{H}^+\)) and hydrogencarbonate (\(\text{HCO}_3^-\)), thus lowering the blood pH. Chemoreceptors located in the medulla oblongata, carotid arteries, and aorta detect this drop in pH (and high \(\text{CO}_2\) levels). They send nerve impulses to the ventilation center in the medulla oblongata, which responds by sending more frequent nerve impulses via the phrenic and intercostal nerves to the diaphragm and external intercostal muscles. This increases both the rate and depth of ventilation to rapidly expel \(\text{CO}_2\) and take in \(\text{O}_2\).

(c) In response to exercise or stress, the sympathetic nervous system stimulates the adrenal glands (specifically the adrenal medulla) to secrete adrenaline into the bloodstream. Adrenaline acts as a chemical messenger and binds to adrenergic receptors on the sinoatrial (SA) node of the heart, increasing the rate of cardiac contraction (heart rate) and increasing stroke volume (strength of contraction). Furthermore, adrenaline causes vasodilation in arterioles supplying skeletal muscles and the heart, while causing vasoconstriction in arterioles supplying non-essential organs (such as the digestive tract and kidneys). This redirects oxygenated blood to the active muscles.

(a) Award [1] for:
- Medulla oblongata [1]

(b) Award [1] for each of the following up to [3 max]:
- Respiration in active muscles produces \(\text{CO}_2\), which dissolves to form carbonic acid / releases \(\text{H}^+\) ions / lowers blood pH [1];
- Chemoreceptors in the medulla / carotid arteries / aorta detect the decrease in pH / increase in \(\text{CO}_2\) [1];
- Nerve impulses are sent to the ventilation center in the medulla, which increases motor impulses to the diaphragm / external intercostal muscles to increase breathing rate and depth [1].

(c) Award [1] for each of the following up to [3 max]:
- Sympathetic stimulation triggers the release of adrenaline from the adrenal glands into the blood [1];
- Adrenaline binds to the sinoatrial (SA) node to increase heart rate and/or increase the force of contraction (stroke volume) [1];
- Adrenaline causes selective vasodilation (to active skeletal muscles) and vasoconstriction (to digestive organs/kidneys) to redistribute blood flow [1].

(a) The hypothalamus functions as the primary thermoregulatory center/thermostat.

(b) When thermoreceptors in the skin and the hypothalamus detect a decrease in ambient or core temperature, the hypothalamus sends sympathetic nerve impulses to the smooth muscle in the walls of arterioles supplying the skin. This triggers vasoconstriction of these arterioles, reducing the volume of blood flowing through the capillary beds near the surface of the skin. As a result, less heat is lost to the environment via radiation, conduction, and convection. Blood is shunted to deeper vessels, conserving core body heat.

(c) Muscular system integration: The hypothalamus sends rapid, involuntary motor impulses to skeletal muscles, causing them to contract and relax rapidly (shivering). This intense muscle activity significantly increases cellular respiration, releasing heat as a metabolic byproduct.
Endocrine system integration: Under prolonged cold exposure, the hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH prompts the thyroid gland to release thyroxin. Thyroxin acts on body cells to increase the basal metabolic rate, which increases heat production through cellular metabolism.

(a) Award [1] for:
- Hypothalamus [1]

(b) Award [1] for each of the following up to [3 max]:
- Thermoreceptors in the skin/hypothalamus detect a drop in temperature [1];
- Sympathetic nerves stimulate the contraction of smooth muscle in the skin arterioles (vasoconstriction) [1];
- Blood flow is diverted away from the skin surface to deeper blood vessels, reducing heat loss by radiation/convection [1].

(c) Award [1] for each of the following up to [3 max]:
- Shivering: involuntary rapid contraction of skeletal muscles increases cell respiration, generating heat as a byproduct [1];
- Prolonged cold stimulates the release of thyroxin (via TRH and TSH pathway) from the thyroid gland [1];
- Thyroxin increases the basal metabolic rate / cellular metabolism of target tissues, producing metabolic heat [1].

(a) Osmoreceptors (located in the hypothalamus).

(b) When blood solute concentration increases (dehydration), osmoreceptors in the hypothalamus detect the change. The hypothalamus synthesizes antidiuretic hormone (ADH), which is transported to and released by the posterior pituitary gland into the bloodstream. ADH travels to the kidneys, where it binds to specific receptors on the cells of the collecting ducts. This triggers an intracellular signaling pathway that causes aquaporin channels to insert into the luminal membranes of the collecting duct cells. This dramatically increases the permeability of the collecting duct to water, allowing water to be reabsorbed by osmosis back into the hypertonic renal medulla and then into the bloodstream. Consequently, blood osmolarity decreases, and a small volume of highly concentrated urine is excreted.

(c) When blood volume drops or blood osmolarity rises, the hypothalamus's thirst center is stimulated (directly by osmoreceptors and indirectly by baroreceptors or hormones like angiotensin II). The nervous system processes these sensory inputs to generate the conscious sensation of thirst. This motivates the behavioral adaptation of seeking and drinking water. Once water is ingested and absorbed in the digestive tract, blood osmolarity decreases and blood volume increases, which reduces the stimulation of osmoreceptors and shuts down the thirst sensation via negative feedback.

(a) Award [1] for:
- Osmoreceptors [1]

(b) Award [1] for each of the following up to [3 max]:
- Osmoreceptors in the hypothalamus detect high blood osmolarity and trigger the release of ADH from the posterior pituitary [1];
- ADH travels to the kidneys and increases the permeability of collecting ducts by inserting aquaporin channels into cell membranes [1];
- Water is reabsorbed by osmosis into the blood, producing concentrated urine / lowering blood solute concentration [1].

(c) Award [1] for each of the following up to [3 max]:
- High osmolarity / low blood volume stimulates the thirst center in the hypothalamus [1];
- This generates a conscious sensation of thirst, driving the behavioral response of seeking and drinking water [1];
- Water absorption in the gut dilutes blood osmolarity, inhibiting further thirst signals (negative feedback) [1].

(a) Anthropogenic sources: Carbon dioxide (\(\text{CO}_2\)) is primarily released by burning fossil fuels (coal, oil, gas) and deforestation. Methane (\(\text{CH}_4\)) is primarily released from agricultural activities (ruminant livestock digestive gases, rice paddies), landfill decomposition, and natural gas leaks.
Warming potential: Methane has a significantly higher global warming potential per molecule than carbon dioxide (approximately 25-30 times greater over a 100-year timescale), though it has a much shorter average lifetime in the atmosphere.

(b) Snow and ice have a high albedo, meaning they reflect a very high percentage of incoming solar radiation back into space. As global temperatures rise, Arctic sea ice melts, exposing the dark ocean water underneath. Dark ocean water has a low albedo and absorbs the vast majority of incoming solar radiation, converting it into heat. This absorbed heat raises regional water temperatures and warms the surrounding atmosphere. This warming, in turn, causes even more sea ice to melt, further reducing Earth's albedo and accelerating the warming cycle. This self-reinforcing process is a positive feedback loop.

(c) Consequences on high-latitude ecosystems include:
- Melting of permafrost, which destabilizes soil structures, changes local hydrology, and releases trapped greenhouse gases (like methane and \(\text{CO}_2\)).
- Loss of specialized habitat and breeding/hunting grounds for arctic species (such as polar bears, seals, and walruses) that rely on stable ice cover.
- Range expansion of sub-arctic or boreal species northward, leading to increased competition with, or displacement of, native arctic species.
- Increased frequency and severity of wildfires and pest infestations (e.g., bark beetles) in boreal forests due to warmer, drier conditions.

(a) Award [1] for each of the following up to [2 max]:
- \(\text{CO}_2\) sources include fossil fuel combustion / deforestation, while \(\text{CH}_4\) sources include cattle farming / rice paddies / landfills [1];
- \(\text{CH}_4\) has a significantly higher warming potential per molecule than \(\text{CO}_2\) (despite its shorter lifespan) [1].

(b) Award [1] for each of the following up to [3 max]:
- Ice has a high albedo/reflectivity, which reflects solar radiation back into space [1];
- Melting of ice exposes dark ocean water, which has a low albedo and absorbs more solar radiation as heat [1];
- The heat absorbed warms the ocean and air, leading to more ice melting, reinforcing the cycle (positive feedback) [1].

(c) Award [1] for each of two distinct consequences up to [2 max]:
- Loss of habitat / decline in populations of ice-dependent species (e.g., polar bears, seals) [1];
- Melting of permafrost causing landscape changes / release of trapped greenhouse gases (positive feedback) [1];
- Northward shift in species distributions / invasion of boreal species into tundra [1];
- Increased frequency of wildfires or pest outbreaks in northern forests [1].

(a) When atmospheric carbon dioxide (\(\text{CO}_2\)) dissolves in seawater, it reacts with water molecules (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)). Carbonic acid is unstable and rapidly dissociates into hydrogen ions (\(\text{H}^+\)) and hydrogen carbonate ions (\(\text{HCO}_3^-\)). The concentration of free hydrogen ions (\(\text{H}^+\)) in the ocean increases. Since pH is a measure of hydrogen ion concentration, an increase in \(\text{H}^+\) concentration directly lowers the pH, making the seawater more acidic.

(b) The excess free hydrogen ions (\(\text{H}^+\)) in the acidified seawater react with dissolved carbonate ions (\(\text{CO}_3^{2-}\)) to form hydrogen carbonate (\(\text{HCO}_3^-\)) ions. This reaction consumes free carbonate ions, significantly reducing their concentration in seawater. Calcifying organisms, such as corals and molluscs, require carbonate ions to combine with calcium ions (\(\text{Ca}^{2+}\)) to synthesize calcium carbonate (\(\text{CaCO}_3\)) for their skeletons and shells. Depleted carbonate levels make it energetically difficult for these organisms to build and maintain their calcium carbonate structures. In extreme cases, the undersaturation of carbonate causes existing shells and skeletons to dissolve.

(c) Ecological consequences of coral reef degradation include:
- Loss of critical habitat, shelter, and breeding grounds for approximately 25% of all marine species, leading to a massive decline in marine biodiversity.
- Collapse of local marine food webs, impacting commercial fisheries and predators that depend on reef organisms.
- Loss of the physical barrier that coral reefs provide, leading to increased coastal erosion and storm surge damage to coastal human communities.

(a) Award [1] for each of the following up to [3 max]:
- \(\text{CO}_2\) reacts with \(\text{H}_2\text{O}\) to form carbonic acid (\(\text{H}_2\text{CO}_3\)) [1];
- Carbonic acid dissociates into hydrogen ions (\(\text{H}^+\)) and hydrogen carbonate/bicarbonate (\(\text{HCO}_3^-\)) [1];
- The increased concentration of \(\text{H}^+\) ions directly reduces the pH of seawater [1].

(b) Award [1] for each of the following up to [3 max]:
- Excess \(\text{H}^+\) ions react with carbonate ions (\(\text{CO}_3^{2-}\)) to form hydrogen carbonate (\(\text{HCO}_3^-\)), reducing the pool of available carbonate ions [1];
- Calcifying organisms need carbonate ions to react with calcium ions to form calcium carbonate (\(\text{CaCO}_3\)) skeletons/shells [1];
- Reduced carbonate availability hinders shell/skeleton growth and can cause existing calcium carbonate structures to dissolve [1].

(c) Award [1] for any of the following:
- Dramatic loss of biodiversity as reef habitats collapse [1];
- Loss of coastal protection, leading to increased coastal erosion from waves/storms [1];
- Disruption of marine food webs / decline in fish stocks [1].

Part (a) Nervous and Endocrine Coordination of Cardiac Output:
During exercise, the cardiovascular control center in the medulla oblongata receives signals from proprioceptors and chemoreceptors. It initiates a sympathetic nervous response, sending impulses along the cardiac nerve to the sinoatrial (SA) node to increase heart rate. Simultaneously, the sympathetic nervous system stimulates the adrenal medulla (endocrine system) to secrete epinephrine (adrenaline) into the bloodstream. Epinephrine travels to the heart and binds to beta-receptors on the SA node and cardiac muscle fibers, causing an increase in both heart rate and stroke volume. This dual integration ensures a rapid and sustained increase in cardiac output.

Part (b) Regulation of Ventilation Rate and Depth:
1. Increased cellular respiration in active muscles produces higher amounts of carbon dioxide (CO2).
2. CO2 diffuses into the blood plasma, where it reacts with water to form carbonic acid (H2CO3), catalyzed by carbonic anhydrase; this dissociates into hydrogen ions (H+) and hydrogen carbonate ions (HCO3-).
3. The rise in H+ concentration lowers blood pH (increases acidity).
4. Central chemoreceptors in the medulla oblongata and peripheral chemoreceptors in the carotid and aortic bodies detect the decrease in pH / increase in CO2 levels.
5. These chemoreceptors send nerve impulses to the respiratory control center in the medulla oblongata.
6. The medulla oblongata increases the frequency of impulses sent via the phrenic nerve to the diaphragm and via intercostal nerves to the external intercostal muscles, leading to faster and deeper breathing (increased ventilation).

Part (c) Integration of Cardiovascular, Respiratory, and Muscular Systems:
- Active muscular contraction exerts pressure on deep veins, facilitating venous return to the heart, which increases cardiac stroke volume via Starling's law.
- Local metabolic activity in active muscles produces heat, CO2, and lactic acid (hydrogen ions). This local acidity and warmth cause local vasodilation of arterioles, directing more blood flow to active tissues while systemic sympathetic vasoconstriction reduces blood flow to non-essential organs (kidneys, gut).
- The high acidity (low pH) and high temperature in working muscle capillary beds shift the oxygen-hemoglobin dissociation curve to the right (the Bohr shift), facilitating the dissociation and release of oxygen from hemoglobin to active muscle fibers.
- The respiratory system increases ventilation to maintain high partial pressure of oxygen and low partial pressure of CO2 in the alveoli, sustaining a steep concentration gradient for gas exchange.
- Rapid pulmonary capillary blood flow coordinates with this steep gradient, maximizing oxygen uptake into the blood and carbon dioxide elimination into the expired air.

Part (a): Maximum [4 marks]
- Medulla oblongata / cardiovascular control center detects onset of exercise (via proprioceptors / chemoreceptors). [1]
- Sympathetic nervous system sends impulses via cardiac nerves to the sinoatrial (SA) node to increase heart rate. [1]
- Sympathetic stimulation triggers the adrenal glands/medulla to release epinephrine (adrenaline) into the blood. [1]
- Epinephrine binds to receptors on the SA node / cardiac muscle cells to further increase heart rate / stroke volume / cardiac output. [1]

Part (b): Maximum [5 marks]
- Active tissues produce CO2 which diffuses into blood plasma. [1]
- CO2 reacts with water to form carbonic acid, lowering blood pH / increasing H+ concentration. [1]
- Chemoreceptors in medulla oblongata / carotid arteries / aorta detect decrease in pH / increase in CO2. [1]
- Nerve impulses are sent to the respiratory control center in the medulla oblongata. [1]
- Medulla sends increased frequency of nerve impulses to diaphragm (via phrenic nerve) and external intercostal muscles (via intercostal nerves). [1]
- Results in an increased rate and depth of ventilation (breathing). [1]

Part (c): Maximum [6 marks]
- Muscle contractions squeeze veins, increasing venous return, which increases stroke volume / cardiac output. [1]
- Local metabolic factors (high CO2, low pH, warmth) in active muscles cause local vasodilation of arterioles to increase muscle blood flow. [1]
- Sympathetic nervous system causes systemic vasoconstriction of arterioles in non-essential organs (e.g., digestive tract, kidneys) to redirect blood to muscles. [1]
- Low pH / high temperature in working muscles causes the Bohr shift / shifts the oxygen-hemoglobin dissociation curve to the right, increasing oxygen release to muscle cells. [1]
- Ventilation rate increases to maintain steep diffusion gradients (for O2 and CO2) between alveoli and pulmonary capillaries. [1]
- Rapid blood flow through pulmonary capillaries ensures continuous loading of O2 and unloading of CO2. [1]

Clarity and Quality of Communication: [1 mark]
- 1 mark is awarded if the response is well-structured, coherent, and uses precise biological terminology consistently (e.g., medulla oblongata, sinoatrial node, epinephrine, chemoreceptors, Bohr shift, vasodilation, vasoconstriction) across all three parts.

(a) Minute ventilation is calculated by multiplying tidal volume by ventilation rate:
At 150 W: \(2.20 \text{ L} \times 32 \text{ breaths/min} = 70.4 \text{ L min}^{-1}\) (or \(70.4 \text{ L/min}\)).

(b) Tidal volume is determined by measuring the amplitude (vertical height) of a single wave/cycle on the spirometer trace, which represents the volume of air inspired or expired in a normal breath. Ventilation rate is determined by counting the number of wave peaks (respiratory cycles) within a specific time period (typically one minute).

(c) During exercise, active muscle tissues increase their rate of cell respiration, which produces more carbon dioxide. This increases the concentration of \(H^+\) ions, lowering blood pH. This change is detected by chemoreceptors, which stimulate the respiratory center in the medulla oblongata to increase ventilation rate to expel excess carbon dioxide and bring in more oxygen.

(a) [2 marks maximum]
- \(70.4 \text{ L min}^{-1}\) (accept \(70.4 \text{ L/min}\) or \(70.4\)) [1]
- Correct working shown: \(2.20 \times 32\) [1]

(b) [2 marks maximum]
- Tidal volume: vertical distance / amplitude of the breathing cycle peaks on the trace [1]
- Ventilation rate: count the number of peaks (breaths) per unit time (per minute) [1]

(c) [1 mark maximum]
- To deliver more oxygen to rapidly respiring muscle cells [1]
- To remove excess carbon dioxide / to prevent blood pH from dropping further [1]

(a) There is an inverse relationship (or negative correlation): as the pH of seawater decreases (becomes more acidic), the percentage mass loss of the mussel shells increases.

(b) Increased atmospheric carbon dioxide dissolves in seawater to form carbonic acid (\(H_2CO_3\)). Carbonic acid then dissociates into hydrogen ions (\(H^+\)) and hydrogen carbonate ions (\(HCO_3^-\)). The release of these hydrogen ions increases the acidity and thus lowers the pH of the seawater.

(c) 1. Place the shells in a drying oven or desiccator until a constant mass is reached before taking both the initial and final weighings, ensuring that residual water does not distort the mass measurements.
2. Use a high-precision electronic balance (e.g., to 0.001 g) to reliably measure very small changes in the shell mass.

(a) [1 mark maximum]
- Inverse relationship / negative correlation / as pH decreases, mass loss increases (or vice versa) [1]

(b) [2 marks maximum]
- Carbon dioxide reacts with water to form carbonic acid / \(CO_2 + H_2O \rightarrow H_2CO_3\) [1]
- Carbonic acid dissociates, releasing hydrogen ions / \(H^+\) which decreases the pH [1]

(c) [2 marks maximum]
- Dry shells completely in an oven or desiccator before weighing (at start and end) [1]
- Use a high-precision electronic digital balance (e.g., three decimal places) [1]
- Wash the shells with distilled water to remove salt residue before drying and final weighing [1]
(Accept any other valid control/accuracy measure related to mass determination)

(a) Initial heart rate at 0 min = 150 bpm; final heart rate at 5 min = 75 bpm.
Heart rate decrease = \(150 - 75 = 75\) bpm.
Percentage decrease = \(\frac{75}{150} \times 100 = 50.0\%\) (or \(50\%\)).

(b) Deep breathing recovery is significantly more effective than active recovery at lowering heart rate. At all time intervals post-exercise, the heart rate under the deep breathing treatment is lower than that of the active recovery treatment (e.g., at 5 minutes, deep breathing reached 70 bpm compared to 95 bpm for active recovery). Additionally, the total decrease in heart rate is greater for deep breathing (79 bpm reduction) than active recovery (57 bpm reduction).

(c) Deep breathing activates the parasympathetic nervous system (via the vagus nerve), which releases acetylcholine at the sinoatrial node to slow down the pacemaker, while simultaneously decreasing sympathetic nervous system activity.

(a) [2 marks maximum]
- \(50.0\%\) or \(50\%\) [1]
- Correct working: \(\frac{150 - 75}{150} \times 100\) [1]

(b) [2 marks maximum]
- Deep breathing is more effective at lowering heart rate than active recovery throughout the entire recovery period [1]
- Deep breathing results in a lower final heart rate after 5 minutes (70 bpm) compared to active recovery (95 bpm) [1]
- Deep breathing shows a larger overall drop in heart rate (79 bpm vs 57 bpm) [1]
(Award max 2 marks)

(c) [1 mark maximum]
- Deep breathing stimulates parasympathetic nervous activity / vagus nerve [1]
- Deep breathing suppresses sympathetic nervous activity [1]
- Leads to increased acetylcholine release at the sinoatrial node [1]
(Accept any valid biological mechanism of autonomic heart control)

During exercise, the cardiovascular control center in the medulla oblongata increases sympathetic nervous system activity and decreases parasympathetic activity.
1. Sympathetic neurons release noradrenaline directly onto the sinoatrial (SA) node, increasing heart rate, and onto cardiac muscle cells, increasing stroke volume.
2. Simultaneously, the sympathetic nervous system stimulates the adrenal medulla to secrete adrenaline (epinephrine) into the blood, reinforcing the increase in heart rate and contractility.
3. Sympathetic stimulation causes vasoconstriction in arterioles supplying non-essential organs (such as the digestive tract and kidneys) via alpha-adrenergic receptors.
4. Local metabolic changes in active skeletal muscles (such as accumulation of CO2, lactic acid, and heat) trigger local vasodilation (active hyperemia), routing the increased cardiac output preferentially to working muscles.

Award up to [5] marks from the following points:
- Sympathetic stimulation initiated by the cardiovascular center in the medulla oblongata [1]
- Noradrenaline/norepinephrine increases heart rate (via the SA node) and contractility [1]
- Adrenaline/epinephrine is released from the adrenal medulla into the bloodstream to prolong and enhance cardiac output [1]
- Vasoconstriction of arterioles serving non-essential organs (e.g., gut, kidneys) shunts blood away from these regions [1]
- Vasodilation of arterioles in skeletal muscles (mediated by local metabolic factors / nitric oxide / autoregulation) increases localized blood flow [1]

As global average temperatures rise, species' physiological tolerances and climate niches shift geographically. Species migrate upslope (to higher altitudes) where temperatures are cooler to track their historical thermal envelopes. For species already living near mountain peaks (the alpine zone), there is no remaining land of higher altitude to colonize. These species face 'mountain-top extinction' as their suitable habitat completely disappears. Furthermore, they face increased competition and predation from generalist species migrating upward from lower elevations, and are highly vulnerable to the loss of unique meltwater-dependent microhabitats.

Award up to [5] marks from the following points:
- Species migrate upslope / to higher altitudes to remain within their physiological temperature tolerances (tracking their ecological niche) [1]
- Alpine / high-altitude species have restricted range expansion potential because there is no higher territory left to colonize / 'mountain-top extinction' concept [1]
- Climate change reduces the total area of the alpine zone (mountains are cone-shaped, so area decreases with altitude) [1]
- High-altitude specialists face increased competition from lower-elevation species expanding their ranges upward [1]
- Loss of specialized habitats, such as those relying on seasonal glacial melt or snowpack, threatens dependent taxa [1]

Alveolar adaptation relies on Fick's Law of Diffusion, minimizing diffusion distance and maximizing surface area:
1. Type I pneumocytes are exceptionally thin, flattened epithelial cells that form the majority of the alveolar wall, minimizing the diffusion distance for oxygen and carbon dioxide.
2. Type II pneumocytes are cuboidal cells that secrete pulmonary surfactant, a lipoprotein mixture that reduces surface tension. This prevents alveolar collapse during expiration and keeps the surface moist, allowing gases to dissolve before diffusing.
3. The alveoli are highly folded into millions of tiny sacs to provide a massive total surface area.
4. Alveoli are surrounded by a dense network of capillaries with thin endothelial walls, ensuring a continuous flow of blood that maintains steep concentration gradients for both oxygen and carbon dioxide.

Award up to [5] marks from the following points:
- Type I pneumocytes are extremely thin/flattened to minimize diffusion distance [1]
- Type II pneumocytes secrete pulmonary surfactant [1]
- Surfactant reduces surface tension to prevent alveolar collapse / keeps the surface moist for gas dissolution [1]
- Massive number of alveoli provides a very large surface area to volume ratio for diffusion [1]
- Dense capillary network maintains a steep concentration gradient by rapidly transporting oxygenated blood away and carbon dioxide to the alveoli [1]

On the contaminated soil, high copper concentrations are toxic to most plants, acting as a strong selective pressure. Random genetic mutations within the *Agrostis capillaris* population generate individuals with varying degrees of copper tolerance. On contaminated soil, non-tolerant individuals die, while copper-tolerant individuals survive, reproduce, and pass their tolerance alleles to their offspring. Over generations, the frequency of copper-tolerance alleles increases dramatically in this localized subpopulation.

Conversely, on uncontaminated soil, there is no copper toxicity. Tolerant plants often have a lower fitness because maintaining the mechanisms for copper detoxification (e.g., producing phytochelatins or active efflux pumps) requires metabolic energy. Therefore, in the absence of copper, non-tolerant individuals grow faster and outcompete tolerant ones, keeping the frequency of the tolerance allele low in the uncontaminated area.

Award up to [5] marks from the following points:
- Genetic variation for copper tolerance exists in the ancestral population due to mutation/recombination [1]
- On contaminated soil, copper acts as a selective pressure / kills non-tolerant plants [1]
- Tolerant individuals have higher survival and reproductive success on contaminated soil [1]
- The allele/gene frequency for copper tolerance increases over generations in the contaminated area [1]
- On uncontaminated soil, copper-tolerant plants have lower relative fitness due to the metabolic cost of maintaining detoxification mechanisms / are outcompeted by non-tolerant plants [1]