username@email.com
username@email.com
You have read that cells get their energy from ATP (adenosine triphosphate). But what is ATP and where does it come from? ATP is the molecule of choice for energy transfer in all cells. ATP stores the energy that is used in cellular processes in the high-energy chemical bonds between its three phosphates. The breakdown of ATP breaks the phosphate bonds, releasing energy and making it available to the cell. The formula for this reaction is:
ATP + H2O → ADP + Pi + energy
Where Pi is inorganic phosphate. Often, the breakdown of ATP does not release inorganic phosphate, but instead transfers it, with the aid of an enzyme, to another molecule. This process is called phosphorylation. Thus, the phosphorylation of ADP creates ATP. Getting ATP from glucose is a multi-step process. The first step is glycolysis, which breaks down glucose, and the second step is respiration, which itself consists of two steps: the Krebs cycle and electron transport chain. Both oxidation (the loss of an electron) and reduction (the addition of an electron) may be used to create energy from glucose.
Glycolysis, which occurs in the cell cytoplasm, is an anaerobic process that entails the splitting, or lysing, of glucose. There is a specific sequence of nine chemical reactions in glycolysis, each catalyzed by a particular enzyme. The overall purpose of glycolysis is to break down the carbon bonds in glucose and to use the released energy to produce fuel, or energy, for the cell (in the form of ATP):
glucose + oxygen → carbon dioxide + water + energy
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
In glycolysis, a phosphate group is transferred from an ATP molecule to a glucose molecule. The glucose molecule then splits apart, and energy is produced as NAD (nicotinamide adenine dinucleotide, a hydrogen carrier). NAD is reduced to NADH (hydrogen having been obtained from PGAL, phosphoglyceraldehyde). Finally, molecules of ADP are phosphorylated to become ATP.
Glycolysis is the lysing of glucose
Glycolysis yields 2 ATP plus 2 molecules of pyruvic acid, so its electrons must be carried across the inner mitochondrial membrane. Once this occurs, more ATP molecules are produced. In total, glycolysis yields 8 ATP. The 9-step process (each product is the next reaction’s reactant) looks like this:
Cellular respiration is the oxidation of food (glucose) by cells. Cellular respiration entails the further breakdown of glucose to fuel cell function. In cellular respiration, a sequence of reactions oxidizes pyruvic acid, which is produced in the later steps of glycolysis to yield energy, carbon dioxide, and water. Cellular respiration occurs in the mitochondria and involves two steps: the Krebs cycle and electron transport chain.
The prelude to the Krebs cycle is often called the transition reaction. The first step in cellular respiration involves the oxidation of pyruvic acid. The carbon is removed from the three-carbon pyruvic acid and forms 2 CO2. Two two-carbon acetyl groups are left (2 CH3CO). The pyruvic acid’s hydrogen atoms are transferred to hydrogen-carrying molecules of NAD to form 4 NADH. Each acetyl group bonds with coenzyme A (a compound made up of nucleotides and forms of vitamin B) to form the substance acetyl coenzyme A, the key compound that links glycolysis with the Krebs cycle. Converting pyruvic acid to coenzyme A yields 6 ATP.
The structure of acetyl coenzyme A is important in its function
In the Krebs cycle carbons from the acetyl group (in acetyl coenzyme A) are oxidized to create carbon dioxide, and hydrogen atoms are transferred in an electron transport process. Coenzyme A is involved both in the oxidation of pyruvic acid and in the Krebs cycle.
When the two-carbon acetyl group enters the Krebs cycle, it is combined with a four-carbon compound (oxaloacetic acid) to form the six-carbon compound citric acid. During this process, two of the six carbons are oxidized to carbon dioxide. Some of the energy released during oxidation and breaking of the carbon-carbon and carbon-hydrogen bonds is used to change ADP to ATP, and some of the energy is used to transform NAD to NADH. Remaining energy is used to reduce another electron carrier, FAD (flavin dinucleotide) into FADH2. Oxygen is not used during the Krebs cycle. All the electrons and protons released are picked up by the NAD+ and FAD. The total number of ATP molecules produced in the Krebs cycle is 24 per molecule of glucose.
oxaloacetic acid + acetyl coenzyme A + ADP + Pi + 3 NAD + FAD → oxaloacetic acid + 2 CO2 + coenzyme A + ATP + 3 NADH + FADH + 3 H+ H2O
The Krebs Cycle is vital to life
In the electron transport chain, electron carriers (NADH and FADH) transfer electrons to lower energy levels, with the energy given up by the electrons being used to create ATP from ADP. The electron transport chain begins with the glucose molecule’s carbon atoms thoroughly oxidized. Some of the energy produced in this process was used to create ATP from ADP. But a good deal of energy remains in the electrons that were removed from the carbon-carbon and carbon-hydrogen bonds, which were transferred to the electron carriers NAD and FAD, now NADH and FADH2. These high-energy electrons are then transferred in a downward process to ever-lower energy levels, bottoming out with oxygen. At each step, an electron carrier moves the electrons along to the next lower energy level. The energy the electrons give up on their downward path is used to re-create ATP from ADP. The process that produces ATP from the energy released as the electrons are moved to each lower level is called oxidative phosphorylation.
Electron transport system
The main carriers in the electron transport chain are known as cytochromes, which are made up of protein and a porphyrin ring. At each step, a different cytochrome, designed specifically to carry an electron at a particular energy level, carries the electron to the next lower level, ending with low-energy oxygen. At the end of the downward slide, the electrons link up with oxygen and then combine with hydrogen ions to yield water.
At each lower step in the chain, the energy released by a pair of electrons is sufficient to transform, or phosphorylate, one ADP to one ATP molecule. Once created, an ATP molecule is moved across the mitochondrial membrane. Simultaneously, an ADP molecule moves into the mitochondrion to begin its phosphorylation into ATP. You can see that the transformation of ADP into ATP, and vice versa, is a perpetual cycle in which a cell creates and uses the fuel that keeps it going.
In summary, glycolysis and cellular respiration yield 38 molecules of ATP to energize the cell.
ATP is created in the electron transfer chain process because
D is the correct answer. Energy is released as electrons are transferred to lower energy levels. This energy is used in the electron transfer chain to add phosphate to ADP to form ATP. Oxygen is not part of the electron transfer chain until the final step in the process, so no energy is released from oxygen. ADP is phosphorylated in the electron transfer chain, but electrons cannot phosphorylate it; only a phosphate group can. Electron transfer provides the energy for phosphorylation, but electrons neither create nor change molecules. Choice C is incorrect because NAD and FAD are electron carriers; they are not created from ADP, which is the precursor to ATP.
The last step in the glycolysis process yields
The correct answer is A. The overall purpose of glycolysis is to break down glucose to produce energy for the cell in the form of ATP. The last step in the glycolysis process yields 2 ATP and 2 molecules of pyruvic acid.
The substance that provides the necessary acetyl group in the Krebs cycle is
The correct answer is B. Acetyl coenzyme A provides the two-carbon acetyl group that enters the Krebs cycle and combines with oxaloacetic acid to form citric acid. Adenosine diphosphate is ADP, the precursor to ATP. Flavin dinucleotide is an electron carrier and does not donate an acetyl group to the Krebs cycle. Oxaloacetic acid joins with the acetyl group that is contributed by coenzyme A, but is not the original donor of this molecule to the Krebs cycle.