Question

All of the following stages are considered aerobic processes EXCEPT (A) the Krebs cycle (B) formation of acetyl CoA (C) glycolysis (D) electron transport chain (E) oxidative phosphorylation

Short answer

Glycolysis (Option C)

Step-by-step solution

Understanding the Terminologies

The first step is to understand what aerobic and anaerobic processes are. Aerobic processes are those that require oxygen to produce energy, while anaerobic processes don't require oxygen. Examples of aerobic processes are the Krebs cycle, the electron transport chain, and oxidative phosphorylation. Anaerobic processes include glycolysis.

Identifying the Aerobic Processes

The Krebs cycle (A), the electron transport chain (D), and oxidative phosphorylation (E) are all aerobic processes. This is because they all occur in the presence of oxygen.

Identifying the Anaerobic Process

Glycolysis (C) is an anaerobic process as it occurs whether oxygen is present or not. So, this is the only process from the given options that is classified as an anaerobic process.

Answering the question

Based on the above analysis, the correct answer to the question 'All of the following stages are considered aerobic processes EXCEPT' is option C, Glycolysis. This is because glycolysis is an anaerobic process and does not need oxygen to occur.

Key concepts

Krebs cycle

The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to release stored energy. This cycle takes place in the mitochondria, which is often referred to as the powerhouse of the cell.

The cycle begins with the molecule acetyl CoA. This molecule is derived from carbohydrates, fats, and proteins after they've been broken down through earlier metabolic processes. In the Krebs cycle, acetyl CoA combines with a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule. Through eight distinct steps, citrate undergoes a transformation.

• Two carbon atoms are released as carbon dioxide.
• High-energy electrons are transferred to NAD+ and FAD, creating NADH and FADH2.
• ATP, the energy currency of the cell, is produced directly at one step.

The NADH and FADH2 generated are vital because they carry electrons to the next stage of cellular respiration, the electron transport chain.

Glycolysis

Glycolysis is a metabolic pathway that converts glucose, a six-carbon sugar molecule, into two three-carbon molecules called pyruvate. This process occurs in the cytoplasm of the cell and is considered anaerobic because it does not require oxygen.

Glycolysis involves several key stages, including:

• Energy Investment Phase – where the cell uses up 2 ATP molecules to kickstart the reaction.
• Payoff Phase – where energy is released in the form of 4 ATP and 2 NADH molecules.

The net gain from glycolysis is 2 ATP and 2 NADH. Importantly, glycolysis is the first step in both aerobic and anaerobic respiration. If oxygen is available, pyruvate will enter the mitochondria to participate in aerobic pathways. If not, it may undergo fermentation in the cytoplasm.

Electron transport chain

The electron transport chain (ETC) is the last stage of cellular respiration, occurring in the inner mitochondrial membrane. It plays a key role in producing a large amount of ATP.

Here’s how it works:

• NADH and FADH2, created in earlier stages, donate electrons to the chain.
• Electrons are passed along a series of proteins embedded in the membrane.
• This passage of electrons allows protons to be pumped across the membrane, creating a gradient.

The proton gradient generated through this process is used by ATP synthase, a specialized enzyme, to produce ATP. Oxygen is crucial here, serving as the final electron acceptor. Without it, the chain cannot function, underscoring why this is an aerobic process.

Oxidative phosphorylation

Oxidative phosphorylation is a crucial part of cellular respiration where ATP is synthesized as a result of electron transport. Although it's often used interchangeably with the electron transport chain, it's worth noting it includes the function of ATP synthase.

During this process:

• Electrons travel down the electron transport chain and energy is used to pump protons out of the mitochondrial matrix.
• This creates a significant proton gradient across the inner mitochondrial membrane.
• ATP synthase harnesses this proton motive force to convert ADP to ATP.

The term "oxidative" denotes the oxygen-dependent movement of electrons, while "phosphorylation" refers to the addition of a phosphate group to ADP, forming ATP. This process is highly efficient, producing around 34 ATP from a single molecule of glucose. Hence, oxidative phosphorylation is considered a primary source of energy in aerobic organisms.