Chapter 9

The salinity (salt concentration) of the ocean’s surface water varies over space and time. Which of the following would result in a higher local sea surface salinity?

• Evaporation

  Yes! Rainfall and continental runoff both provide inputs of fresh water, which lowers the sea surface salinity. Evaporation removes water, thereby concentrating the salts that are left behind. (Waves have no effect on salt concentration either way.)

Mussels are bivalve (two-shelled) filter-feeding mollusks. Marine mussels are osmoconformers that break proteins into individual free amino acids to increase their internal solute concentration, and dump the free amino acids to the environment to decrease their internal solute concentration. Mussels live in a variety of habitats in the surface ocean, including estuaries: waterways where freshwater from a river mixes with the saltwater of the sea. Estuaries become more salty as the ocean tide pushes in, and become less salty as the tide draws back out. Where within an estuary would mussels experience the greatest cost for their osmoconformation?

In the interior of the estuary, where the influx and efflux of the tide has a greater influence on the salinity

That’s right! Mussels must break down proteins when salinity increases and dump amino acids to the environment when it decreases. This cost of energy and materials is less at the mouth of the estuary, where salinity doesn’t change as much.

Excessive heat can physically damage enzymes, resulting in a catastrophic and often irreversible loss of biochemical function. What happens if an organism instead experiences internal conditions that are too cold?

Their biochemical processes will occur at a slower rate, but will not be irreversibly damaged

Correct! Colder conditions do not pose a threat to enzymes but do result in less vigorous collisions between molecules, causing the rate of biochemical reactions to decrease, to potentially dangerously low levels (too slow to meet demand).

Both hyperthermia (increase in core body temperature) or hypothermia (decrease in core body temperature) are dangerous to humans. Which is more likely to happen suddenly with permanent damage to our bodies?

Hyperthermia

Yes. Excessive heat is more likely to cause sudden and irreversible damage, as occurs in heat stroke. Hypothermia is also dangerous, but due to a progressive biochemical slowdown rather than abrupt and permanent failure.

Which of our body structures will exchange heat more rapidly with the environment: our trunk (main body) or limbs (arms and legs)?

• Limbs

  That’s right. Our limbs are long thin structures, which means a relatively large part of their mass is in close contact to the environment (they have a relatively high surface area to volume ratio).

In extremely cold conditions, the blood vessels that supply our limbs may constrict in order to reduce blood circulation through them, to reduce the rate of heat loss from the blood and keep more of the heat in the body core. This can cause blood circulation to our fingers and toes to slow down so much as to put them at risk of not getting enough oxygen etc. Why should you not place somebody who is suffering from hypothermia in a hot bath to warm them up?

• Because the blood vessels to the limbs may dilate in response to the sudden heat thereby allowing cold blood to flood from the limbs into the core

  Exactly! The sudden flush of warmth to the limbs can ironically cause the body core temperature to drop as the cold blood is released into the core. It’s thus better to warm the body and especially the limbs somewhat slowly.

Why do active thermoregulators have to generate their own heat to maintain thermal homeostasis?

Because it’s easier to warm up than cool off

Correct! It’s possible to actively generate heat (as a byproduct of biochemistry), but not to actively generate cold. Therefore, the only viable option to stay thermally constant is to generate heat to maintain a body temperature above typical ambient.

Which of the following statements is correct about the thermal strategies and classifications of animals?

It is possible to be an ectothermic homeotherm, but not an endothermic poikilotherm

Yes! Endotherms are strictly homeothermic, by definition. Ectotherms, however, may be poikilothermic or exhibit some level of homeothermy (though not as capably as endotherms).

Endothermy offers advantages, but those advantages are not always worth the energy cost. Why is ectothermy generally more advantageous than endothermy in the ocean?

Because the ocean doesn't change temperature much there isn't much need to deal with widely varying temperature conditions

Correct! Because the temperature of the ocean typically varies little in a given location, the animals that live there can simply optimize to the relatively constant prevailing temperature, to achieve whatever maximum metabolic rate it allows.

Where are ectotherms in the ocean likely to better thrive: in cold or warm water?

Warm water

You got it! Because warmer conditions drive more vigorous biochemistry, colder water lowers the ceiling on how active the biochemistry can be, no matter how finely tuned it may be to the prevailing temperature.

Why are all marine mammals endothermic?

• 
  Probably because they first evolved on land

  That’s right. Mammals evolved endothermy on land, where it is extremely advantageous, and the ancestors of the marine mammals kept it with them when they returned to the sea, despite its cost. It has evidently remained worthwhile enough to persist.

In general, biodiversity is highest in the tropics and decreases toward the poles. However, marine mammal and bird diversity exhibit the reverse trend. In 2019, a team of researchers led by John Grady argued that the explanation for this oddity may be surprisingly simple: it is because marine mammals and birds are endotherms, whereas the fish and reptiles they eat and compete with are ectotherms. Why would this matter—how could this explain the trend?

Endothermy becomes increasingly advantageous over ectothermy as conditions get colder

Good job! The maximum possible biochemical rates for ectotherms are highest in the warm waters of the tropics, but decrease as the water cools toward the poles—where the cost of maintaining high rates through endothermy becomes increasingly worth it.

Some phytoplankton (aquatic single-celled photosynthetic organisms) produce a compound called dimethylsulfide (DMS). DMS is a gas that, once in the atmosphere, acts as a cloud nucleation particle: it encourages water vapor to condense into clouds. Clouds, in turn, block the sun, thereby lowering the amount of solar radiation that reaches the Earth. Suppose that a warming Earth spurs an increase in phytoplankton growth, causing an increase in DMS production. What effect is this likely to have on global climate?

• It will cause the Earth to warm less on average

  Exactly! DMS results in more clouds, which reflects more of the sun’s energy away from the Earth rather than allowing it to be absorbed, which slows global warming.

More global warming --> more phytoplankton growth --> more DMS production --> more cloud cover --> more solar radiation being reflected away from the Earth --> less global warming. Is this a positive or negative feedback in Earth’s climate system?

Negative

• Yes! The outcome of the warming itself reduces the warming, so this is a negative feedback.

Body warms --> sweat produced --> evaporative cooling occurs --> body cools. How will humidity affect this negative feedback?

The feedback will be weaker in higher humidity

That’s right. It is harder for sweat to evaporate in higher humidity (as water vapor is more likely to re-condense even as it evaporates), thus reducing its effectiveness.

Suppose that a cell has two different receptors for a hormone, one of which acts to inhibit a certain response when it binds the hormone and the other of which stimulates the response when it binds the hormone. Which of the following scenarios could result in the hormone inhibiting the cell response when present at low levels, but stimulating the response when it is present at high levels?

The inhibitory receptor has the higher affinity (greater sensitivity) for the hormone

You got it! This is one model for the ovulation cycle in the human reproductive system: egg release is inhibited at low hormone concentrations, but stimulated once the hormones build to a high enough level for the stimulatory receptor to activate.

Which kind of hormone diversifies through evolution most easily?

Peptides

Yes! A new peptide can be generated from as little as a single amino acid substitution in the polypeptide chain, meaning a single nucleotide change in the relevant gene can do it. New monoamines and steroids require whole new genes coding for new enzymes.

Which of the following represents proper homeostatic control of the relevant system?

Group of answer choices

Blood cells shrink when placed in a solution of salt and water

The core body temperature of a runner rises gradually as she exercises

Blood pressure increases in response to an increase in blood volume (such as from drinking more water)

The kidneys allow more salt to leave in the urine when the body's salt levels rise

The kidneys allow more salt to leave in the urine when the body's salt levels rise.

Explanation

Homeostasis is the process by which an organism maintains a relatively stable internal environment despite changes inside or outside the body.

Homeostasis typically works through negative feedback, meaning the body detects a change and responds in a way that counteracts it.

Here's how Option 4 demonstrates homeostasis:

1. Blood salt levels increase above their normal range.

2. The body detects the change in salt concentration.

3. The kidneys increase salt excretion through urine.

4. Salt levels decrease toward their normal range.

5. Homeostasis is restored.

Why the Other Answers Are Incorrect

• Option 1 — Blood cells shrink in salt water: ❌ This describes osmosis, not homeostatic regulation. Water leaves cells when they are placed in a hypertonic solution.

• Option 2 — Core body temperature rises during exercise: ❌ This describes a change in body temperature rather than a corrective response. Homeostasis would involve sweating and increased blood flow to the skin to prevent excessive overheating.

• Option 3 — Blood pressure increases when blood volume increases: ❌ This describes a direct consequence of increased blood volume, not a homeostatic response. Homeostatic mechanisms would work to reduce excess blood volume and bring blood pressure back toward normal.

• Option 4 — Kidneys excrete more salt when salt levels rise: ✅ This is negative feedback because the response counteracts the original change.

Which of the following statements about thermoregulation is correct?

Group of answer choices

Organisms can generally tolerate increases in temperature above the optimum better than they can decreases in temperature below the optimum.

Organisms that utilize morphological adaptations to retain heat never also actively generate heat via biochemical or physiological mechanisms.

Ectotherms are thermoconformers.

Endotherms may be either poikilotherms or homeotherms.

Endotherms may be either poikilotherms or homeotherms.

Explanation

To understand this question, you need to distinguish how an organism gets its heat from how stable its body temperature remains.

• Endotherms: Generate most of their body heat internally through metabolism.

• Ectotherms: Depend primarily on external sources of heat.

• Homeotherms: Maintain a relatively constant body temperature.

• Poikilotherms: Have a body temperature that varies considerably.

These classifications describe different characteristics, so they can overlap.

For example:

• Most mammals are endothermic homeotherms because they generate internal heat and maintain a relatively stable body temperature.

• Hibernating mammals can become endothermic poikilotherms because their body temperature varies substantially during hibernation.

Why the Other Answers Are Incorrect

Option 1: ❌ Organisms generally tolerate temperatures below their optimum better than temperatures above it. Excessive heat can rapidly disrupt proteins and cellular processes.

Option 2: ❌ Organisms can use multiple thermoregulatory strategies simultaneously. For example, mammals use fur to retain heat and shivering to generate heat.

Option 3: ❌ Ectotherms are not necessarily thermoconformers. Some ectotherms actively regulate their body temperature through behaviors such as basking in sunlight or seeking shade.

Option 4: ✅ Endotherms can maintain stable body temperatures or experience substantial temperature fluctuations.

Why might an ectotherm be more likely than an equally sized endotherm to survive an extended period of moderate cold without food?

Group of answer choices

Because the ectotherm has greater insulation over its surface

Because the ectotherm is better able to maintain a high metabolic rate even in the cold

Because the ectotherm does not expend some of its available energy stores on keeping warm

Because the ectotherm maintains a higher basal metabolic rate

Because the ectotherm does not expend some of its available energy stores on keeping warm.

Explanation

Ectotherms rely primarily on external heat sources to regulate their body temperature, while endotherms generate heat internally through metabolism.

During an extended period of moderate cold without food:

1. Endotherms must expend energy to stay warm. Animals such as mammals and birds use metabolic processes to maintain their body temperature.

2. Ectotherms do not need to generate as much metabolic heat. Their body temperature generally decreases as the environment becomes colder.

3. Lower temperatures slow their metabolism. This reduces the amount of energy they need to survive.

4. Their energy reserves last longer. Because they use less energy, ectotherms may survive longer without food.

Why the Other Answers Are Incorrect

• Option 1 — Greater insulation: ❌ Ectotherms do not necessarily have better insulation than endotherms. Many endotherms have fur, feathers, or fat that help retain heat.

• Option 2 — Maintain a high metabolic rate: ❌ Ectotherms generally experience a decrease in metabolic rate as their body temperature decreases.

• Option 3 — Do not expend energy stores on keeping warm: ✅ Ectotherms generally spend much less metabolic energy generating heat, allowing them to conserve energy when food is unavailable.

• Option 4 — Higher basal metabolic rate: ❌ Endotherms generally have higher resting metabolic rates than similarly sized ectotherms.

A certain species of animal effectively maintains a fairly constant temperature behaviorally, for example by moving from sun into shade. However it lacks metabolic/physiological systems for generating heat. Which of the following best describes this animal?

Group of answer choices

It is a best described as a homeothermic poikilotherm

It is a best described as a homeothermic endotherm

It is a best described as a homeothermic ectotherm

It is a best described as a poikilothermic endotherm

It is a best described as a poikilothermic ectotherm

It is best described as a homeothermic ectotherm.

Explanation

This animal has two important characteristics:

1. It is homeothermic because it maintains a relatively constant body temperature.

• Homeotherms maintain a fairly stable body temperature.

• This animal moves between sunlight and shade to prevent its body temperature from changing significantly.

• It doesn't matter whether temperature regulation is behavioral or physiological.

2. It is ectothermic because it relies on external heat sources.

• Ectotherms obtain most of their heat from their environment.

• This animal cannot generate substantial metabolic heat to regulate its temperature.

• Instead, it uses environmental conditions to maintain its body temperature.

Why the Other Answers Are Incorrect

• Option 1 — Homeothermic poikilotherm: ❌ These terms describe opposing patterns of body temperature stability.

• Option 2 — Homeothermic endotherm: ❌ The animal does not generate substantial internal metabolic heat.

• Option 3 — Homeothermic ectotherm: ✅ It maintains a stable temperature using external heat sources and behavioral regulation.

• Option 4 — Poikilothermic endotherm: ❌ The animal neither has a highly variable body temperature nor relies on metabolic heat production.

• Option 5 — Poikilothermic ectotherm: ❌ Although it is ectothermic, its body temperature remains relatively constant.

Heat shock proteins protect organisms against thermal stress. How?

Group of answer choices

By shielding key biomolecules from molecular impacts

By shuttling key biomolecules into protective organelles

By binding to free water molecules

By binding to free ions

By shielding key biomolecules from molecular impacts.

Explanation

Heat shock proteins (HSPs) protect cells from damage caused by high temperatures.

When temperatures rise, molecules move more rapidly, increasing the likelihood that proteins will unfold or lose their normal three-dimensional shape.

Heat shock proteins help protect cells by:

1. Binding to vulnerable proteins: They stabilize proteins that are at risk of unfolding.

2. Preventing protein aggregation: They stop damaged or unfolded proteins from sticking together.

3. Assisting protein refolding: Many heat shock proteins act as molecular chaperones, helping proteins regain their proper shape.

4. Reducing cellular damage: They help proteins remain functional during thermal stress.

Why the Other Answers Are Incorrect

• Option 1 — Shielding key biomolecules: ✅ This is the best answer because heat shock proteins help protect other proteins from damage caused by thermal stress.

• Option 2 — Shuttling biomolecules into protective organelles: ❌ Their primary function is not to move proteins into protective organelles.

• Option 3 — Binding to free water molecules: ❌ Heat shock proteins do not protect cells by binding free water.

• Option 4 — Binding to free ions: ❌ Their primary protective mechanism does not involve binding free ions.

What is the difference between facultative and regional endothermy?

Group of answer choices

Facultative endothermy occurs only during certain times, whereas regional endothermy occurs only in certain tissues.

Facultative endothermy occurs all of the time, whereas regional endothermy occurs only during certain seasons.

Facultative endothermy occurs only in certain tissues, whereas regional endothermy occurs only in certain environments.

Facultative endothermy utilizes active mechanisms, whereas regional endothermy is achieved only via passive mechanisms.

Facultative endothermy occurs only during certain times, whereas regional endothermy occurs only in certain tissues.

Explanation

Both facultative endothermy and regional endothermy involve generating metabolic heat, but they differ in when and where that heat is used.

1. Facultative Endothermy = WHEN

• An organism produces additional metabolic heat only when needed.

• Heat production may occur during specific activities or environmental conditions.

• Example: Some insects, such as moths, warm their flight muscles before flying.

2. Regional Endothermy = WHERE

• An organism maintains certain body regions at temperatures higher than the surrounding environment or other body tissues.

• Rather than warming the entire body, it concentrates heat in specific areas.

• Example: Tuna and some sharks retain metabolic heat in their swimming muscles, helping those muscles function efficiently in cold water.

Why the Other Answers Are Incorrect

• Option 1: ✅ Facultative endothermy describes when heat production occurs, while regional endothermy describes where heat is maintained.

• Option 2: ❌ Facultative endothermy is not continuous, and regional endothermy is not restricted to particular seasons.

• Option 3: ❌ This reverses the definitions.

• Option 4: ❌ Regional endothermy can involve active metabolic heat production as well as heat-retaining mechanisms, such as countercurrent heat exchange.

Which of the following statements is correct?

Group of answer choices

Poikilothermy requires an open circulatory system.

Shivering is an example of a morphological thermoregulatory adaptation.

Water inside living things acts to slow internal temperature change.

Evaporative cooling is an active process that requires continuous inputs of energy to occur.

Water inside living things acts to slow internal temperature change.

Explanation

Water has a high specific heat capacity, meaning it takes a relatively large amount of energy to change its temperature.

Because living organisms contain large amounts of water, this property helps them resist rapid changes in body temperature.

For example:

1. When an organism absorbs heat, the water in its body warms relatively slowly.

2. When the environment becomes colder, the water in its body also cools relatively slowly.

3. This helps maintain a more stable internal temperature.

Why the Other Answers Are Incorrect

• Option 1 — Poikilothermy requires an open circulatory system: ❌ Poikilothermy means having a variable body temperature. It does not depend on the type of circulatory system.

• Option 2 — Shivering is a morphological adaptation: ❌ Shivering is a physiological response involving involuntary muscle contractions that generate heat. Morphological adaptations are physical structures, such as fur or thick layers of fat.

• Option 3 — Water slows internal temperature change: ✅ Water's high specific heat capacity helps organisms resist rapid temperature fluctuations.

• Option 4 — Evaporative cooling requires continuous energy inputs: ❌ Evaporation itself is a passive physical process that absorbs heat from the organism. However, producing sweat and transporting water to the skin can require metabolic energy.

Which of the following molecules reacts with water to produce a chemical buffering system in our blood?

Group of answer choices

CO₂

O₂

Glucose

Hemoglobin

Urea

✅ Correct Answer: Option 1 — CO₂ (Carbon Dioxide).

Explanation

Carbon dioxide reacts with water in the blood to form carbonic acid (H₂CO₃), which can then dissociate into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).

This creates the bicarbonate buffering system, one of the body's most important mechanisms for maintaining blood pH.

How It Works

Step 1: Carbon dioxide reacts with water.

CO₂ + H₂O ⇌ H₂CO₃

Step 2: Carbonic acid dissociates.

H₂CO₃ ⇌ H⁺ + HCO₃⁻

Overall reaction:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Because these reactions are reversible, the system helps resist changes in blood pH.

• When blood becomes too acidic: Bicarbonate can bind excess H⁺, helping reduce acidity.

• When blood becomes too basic: Carbonic acid can release H⁺, helping lower pH.

Why the Other Answers Are Incorrect

• Option 1 — CO₂: ✅ Reacts with water to form carbonic acid, creating the bicarbonate buffering system.

• Option 2 — O₂: ❌ Oxygen is essential for cellular respiration but does not form this buffering system.

• Option 3 — Glucose: ❌ Glucose provides energy but does not form the bicarbonate buffer.

• Option 4 — Hemoglobin: ❌ Hemoglobin can help buffer blood pH by binding hydrogen ions, but it does not react with water to produce the bicarbonate buffering system.

• Option 5 — Urea: ❌ Urea is a nitrogenous waste product excreted by the kidneys.

A lower incidence of illness and inflammation or a more climate controlled, sedentary lifestyle have been hypothesized to explain what observation in humans?

Group of answer choices

A lower average body temperature today than in the 1850s

More widespread resistance to new viruses today than in the 1850s

Less widespread resistance to new viruses today than in the 1850s

A higher average body temperature today than in the 1850s

A lower average body temperature today than in the 1850s.

Explanation

Research suggests that the average human body temperature has gradually decreased since the 1800s.

Although 98.6°F (37°C) is traditionally considered normal body temperature, modern population averages are often slightly lower.

Scientists have proposed several explanations for this change.

1. Lower rates of illness and inflammation

• In the 1850s, infectious diseases were more common.

• Frequent infections and chronic inflammation could elevate body temperature.

• Modern medicine, sanitation, and improved hygiene have reduced many sources of inflammation.

• This may contribute to lower average body temperatures.

2. Climate-controlled environments

• People today spend more time in buildings with heating and air conditioning.

• These environments reduce the need for the body to generate extra heat or work as hard to maintain its temperature.

• This may contribute to changes in average body temperature.

3. More sedentary lifestyles

• Many people today engage in less physical activity throughout the day.

• Reduced activity may contribute to lower metabolic heat production.

Why the Other Answers Are Incorrect

• Option 1 — Lower average body temperature: ✅ This is the observation researchers are trying to explain.

• Option 2 — More resistance to new viruses: ❌ The proposed explanations concern body temperature, not viral resistance.

• Option 3 — Less resistance to new viruses: ❌ This is unrelated to the observed temperature trend.

• Option 4 — Higher average body temperature: ❌ Research suggests the opposite trend.

Which of the following would represent a positive feedback?

Group of answer choices

As the human body warms, sweat is produced which carries heat out of the body, causing the body to cool

As a population increases in density, disease becomes a greater problem causing the population size to decline

As the abundance of a species increases in a biological community, the abundance of another species in the community decreases in response

As the earth warms, frozen methane deposits are liberated, causing the earth to warm even more

As the Earth warms, frozen methane deposits are liberated, causing the Earth to warm even more.

Explanation

Positive feedback occurs when an initial change triggers a response that amplifies the original change rather than reversing it.

Here's how Option 4 demonstrates positive feedback:

1. Earth's temperature increases.

2. Rising temperatures cause frozen methane deposits to thaw and release methane.

3. Methane is a powerful greenhouse gas that traps heat in the atmosphere.

4. The additional methane causes Earth's temperature to rise further.

5. More warming can release even more methane, continuing the cycle.

Positive feedback cycle:

🌡 Earth warms → Methane released → More heat trapped → Earth warms even more → More methane released

Why the Other Answers Are Incorrect

• Option 1 — Sweating when body temperature rises: ❌ Negative feedback. Sweating cools the body, counteracting the original temperature increase.

• Option 2 — Increased population density causes more disease: ❌ Negative feedback. Disease reduces population growth or population size, counteracting the initial increase.

• Option 3 — One species increases while another decreases: ❌ This describes an ecological interaction, such as competition or predation. It does not necessarily establish a positive feedback loop.

• Option 4 — Warming releases methane, causing more warming: ✅ Positive feedback. The response amplifies the original temperature increase.

Why aren't positive feedback loops common in the homeostatic control mechanisms of animals?

Group of answer choices

Because positive feedback loops drive the system away from a setpoint, rather than toward it

Because positive feedback loops do not involve enough genes and associated regulatory sequences to allow for proper complex control

Because positive feedback loops are energetically costly, thus requiring extremely beneficial outcomes to be worthwhile enough to be selected for

Because positive feedback loops require a large number of genes and associated regulatory sequences to complete, and thus are unlikely to evolve

Because positive feedback loops drive the system away from a setpoint, rather than toward it.

Explanation

Homeostasis is the process of maintaining stable internal conditions, such as body temperature, blood pressure, and blood glucose levels.

Most homeostatic mechanisms use negative feedback because it counteracts changes and returns the body toward its normal setpoint.

Positive feedback does the opposite: It amplifies the original change, pushing the system further away from its starting condition.

For example:

• Negative feedback: Body temperature rises → Sweating increases → Body temperature decreases toward normal.

• Positive feedback: Body temperature rises → A response causes body temperature to rise even more.

Because homeostasis requires stability, positive feedback is generally unsuitable for maintaining stable internal conditions.

Why the Other Answers Are Incorrect

• Option 1: ✅ Positive feedback amplifies changes rather than restoring a setpoint.

• Option 2: ❌ Positive feedback is not inherently limited by the number of genes involved.

• Option 3: ❌ Energy cost is not the primary reason positive feedback is uncommon in homeostatic regulation.

• Option 4: ❌ Positive feedback loops do not necessarily require large numbers of genes, nor are they inherently unlikely to evolve.

Which of the following statements is correct?

Group of answer choices

Insulin is produced in response to low blood glucose, and prompts cells to take up glucose.

The biochemical machinery of life generally runs faster as it gets warmer, up until the point the biomolecules begin to be damaged by heat.

A vascular countercurrent exchange system, as found in the fins of dolphins and feet of geese, helps to prevent excessive build-up of carbon dioxide in the tissues and blood.

UCP1 (uncoupling protein 1) makes the inner membrane of the mitochondrion permeable to protons in order to eliminate excess ATP.

The biochemical machinery of life generally runs faster as it gets warmer, up until the point the biomolecules begin to be damaged by heat.

Explanation

Temperature has a major influence on the rate of biochemical reactions.

As temperature increases:

1. Molecules gain kinetic energy, meaning they move faster.

2. Molecular collisions become more frequent and energetic, increasing the rates of many biochemical reactions.

3. Enzyme activity generally increases as temperature rises toward an optimal temperature.

4. Excessive heat damages proteins and enzymes, causing them to lose their proper structure and function.

5. As a result, biochemical reaction rates eventually decrease when temperatures become too high.

Key concept: Biochemical reactions generally become faster with increasing temperature, but only up to a certain point.

Why the Other Answers Are Incorrect

Option 1 — Insulin is produced in response to low blood glucose: ❌

Insulin is released when blood glucose is high, not low.

• High blood glucose → Insulin released → Cells take up glucose → Blood glucose decreases.

• Low blood glucose → Glucagon released → Liver releases glucose → Blood glucose increases.

Option 2 — Biochemical reactions speed up as temperature increases: ✅

Higher temperatures generally increase reaction rates until excessive heat damages proteins and enzymes.

Option 3 — Countercurrent exchange prevents carbon dioxide buildup: ❌

Countercurrent heat exchange in dolphin fins and goose feet primarily helps conserve body heat.

Warm arterial blood transfers heat to cooler venous blood returning from the extremities, reducing heat loss to the environment.

Option 4 — UCP1 eliminates excess ATP: ❌

UCP1 is found in the inner mitochondrial membrane of brown adipose tissue (brown fat).

It allows protons to flow back into the mitochondrial matrix without passing through ATP synthase.

This:

• Reduces ATP production.

• Releases energy as heat.

• Helps organisms maintain body temperature.

UCP1 does not eliminate excess ATP; it redirects energy away from ATP production toward heat generation.

What are transcription factors?

Group of answer choices

Proteins that modify other biomolecules in ways that alter their function

Environmental conditions that trigger particular homeostatic responses

Molecules that regulate gene expression

Hormones that participate in feedback loops

Molecules that regulate gene expression.

Explanation

Transcription factors are proteins that help control which genes are expressed and how much they are expressed.

They typically work by binding to specific DNA sequences and influencing transcription, the process of making RNA from DNA.

Transcription factors can:

1. Activate gene expression: Increase transcription of a particular gene.

2. Repress gene expression: Decrease or prevent transcription of a particular gene.

3. Respond to signals: Help cells change gene expression in response to hormones, environmental conditions, or other signals.

Why the Other Answers Are Incorrect

• Option 1 — Proteins that modify other biomolecules: ❌ This primarily describes enzymes that chemically modify molecules, such as kinases.

• Option 2 — Environmental conditions: ❌ Temperature, light, and other environmental factors can influence transcription factors, but they are not transcription factors themselves.

• Option 3 — Molecules that regulate gene expression: ✅ Transcription factors regulate the transcription of genes.

• Option 4 — Hormones that participate in feedback loops: ❌ Hormones can influence transcription factors, but transcription factors are regulatory proteins, not hormones.

Which of the following statements is correct regarding hormones?

Group of answer choices

A given hormone will typically elicit the same response across all different kinds of living things.

A given hormone will typically elicit the same response in all of the cells of an organism.

The basic hormone structures are found only in multicellular organisms.

Of the three classes of hormones, steroids cross the membranes of cells most easily.

Of the three classes of hormones, steroids cross the membranes of cells most easily.

Explanation

Hormones are chemical messengers that regulate processes such as growth, metabolism, reproduction, and homeostasis.

The three major classes of hormones are:

1. Peptide/protein hormones: Made of amino acid chains and generally cannot cross the cell membrane easily.

2. Amine hormones: Derived from amino acids. Their ability to cross membranes varies depending on the hormone.

3. Steroid hormones: Derived from cholesterol and are lipid-soluble, allowing them to diffuse through the phospholipid bilayer.

Why steroids cross membranes easily:

Cell membranes are composed primarily of phospholipids, which have a hydrophobic interior.

Because steroid hormones are lipid-soluble, they can pass through this hydrophobic region relatively easily.

Steroid hormones often bind to receptors inside cells, where they can regulate gene expression.

Why the Other Answers Are Incorrect

• Option 1: ❌ The same hormone can produce different effects in different organisms because their receptors and cellular responses may differ.

• Option 2: ❌ A hormone does not necessarily produce the same response in every cell. Its effects depend on whether a cell has the appropriate receptor and how that cell responds to receptor activation.

• Option 3: ❌ Hormonal signaling is not exclusive to multicellular organisms. Single-celled organisms also use chemical signaling molecules.

• Option 4: ✅ Steroid hormones are lipid-soluble and readily cross cell membranes.

Hormones bind to what structures to elicit a response?

Group of answer choices

Messenger RNA

Cell receptors

Other hormones

Glandular vesicles

Cell receptors.

Explanation

Hormones are chemical messengers that bind to specific receptors on or inside target cells to trigger a response.

Here's how it works:

1. A gland releases a hormone into the bloodstream.

2. The hormone travels throughout the body.

3. The hormone binds to a specific receptor on or inside a target cell.

4. The receptor activates a signaling pathway that produces a cellular response.

Important: Not every cell responds to every hormone. A cell must have the appropriate receptor to detect and respond to that hormone.

Why the Other Answers Are Incorrect

• Option 1 — Messenger RNA: ❌ mRNA carries genetic instructions for protein production. It is not the structure hormones typically bind to.

• Option 2 — Cell receptors: ✅ Hormones bind to specific receptors to initiate cellular responses.

• Option 3 — Other hormones: ❌ Hormones generally communicate with target cells by binding to receptors, not other hormones.

• Option 4 — Glandular vesicles: ❌ Vesicles store and transport certain hormones before secretion, but they are not the receptors responsible for detecting hormones.

Which of the following is true of amine hormones?

Group of answer choices

They are composed of amino acids linked together in a particular sequence

They typically have names ending in -ol or -one, such as cholesterol or testosterone

They cross the membranes of cells easily

They are produced by modification of amino acids

They are produced by modification of amino acids.

Explanation

Amine hormones are hormones made by chemically modifying individual amino acids, particularly tyrosine and tryptophan.

Examples include:

• Epinephrine (adrenaline): Derived from tyrosine and involved in the fight-or-flight response.

• Norepinephrine: Derived from tyrosine and involved in stress responses.

• Thyroid hormones (T3 and T4): Derived from tyrosine and help regulate metabolism.

• Melatonin: Derived from tryptophan and helps regulate sleep-wake cycles.

Why the Other Answers Are Incorrect

Option 1 — Amino acids linked together in a sequence: ❌ This describes peptide and protein hormones, which consist of chains of amino acids.

Option 2 — Names ending in -ol or -one: ❌ This is more characteristic of certain steroid hormones, such as testosterone and cortisol. However, hormone names alone do not reliably identify their chemical class.

Option 3 — Cross cell membranes easily: ❌ This is generally characteristic of steroid hormones. Some amine hormones, such as thyroid hormones, can cross membranes with the help of transport proteins, but many others cannot readily cross the lipid bilayer.

Option 4 — Produced by modification of amino acids: ✅ This is the defining characteristic of amine hormones.

Going only by their names, which of the following human hormones is most likely to be a steroid?

Group of answer choices

Epinephrine

Oxytocin

Aldosterone

Antidiuretic hormone

Ghrelin

Aldosterone.

Explanation

Aldosterone is a steroid hormone produced by the adrenal cortex.

Steroid hormones are derived from cholesterol, and many have names ending in -one or -ol.

Aldosterone ends in -one, making it the best choice based on its name.

Why the Other Answers Are Incorrect

• Option 1 — Epinephrine: ❌ An amine hormone derived from the amino acid tyrosine.

• Option 2 — Oxytocin: ❌ A peptide hormone involved in childbirth and milk ejection.

• Option 3 — Aldosterone: ✅ A steroid hormone that helps regulate sodium, potassium, and blood pressure.

• Option 4 — Antidiuretic hormone (ADH): ❌ A peptide hormone that promotes water reabsorption in the kidneys.

• Option 5 — Ghrelin: ❌ A peptide hormone that stimulates hunger.

Depending on how you count them, there are about three times more peptide hormones produced by the human body than there are amine and steroid hormones combined. Which of the following is the best explanation for why?

Group of answer choices

Peptides cannot persist as long in the body

Peptide hormones can't pass through membranes without a transport channel

Peptide hormones generally cannot prompt as many different kinds of responses

Peptides diversify (evolve) more easily

Peptides diversify (evolve) more easily.

Explanation

Peptide hormones are made of chains of amino acids. Because there are 20 standard amino acids that can be arranged in many different sequences, peptides can evolve into a tremendous variety of structures.

Here's why this matters:

1. Different amino acid sequences produce different peptides.

2. Changes in DNA can alter the amino acid sequence of a peptide hormone.

3. These changes can affect the hormone's shape and how it interacts with receptors.

4. Over evolutionary time, gene duplication and mutations can produce new peptide hormones with different functions.

This makes peptide hormones particularly capable of evolutionary diversification.

Why the Other Answers Are Incorrect

• Option 1 — Peptides cannot persist as long: ❌ Peptide hormones often have relatively short half-lives, but this does not explain why there are more types of them.

• Option 2 — Peptide hormones can't pass through membranes: ❌ Most peptide hormones cannot readily cross cell membranes, but this does not explain their greater diversity.

• Option 3 — Peptide hormones cannot prompt as many responses: ❌ Peptide hormones can trigger many different cellular responses through receptor-mediated signaling pathways.

• Option 4 — Peptides diversify more easily: ✅ Their amino acid sequences can change through mutations, allowing many different hormone structures and functions to evolve.

Which vertebrate body system has most specifically to do with hormone production and regulation?

Group of answer choices

The lymphatic system

The excretory system

The reproductive system

The endocrine system

The circulatory system

The endocrine system.

Explanation

The endocrine system is responsible for producing, releasing, and regulating hormones throughout the body.

Hormones are chemical messengers that help control important processes, including metabolism, growth, reproduction, stress responses, and homeostasis.

How the endocrine system works:

1. Endocrine glands produce hormones.

2. Hormones are released into the bloodstream.

3. They travel to target cells that have the appropriate receptors.

4. Hormones bind to these receptors and trigger specific responses.

5. Negative feedback mechanisms often regulate hormone production to maintain homeostasis.

Why the Other Answers Are Incorrect

• Option 1 — Lymphatic system: ❌ Primarily involved in immune defense and returning excess tissue fluid to the bloodstream.

• Option 2 — Excretory system: ❌ Primarily removes metabolic wastes and regulates water and electrolyte balance.

• Option 3 — Reproductive system: ❌ Produces certain hormones, such as estrogen and testosterone, but hormone regulation is not its sole or primary function.

• Option 4 — Endocrine system: ✅ Specifically responsible for hormone production and regulation.

• Option 5 — Circulatory system: ❌ Transports hormones through the bloodstream but is not primarily responsible for producing them.

Which of the following statements is correct regarding the human female reproductive system?

Group of answer choices

Ovulation occurs when hormone levels are constrained by a negative feedback

The corpus luteum forms before ovulation occurs

After follicles grow on the ovary, estradiol switches from inhibiting LH production to stimulating LH production

Low levels of estradiol stimulate LH and FSH production

After follicles grow on the ovary, estradiol switches from inhibiting LH production to stimulating LH production.

Explanation

The menstrual cycle is regulated by hormones, including estradiol (estrogen), luteinizing hormone (LH), and follicle-stimulating hormone (FSH).

Estradiol can produce either negative feedback or positive feedback, depending on its concentration and how long it remains elevated.

Here's how it works:

1. Follicles begin developing: FSH stimulates ovarian follicles to grow.

2. Estradiol levels increase: Growing follicles produce increasing amounts of estradiol.

3. Negative feedback occurs initially: At low to moderate concentrations, estradiol generally suppresses FSH and LH secretion.

4. Positive feedback begins: When estradiol remains sufficiently elevated, it stimulates a surge in LH production.

5. Ovulation occurs: The LH surge triggers the release of an egg from the mature follicle.

Why the Other Answers Are Incorrect

• Option 1 — Ovulation occurs during negative feedback: ❌ Ovulation is triggered by an LH surge resulting from positive feedback.

• Option 2 — The corpus luteum forms before ovulation: ❌ The corpus luteum forms after ovulation, from the remains of the ruptured follicle. It produces progesterone and estradiol.

• Option 3 — Estradiol switches from inhibiting to stimulating LH: ✅ Sustained high estradiol levels trigger positive feedback, causing the LH surge that leads to ovulation.

• Option 4 — Low estradiol stimulates LH and FSH: ❌ Low to moderate estradiol generally exerts negative feedback. Although very low estradiol reduces that inhibition, the major positive-feedback response occurs when estradiol is sustained at high levels.

Why must living organisms maintain suitable internal conditions?

Their biochemical machinery requires appropriate internal conditions to function properly and keep life processes within operational limits.

What happens when an organism's internal conditions move outside its tolerable range?

Its biochemical machinery begins to malfunction, resulting in reduced performance (stress) or potentially catastrophic failure (death).

Do all organisms tolerate the same range of internal conditions?

No. The range of conditions an organism can tolerate varies among living things.

What is homeostasis?

The maintenance of sufficiently constant internal conditions within limits that allow an organism's biochemical machinery to function properly.

What does the word homeostasis mean?

"Standing the same."

Why is homeostasis essential for survival?

It prevents internal conditions from moving beyond the limits required for normal biochemical function.

What happens when homeostasis is not maintained?

The organism may experience stress, reduced metabolic performance, or death if conditions become sufficiently unfavorable.

Why must all living organisms have systems for maintaining homeostasis?

Because all organisms depend on biochemical processes that function only within certain physical and chemical conditions.

What two general strategies can organisms use to maintain biochemical function under changing environmental conditions?

Develop biochemical machinery capable of tolerating a wide range of conditions. Develop highly specialized biochemical machinery optimized for narrow conditions and maintain those conditions.

What is the advantage of having biochemical machinery that tolerates a wide range of conditions?

It allows organisms to withstand environmental variation without requiring large energy expenditures.

What is the disadvantage of having biochemical machinery that tolerates a wide range of conditions?

The machinery cannot be optimized for maximum efficiency at any particular condition, limiting metabolic performance.

What is the advantage of having biochemical machinery specialized for narrow conditions?

It allows greater biochemical efficiency and higher maximum metabolic performance.

What is the disadvantage of having highly specialized biochemical machinery?

It functions optimally only within a narrow range of conditions, making the organism less tolerant of environmental deviations.

Why does broad environmental tolerance often come at the expense of metabolic performance?

Biochemical machinery adapted to function across many conditions cannot be optimized for maximum efficiency under any single condition.

Why does biochemical specialization increase an organism's sensitivity to environmental changes?

Specialized machinery has a smaller range of conditions under which it can function efficiently.

What must organisms maintain to benefit from highly optimized biochemical machinery?

A relatively constant internal environment that supports the conditions for which the machinery is specialized.

What are two ways an organism can experience constant internal conditions?

Living in an environment where external conditions remain constant. Actively maintaining constant internal conditions despite environmental changes.

What is the central tradeoff involved in biochemical specialization?

Organisms must balance the benefits of improved metabolic performance against the energy costs of maintaining the conditions needed for that performance.

What is the central tradeoff involved in broad biochemical tolerance?

Organisms conserve energy by tolerating changing conditions but sacrifice some metabolic efficiency and maximum performance.

What three factors influence which homeostatic strategy is most beneficial?

The energy cost of maintaining the internal condition in a particular environment. How important that condition is for optimal biochemical performance. How important optimal performance is to the organism's overall success.

Why might different organisms evolve different homeostatic strategies?

Because the benefits of biochemical performance and the costs of maintaining internal conditions differ depending on the organism and its environment.

Why can maintaining homeostasis require substantial energy?

Because organisms may need to continuously prevent internal conditions from changing in response to the surrounding environment.

How does maintaining constant internal conditions improve metabolic performance?

It allows an organism's biochemical machinery to become more specialized and efficient under those conditions.

Why might organisms invest considerable energy in homeostasis?

Because maintaining favorable internal conditions can improve metabolic performance, survival, and overall success.

What limits how much energy an organism can invest in homeostasis?

The total amount of energy the organism can produce.

What is the tradeoff between homeostasis and reproduction?

Energy spent maintaining homeostasis cannot also be used for reproduction.

Why is maintaining perfectly constant internal conditions not always the best strategy?

Because doing so can require excessive energy, and the benefits may not justify the costs.

What does the point of diminishing returns mean in homeostasis?

Beyond a certain point, additional energy invested in maintaining more constant internal conditions provides progressively smaller benefits.

Why must organisms balance homeostatic stability with energy conservation?

They have limited energy resources that must also support other biological processes, including reproduction.

What are the two general categories of homeostatic strategies?

Active and passive strategies.

What are active homeostatic strategies?

Strategies that require continuous energy input to keep internal conditions within a narrow range.

What are passive homeostatic strategies?

Strategies that maintain or tolerate internal conditions without requiring continuous energy input.

Do organisms rely exclusively on active or passive homeostatic strategies?

No. All living organisms use a combination of both.

Why do species differ in the degree of homeostasis they maintain?

They differ in how much energy they invest in homeostatic mechanisms.

What is the relationship between energy investment and the degree of homeostasis?

Greater energy investment can allow organisms to maintain more constant internal conditions.

What is active regulation?

A homeostatic strategy in which an organism continuously expends energy to maintain an internal condition near a constant setpoint despite environmental changes.

What is a setpoint?

A particular internal condition or value that an organism works to maintain.

What are regulators?

Organisms that actively maintain particular internal conditions near constant setpoints regardless of external conditions.

Why is regulation considered an active homeostatic strategy?

It requires continuous energy input to prevent internal conditions from equilibrating with the surrounding environment.

What does it mean to maintain an internal condition against equilibration with the environment?

The organism uses energy to prevent its internal condition from simply matching the external condition.

What is the main disadvantage of active regulation?

It requires substantial ongoing energy expenditure.

What is the main biochemical advantage of active regulation?

It allows organisms to develop highly specialized biomolecules that function with maximum efficiency under stable internal conditions.

How does active regulation influence metabolic performance?

It allows organisms to maintain consistently high metabolic performance even when external conditions change.

How does active regulation affect the habitats an organism can occupy?

It allows organisms to survive in environments that would otherwise be too inhospitable.

Why can regulators survive in highly variable environments?

Because they maintain relatively constant internal conditions despite changes in the surrounding environment.

What are the two major advantages of active regulation?

Consistent, highly efficient metabolic performance. The ability to survive in a wider variety of environments.

What is the major tradeoff associated with active regulation?

Organisms gain metabolic efficiency and environmental independence but must expend considerable energy to maintain them.

Why does active regulation allow organisms to specialize their biochemical machinery?

Because the internal conditions experienced by that machinery remain relatively constant.

What is conformation?

A homeostatic strategy in which an organism allows an internal condition to equilibrate with the surrounding environment.

What is another name for conformation?

Conformity.

What are conformers?

Organisms that allow particular internal conditions to match their surrounding environment rather than actively maintaining constant setpoints.

Why is conformation considered a passive strategy?

It does not require continuous energy expenditure to maintain the relevant internal condition.

What type of biochemical machinery must conformers possess?

Biochemical machinery capable of functioning across the full range of environmental conditions they are likely to encounter.

What is the main advantage of conformation?

It conserves energy by allowing internal conditions to equilibrate with the environment.

What is the main disadvantage of conformation?

Metabolic performance may fluctuate with environmental conditions and generally has a lower maximum.

Why can conformation result in uneven metabolic performance?

Because internal conditions change with the environment, causing biochemical machinery to operate under different conditions.

Why does conformation generally limit maximum metabolic performance?

Biochemical machinery must tolerate a range of conditions rather than being optimized for one narrow set of conditions.

How does the rate of environmental change affect the success of conformation?

Faster environmental changes generally make conformation more problematic.

How does the frequency of environmental change affect conformation?

More frequent changes make it harder for conformers to maintain adequate biochemical performance.

How does the magnitude of environmental change affect conformation?

Larger changes increase the risk that internal conditions will move outside the organism's tolerable range.

Under what environmental conditions is conformation most advantageous?

When the environment remains highly uniform and stable, and its conditions are suitable for biochemical function.

Why is conformation particularly effective in stable environments?

Because internal conditions remain relatively constant without requiring the organism to expend energy maintaining them.

Why might conformation be disadvantageous in highly variable environments?

Because internal conditions follow external changes, potentially reducing metabolic performance or exceeding tolerable limits.