Biology 3.15: The Krebs Cycle

Krebs cycle

A cyclical set of reactions in which pyruvic acid is metabolized and ATP, NADH, and FADHS are formed

The Krebs cycle begins and ends with specific molecules

For each molecule do glucose in food, glycolysis produces 2 molecules of pyruvic acid. -for each pyruvic molecule, 1acetyl-CoA molecule is generated The Krebs cycle, therefore, goes through 2 passes for each molecule of glucose that enters cellular respiration Each pass of the Krebs cycle produces: -3 molecules of NADH -1 molecule of FADH2 -1 molecule of ATP -2 molecules of CO2

Sir Hans Adolf Krebs discovered the details of the citric acid cycle

In 1937, the German biochemist discovered the chemical compounds that produce NADH and FADH2. The Krebs cycle is also known as the citric acid cycle The discovery of citric acid as the first compound in the cycle filled in gaps in scientists knowledge about this essential process in the production of usable energy In 1953, 16 years after his discovery, Krebs won the nobel prize in physiology or medicine. He lived until 1981, long enough to witness great advancements in the field of biology

Glycolysis begins with glucose and ends with pyruvic acid

In the cytosol of a cell, each glucose molecule is broken down into 2 molecules of pyruvic acid, and small amounts of ATP and NADH are also generated. When oxygen is available and when the cell has the ability, the pyruvic acid continues along the energy-producing pathway of cellular respiration, also called aerobic respiration. Before the Krebs cycle begins, the pyruvic acid molecules move out of the cytosol and into the mitochondria of the cells.

Electron carrier

Molecules such as FADH2 and NADH that carry electrons from one part of a chemical process to another

Mitochondria

Organelles that are the sites of cellular respiration

Most of the energy that organisms use comes from the sun

Plants capture radiant energy from the sun and transfer it to the chemical bonds in glucose. Animals eat the plants. I'm the cytosol of the cells of plants, animals, and other organisms, glycolysis breaks down glucose to transfer the energy to the chemical bonds of pyruvic acid. But pyruvic acid is still not a form of energy that cells can use. -more chemical reactions must take place before suns energy is ultimately transformed into usable energy

The Krebs cycle uses the product of glycolysis- pyruvic acid- to generate electron carriers for the final stage of cellular respiration

Pyruvic acid from glycolysis—enters—the krebs cycle—

The Krebs cycle takes place in five main steps

STEP 1: Acetyl-CoA combines with a 4 carbon compound (oxaloacetic acid) already present in the cycle and forms a 6 carbon compound (citric acid) STEP 2: carbon dioxide is released from the citric acid molecule, so the molecule now has 5 carbon atoms. A molecule of NAD+ accepts electrons to form 1 molecule of NADH STEP 3: CO2 is again released, producing a 4-carbon compound. A molecule of NAD+ accepts electrons, forming 1 molecule of NADH. A phosphate group joins ADP to form 1 molecule of ATP STEP 4: The 4-carbon compound is altered, but it keeps its 4 carbon atoms. FAD gains electrons to form 1 molecule of FADH2 STEP 5: the 4-carbon compound is altered again, this time back into oxaloacetic acid, the original molecule that bonded with acetyl-CoA. Another molecule of NADH is produced, and the cycle is ready to begin again. The cycle is now ready for another CoA molecule

Glycolysis

Takes place in the cytosol of a cell. Glucose molecules move into the cytosol, where a series of chemical reactions take place to produce molecules of pyruvic acid

Krebs cycle

Takes place in the matrix of the mitochondria. Through a series of chemical reactions, the energy in the pyruvic acid is transferred to molecules of NADH and FADH2

Acetyl-CoA

The beginning molecule for the Krebs cycle

Electron carriers store energy that's used in the production of ATP

The molecule FADH2 can donate electrons in a chemical reaction. -when it loses electrons, it becomes FAD During the Krebs cycle, the chemical energy that was part of a glucose molecule becomes part of the structure of 2 electron carriers- NADH and FADH2

Carbon dioxide is an indication that cells are discharging waste during the Krebs cycle

When an animal exhales, it releases carbon dioxide. -each exhalation of carbon dioxide is an indication that millions of cells have gone through the first few steps in the Krebs cycle and are discharging the waste from the process

In the mitochondria of your cells, pyruvic acid is converted into acetyl-CoA, the starting material of the Krebs cycle

When you breathe, oxygen floods into your lung and then into your cells. In the mitochondria of your cells, some of that fresh oxygen reacts with the product of glycolysis-pyruvic acid In the presence of oxygen, pyruvic acid is converted into a 2-carbon compound called acetyl-coenzyme A