Respiration

To solve this, remember what aerobic respiration is and what it is for in Metabolism: Cell respiration. Aerobic respiration is how cells release lots of usable energy (ATP) by breaking down glucose when oxygen is available.

A direct way to answer is to recall the balanced summary equation for complete glucose oxidation:

$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{Energy (ATP)}$

This shows the atoms in glucose end up as carbon dioxide and water, not as other organic products.

Because the final products are simple inorganic substances ($C{O}_2$ and $H_{2}O$), we say glucose has been completely broken down (fully oxidized). That complete breakdown is what separates aerobic respiration from anaerobic routes.

Choices mentioning lactic acid or ethanol describe pathways used when oxygen is not available. In animals, anaerobic respiration can lead to lactic acid, and in yeast, fermentation produces ethanol and carbon dioxide—these are not the aerobic end products.

The statement about pyruvate in the cytoplasm describes glycolysis. Glycolysis happens before the mitochondrial stages and does not represent the whole of aerobic respiration. The complete aerobic process continues in mitochondria to finish breaking glucose down.

Using the overall equation, we confirm that aerobic respiration results in glucose being fully converted into carbon dioxide and water.

The question is asking specifically about aerobic respiration. Using Elimination of Anaerobic Pathways, we cross out any choice that describes respiration without oxygen or that only covers an incomplete breakdown of glucose.

Lactic acid is made when animal cells respire without oxygen (anaerobic conditions). Since aerobic respiration requires oxygen and has different end products, the lactic acid option cannot be correct.

Ethanol and carbon dioxide are produced during fermentation in yeast (and some plant tissues) when oxygen is absent. This is an anaerobic process and does not describe the full, high-ATP aerobic pathway.

Glycolysis occurs in the cytoplasm and turns glucose into pyruvate, but that is only the first part of respiration. Aerobic respiration continues in the mitochondria to complete the oxidation, so this choice is too narrow.

After removing anaerobic and incomplete descriptions, the remaining correct idea is the key feature of aerobic respiration: glucose is fully oxidized. With oxygen present, carbon from glucose leaves as $C{O}_2$ and hydrogen ultimately forms $H_{2}O$.

The overall word equation is: $\text{glucose}+\text{oxygen}\to \text{carbon dioxide}+\text{water}+(\text{large amount of ATP})$

We need to tell the difference between aerobic respiration and anaerobic respiration. The task is to pick a molecule that is a reactant (a starting ingredient) for the aerobic route, but that the anaerobic route does not need in order to get started.

Use the standard summary equations for each pathway from Metabolism: Cell respiration.

Aerobic respiration: $\text{Glucose}+\text{Oxygen}\to \text{Carbon Dioxide}+\text{Water}+\text{Energy (ATP)}$

Anaerobic respiration (fermentation):

• In yeast: $\text{Glucose}\to \text{Ethanol}+\text{Carbon Dioxide}+\text{Energy (ATP)}$
• In mammals: $\text{Glucose}\to \text{Lactic Acid}+\text{Energy (ATP)}$

Now we can check which substances appear as inputs in one pathway but not the other.

In both sets of equations, glucose is the molecule being broken down to release energy. Since aerobic and anaerobic respiration both start by using glucose, it cannot be the reactant that only aerobic respiration needs.

Check what role each option plays:

• Ethanol is made during fermentation in yeast, so it appears on the products side, not as a starting reactant.
• Water is formed during aerobic respiration, so it is also on the products side rather than being required to begin the pathway.

Because the question asks for a reactant, neither of these can be correct.

Only the aerobic equation includes oxygen as an input. The anaerobic equations do not include oxygen at all—anaerobic processes occur when oxygen is not available. Therefore, oxygen is required for aerobic respiration but not needed for the anaerobic pathway to start.

The correct choice must satisfy both parts of the prompt:

1. It is a reactant (starting substance) in aerobic respiration.
2. It is not necessary for anaerobic respiration to begin.

This connects to Topic: Cell respiration in the Metabolism section, where you learn the overall equations and what each pathway requires.

Aerobic and anaerobic respiration both begin with glycolysis in the cytoplasm. Glycolysis uses glucose as its main reactant to start releasing energy (ATP).

Because glucose is needed to begin both pathways, it does not match the requirement of being unnecessary for anaerobic respiration.

During anaerobic respiration in yeast (and some plant cells), ethanol is formed after glucose is partially broken down. Since ethanol is produced, not used up at the start, it cannot be the reactant the question is looking for.

In the overall aerobic equation:

$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{ATP}$

Water ($H_{2}O$) is shown on the products side, meaning it is made during aerobic respiration rather than required as an input. Even though water is common in cells, it is not the key reactant that separates aerobic from anaerobic respiration.

Aerobic means “using oxygen.” Oxygen ($O_2$) acts as the final electron acceptor in aerobic respiration, which is why aerobic respiration can produce a much larger amount of ATP.

In contrast, anaerobic respiration (fermentation) happens when oxygen is not present. So oxygen is required for aerobic respiration but not required for the anaerobic pathway to begin.

We are looking for a substance that aerobic respiration uses as a reactant, but that is not needed to start anaerobic respiration. The names of the pathways themselves can help, which links to Metabolism: Cell respiration.

Use Etymological Analysis by splitting the word into its roots. Anaerobic comes from Greek parts:

1. an-: meaning “not” or “without”
2. aero: meaning “air” (in biology, this points to oxygen)
3. bios: meaning “life”

So, anaerobic can be understood as “life/process without air (oxygen).”

If anaerobic respiration happens “without oxygen,” then oxygen is the molecule that is specifically not required for that pathway to run or begin.

Using that definition, the other options don’t match:

• Glucose: Both pathways start by breaking down glucose in glycolysis, so it is needed either way.
• Ethanol: This is formed during fermentation in yeast; it is not an input reactant.
• Water: Water is involved in many reactions, but it is not the key feature that separates aerobic from anaerobic conditions.

From the word roots, anaerobic means the process occurs without oxygen. That makes Oxygen the reactant required for aerobic respiration but not needed for the anaerobic pathway to begin.

The key phrase is an energy-requiring process. Cellular respiration releases energy from glucose and captures it in ATP (adenosine triphosphate). ATP is the cell’s usable energy source, driving reactions and activities that won’t proceed on their own without an energy input.

Check the choices about substances moving into or out of cells.

• Osmosis is water moving across a partially permeable membrane from higher to lower water potential.
• Diffusion is particles (such as oxygen) moving down a concentration gradient.

Both are passive processes, meaning they happen without the cell spending ATP. So the first two options can be removed.

The option about food being physically broken up refers to mechanical digestion. Although stomach muscle contractions do use energy, the phrase describes a physical grinding/mixing action rather than a specific ATP-driven biochemical reaction inside cells that respiration is typically linked to in metabolism questions. So we eliminate that option here.

Metabolism includes catabolism (breaking molecules down) and anabolism (building larger molecules).

Making proteins is a classic anabolic process: many small units (amino acids) are joined to form a large molecule (a protein) through condensation reactions. Forming these new bonds requires energy, and cells pay for it using ATP produced by respiration.

After removing passive transport and mechanical digestion, the remaining option—protein building from amino acids—is the only one that clearly represents a cellular metabolic process that needs ATP input from respiration.

We are looking for an energy-requiring process supported by respiration. In cells, that usually means a process that uses ATP (adenosine triphosphate) made during cellular respiration. We’ll find it by using Elimination of Passive Processes.

Start with transport ideas. Osmosis and diffusion are passive: particles move from high to low concentration (or high to low water potential) without the cell needing to spend ATP.

Because they don’t require ATP from respiration, we eliminate options A and B.

Next consider the option about physical breakdown of food. That describes mechanical digestion (such as stomach churning). Even though muscles require energy, this is not the kind of ATP-powered cellular biochemical process the question is targeting when linking respiration to a cell’s metabolic needs. So we eliminate it.

Now look at protein synthesis. This is an anabolic process where many amino acids are joined to make a larger polymer (a protein). Creating those chemical bonds needs a substantial energy input, provided as ATP.

That matches an energy-requiring metabolic process supported by respiration.

Once passive transport and mechanical digestion are eliminated, protein synthesis is the only choice left that clearly depends on ATP produced by respiration, so it is the correct answer.

To predict what gases a plant exchanges, we need to consider two main biological processes: cellular respiration and photosynthesis.

1. Cellular respiration: This happens in every living cell all the time (day and night) to release usable energy. A simple word equation is: $\text{Glucose}+\text{Oxygen}\to \text{Carbon Dioxide}+\text{Water}+\text{Energy (ATP)}$

2. Photosynthesis: This process can only run when light energy is available. A simple word equation is: $\text{Carbon Dioxide}+\text{Water}+\text{Light Energy}\to \text{Glucose}+\text{Oxygen}$

The plant is kept in a completely dark room. Since light energy is required to power photosynthesis, photosynthesis cannot take place in total darkness.

So, the plant will not be carrying out the photosynthesis gas pattern (taking in carbon dioxide and releasing oxygen). That lets us rule out that choice right away.

Even without light, the plant is still alive. Living cells must keep doing cellular respiration continuously to get energy, whether it is light or dark.

Because respiration continues, the plant must still exchange gases with its surroundings. Therefore, the idea that there is no gas exchange at all cannot be correct.

Plants can lose water vapor by transpiration, but that is not the only exchange happening. Respiration involves specific gases that must be taken in and released.

From the respiration equation, the key gas movement is: $\text{Oxygen (In)}\to \text{Carbon Dioxide (Out)}$

In darkness, this respiration exchange is the main metabolic gas pattern. The statement about releasing only water vapor is incomplete because it leaves out oxygen and carbon dioxide.

With photosynthesis stopped but respiration still running, the plant will take in oxygen from the air and release carbon dioxide as a waste product of respiration.

The key detail is that the plant is in a completely dark room. That means photosynthesis cannot happen because it needs light energy.

But the plant is still alive, so cellular respiration must continue to provide energy. Respiration occurs in mitochondria all day and all night, regardless of light.

Option A claims there will be no gas exchange. That cannot be true because living organisms must exchange gases to keep respiration going. Even in the dark, the plant’s cells are breaking down stored sugars for energy, which requires gas movement.

Option D describes taking in $C{O}_2$ and releasing $O_2$, which is the overall gas exchange for photosynthesis. Since the plant is in complete darkness, photosynthesis cannot occur, so this option must be wrong.

Plants do lose water vapor by transpiration, but the word “only” makes this option incorrect. The plant must also exchange $O_2$ and $C{O}_2$ for respiration to keep its cells functioning.

After eliminating the other choices, Option C is left. In darkness, the main process controlling gas exchange is cellular respiration.

During respiration, oxygen is used and carbon dioxide is produced:

$\text{Glucose}+6O_2\to 6C{O}_2+6H_{2}O+\text{ATP (energy)}$

This matches the remaining option.

We need to name one metabolic process. The clues in the question are:

1. It uses energy from respiration.
2. It makes complex enzymes and structural proteins.
3. It starts with simpler amino acids as the raw materials. These details point to a building (anabolic) process that assembles amino acids into proteins.

Check which choices are not describing the chemical building of proteins:

• Cell division: This is when one cell becomes two (mitosis/meiosis). It needs energy, but it isn’t the specific reaction where amino acids are connected into a protein chain.
• Muscle contraction: This uses ATP for movement as muscle filaments interact, but it is about using existing proteins to do work, not constructing new proteins from amino acids.
• Active transport: This spends energy to move particles across membranes against a gradient. It’s a transport mechanism, not the assembly of amino acids into proteins.

The process where amino acids are linked into polypeptide chains to form proteins (including enzymes and structural proteins like collagen) is known as protein synthesis.

It takes place on ribosomes and requires lots of ATP, which cells obtain from respiration.

Because the description is about using energy to build proteins from amino acids, the correct option is the one describing protein synthesis.

We’re looking for a metabolic process that uses energy (typically $ATP$ produced by respiration) to join amino acids into enzymes and structural proteins.

That describes an anabolic pathway: building big molecules from smaller subunits, linked to respiration because it supplies the ATP.

Cell division uses energy for many tasks (copying DNA, separating chromosomes, forming new cells), but it is not the specific biochemical process that bonds amino acids together into a protein.

Muscle contraction requires lots of $ATP$ so actin and myosin can slide and generate force. That’s mechanical activity using existing structures, not a synthesis reaction that creates new protein molecules from monomers.

Active transport spends energy to push substances across membranes against their concentration gradient. It relocates molecules that already exist; it doesn’t assemble amino acids into proteins.

The remaining choice is protein synthesis, which includes transcription and translation. During translation, amino acids are joined by peptide bonds to form polypeptides that fold into the enzymes and structural proteins the cell needs.

Anaerobic respiration is when cells get energy from glucose without oxygen. This fits the Metabolism: Cell respiration part of the course. Yeast cells and human muscle cells can both do this when oxygen is limited, but the end products they leave behind are not the same.

In yeast, anaerobic respiration happens as alcoholic fermentation. Glucose is changed into ethanol and carbon dioxide ($C{O}_2$), releasing a small amount of usable energy.

Word equation: $\text{Glucose}\to \text{Ethanol}+\text{Carbon Dioxide}+\text{Energy (2 ATP)}$

In human muscles, when oxygen supply can’t keep up (for example, during a sprint), cells carry out lactic acid fermentation. Here, glucose is turned into lactic acid (lactate). A key point is that this pathway does not produce carbon dioxide gas.

Word equation: $\text{Glucose}\to \text{Lactic Acid}+\text{Energy (2 ATP)}$

Now compare the two sets of products to reject the incorrect statements:

• Option B: Under anaerobic conditions, both yeast and muscle cells end up with about the same energy gain (2 ATP per glucose), so this is not a product difference.
• Option C: Water is mainly produced in aerobic respiration, not fermentation. Also, muscle fermentation does not give off $C{O}_2$.
• Option D: The products are reversed—humans produce lactic acid, while yeast produces ethanol (and $C{O}_2$).

Putting the two pathways side-by-side shows that yeast produces $C{O}_2$ (the gas that can make dough expand), but human muscle fermentation produces lactic acid without releasing a gas. That matches Option A.

You are being asked about a difference in byproducts from anaerobic respiration (fermentation) in yeast compared with human muscle cells. This is part of Metabolism: Cell respiration.

We can find the correct option by crossing out statements that contradict the biology.

Without oxygen, glucose is not fully broken down. A major distinction is that water ($H_{2}O$) is produced in aerobic respiration, not in typical anaerobic fermentation pathways.

So if an option claims yeast makes water during anaerobic respiration, it should be rejected.

Check what each organism actually makes:

Humans (muscle): lactic acid fermentation $\text{Glucose}\to \text{Lactic Acid}+2\text{ ATP}$

Yeast: ethanol fermentation $\text{Glucose}\to \text{Ethanol}+\text{Carbon Dioxide}+2\text{ ATP}$

Any option claiming humans make ethanol while yeast makes lactic acid has the products swapped, so it must be eliminated.

Another option suggests one organism gets more energy per glucose in anaerobic conditions. But both yeast and muscle cells rely on glycolysis when oxygen is absent.

In both cases, the net energy gain is 2 ATP per glucose, so this is not a valid difference. Eliminate that option.

The remaining choice states that:

1. Yeast produces ethanol and carbon dioxide ($C{O}_2$).
2. Human muscle cells produce lactic acid only, with no carbon dioxide released.

Therefore, the correct difference is that yeast produces carbon dioxide while human muscles do not.

A helpful way to think about ATP is as the cell’s energy currency. Glucose is like having one very valuable, large item (like a big gold ingot): it contains lots of energy, but it’s awkward to use for tiny purchases. ATP is more like small bills or coins—easy to “pay” for individual jobs in the cell without releasing a huge amount of energy all at once (which would mostly turn into heat).

Choice A says ATP is made to stop glucose reacting with oxygen in the bloodstream. But the whole point of respiration is that cells carry out controlled reactions (using enzymes) that ultimately involve oxygen inside cells to release energy. ATP is produced during these cellular reactions; it isn’t a safety step to keep blood glucose from reacting with oxygen.

Choice B claims glucose is too big to enter mitochondria. It’s true that glucose is first converted into pyruvate in the cytoplasm before pyruvate enters the mitochondrion, but that’s describing part of the pathway, not explaining why ATP exists. Even if transport were different, the key issue is that glucose contains a lot of energy and releasing it in one step would be inefficient and damaging.

Choice D suggests ATP is a stable, months-long energy store. Actually, ATP is relatively unstable and is typically used very soon after it is produced. For storing energy over long periods, organisms rely on more stable reserves such as glycogen or lipids (fats).

A single glucose molecule contains a large amount of chemical energy (about $2800\text{ kJ/mol}$). If that energy were released all at once to power a small cellular job (like moving a molecule or changing a protein’s shape), most of the energy would spill out as too much heat, risking damage to the cell.

By transferring energy into many ATP molecules (each around $30.5\text{ kJ/mol}$), cells can release energy in small, usable amounts that fit the needs of specific reactions.

This question is about why respiration transfers energy from glucose into ATP before the cell uses it. In IB Biology, ATP is often described as the cell’s "energy currency." Using Elimination of Distractors, we can remove choices that don’t match how respiration and energy storage actually work.

Option A implies glucose would otherwise react with oxygen in the bloodstream. But blood normally carries glucose (blood sugar) without it suddenly reacting with oxygen. The breakdown of glucose with oxygen is a carefully controlled, enzyme-driven process that takes place in cells during respiration, not something that must be prevented in the blood.

Option B says glucose can’t enter mitochondria because it’s too large. While glucose ($C_6{H}_{12}{O}_6$) is indeed converted to pyruvate in the cytoplasm before pyruvate enters mitochondria, that is simply one stage of the metabolic pathway. The purpose of ATP production is to make energy easy to use in controlled amounts, not to solve a “glucose size” problem.

Option D claims ATP is stable for long-term storage. This is incorrect: ATP is designed for rapid turnover and is often used within seconds after being made. For energy storage over weeks or months, cells store energy as glycogen or as lipids (fats), which are much more stable.

Glucose holds far more energy than most single cellular processes require. By converting that energy into many ATP molecules, the cell can provide energy in small, manageable “chunks” (about $7.3\text{ kcal/mol}$ per ATP) suitable for specific tasks such as active transport, muscle contraction, or building large molecules.

First, remember the usual word equation for aerobic respiration. In this process, cells use oxygen to break down a food molecule and transfer energy into ATP.

The equation is: $\text{glucose}+\text{oxygen}\to \text{carbon dioxide}+\text{water}+\text{energy (ATP)}$

This links to the Metabolism: Cell respiration part of your syllabus.

In any equation, the reactants are written on the left of the arrow—these are the materials you begin with.

From the equation, the two essential reactants are:

1. Glucose (the energy-rich fuel molecule)
2. Oxygen (needed to allow complete breakdown)

Next, compare the options to the equation. Carbon dioxide appears on the right side, meaning it is formed during respiration. That makes it a product, not a reactant.

Ethanol and lactic acid are not typical outputs of aerobic respiration. They are produced when respiration happens without oxygen (anaerobic respiration), so they cannot be the needed reactants for the aerobic process.

After recalling the equation and removing products/anaerobic substances, glucose is confirmed as one of the two starting reactants for aerobic respiration.

Using the Elimination of Products approach, start by recalling what aerobic respiration means. It is when cells release energy by breaking down a fuel molecule in the presence of oxygen.

The word equation is: $\text{Glucose}+\text{Oxygen}\to \text{Carbon Dioxide}+\text{Water}+\text{Energy (ATP)}$

A reactant is something present at the beginning of the reaction (on the left of the arrow).

Look again at the equation: carbon dioxide is written on the right of the arrow. That placement shows it is produced by cells as a waste product, not taken in to start the reaction. So it can be removed.

Ethanol and lactic acid are also products, but they come from anaerobic routes (no oxygen):

• Lactic acid forms in animal muscle cells during anaerobic respiration.
• Ethanol forms during fermentation in yeast and in some plant cells.

Because these are made in different conditions, they are not reactants for aerobic respiration.

Once ethanol, lactic acid, and carbon dioxide are eliminated, the remaining option is glucose.

Glucose is the main sugar cells use as a fuel. In aerobic respiration it reacts with oxygen, allowing the cell to release energy and make ATP for life processes.