Question

What molecule is the end product of glycolysis? How are the carbons of this molecule used in stage 1 of cellular respiration? In what form is most of the energy from the Krebs cycle captured?

Short answer

The end product of glycolysis is pyruvate. In stage 1 of cellular respiration, pyruvate undergoes pyruvate decarboxylation, losing one carbon as carbon dioxide and forming acetyl-CoA, which then enters the Krebs cycle. Most of the energy from the Krebs cycle is captured in the form of ATP and reduced electron carriers, such as NADH and FADH2, which later transfer these high-energy electrons to the electron transport chain for ATP generation via oxidative phosphorylation.

Step-by-step solution

Identify the end product of glycolysis

Glycolysis is the first stage of cellular respiration in which one glucose molecule is broken down into two molecules of pyruvate. The end product of glycolysis is pyruvate.

Explain the use of carbons in stage 1 of cellular respiration

In stage 1 of cellular respiration, the pyruvate molecules produced during glycolysis are transported into the mitochondria. Here, they undergo a process called pyruvate decarboxylation (also known as pyruvate oxidation), which occurs in the mitochondrial matrix. During this process, each 3-carbon pyruvate molecule loses one carbon in the form of carbon dioxide. The remaining 2-carbon molecule combines with a molecule of coenzyme A (CoA) to form acetyl-CoA, which will then enter the Krebs cycle.

Describe the Krebs cycle and its energy capture

The Krebs cycle (also called the citric acid cycle or the tricarboxylic acid cycle) is the second stage of cellular respiration. Acetyl-CoA, produced in stage 1, enters the Krebs cycle and combines with oxaloacetic acid (a 4-carbon molecule) to form citric acid (a 6-carbon molecule). Throughout the Krebs cycle, this citric acid molecule undergoes various transformations which release energy and reduce electron carrier molecules, mainly NAD+ and FAD. The energy from the Krebs cycle is mostly captured in the form of ATP and reduced electron carriers, such as NADH and FADH2. These electron carriers will then deliver these high-energy electrons to the electron transport chain (the final stage of cellular respiration), where most of the ATP will be generated in a process called oxidative phosphorylation.

Key concepts

Glycolysis

Glycolysis is often referred to as the gateway to cellular respiration, marking the initial breakdown of glucose, a six-carbon sugar, into more manageable compounds. This process is fascinating in its simplicity and efficiency, unfolding in the cytosol of the cell. During glycolysis, glucose undergoes a 10-step enzymatic division, culminating in the production of two three-carbon molecules called pyruvate. It's essential to understand that this conversion happens without the need for oxygen, making glycolysis an anaerobic process.

Through glycolysis, not only is pyruvate formed, but also a small net gain of ATP — specifically, 2 ATP molecules per glucose molecule. Additionally, 2 NAD+ molecules are reduced to 2 NADH, gearing them up for a critical role in later stages of cellular respiration. This energy currency, though modest compared to later stages, is vital for various cellular activities.

Pyruvate Oxidation

Upon the successful completion of glycolysis, pyruvate finds its way to the mitochondrial matrix, if oxygen is present, where it is primed for the subsequent stage in cellular respiration: pyruvate oxidation. This pivotal phase serves as a bridge between glycolysis and the Krebs cycle. Each pyruvate molecule is meticulously stripped of one carbon, which is then released as carbon dioxide. What remains is a two-carbon compound that, when combined with coenzyme A (CoA), forms acetyl-CoA.

This decarboxylation process not only contributes to the carbon cycle but also produces more NADH, which is packed with electrons set to be utilized in the electron transport chain. The irreversible conversion sets the stage for the Krebs cycle, allowing the two-carbon acetyl group to enter and feed the cycle.

Krebs Cycle

The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a graceful dance of biochemical reactions that occurs in the mitochondrial matrix. This cycle begins when acetyl-CoA merges with a four-carbon acceptor molecule, oxaloacetate, forming a six-carbon molecule: citric acid.

During the cycle's turns, citric acid is systematically dismantled and reconstructed, with two carbon dioxide molecules escaping along the way. However, it is not just carbon that's being shuffled around; electrons are deftly extracted and transferred to electron carriers NAD+ and FAD, forming NADH and FADH2. This energetically flourishing process yields 1 ATP (or the equivalent in GTP) per turn directly. However, the true significance lies in the loaded NADH and FADH2 molecules produced, primed to launch a surplus of ATP in the final act of cellular respiration.

ATP Production

ATP (adenosine triphosphate) acts as the principal energy carrier within the cell, akin to currency. The production of ATP throughout cellular respiration can be likened to charging a battery; each step incrementally adds energy until it is fully 'charged' at the end of the process.

The cumulative gain of ATP directly from glycolysis and the Krebs cycle is comparatively modest, topping out at about 4 ATP per glucose molecule. However, the real power surge ensues during the electron transport chain, powered by the NADH and FADH2 synthesized in previous steps. Here, these molecules part with their electrons, enabling the synthesis of up to approximately 34 additional ATP molecules through oxidative phosphorylation. The total potential yield hovers around 38 ATP molecules from one glucose, although variations in efficiency can bring this number down slightly in living cells.

Electron Transport Chain

The electron transport chain (ETC) can be considered the crescendo of cellular respiration, where the oxygen we breathe plays a pivotal role. This complex sequence occurs along the inner mitochondrial membrane and is driven by the electrons donated from NADH and FADH2. As these electrons glide through various protein complexes, a remarkable process known as chemiosmosis comes into play.

Protons are pumped from the mitochondrial matrix to the intermembrane space, creating a concentration gradient. Harnessing the potential energy of this gradient, ATP synthase, a molecular turbine, produces ATP as protons flow back into the matrix. This coupling of electron transport and ATP generation is termed oxidative phosphorylation. Oxygen, essential to life, reveals its purpose as the ultimate electron acceptor, uniting with protons to form water – a benign and vital by-product.