Cellular Respiration — Print · Grade 10 Science

Chapter: Matter & Energy in Living Systems

Cellular Respiration

Learning goals

Readings for this unit

Before this unit

Distinguish ATP generation from the matter in food.

Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain. Trace the explained visual parts and write a first model, then attempt the illustrated and changed independent cases. In the third session explain a correction and revise the opening account using evidence.

Start with something to explain

How can respiration continue as a linked process rather than a single oxygen-consuming event?

Illustrative evidence case. Cytosolic transformations, mitochondrial electron transfer and a proton gradient have distinct roles; chamber records measure gas rather than ATP directly.

Respiration links several pathways rather than one burning stepRespiration links several pathways rather than one burning stepGlycolysisin cytosolMitochondrial pathways in this eukaryotic modelMatrix + inner membraneOrganic-carbon products and energy transfers both need accounting.
Original typical aerobic eukaryotic compartment model, not a universal organization of all respiration.

Notice and wonder

Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.

How can respiration continue as a linked process rather than a single oxygen-consuming event?

Your first model

Draw material and carrier steps separately from proton flow and ATP synthesis.

Build the language from the idea

glycolysis
A cytosolic pathway converting glucose to pyruvate with a net gain of two ATP per glucose.
citric acid cycle
A cycle oxidizing acetyl groups, releasing CO₂ and supplying electron carriers in a typical respiration model.
oxidative phosphorylation
ATP production coupled to electron transport and a proton gradient.
fermentation
A pathway regenerating NAD⁺ to sustain glycolysis without adding further ATP.
aerobic respiration
Energy-transfer metabolism using oxygen as the final electron acceptor.

In aerobic respiration, organic molecules are oxidized and oxygen is reduced, with energy transferred to ATP production and heat. For glucose the simplified net equation is C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O. This is a net accounting model, not one reaction or photosynthesis’s pathway run backward.

Glycolysis in the cytosol yields pyruvate and a net two ATP per glucose. In a typical eukaryotic model, pyruvate processing and the citric acid cycle in mitochondria release carbon dioxide and transfer electrons to carriers. Electron transport on the inner mitochondrial membrane helps build a proton gradient; ATP synthase uses the gradient. Oxygen accepts electrons and helps form water. The electron-transport chain does not make the carbon dioxide.

Respiration links several pathways rather than one burning stepRespiration links several pathways rather than one burning stepGlycolysisin cytosolMitochondrial pathways in this eukaryotic modelMatrix + inner membraneOrganic-carbon products and energy transfers both need accounting.
Original typical aerobic eukaryotic compartment model, not a universal organization of all respiration.

Many modern eukaryotic estimates are approximately 30–32 ATP per fully oxidized glucose, with yield depending on transport and coupling. Use a stated value for a model calculation rather than memorizing one universal count. Without oxygen, some cells use fermentation to regenerate NAD⁺ and sustain glycolysis’s net two ATP; fermentation itself adds no additional ATP.

A proton gradient links electron transfer with ATP synthesisA proton gradient links electron transfer with ATP synthesisHigher H⁺ concentrationLower H⁺ concentrationATP synthaseElectron transfer supports pumping; downhill H⁺ flow supports synthesis.
Original selected proton-gradient mechanism with representative markers, not a membrane potential measurement or exact ATP stoichiometry.

Plant and animal cells commonly use aerobic respiration; some cells rely on other pathways or lack mitochondria. In working muscles, glycolysis and lactate production can rise alongside ongoing aerobic respiration. Fatigue is multifactorial; lactate should not be taught as the single cause of muscle burning or later soreness.

Cellular respiration transfers energy from organic molecules through linked reactions that regenerate ATP. In aerobic respiration, oxygen serves as the final electron acceptor in the electron-transport pathway. Carbon dioxide and water are products in the overall accounting.

Glycolysis occurs in the cytosol, while much of aerobic respiration in eukaryotes involves mitochondria. Actual ATP yields vary with conditions and transport costs; a single fixed yield should not be taught as universal. Prokaryotes can carry out respiration without mitochondria.

Fermentation helps regenerate electron carriers so glycolysis can continue when the aerobic pathway is unavailable. It produces much less ATP per glucose in the standard model. Breathing and cellular respiration are related through gas supply but are distinct processes at different scales.

Fermentation can keep glycolysis supplied with a required carrierFermentation can keep glycolysis supplied with a required carrierGlycolysisuses NAD⁺Fermentationregenerates NAD⁺Net glycolytic ATP in the supplied glucose model: 2 ATP.Fermentation supports carrier recycling, not a second large ATP yield.
Original carrier-recycling abstraction; the two-ATP count belongs to the stated glycolysis model, not every anaerobic process.

Build your understanding

Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain.

Part 1 · One summary equation, three stages

Glycolysis splits glucose through cytosolic steps, producing pyruvate and carriers. In a typical aerobic eukaryotic pathway, further processing and the citric-acid cycle release carbon dioxide in mitochondria; an electron transport chain and ATP synthase involve the inner membrane. Oxygen is the final electron acceptor in this selected aerobic chain, not a reactant required in every glycolysis step. Prokaryotic organizations and alternative pathways differ. The overall glucose/oxygen equation summarizes material products without showing the full sequence or proving cellular combustion is an ordinary flame.

Respiration links several pathways rather than one burning stepRespiration links several pathways rather than one burning stepGlycolysisin cytosolMitochondrial pathways in this eukaryotic modelMatrix + inner membraneOrganic-carbon products and energy transfers both need accounting.
Original typical aerobic eukaryotic compartment model, not a universal organization of all respiration.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

Why can glycolysis occur when mitochondrial aerobic electron transport cannot continue?

Visual reasoning — after your attempt

Its steps do not directly require oxygen as their final acceptor; continued glycolysis also requires suitable carrier regeneration.

Aerobic cellular respiration is summarized as glucose plus oxygen yielding carbon dioxide and water, with energy captured in ATP. Like photosynthesis, the summary hides a sequence. Glycolysis splits glucose into two three-carbon molecules in the cytoplasm, producing a small net yield of ATP and requiring no oxygen. The products then enter the mitochondrion, where a cycle of reactions releases carbon dioxide and loads electron carriers.

The third stage, the electron transport chain in the mitochondrion's inner membrane, produces most of the ATP. Electrons passed along the chain drive hydrogen ions across the membrane, and their return flow powers ATP synthesis. Oxygen's role is at the end of this chain: it accepts the spent electrons and combines with hydrogen ions to form water.

Pause and explain

Oxygen does not appear until the final stage. Why does its absence stop the earlier mitochondrial stage as well?

Check your reasoning after answering

Without oxygen to accept electrons, the transport chain backs up and the electron carriers stay loaded. The earlier cycle cannot continue because it depends on unloaded carriers, so the whole mitochondrial sequence halts and only glycolysis can proceed.

Part 2 · Why the yield is model-dependent

Chemiosmosis couples a proton, H⁺, gradient across a membrane to ATP production through ATP synthase. Electron transfer can support proton pumping; return flow through the enzyme is linked to ATP synthesis. This separates electron movement, proton movement and ATP chemistry instead of calling them one arrow. Yield depends on carrier entry, coupling, proton leaks and other demands. Therefore a single fixed ATP total per glucose is an approximation for a specified pathway, not a measured guarantee for every cell and condition.

A proton gradient links electron transfer with ATP synthesisA proton gradient links electron transfer with ATP synthesisHigher H⁺ concentrationLower H⁺ concentrationATP synthaseElectron transfer supports pumping; downhill H⁺ flow supports synthesis.
Original selected proton-gradient mechanism with representative markers, not a membrane potential measurement or exact ATP stoichiometry.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

Why can the same glucose input give different captured ATP output under different coupling conditions?

Visual reasoning — after your attempt

Energy transfer, leaks, carrier pathways and competing demands change the fraction captured through ATP production.

Glycolysis gives a net two ATP per glucose. For the rest, many modern eukaryotic estimates fall around 30 to 32 ATP per fully oxidized glucose, and the figure depends on transport costs and on how tightly electron transport is coupled to ATP synthesis. There is no single universal count to memorize, and older textbooks quoting a higher fixed number were stating an assumption rather than a measurement.

The way to use this honestly is to state a model yield and calculate from it. Taking 30 ATP per glucose aerobically against the net 2 available when only glycolysis runs, the aerobic route recovers about fifteen times as much from the same glucose. That order of magnitude is what explains why breathing and circulation respond so quickly to exercise, why tissues deprived of blood flow fail rapidly, and why anaerobic effort cannot be sustained — and it holds whichever estimate in the 30 to 32 range you adopt.

Pause and explain

A muscle switches from aerobic respiration to fermentation at the same ATP demand. Using a stated model yield of 30 ATP per glucose, what must happen to its glucose consumption?

Check your reasoning after answering

It must rise roughly fifteenfold, since only the net 2 ATP from glycolysis remain available per glucose. That rate of consumption cannot be sustained, which is why anaerobic effort is brief.

Part 3 · Fermentation: a small yield that keeps glycolysis going

NAD⁺ is an electron carrier used in glycolysis, becoming reduced NADH. Fermentation regenerates NAD⁺ by transferring electrons to selected organic products, allowing glycolysis to continue under suitable conditions. In the simple glucose-to-glycolysis account, the net ATP yield is two per glucose; the fermentation conversion is not an additional large ATP-producing electron-transport system. Different fermentation pathways produce different materials, such as lactate or ethanol plus CO₂. “Anaerobic” does not name one identical pathway for every organism.

Fermentation can keep glycolysis supplied with a required carrierFermentation can keep glycolysis supplied with a required carrierGlycolysisuses NAD⁺Fermentationregenerates NAD⁺Net glycolytic ATP in the supplied glucose model: 2 ATP.Fermentation supports carrier recycling, not a second large ATP yield.
Original carrier-recycling abstraction; the two-ATP count belongs to the stated glycolysis model, not every anaerobic process.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

What would limit ongoing glycolysis if the needed oxidized carrier were not regenerated?

Visual reasoning — after your attempt

The carrier supply would become insufficient; regeneration enables continued pathway flow under suitable conditions.

When oxygen is unavailable, cells use fermentation. In human muscle, the three-carbon product of glycolysis is converted to lactate; in yeast, to ethanol and carbon dioxide. Neither pathway produces additional ATP itself. Its purpose is to regenerate the electron carriers that glycolysis requires, so that glycolysis — and its small ATP yield — can continue.

Seeing fermentation as a maintenance step rather than an energy-producing one resolves a common confusion. The ATP comes from glycolysis; fermentation merely resets the carriers. That is why the yield is 2 ATP per glucose whether or not fermentation follows, and why the products are waste rather than useful stores.

Pause and explain

If fermentation produces no ATP, why can a cell not simply skip it when oxygen is absent?

Check your reasoning after answering

Glycolysis needs a supply of unloaded electron carriers. Without oxygen, the mitochondrial chain cannot unload them, so fermentation does that job instead; skipping it would stop glycolysis and end even the small ATP yield.

Part 4 · Measuring respiration

A respirometry model tracks gas exchange within a defined system. Chamber oxygen declines by 18 units in the living-material example and by six in a reference over 15 minutes. Subtracting the matched reference change gives a net comparison of 12 units, or 0.8 unit/min, under the supplied correction. Temperature, chamber leaks, sample mass and other oxygen-using processes matter. A gas curve is not itself an ATP meter, and a sample’s total rate must be distinguished from rate per unit mass.

Gas change needs a reference, conditions and a stated boundaryGas change needs a reference, conditions and a stated boundaryO₂ amount in model chamber (units)8090100051015Time (min)Living materialReference chamber
Original illustrative chamber records with arbitrary gas units, not a real organism’s measured metabolism.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

Why is the raw chamber decline not automatically the living material’s corrected respiration rate?

Visual reasoning — after your attempt

The reference indicates other changes or measurement effects; matched correction and conditions are needed before attributing the full decline.

Respiration is measured indirectly: oxygen consumed, carbon dioxide released, or heat produced. Germinating seeds or small invertebrates in a sealed vessel with a carbon-dioxide absorber allow oxygen use to be read as a volume change; a carbon-dioxide indicator such as bromothymol blue changes colour as the gas accumulates. Each method measures one part of the process and assumes the rest follows.

Controls carry the interpretation. A vessel of dead or boiled seeds shows what the apparatus does without respiration, and a vessel with no organisms shows the effect of temperature or pressure changes on the gas volume. Without these, a reading cannot be attributed to respiration, because warming air alone produces a similar change.

Pause and explain

Why does a respirometer experiment need a vessel containing boiled seeds?

Check your reasoning after answering

It provides a control for everything the apparatus does apart from respiration — temperature changes, pressure changes, and absorption by the equipment — so that any additional change in the live vessel can be attributed to the seeds' respiration.

Apply the whole unit · Account for a respirometer record

Illustrative respirometer data over 20 minutes at 22 °C, reported as oxygen consumed in mL. Germinating peas: 0.42, 0.45, 0.39 (three replicates). Dry dormant peas: 0.03, 0.02, 0.04. Boiled peas: 0.01, 0.00, 0.01. Empty vessel with absorber only: 0.01, 0.01, 0.00. Each vessel held carbon dioxide absorber and equal pea volume where peas were present.

Your task

Calculate the mean for each vessel, state the corrected oxygen consumption attributable to respiration in the germinating peas, explain what the dormant and boiled vessels each establish, and calculate the rate in mL per minute. Then explain why oxygen consumption is a valid proxy for ATP production here and name one limit of that inference.

What your response should show

  • Choose the appropriate biological model or relationship and state the conditions it assumes.
  • Show the counts, rates, ratios, or intermediate reasoning the question requires.
  • Use the supplied evidence to justify the conclusion and separate observation from inference.
  • Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt

  1. Means: germinating 0.42 mL, dormant 0.03 mL, boiled 0.007 mL, empty 0.007 mL.

  2. Correct for apparatus effects: subtract the empty-vessel mean, giving about 0.41 mL for germinating peas and about 0.02 mL for dormant peas.

  3. Interpret the controls: the empty vessel shows the apparatus itself produces almost no volume change, and the boiled peas show that dead tissue of the same volume consumes no oxygen, so the signal is biological and not physical.

  4. Interpret the dormant peas: they respire at roughly one twentieth the germinating rate, which shows respiration continues at a low level in dormant tissue rather than being absent.

  5. Rate: 0.41 mL / 20 min ≈ 0.021 mL per minute for the germinating peas.

  6. Justify the proxy: in aerobic respiration oxygen consumption is stoichiometrically tied to glucose oxidation and therefore to ATP production, so the volume consumed scales with ATP generated.

  7. State the limit: the proxy fails if any tissue is fermenting, since fermentation produces ATP without consuming oxygen, so oxygen use would understate total ATP production.

The germinating peas consumed a mean of 0.42 mL of oxygen, and after subtracting the 0.007 mL seen in the empty vessel, about 0.41 mL is attributable to respiration — a rate of roughly 0.021 mL per minute. The boiled peas and empty vessel together establish that neither dead tissue of the same volume nor the apparatus accounts for the change, so the signal is biological. The dormant peas' 0.02 mL shows respiration at about one twentieth the germinating rate, which is a low level rather than none, consistent with dormant tissue maintaining minimal metabolism. Oxygen consumption works as a proxy for ATP production because aerobic respiration consumes oxygen in fixed proportion to the glucose it oxidizes. The inference fails if part of the tissue is fermenting, since fermentation yields ATP without using oxygen, so the measurement would understate total ATP production; measuring carbon dioxide release alongside oxygen use would reveal that discrepancy.

Scientific references and adapted source figures

Parent / teacher teaching notes

The 36-against-2 comparison is the single most useful number in this unit, and it should be reachable without notes, because it explains exercise physiology, tissue damage from interrupted blood flow, and why fermentation is a stopgap. Be precise that fermentation produces no ATP of its own; students almost always say it does. The respirometer controls are a good place to reinforce experimental design from the first chapter, since the boiled-seed vessel is exactly the kind of control students omit. Ask for an explanation of each label, arrow and axis before calculation. Keep first and revised models; attempt the changed case before consulting guidance. Distinguish invented instructional records from named historical sources. Source chromosome symbols are allele-category models, not DNA sequences. These independently authored high-school supplements are not the full OpenSciEd teacher editions or an endorsement.

The key idea

Aerobic respiration transfers energy through glycolysis, pyruvate processing, the citric acid cycle, and oxidative phosphorylation. ATP yield is model-dependent; fermentation supports glycolysis without adding ATP.

Aerobic respiration (simplified net equation)

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

Energy is transferred to ATP production and heat. Use an explicitly stated model yield for numerical work.

Worked examples

Example 1

Use a model yield of 30 ATP per glucose for aerobic respiration and 2 from glycolysis supported by fermentation. Compare.
  1. 30 ÷ 2 = 15.
  2. State that 30 is the model assumption, not a universal yield.
Answer: The model aerobic yield is 15 times as large.

Example 2 · Worked application

A cell uses oxygen while oxidizing food-derived molecules and produces CO₂ and water. Explain the energy result.
  1. Chemical reactions transfer energy from organic molecules.
  2. Some energy supports ATP generation; some transfers as heat.
  3. Oxygen serves as a final electron acceptor in aerobic respiration.
Answer: Aerobic respiration supports ATP production with CO₂, water and thermal outputs.
Check the reasoning

Atoms and energy must be accounted for separately.

Example 3

Why can a prokaryote obtain energy by respiration without mitochondria?
  1. Respiration requires linked biochemical pathways and suitable membranes.
  2. Prokaryotes can use their cell membrane for relevant electron-transport processes.
Answer: Mitochondria are the eukaryotic compartment, not a universal requirement for respiration.

Example 4 · Find and fix the mistake

Context: A cell uses oxygen while oxidizing food-derived molecules and produces CO₂ and water. Explain the energy result. A student claims: “Respiration is only breathing air in and out.” Evaluate the claim and write a corrected explanation.
  1. Test the claim against the stated conditions: Atoms and energy must be accounted for separately.
  2. Breathing exchanges gases at organism level; cellular respiration is a set of metabolic reactions in cells.
Answer: Breathing exchanges gases at organism level; cellular respiration is a set of metabolic reactions in cells.
Check the reasoning

Atoms and energy must be accounted for separately.

Example 5 · Reason from the visual evidence

Fermentation can keep glycolysis supplied with a required carrierFermentation can keep glycolysis supplied with a required carrierGlycolysisuses NAD⁺Fermentationregenerates NAD⁺Net glycolytic ATP in the supplied glucose model: 2 ATP.Fermentation supports carrier recycling, not a second large ATP yield.
Original carrier-recycling abstraction; the two-ATP count belongs to the stated glycolysis model, not every anaerobic process.
What would limit ongoing glycolysis if the needed oxidized carrier were not regenerated?
  1. NAD⁺ is an electron carrier used in glycolysis, becoming reduced NADH. Fermentation regenerates NAD⁺ by transferring electrons to selected organic products, allowing glycolysis to continue under suitable conditions. In the simple glucose-to-glycolysis account, the net ATP yield is two per glucose; the fermentation conversion is not an additional large ATP-producing electron-transport system. Different fermentation pathways produce different materials, such as lactate or ethanol plus CO₂. “Anaerobic” does not name one identical pathway for every organism.
  2. The carrier supply would become insufficient; regeneration enables continued pathway flow under suitable conditions.
Answer: The carrier supply would become insufficient; regeneration enables continued pathway flow under suitable conditions.
Check the reasoning

Original carrier-recycling abstraction; the two-ATP count belongs to the stated glycolysis model, not every anaerobic process.

Avoid these mistakes

Check your understanding

Why can a prokaryote obtain energy by respiration without mitochondria?

Discuss the answer

Mitochondria are the eukaryotic compartment, not a universal requirement for respiration.

Why should ATP yield be treated as condition dependent rather than one universal number?

Discuss the answer

Pathways, carrier entry, coupling, proton leaks and other cellular demands alter ATP capture. A supplied model yield is not a universal measured total.

Return to the phenomenon

Return to the initial case. How can respiration continue as a linked process rather than a single oxygen-consuming event? Revise your drawing or explanation using two named ideas and one stated limitation.

Compare explanations after your attempt

Pathways, acceptors and carrier regeneration determine flow; corrected gas measurements require references and do not fix universal ATP yield.

What should we investigate next?

Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.

Investigate, explain and revise

Complete these tasks after the teaching parts. Preserve your first model and a supported revision; use guidance after a real attempt.

Activity 1 · Return to the opening case

Assigned reading and preparation

Use the opening question, defined terms, Parts 1–4 and your notebook. The supplied-data route needs no equipment or external account.

Your task

Return to the initial case. How can respiration continue as a linked process rather than a single oxygen-consuming event? Revise your drawing or explanation using two named ideas and one stated limitation.

Success criteria

  • Use relevant stated observations.
  • Explain the mechanism with a labeled model.
  • Distinguish quantities and assumptions.
  • Describe a revision and a useful next test.
Hint

Follow objects, materials and information separately. Check what each measured quantity means.

Response guidance — discuss after your attempt

Pathways, acceptors and carrier regeneration determine flow; corrected gas measurements require references and do not fix universal ATP yield.

Activity 2 · Transfer to a changed case

Assigned reading and preparation

Cover the worked guidance and use the unit explanations/figures to solve a changed problem.

Your task

A sample chamber loses 24 oxygen units in 10 min while a matched reference loses 4. Calculate corrected rate and explain why it does not alone provide an exact ATP count.

Success criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
Hint

Name the stated conditions, select the relevant mechanism and show intermediate reasoning.

Response guidance — discuss after your attempt

The corrected decline is 20 units, giving 2 units/min. Pathway coupling, oxygen-to-substrate relations and other demands must be specified before converting gas exchange into ATP yield.

Words to know

glycolysis
A cytosolic pathway converting glucose to pyruvate with a net gain of two ATP per glucose.
citric acid cycle
A cycle oxidizing acetyl groups, releasing CO₂ and supplying electron carriers in a typical respiration model.
oxidative phosphorylation
ATP production coupled to electron transport and a proton gradient.
fermentation
A pathway regenerating NAD⁺ to sustain glycolysis without adding further ATP.
aerobic respiration
Energy-transfer metabolism using oxygen as the final electron acceptor.

Evaluate a model, not only a slogan

Aerobic respiration transfers energy through glycolysis, pyruvate processing, the citric acid cycle, and oxidative phosphorylation. ATP yield is model-dependent; fermentation supports glycolysis without adding ATP. Describe one observation that would test a prediction, and one simplification that could change the result.

Check your mechanism

State the location, reactants, products, and evidence supporting a process. Explain what your model leaves out before using it to predict a result.

In the real world

Fermentation quietly powers industries worth billions: yeast's alcoholic fermentation makes bread rise and brews beer, while bacterial fermentation turns milk into yogurt and cabbage into sauerkraut — all cells making ATP without oxygen.

Exit check

What roles do oxygen and carbon dioxide have in aerobic respiration?

Check your explanation after answering

Oxygen accepts electrons near the end of the respiratory chain and is reduced to water. Carbon dioxide is released during pyruvate processing and the citric acid cycle; the chain itself does not produce CO₂.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

Which pair are the reactants of cellular respiration?
  1. Carbon dioxide and water
  2. ATP and sunlight
  3. Glucose and carbon dioxide
  4. Glucose and oxygen
Answer and explanation

Glucose and oxygen

Cellular respiration breaks down glucose using oxygen, producing carbon dioxide and water: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.

Problem 2

Most of a cell's ATP from aerobic respiration is produced in the
  1. electron transport chain on the inner mitochondrial membrane
  2. cytoplasm during glycolysis
  3. nucleus
  4. ribosomes
Answer and explanation

electron transport chain on the inner mitochondrial membrane

Oxidative phosphorylation coupled to electron transport makes most of the ATP in this eukaryotic model.

Problem 3

How many ATP molecules does glycolysis produce as a NET gain from one glucose?
Answer and explanation

2 ATP

Glycolysis makes 4 ATP but spends 2 to get started, for a net gain of 2 ATP.

Problem 4

Using model yields of 30 ATP per glucose aerobically and 2 from glycolysis supported by fermentation, how many times as much ATP is obtained aerobically?
Answer and explanation

15 times

30 ÷ 2 = 15 under the stated model.

Problem 5

Explain why you breathe faster during hard exercise, in terms of what the electron transport chain needs and what it produces.
Answer and explanation

Exercise raises demand for ATP and material transport. Oxygen supports the end of electron transport, while CO₂ is produced during pyruvate processing and the citric acid cycle. Ventilation and circulation help meet those demands.

Problem 6

Why can a prokaryote obtain energy by respiration without mitochondria? Explain the reasoning in your own words.
Answer and explanation

Mitochondria are the eukaryotic compartment, not a universal requirement for respiration.

Respiration requires linked biochemical pathways and suitable membranes.

Prokaryotes can use their cell membrane for relevant electron-transport processes.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Why can fermentation help glycolysis continue when oxygen-dependent pathways are unavailable? Show the reasoning, not just the conclusion.
Hints

Atoms and energy must be accounted for separately.

Answer and explanation

It regenerates NAD⁺ needed for glycolysis, with products depending on the organism/pathway.

Problem 8

Does anaerobic metabolism always produce the same amount of ATP as aerobic respiration?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Yields and pathways differ; simple fermentation typically yields much less ATP per glucose than aerobic respiration.

Problem 9

Fermentation can keep glycolysis supplied with a required carrierFermentation can keep glycolysis supplied with a required carrierGlycolysisuses NAD⁺Fermentationregenerates NAD⁺Net glycolytic ATP in the supplied glucose model: 2 ATP.Fermentation supports carrier recycling, not a second large ATP yield.
Original carrier-recycling abstraction; the two-ATP count belongs to the stated glycolysis model, not every anaerobic process.
Interpret the supplied visual. What would limit ongoing glycolysis if the needed oxidized carrier were not regenerated?
Answer and explanation

The carrier supply would become insufficient; regeneration enables continued pathway flow under suitable conditions.

NAD⁺ is an electron carrier used in glycolysis, becoming reduced NADH. Fermentation regenerates NAD⁺ by transferring electrons to selected organic products, allowing glycolysis to continue under suitable conditions. In the simple glucose-to-glycolysis account, the net ATP yield is two per glucose; the fermentation conversion is not an additional large ATP-producing electron-transport system. Different fermentation pathways produce different materials, such as lactate or ethanol plus CO₂. “Anaerobic” does not name one identical pathway for every organism.

The carrier supply would become insufficient; regeneration enables continued pathway flow under suitable conditions.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

A sample chamber loses 24 oxygen units in 10 min while a matched reference loses 4. Calculate corrected rate and explain why it does not alone provide an exact ATP count.
Answer and explanation

The corrected decline is 20 units, giving 2 units/min. Pathway coupling, oxygen-to-substrate relations and other demands must be specified before converting gas exchange into ATP yield.

The corrected decline is 20 units, giving 2 units/min. Pathway coupling, oxygen-to-substrate relations and other demands must be specified before converting gas exchange into ATP yield.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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