Glucose is a simple sugar that is the main energy constituent of the blood. It flows through the blood and supplies cells with a means to make adenosine triphosphate, or ATP, which is the primary energy carrier of the cell. Without oxygen, glucose is metabolized to lactic acid with the net synthesis of two ATP units per glucose; this is called anaerobic metabolism. In the presence of oxygen, the cellular machinery can use glucose to synthesize about 38 ATP per glucose through aerobic metabolism.
Glucose is a single unit of sugar that can be linked together with other units in long chains, such as found in starches like pasta or breads. Foods are mechanically processed in the mouth and exposed to the enzyme amylase in the saliva, which cleaves the chains of glucose into smaller units. Carbohydrates are further broken down into single glucose units in the intestine through the action of pancreatic enzymes. The cells of the intestine take up glucose and deposit it into the blood. Insulin is released from the pancreas into the blood, and signals the cells to take up glucose.
In glycolysis, the six-carbon glucose molecule is broken in half into two three-carbon molecules. This process uses two ATP units and creates four ATP units, for a net production of two ATP molecules. At the end of glycolysis, the molecules can become lactic acid, or lactate, and exit the cell, or become pyruvate and enter the next part of anaerobic glycolysis called the tricarboxylic acid cycle.
Tricarboxylic Acid Cycle
The tricarboxylic acid cycle, TCA cycle, or Krebs cycle, breaks the pyruvate formed from glycolysis into carbon dioxide to form more energy-carrying molecules such as FADH2, NADH and more ATP. The FADH2 and NADH are then taken into a large cellular structure called the mitochondria. The mitochondria is the energy producer of the cell. Here, the process of oxidative phosphorylation makes much more ATP.
Oxidative phosphorylation uses the energy in NADH and FADH2 to generate an electrical and chemical current that synthesizes ATP. NADH and FADH2 send electrons down a series of chemical carriers to oxygen, which is split to form water. This process releases energy that is used to generate the electrical current. This current passes through a special protein called ATP synthase, which makes large amounts of ATP.
Cori Cycle and Gluconeogenesis
At times, such as in the absence of oxygen, glucose only goes through glycolysis and forms lactic acid. The lactic acid leaves the cell and goes to the liver and kidneys. These organs can run glycolysis basically in reverse, in the process of gluconeogenesis, to form glucose again, and give it back to the blood. The cycle of lactic acid turning into glucose and then being deposited back into the blood is the Cori cycle.
- Lehninger Principles of Biochemistry; David L. Nelson et al.
- Molecular Biology of the Cell; Bruce Alberts et al.
- Physiology; Linda S.Costanzo
- Chad Baker/Photodisc/Getty Images
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