Cell Respiration - High School Biology

Lactic acid fermentation occurs in the absence of oxygen, and involves an enzyme that converts pyruvate from glycolysis to lactic acid via the transfer of two hydrogen atoms from NADH and H+. The NADH is oxidized to form NAD+, a required component to catalyze glycolysis.

During anaerobic respiration the Krebs cycle and electron transport chain are not functional, leaving all cell metabolism reliant on glycolysis. Without NAD+, glycolysis would also become non-functional and cell metabolism would completely stop. Both alcoholic fermentation and lactic acid fermentation serve this purpose or replenishing glycolysis reactants, but occur in different organisms. Lactic acid fermentation is most common in animals while alcoholic fermentation occurs in unicellular organisms, such as yeast.

Cellular respiration is the metabolic process used to generate energy, in the form of ATP, that can power cellular functions. During cellular respiration, glucose is broken down and used to generate NADP and FADH2. These molecules then donate electrons to the electron transport chain, power the proton gradient that is responsible for producing ATP through ATP synthase.

Glucose and oxygen are consumed during this process, while water and carbon dioxide are produced, along with ATP. Sulfur dioxide and carbon monoxide are not involved in the processes of cellular respiration.

Glycolysis is the first step of cellular respiration, and takes place in the cytosol. The citric acid cycle and electron transport chain are both located in the mitochondria.

Glycolysis will split a glucose molecule into two molecules of pyruvate. This process will result in a net gain of 2 molecules of both ATP and NADH. Lactate is not a product of glycolysis, and will only be made in the event that oxygen is not available and fermentation must be used to reduce pyruvate into lactate.

In glycolysis, which takes place in the cytoplasm, 4 ATP are made and 2 ATP are spent. This means that there is a net gain of 2 ATP, which are produced via substrate level phosphorylation, which involves adding a phosphate to ADP to yield ATP.

The overall process of glycolysis is:

The net gain of pyruvate, NADH, and ATP during glycolysis is 2 pyruvate, 2 NADH, and 2 ATP per molecule of glucose. Although the process of glycolysis yields 4 ATP, the early steps of glycolysis use 2 ATP to convert glucose into 2 phosphoglyceraldehydes (note: phosphoglyceraldehyde is a 3 carbon molecule) leading to a net gain of 2 ATP.

The process of lactic acid fermentation is:

Pyruvate + NADH + Lactate +

(under hypoxic or partially anaerobic conditions)

Lactic acid fermentation is a biological process where pyruvate is converted into and lactate under hypoxic conditions. It occurs in some bacterial cells, and some animal cells such as muscles. This process is catalyzed by the enzyme lactate dehydrogenase. If oxygen is present in the cell, many organisms will bypass fermentation and undergo cellular respiration. (Fun fact: lactic acid fermentation is utilized to produce kimchi, sauerkraut, and yogurt). Animal muscle cells will undergo lactic acid fermentation, when starved of oxygen. This process is a last resort for energy and cannot be tolerated for long periods of time.

The process of glycolysis (glucose to pyruvate) occurs in the cytosol.

Once formed, pyruvate can have numerous fates. In yeasts it can remain in the cytosol and undergo alcoholic fermentation. Pyruvate can also undergo lactic acid fermentation (under hypoxic conditions) within the cytosol of red blood cells and active muscles. Additionally, pyruvate can undergo cellular respiration in the mitochondria where it is oxidized completely into carbon dioxide and water.

Energy from catabolism—exergonic or energy-yielding processes—is used to drive the formation of ATP from ADP and inorganic phosphate because ATP formation requires energy to be made. Catabolism is a pathway, which breaks down a larger molecule into smaller ones. Glycolysis is an example of a catabolic pathway.

In glycolysis, a glucose molecule is broken down into two pyruvate molecules with a net gain of 2 ATP. Oxygen is not needed for this process, making glycolysis both an aerobic and anaerobic process. The citric acid cycle does not directly require oxygen, however it does require by-products from the electron transport chain, which does require oxygen to be the final electron acceptor. The electron transport chain is an aerobic process, and because the citric acid cycle relies on electron transport chain by-products, it is an aerobic process as well.

The citric acid cycle is the process by which acetyl-CoA (a two-carbon molecule) is completely broken down to carbon dioxide and energy. Acetyl-CoA loses its CoA and is attached to oxaloacetate (OAA) to produce citrate, which is converted to isocitrate. From there the following occurs:

• Isocitrate (6C) is converted to -ketoglutarate (5C), 1 CO2, and 1 NADH
• -ketoglutarate (5C) is converted to succinyl-CoA (4C), 1 CO2, and 1 NADH
• Succinyl-CoA (4C) is converted to succinate (4C) and 1 GTP (similar to ATP)
• Succinate (4C) is converted to fumarate (4C) and 1 FADH2
• Fumarate (4C) is converted to malate (4C)
• Malate (4C) is converted to OAA (4C) and 1 NADH

The net result is 3 NADH, 2 CO2, 1 FADH2, and 1 GTP (similar to ATP) per round. Since one glucose molecule produces two pyruvate molecules, which produce two Acetyl-CoA, the cycle occurs twice per glucose molecule.

Glycolysis is the first step in extracting energy from a sugar molecule. It converts a 6-carbon sugar molecule, such as glucose, into two three-carbon pyruvate molecules. It does not require oxygen, and is the first step in both aerobic and anaerobic respiration. Glycolysis produces two net ATP per sugar molecule.

If oxygen is present, the pyruvate molecules are broken down into acetyl-CoA and translocated into the mitochondria, where they undergo the Krebs cycle in the mitochondrial matrix. The Krebs cycle products NADH and FADH2, which are used to make ATP in the electron transport chain, which uses oxygen and hydrogen ions to create water. The electron transport chain creates an additional 34 ATP per original sugar molecule.

If oxygen is not present, pyruvate from glycolysis can be converted to lactic acid through fermentation, which regenerates the NAD+ required for more glycolysis cycles. The Krebs cycle and electron transport chain cannot function in anaerobic conditions (no oxygen).

Prior to entering the citric acid cycle, pyruvate (a three-carbon molecule) is processed and converted into acetyl CoA (a two-carbon molecule).

This will then enter the citric acid cycle and combine with oxaloacetate (a four-carbon molecule) in order to make citrate, a six-carbon molecule.

All organisms, including plants, undergo cellular respiration. Some students get confused when discussing both cellular respiration and photosynthesis because they assume that plants photosynthesize and animals respire. One way to remember that all organisms respire is to understand what the two processes do. Photosynthesis is the process that creates glucose which is a form of energy storage. Cellular respiration is the process that breaks down glucose piece by piece into small packets of energy called ATP which is the usable form of energy in cells. When thinking about both processes, it becomes apparent why all organisms must undergo cellular respiration in order to convert stored energy to usable energy.

After 2 rounds of the Krebs cycle per glucose are completed, , and are produced. Water is produced during one step in the Krebs cycle, but it is consumed during three steps. Thus, water is a reactant, not a product of the Krebs cycle.

Although the citric acid cycle does synthesize two ATP per round, its main purpose is to produce NADH for the electron transport chain that makes ATP much more efficiently. Since the electron transport chain is located in the inner mitochondrial membrane, it is most efficient for the cell to produce the NADH in the mitochondrial matrix where it can be used immediately for its purpose, rather than having to use time and resources to transport it there.

The electron transport chain is used to pump protons into the intermembrane space. This establishes a proton gradient, allowing protons to be pumped through ATP synthase in order to create ATP. This method of ATP production is called oxidative phosphorylation.

The other two metabolic processes, glycolysis and the citric acid cycle, use substrate-level phosphorylation to generate ATP. Substrate-level phosphorylation uses enzymes to create ATP from ADP, while oxidative phosphorylation uses the proton gradient to drive ATP synthase.

NADH and FADH2 are produced during glycolysis and the citric acid cycle. Electrons from these molecules are donated to proteins of the electron transport chain. The electrons interact with the proteins, helping them push protons into the intermembrane space. This accumulation of protons is what powers ATP synthesis via oxidative phosphorylation. When the electron reaches the final protein of the electron transport chain, it binds with oxygen to produce water.

Pyruvate decarboxylation convert pyruvate to acetyl CoA before the citric acid cycle. The TCA cycle is another name for the citric acid cycle.

The electron transport chain is primarily used to send protons across the membrane into the intermembrane space. This create a proton-motive force, which will drive ATP synthase in the final step of cellular respiration to create ATP from ADP and a phosphate group. This final process is known as oxidative phosphorylation.

The electron is passed through proteins in the electron transport chain. With each subsequent protein, more electrons are transferred to the intermembrane space of the mitochondrion. Though oxygen is the final electron acceptor, generating water from oxygen is not the primary function of the electron transport chain.