▶️ Answer/Explanation
Correct Option: C
Glycolysis is the initial stage of cellular respiration occurring in the cytoplasm.
It involves the breakdown of $1$ molecule of glucose into $2$ molecules of pyruvate.
This process occurs in both aerobic and anaerobic conditions.
A net gain of $2$ $\text{ATP}$ and $2$ $\text{NADH}$ is produced during this phase.
Fermentation (Options A and B) only occurs if oxygen is absent after glycolysis.
The Electron Transport Chain (Option D) is the final stage of aerobic respiration.
▶️ Answer/Explanation
The correct options are A and B (Note: The question asks for carriers for both energy and high-energy electrons).
$\text{ATP}$ acts as the primary energy carrier produced during the payoff phase of glycolysis.
$\text{NADH}$ is the high-energy electron carrier formed by the reduction of $\text{NAD}^+$.
$\text{FADH}_2$ is not produced in glycolysis; it is specific to the Krebs cycle.
$\text{NADPH}$ is typically involved in anabolic reactions and photosynthesis, not glycolysis.
Glycolysis yields a net gain of $2$ $\text{ATP}$ and $2$ $\text{NADH}$ per molecule of glucose.
These molecules carry chemical energy and electrons to subsequent stages of cellular respiration.
▶️ Answer/Explanation
The correct option is C. fermentation.
In the absence of oxygen ($\text{O}_2$), the cell cannot perform aerobic respiration.
Glycolysis produces $2$ molecules of pyruvate and $2$ molecules of $\text{NADH}$.
Without oxygen, the Krebs cycle and electron transport chain ($ETC$) cannot operate.
Fermentation occurs to regenerate $\text{NAD}^+$ from $\text{NADH}$.
This regeneration allows glycolysis to continue producing a small amount of $\text{ATP}$.
In animals, this typically results in lactic acid; in yeast, it results in ethanol and $\text{CO}_2$.
▶️ Answer/Explanation
Glycolysis begins with a single $6$-carbon glucose molecule ($C_{6}H_{12}O_{6}$).
Through a series of enzymatic reactions, glucose is split into two $3$-carbon sugars.
These sugars are further oxidized and rearranged to form pyruvic acid (also known as pyruvate).
The chemical formula for the resulting pyruvic acid is $CH_{3}COCOOH$.
This process yields a net gain of $2$ $ATP$ and $2$ $NADH$ molecules per glucose.
Therefore, the correct terminal $3$-carbon product of glycolysis is pyruvic acid.
▶️ Answer/Explanation
The correct answer is B. anaerobic.
The term anaerobic literally means “without air” or “without oxygen.”
Fermentation is a metabolic process that extracts energy from carbohydrates.
This process occurs in the absence of $O_2$ (free oxygen).
In contrast, aerobic processes require oxygen to proceed.
During fermentation, $NADH$ is oxidized back to $NAD^+$ to keep glycolysis running.
Common products of this anaerobic pathway include lactic acid or ethanol and $CO_2$.
▶️ Answer/Explanation
The correct answer is A. $\text{NAD}^+$.
In glycolysis, $\text{NAD}^+$ is reduced to $\text{NADH}$ as glucose is broken down.
Without oxygen, the electron transport chain cannot oxidize $\text{NADH}$ back to $\text{NAD}^+$.
Fermentation steps in to oxidize $\text{NADH}$, converting it back into $\text{NAD}^+$.
This regenerated $\text{NAD}^+$ is essential for the glyceraldehyde-3-phosphate dehydrogenase reaction in glycolysis.
By recycling this carrier, the cell can continue to produce a net of $2$ $\text{ATP}$ molecules per glucose.
Without this regeneration, glycolysis would stop due to a lack of available electron acceptors.
▶️ Answer/Explanation
The correct sequence is Option D.
1. Glycolysis occurs first in the cytosol, breaking down $1$ molecule of glucose into $2$ molecules of pyruvate.
2. The Krebs cycle (Citric Acid Cycle) follows in the mitochondrial matrix, processing derivatives of pyruvate.
3. This cycle generates high-energy electron carriers, namely $\text{NADH}$ and $\text{FADH}_2$.
4. The Electron transport chain is the final stage located in the inner mitochondrial membrane.
5. It uses those electrons to create a proton gradient and synthesize the bulk of $\text{ATP}$.
6. Therefore, the metabolic pathway follows the order: Glycolysis $\rightarrow$ Krebs cycle $\rightarrow$ Electron transport chain.
▶️ Answer/Explanation
The correct answer is B.
Pyruvic acid ($C_3H_4O_3$) undergoes decarboxylation before and during the Krebs cycle.
One carbon is released as $CO_2$ during the conversion of pyruvate to Acetyl-CoA.
The remaining two carbons enter the cycle and are released as $2$ molecules of $CO_2$.
These $CO_2$ molecules diffuse out of the mitochondria and the cell.
Ultimately, they are transported to the lungs and exhaled into the atmosphere.
The carbon atoms do not enter the electron transport chain; only electrons and protons do.
▶️ Answer/Explanation
The correct answer is A. aerobic.
The term “aerobic” originates from the Greek words aer (air) and bios (life).
In biology, processes that utilize $\text{O}_2$ as a final electron acceptor are classified as aerobic.
Cellular respiration uses oxygen to break down glucose into $\text{CO}_2$ and $\text{H}_2\text{O}$.
This process yields a high amount of energy, approximately $30$-$32$ molecules of $\text{ATP}$ per glucose.
In contrast, “anaerobic” refers to biological processes that occur in the absence of oxygen.
Therefore, because this specific pathway requires oxygen, it is defined as an aerobic process.
▶️ Answer/Explanation
Glycolysis produces a net gain of $2$ ATP molecules in the cytosol.
The Krebs cycle (Citric Acid Cycle) produces $2$ ATP (or GTP) per glucose molecule.
The Electron Transport Chain and Chemiosmosis generate approximately $32$ to $34$ ATP.
Summing these stages: $2$ (Glycolysis) + $2$ (Krebs) + $32/34$ (ETC) = $36$ to $38$ ATP.
In many eukaryotic cells, the transport of NADH into the mitochondria reduces the net yield to $36$.
Therefore, $36$ is the standard theoretical total for complete aerobic respiration.
The correct option is D) $36$.
▶️ Answer/Explanation
The correct answer includes all of the options (A, B, C, and D).
For every turn of the cycle (per Acetyl-$\text{CoA}$ molecule):
Two molecules of $\text{CO}_2$ are released as waste products.
Three molecules of $\text{NAD}^+$ are reduced to $\text{NADH}$.
One molecule of $\text{FAD}$ is reduced to $\text{FADH}_2$.
One molecule of $\text{ATP}$ (or $\text{GTP}$) is produced via substrate-level phosphorylation.
Since one glucose molecule powers two cycles, these yields are doubled per glucose.
Therefore, $\text{ATP}$, $\text{NADH}$, $\text{FADH}_2$, and $\text{CO}_2$ are all essential outputs.
▶️ Answer/Explanation
The correct answer is A. oxygen.
In aerobic respiration, the electron transport chain (ETC) transfers electrons through a series of complexes.
At the final step (Complex IV), electrons are passed to molecular oxygen ($O_2$).
Oxygen has a high electronegativity, making it an ideal final electron acceptor.
Upon accepting electrons and pickng up protons ($H^+$), oxygen is reduced to form water ($H_2O$).
The chemical reaction can be summarized as: $\frac{1}{2} O_2 + 2e^- + 2H^+ \rightarrow H_2O$.
Without oxygen, the chain stalls, preventing the regeneration of $\text{NAD}^+$ and stopping ATP production.
▶️ Answer/Explanation
The correct answer is C. $\text{H}^+$ ions.
During cellular respiration, the electron transport chain pumps $\text{H}^+$ ions into the intermembrane space.
This creates a high electrochemical gradient (proton-motive force) relative to the mitochondrial matrix.
$\text{H}^+$ ions flow back into the matrix through the $\text{F}_0$ subunit of the ATP synthase enzyme.
This flow (chemiosmosis) triggers the rotation of the enzyme’s central shaft and the $\text{F}_1$ catalytic unit.
The mechanical energy from this “spinning” converts $\text{ADP} + \text{P}_i$ into $\text{ATP}$.
This entire process is fundamentally known as oxidative phosphorylation.
▶️ Answer/Explanation
The correct answer is C. Electron transport.
Glycolysis occurs in the cytosol and produces a net gain of $2$ $ATP$ molecules.
The Krebs cycle (Citric Acid Cycle) produces $2$ $ATP$ molecules per glucose molecule.
The Electron Transport Chain (ETC) and oxidative phosphorylation produce approximately $26$ to $28$ $ATP$.
Acetyl-CoA charging (the link reaction) produces $0$ $ATP$ directly.
Therefore, the electron transport stage is responsible for the bulk of energy production.
The total theoretical yield for aerobic respiration is roughly $30$–$32$ $ATP$.
▶️ Answer/Explanation
As electrons move through the protein complexes, they release free energy used to pump protons ($H^+$).
These $H^+$ ions are moved from the mitochondrial matrix into the intermembrane space.
This process creates an electrochemical gradient, often referred to as the proton-motive force.
While water is produced at the final step, the movement down the chain specifically drives proton transport.
Option C describes chemiosmosis, which occurs after the gradient is established by the chain.
Carbon dioxide (Option B) is a byproduct of the Krebs cycle, not the Electron Transport Chain.
Therefore, the most direct action occurring as electrons pass down the chain is the pumping of $H^+$.
Correct Option: A
▶️ Answer/Explanation
Correct Option: C. Coenzyme A
The process shown is the decarboxylation of pyruvate within the mitochondrial matrix.
A carbon atom is removed from pyruvic acid ($3\text{C}$) and released as $\text{CO}_2$.
The remaining $2$-carbon acetyl group is oxidized while $\text{NAD}^+$ is reduced to $\text{NADH}$.
Coenzyme A (CoA) then attaches to the acetyl group to form Acetyl-CoA.
This molecule subsequently enters the Krebs cycle by reacting with oxaloacetate.
ATP is a product of later stages, and $\text{NADP}^+$ is primarily used in photosynthesis.
▶️ Answer/Explanation
Correct Option: A
The $NADH$ produced is a high-energy electron carrier.
In the presence of oxygen, it travels to the inner mitochondrial membrane.
It donates its electrons to Complex I of the Electron Transport Chain (ETC).
These electrons move through the chain to power the pumping of protons ($H^+$).
This creates a gradient used by $ATP$ synthase to produce $ATP$ via oxidative phosphorylation.
Oxygen acts as the final electron acceptor, combining with electrons and $H^+$ to form $H_2O$.
▶️ Answer/Explanation
The correct answer is D. citric acid.
The $2$-carbon acetyl group from Acetyl-$\text{CoA}$ combines with a $4$-carbon molecule called oxaloacetate.
This condensation reaction produces a $6$-carbon compound known as citric acid (or citrate).
During this process, the $\text{Coenzyme A}$ ($\text{CoA}$) is released to be reused in the link reaction.
This specific step is why the Krebs cycle is also frequently referred to as the Citric Acid Cycle.
Pyruvic acid is a $3$-carbon precursor, and glucose is a $6$-carbon sugar processed earlier in glycolysis.