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Science 30 practice questions

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Question 1Living Systems Respond to Their Environment

Which statement best describes the role of a receptor protein in a cell signalling pathway?

  1. It binds a specific signalling molecule and initiates a response inside the cell Correct answer.
  2. It destroys all molecules that enter the bloodstream
  3. It stores genetic information for the cell
  4. It manufactures the signalling molecule before release

Explanation

Receptor proteins have binding sites shaped to fit particular signalling molecules, such as hormones or neurotransmitters. When the signal binds, the receptor changes shape and triggers events inside the cell, often through a cascade of reactions. This specificity is why a hormone affects only cells that carry its matching receptor.

Question 2Living Systems Respond to Their Environment

A person becomes dehydrated and their blood becomes more concentrated. Which response helps restore water balance?

  1. Increased release of insulin, so cells take up more glucose
  2. Increased release of antidiuretic hormone, so the kidneys reabsorb more water Correct answer.
  3. Decreased release of aldosterone, so the kidneys reabsorb more sodium
  4. Decreased release of antidiuretic hormone, so the kidneys excrete more water

Explanation

Osmoreceptors detect the rise in solute concentration and stimulate release of antidiuretic hormone from the pituitary. This hormone makes the collecting ducts of the kidneys more permeable to water, so more water is reabsorbed into the blood. Blood becomes more dilute and urine becomes more concentrated, restoring homeostasis.

Question 3Living Systems Respond to Their Environment

In a negative feedback loop, what happens when a controlled variable returns to its set point?

  1. The corrective response continues at full strength indefinitely
  2. The set point is permanently raised to a new value
  3. The corrective response is reduced or switched off Correct answer.
  4. The sensor stops detecting the variable forever

Explanation

Negative feedback works by opposing a deviation from the set point. Once the variable is back within the normal range, the stimulus for the corrective response fades, so the response is reduced. This prevents overshoot and keeps the variable oscillating close to the set point.

Question 4Living Systems Respond to Their Environment

Which sequence correctly describes the path of a reflex arc?

  1. Effector, motor neuron, interneuron, sensory neuron, receptor
  2. Receptor, motor neuron, interneuron, sensory neuron, effector
  3. Sensory neuron, receptor, effector, interneuron, motor neuron
  4. Receptor, sensory neuron, interneuron, motor neuron, effector Correct answer.

Explanation

A reflex arc begins when a receptor detects a stimulus and generates a nerve impulse. The impulse travels along a sensory neuron to the spinal cord, where an interneuron relays it to a motor neuron. The motor neuron carries the impulse to an effector such as a muscle, which produces the response.

Question 5Living Systems Respond to Their Environment

A student claims that hormones and nerve impulses both travel at the same speed to their targets. Which comparison is accurate?

  1. Hormones act faster than nerve impulses because they travel in the blood
  2. Hormones and nerve impulses both act only on the cell that released them
  3. Nerve impulses reach every cell in the body, while hormones reach only one cell
  4. Nerve impulses act quickly over short distances, while hormones act more slowly but can reach distant targets Correct answer.

Explanation

Nerve impulses are electrical signals carried along axons and can produce responses within milliseconds, but they act on a limited set of targets. Hormones are chemical signals released into the blood, so they travel more slowly yet can affect many distant tissues that carry the matching receptor. The two systems differ in speed, range, and duration of effect.

Question 6Living Systems Respond to Their Environment

Which structure of the human eye changes shape to focus light from objects at different distances?

  1. The retina
  2. The sclera
  3. The cornea
  4. The lens Correct answer.

Explanation

The lens is flexible and its curvature is adjusted by the ciliary muscles, a process called accommodation. Changing the lens shape alters how strongly light is refracted so that a sharp image falls on the retina. The cornea provides most of the fixed refraction but does not change shape.

Question 7Living Systems Respond to Their Environment

A sound wave of greater amplitude reaches the ear. What change does the listener perceive?

  1. A louder sound Correct answer.
  2. A higher-pitched sound
  3. A lower-pitched sound
  4. A sound of different timbre

Explanation

Amplitude corresponds to the size of the pressure variation in the sound wave. Larger amplitude vibrations displace the eardrum and ossicles more strongly, and the brain interprets this as greater loudness. Pitch depends on frequency, not amplitude.

Question 8Living Systems Respond to Their Environment

Which part of a neuron receives incoming signals from other neurons?

  1. The dendrites Correct answer.
  2. The synaptic vesicles
  3. The myelin sheath
  4. The axon

Explanation

Dendrites are branching extensions that carry receptor sites for neurotransmitters released by neighbouring neurons. They collect incoming signals and pass them toward the cell body, where the impulse may be generated. The axon then conducts the outgoing impulse away from the cell body.

Question 9Living Systems Respond to Their Environment

A blood glucose level rises after a meal. Which hormonal response lowers it?

  1. The pancreas releases insulin, which stimulates cells to take up glucose Correct answer.
  2. The pancreas releases glucagon, which stimulates the liver to release glucose
  3. The thyroid releases thyroxine, which increases metabolic rate
  4. The adrenal glands release adrenaline, which raises blood glucose

Explanation

Insulin is secreted by beta cells of the pancreas when blood glucose rises. It increases glucose uptake by cells and promotes storage of glucose as glycogen in the liver and muscles, lowering blood glucose toward the set point. Glucagon has the opposite effect and is released when glucose is low.

Question 10Living Systems Respond to Their Environment

Which statement describes the role of the myelin sheath in a myelinated neuron?

  1. It insulates the axon and increases the speed of impulse conduction Correct answer.
  2. It produces neurotransmitters for release at the synapse
  3. It generates the resting potential of the neuron
  4. It connects the neuron directly to a muscle fibre

Explanation

Myelin is a fatty layer wrapped around the axon by supporting cells. It acts as an electrical insulator so the impulse jumps between gaps called nodes of Ranvier, a process known as saltatory conduction. This greatly increases the speed of transmission along the axon.

Question 11Living Systems Respond to Their Environment

A patient's immune system mistakenly attacks the insulin-producing beta cells of the pancreas. Which consequence is most likely?

  1. Digestion of proteins in the stomach stops completely
  2. The kidneys can no longer filter blood
  3. Blood glucose falls dangerously low because too much insulin is released
  4. Blood glucose remains persistently high because insulin cannot be produced Correct answer.

Explanation

Beta cells of the pancreatic islets secrete insulin in response to rising blood glucose. If these cells are destroyed, insulin production falls and cells cannot take up glucose efficiently, so blood glucose stays elevated. This illustrates how damage to one component of a feedback loop disrupts homeostasis.

Question 12Living Systems Respond to Their Environment

Why does a person with a damaged cerebellum have difficulty with balance and coordinated movement?

  1. The cerebellum controls the release of hormones from the pituitary
  2. The cerebellum coordinates muscular activity and maintains posture and balance Correct answer.
  3. The cerebellum is the main centre for conscious thought and memory
  4. The cerebellum regulates the rate of breathing and heart contraction

Explanation

The cerebellum integrates sensory information about body position and compares it with intended movements. It fine-tunes muscle activity so movements are smooth and the body stays balanced. Damage therefore produces uncoordinated movement and poor balance rather than paralysis.

Question 13Living Systems Respond to Their Environment

Which mechanism allows a single hormone to produce different responses in different target tissues?

  1. The target tissues contain different receptors and different intracellular signalling pathways Correct answer.
  2. The hormone changes its chemical structure after it enters the blood
  3. The hormone is transported to a single organ and cannot reach other tissues
  4. The hormone is only released when the nervous system is inactive

Explanation

A hormone circulates throughout the body, so specificity comes from the receptors present on cells. Tissues with different receptor types or different downstream pathways can respond to the same hormone in distinct ways. Cells lacking the receptor do not respond at all.

Question 14Living Systems Respond to Their Environment

A person hyperventilates, causing blood carbon dioxide to fall below normal. Which change is expected?

  1. Blood pH falls because more carbonic acid forms
  2. Blood oxygen concentration falls to zero
  3. Blood glucose concentration rises sharply
  4. Blood pH rises because less carbonic acid forms Correct answer.

Explanation

Carbon dioxide reacts with water to form carbonic acid, which releases hydrogen ions and lowers pH. When carbon dioxide is lost through rapid breathing, less carbonic acid forms, so hydrogen ion concentration falls and pH rises. This condition is called respiratory alkalosis.

Question 15Living Systems Respond to Their Environment

Which description correctly matches a plant hormone with its main effect?

  1. Cytokinin promotes leaf fall and seed dormancy
  2. Abscisic acid promotes seed germination and stem growth
  3. Ethylene promotes cell elongation and prevents fruit ripening
  4. Auxin promotes cell elongation and contributes to phototropism Correct answer.

Explanation

Auxin is produced in shoot tips and moves away from light, causing cells on the shaded side to elongate more. This unequal growth bends the shoot toward the light, a response called phototropism. Other plant hormones have different, often opposing, roles.

Question 16Living Systems Respond to Their Environment

A plant is placed so that light strikes it from one side only. Which response is expected?

  1. The root bends toward the light because auxin promotes root elongation
  2. The shoot stops growing until the light becomes uniform
  3. The shoot bends toward the light because auxin accumulates on the shaded side Correct answer.
  4. The shoot bends away from the light because auxin accumulates on the lit side

Explanation

Auxin produced in the shoot tip is redistributed so that more accumulates on the shaded side. Higher auxin concentration stimulates greater cell elongation on that side, so the shoot curves toward the light source. This directional growth response is positive phototropism.

Question 17Living Systems Respond to Their Environment

Which feature of the alveoli makes gas exchange in the lungs efficient?

  1. A small surface area that concentrates oxygen
  2. A thick muscular wall that actively pumps gases
  3. A large total surface area combined with a thin, moist wall Correct answer.
  4. A dry lining that prevents gases from dissolving

Explanation

Millions of alveoli provide a very large surface area within a compact lung volume. Their walls are one cell thick and lined with moisture so oxygen and carbon dioxide can diffuse rapidly across a short distance. A rich capillary network maintains steep concentration gradients.

Question 18Living Systems Respond to Their Environment

During exercise, heart rate and breathing rate both increase. How does this help maintain homeostasis?

  1. It decreases glucose supply to the muscles
  2. It lowers body temperature by reducing blood flow to the skin
  3. It reduces the amount of oxygen needed by the muscles
  4. It delivers more oxygen and removes carbon dioxide faster from active tissues Correct answer.

Explanation

Active muscles respire faster, consuming oxygen and producing extra carbon dioxide. Raising heart and breathing rates increases blood flow and ventilation, speeding delivery of oxygen and removal of carbon dioxide. This keeps blood gas concentrations and pH near their set points.

Question 19Living Systems Respond to Their Environment

Which statement about the transmission of an impulse across a synapse is correct?

  1. The impulse reverses direction and travels back along the same axon
  2. The synaptic gap is bridged by myelin that conducts the impulse
  3. The electrical impulse jumps directly across the gap without any chemical release
  4. A neurotransmitter is released into the synaptic gap and binds to receptors on the next neuron Correct answer.

Explanation

When an impulse reaches the axon terminal, vesicles release neurotransmitter into the synaptic gap. The neurotransmitter diffuses across the gap and binds to receptors on the postsynaptic membrane, triggering a new impulse if the signal is strong enough. Transmission is therefore chemical and one-directional.

Question 20Living Systems Respond to Their Environment

A drug blocks the receptors that detect antidiuretic hormone in the kidney tubules. Which effect is most likely?

  1. The kidneys reabsorb excessive water and blood becomes too dilute
  2. Blood glucose rises sharply because insulin is blocked
  3. Large volumes of dilute urine are produced and dehydration may develop Correct answer.
  4. The kidneys stop filtering blood completely

Explanation

Antidiuretic hormone normally increases water reabsorption by making the collecting ducts more permeable. If its receptors are blocked, water stays in the tubules and is excreted, producing a large volume of dilute urine. Unchecked water loss can lead to dehydration and rising blood solute concentration.

Question 21Living Systems Respond to Their Environment

Which structure of the human ear converts sound vibrations into nerve impulses?

  1. The auditory canal
  2. The Eustachian tube
  3. The pinna
  4. The cochlea Correct answer.

Explanation

Vibrations pass through the ossicles to the cochlea, a fluid-filled spiral structure. Movement of fluid bends hair cells lining the cochlea, and these cells generate nerve impulses that travel along the auditory nerve to the brain. The other structures collect or channel sound rather than convert it.

Question 22Living Systems Respond to Their Environment

Why do the two organ systems responsible for coordination need to interact rather than work independently?

  1. Hormones travel along axons so they can reach every cell directly
  2. Nerve impulses control long-term growth while hormones control only reflexes
  3. The nervous system can trigger hormone release so responses are both rapid and sustained Correct answer.
  4. The two systems never affect the same physiological variable

Explanation

The nervous system reacts within milliseconds, while hormones produce slower, longer-lasting adjustments. Linkages such as the hypothalamus controlling the pituitary allow a nerve signal to trigger hormone release, combining speed with sustained effect. Many variables, including blood glucose and water balance, are regulated by both systems together.

Question 23Living Systems Respond to Their Environment

A stimulus produces a larger depolarisation than usual in a sensory neuron. What does this mean for the resulting nerve impulses?

  1. Each impulse travels much faster than before
  2. The impulses are generated more frequently, increasing the perceived intensity Correct answer.
  3. Each impulse has a much larger voltage
  4. The impulses travel in the opposite direction

Explanation

Nerve impulses are all-or-nothing events, so a stronger stimulus does not make each impulse larger. Instead, a stronger stimulus triggers impulses more often, and the higher frequency is interpreted as a more intense sensation. Conduction speed depends mainly on axon properties such as myelination and diameter.

Question 24Living Systems Respond to Their Environment

Which response helps the body cool down when core temperature rises above the set point?

  1. Sweat glands secrete more sweat and blood vessels near the skin dilate Correct answer.
  2. Sweat glands secrete less sweat and skin blood vessels constrict
  3. Shivering begins and metabolic rate increases
  4. The thyroid releases more thyroxine to raise metabolism

Explanation

Sweating cools the body as water evaporates from the skin, carrying heat away. Dilation of skin blood vessels increases blood flow near the surface, so more heat is lost to the surroundings. Both responses lower core temperature back toward the set point.

Question 25Living Systems Respond to Their Environment

A plant's roots grow downward into the soil while its shoots grow upward. Which explanation is most consistent with plant responses to gravity?

  1. Auxin accumulates on the upper side of roots and stimulates their elongation, so roots bend downward
  2. Shoots grow upward because light destroys all auxin in the shoot tip
  3. Auxin accumulates on the lower side of roots and inhibits their elongation, so roots bend downward Correct answer.
  4. Roots grow toward gravity because they contain no auxin at all

Explanation

Gravity causes auxin to settle on the lower side of horizontal roots and shoots. In roots, high auxin concentration inhibits cell elongation, so the upper side grows faster and the root curves downward. In shoots, the same accumulation stimulates elongation, producing upward growth.

Question 26Living Systems Respond to Their Environment

Which type of sensory receptor in the skin detects a light touch on the forearm?

  1. Chemoreceptor
  2. Thermoreceptor
  3. Photoreceptor
  4. Mechanoreceptor Correct answer.

Explanation

Touch is a mechanical stimulus, so the receptors that respond to pressure and deformation of the skin are mechanoreceptors. Photoreceptors respond to light, chemoreceptors to dissolved chemicals, and thermoreceptors to temperature change.

Question 27Living Systems Respond to Their Environment

A person steps from a dark room into bright sunlight. Which change occurs in the eye?

  1. The pupils constrict to reduce light entering. Correct answer.
  2. The lens becomes thicker to increase light entering.
  3. The rods become the main photoreceptors used for colour vision.
  4. The optic nerve stops transmitting impulses temporarily.

Explanation

In bright light the circular muscles of the iris contract so the pupil narrows, limiting the amount of light reaching the retina and protecting it from overstimulation. Lens shape adjusts focus rather than total light entry.

Question 28Living Systems Respond to Their Environment

Which part of the human ear is responsible for detecting the orientation of the head and helping maintain balance?

  1. The semicircular canals Correct answer.
  2. The cochlea
  3. The tympanic membrane
  4. The Eustachian tube

Explanation

The semicircular canals contain fluid and hair cells that detect rotational movement of the head, providing information used for balance. The cochlea handles hearing, while the tympanic membrane and Eustachian tube serve sound transmission and pressure equalisation.

Question 29Living Systems Respond to Their Environment

Which statement best describes the function of a reflex arc?

  1. It produces a rapid response that bypasses conscious processing in the brain. Correct answer.
  2. It permanently stores learned motor skills for later use.
  3. It transmits information only from the brain to the muscles.
  4. It allows a voluntary decision to be made before any movement occurs.

Explanation

A reflex arc routes sensory input through the spinal cord or lower brain centres to motor neurons, producing a fast protective response without waiting for conscious decision-making. This shortens reaction time for harmful stimuli.

Question 30Living Systems Respond to Their Environment

A person's blood becomes more concentrated because of water loss. Which response is expected?

  1. Increased release of insulin, causing cells to take up more water
  2. Decreased release of glucagon, causing the liver to store more water
  3. Decreased release of antidiuretic hormone, causing more urine to be produced
  4. Increased release of antidiuretic hormone, causing more water reabsorption in the kidneys Correct answer.

Explanation

Osmoreceptors detect the rise in blood solute concentration and trigger release of antidiuretic hormone, which increases water reabsorption in the kidney tubules and concentrates the urine. This restores blood water balance.

Question 31Living Systems Respond to Their Environment

Which characteristic of a hormone's action is illustrated by the fact that a small amount of hormone can produce a large response in a target cell?

  1. Signal amplification through a cascade of enzyme reactions Correct answer.
  2. Permanent alteration of the target cell's DNA
  3. Direct conversion of the hormone into ATP
  4. Immediate destruction of the target cell membrane

Explanation

Hormone binding can activate an enzyme that activates many more molecules, so a small signal is amplified into a large cellular response. This cascade is a common feature of hormone action.

Question 32Living Systems Respond to Their Environment

Which statement correctly describes an example of negative feedback in the human body?

  1. Childbirth contractions intensify until delivery is complete.
  2. Blood clotting continues until the entire vessel is sealed.
  3. A rise in body temperature triggers responses that lower it back toward the set point. Correct answer.
  4. A rise in body temperature triggers responses that raise it further.

Explanation

Negative feedback reverses a deviation from the set point, so a temperature rise triggers sweating and vasodilation that cool the body. The other examples describe positive feedback, where a change is amplified.

Question 33Living Systems Respond to Their Environment

A plant is grown in soil that is dry on one side and moist on the other. Which response is expected?

  1. Shoots bend toward the dry side.
  2. Roots grow preferentially toward the moist side. Correct answer.
  3. Roots stop growing in both directions equally.
  4. Roots grow preferentially toward the dry side.

Explanation

Roots show hydrotropism, growing toward regions of greater water availability, which helps the plant obtain water. Shoots respond mainly to light and gravity rather than to soil moisture gradients.

Question 34Living Systems Respond to Their Environment

A person with a hormonal disorder produces normal amounts of thyroid hormone but lacks receptors for it in most body tissues. Which outcome is most likely?

  1. Symptoms of low thyroid hormone activity despite normal hormone levels Correct answer.
  2. Symptoms of excessive thyroid hormone activity in all tissues
  3. No effect at all, because hormone concentration is normal
  4. Immediate loss of all nervous system function

Explanation

A hormone produces effects only when it binds to functional receptors on target cells, so normal hormone levels with missing receptors produce signs of deficiency. The hormone is present but cannot deliver its signal.

Question 35Living Systems Respond to Their Environment

Which comparison of nervous and endocrine signalling is correct?

  1. Both systems release their chemical messengers into synapses only.
  2. Both systems use electrical impulses travelling along blood vessels.
  3. Nervous signals act quickly over short distances, while endocrine signals act more slowly and can affect many tissues. Correct answer.
  4. Nervous signals act slowly over long distances, while endocrine signals act instantly.

Explanation

Nervous signalling uses fast electrical impulses and neurotransmitters acting over short distances, while hormones travel in the blood to reach widespread targets with slower, longer-lasting effects. The two systems differ in speed, route, and duration.

Question 36Living Systems Respond to Their Environment

During a stressful situation, adrenaline is released. Which combination of effects is most consistent with its role?

  1. Decreased breathing rate and reduced blood glucose
  2. Increased urine production and reduced alertness
  3. Decreased heart rate and increased digestion
  4. Increased heart rate and increased blood flow to skeletal muscles Correct answer.

Explanation

Adrenaline prepares the body for rapid action by raising heart rate, widening airways, and directing blood flow to skeletal muscles while mobilising glucose. These changes support the fight-or-flight response.

Question 37Living Systems Respond to Their Environment

A toxin prevents the release of neurotransmitters from the presynaptic neuron at a neuromuscular junction. Which effect is most likely?

  1. The sensory neuron stops transmitting touch information.
  2. The muscle fails to contract because no signal crosses the synapse. Correct answer.
  3. The muscle contracts continuously because the synapse is overstimulated.
  4. The muscle contracts normally because the signal travels directly through the synaptic gap.

Explanation

At a chemical synapse the impulse must be carried across the gap by neurotransmitter, so blocking release prevents the signal from reaching the muscle fibre. Without that signal, contraction cannot be triggered.

Question 38Living Systems Respond to Their Environment

A person is given a drug that blocks the reuptake of a neurotransmitter in a synapse. What is the most direct effect?

  1. The presynaptic neuron stops producing the neurotransmitter entirely.
  2. The neurotransmitter is destroyed before it can act on the postsynaptic cell.
  3. The neurotransmitter remains in the synaptic gap longer, prolonging stimulation of the postsynaptic cell. Correct answer.
  4. The postsynaptic receptors are permanently removed from the membrane.

Explanation

Reuptake normally removes neurotransmitter from the gap to end the signal. Blocking reuptake leaves more neurotransmitter available to bind receptors, so the postsynaptic response is prolonged.

Question 39Living Systems Respond to Their Environment

A person's blood glucose falls well below normal. Which hormonal response is expected?

  1. Antidiuretic hormone is released and increases water reabsorption.
  2. Insulin is released and stimulates cells to take up glucose.
  3. Glucagon is released and stimulates the liver to break down glycogen. Correct answer.
  4. Thyroxine is released and increases the metabolic rate of all cells.

Explanation

Low blood glucose stimulates glucagon release, which promotes glycogen breakdown in the liver and releases glucose into the blood. Insulin has the opposite effect and would lower glucose further.

Question 40Living Systems Respond to Their Environment

Which statement best explains why the sinoatrial node is described as the pacemaker of the heart?

  1. It stores oxygenated blood for later release.
  2. It controls the opening of the heart valves mechanically.
  3. It generates electrical impulses that set the rhythm of the heartbeat. Correct answer.
  4. It pumps blood directly into the aorta.

Explanation

The sinoatrial node is a group of specialised cells that spontaneously generate electrical impulses, setting the rate at which the heart contracts. Other heart structures respond to these impulses rather than initiating rhythm.

Question 41Living Systems Respond to Their Environment

Both breathing rate and heart rate increase during vigorous exercise. Which explanation best describes how this supports homeostasis?

  1. It increases delivery of oxygen and removal of carbon dioxide while the muscles respire faster. Correct answer.
  2. It lowers the body's metabolic rate to conserve energy.
  3. It decreases blood flow to the lungs so more blood stays in the muscles.
  4. It reduces the amount of oxygen reaching the muscles to prevent overheating.

Explanation

Exercising muscles consume oxygen and produce carbon dioxide faster, so increased breathing and circulation speed gas exchange and delivery to tissues. This keeps blood gases and pH within workable limits.

Question 42Living Systems Respond to Their Environment

A plant is kept in continuous darkness, yet its shoot continues to grow upward. Which explanation is most consistent with this observation?

  1. The shoot is following a fixed spiral pattern unrelated to any stimulus.
  2. The shoot responds to gravity, a response that does not require light. Correct answer.
  3. The shoot is growing toward the warmest side of the container.
  4. The shoot is responding to light that is invisible to the eye but present in darkness.

Explanation

Negative gravitropism causes shoots to grow away from the pull of gravity even when light is absent, because gravity is still a directional stimulus. Light is not required for this response.

Question 43Living Systems Respond to Their Environment

A gardener removes the growing tip of a young plant. Which change is most likely to occur in the plant's growth pattern?

  1. The plant stops growing entirely because all growth hormones are lost.
  2. The stem becomes shorter because auxin production increases.
  3. The roots stop absorbing water because the tip produced all root hormones.
  4. Lateral buds grow more because the supply of auxin from the tip is reduced. Correct answer.

Explanation

Auxin produced in the shoot tip suppresses the growth of lateral buds, a phenomenon called apical dominance. Removing the tip reduces this suppression, allowing side branches to develop.

Question 44Living Systems Respond to Their Environment

A person living at high altitude has a lower oxygen concentration in the air. Which long-term response is expected?

  1. Decreased breathing rate to conserve oxygen
  2. Increased production of red blood cells to improve oxygen transport Correct answer.
  3. Increased production of digestive enzymes to extract more oxygen
  4. Reduced heart rate to lower oxygen demand

Explanation

Low oxygen availability stimulates the production of more red blood cells, increasing the blood's oxygen-carrying capacity over time. This is an adaptive response that helps maintain oxygen supply to tissues.

Question 45Living Systems Respond to Their Environment

Which example best illustrates the role of a feedback loop in maintaining a stable internal environment?

  1. Sweating increases when body temperature rises, and the resulting cooling causes more sweating.
  2. Sweating occurs only when body temperature falls below normal.
  3. Body temperature changes randomly with no relationship to sweating.
  4. Sweating increases when body temperature rises, and the resulting cooling reduces sweating. Correct answer.

Explanation

In negative feedback, the response counteracts the initial change; cooling the body reduces the stimulus that triggered sweating. This cycle keeps temperature close to the set point.

Question 46Living Systems Respond to Their Environment

A nerve impulse travels along an axon. Which statement about the refractory period is correct?

  1. It increases the speed of the impulse at the synapse.
  2. It allows the impulse to travel in both directions simultaneously.
  3. It ensures the impulse travels in one direction along the axon. Correct answer.
  4. It prevents the neuron from ever firing again.

Explanation

During the refractory period the membrane cannot immediately be re-excited, so the region just behind an impulse cannot fire again. This enforces one-way propagation of the impulse.

Question 47Living Systems Respond to Their Environment

Two hormones act on the same target tissue but produce opposite effects on blood glucose. Which term best describes their relationship?

  1. Synergistic
  2. Permissive
  3. Trophic
  4. Antagonistic Correct answer.

Explanation

Antagonistic hormones have opposing actions on the same variable, allowing fine control in both directions. Insulin lowers blood glucose while glucagon raises it, which is a classic antagonistic pair.

Question 48Living Systems Respond to Their Environment

A patient's kidneys fail to respond to aldosterone. Which change in blood composition is most likely?

  1. Complete loss of the ability to produce urine
  2. Increased loss of sodium in urine, with disturbed salt and water balance Correct answer.
  3. Immediate increase in red blood cell production
  4. Increased reabsorption of sodium, causing high blood sodium

Explanation

Aldosterone normally promotes sodium reabsorption in the kidney tubules, with water following osmotically. If the kidneys do not respond, sodium is lost in urine and salt and water balance is disturbed.

Question 49Living Systems Respond to Their Environment

A drug increases the permeability of the kidney collecting duct to water even when antidiuretic hormone levels are low. Which effect is most likely?

  1. Reduced blood volume because water leaves the blood
  2. More dilute urine and greater water loss
  3. More concentrated urine and greater water retention Correct answer.
  4. Increased sodium loss without any change in water balance

Explanation

Water permeability in the collecting duct allows water to be reabsorbed from the filtrate, concentrating the urine and retaining water in the blood. This effect occurs regardless of the hormone signal.

Question 50Living Systems Respond to Their Environment

A sensory receptor adapts to a constant stimulus. Which observation is most consistent with this adaptation?

  1. The frequency of impulses from the receptor decreases over time despite the stimulus continuing. Correct answer.
  2. The receptor begins to send impulses more frequently as the stimulus continues.
  3. The receptor sends impulses of much greater amplitude to the brain.
  4. The receptor stops responding to all future stimuli permanently.

Explanation

Adaptation means the receptor becomes less responsive to an unchanging stimulus, so impulse frequency declines even though the stimulus persists. This is why a constant smell or pressure fades from awareness.