Interactive Summaries

Spotlight on Metabolism

Energy: Fuel for Work

Our cells get their energy from energy held in molecular bonds of carbohydrates, fats and proteins. Green plants use light energy from the sun to make carbohydrate in a process called . Our bodies extract energy from food in three stages. Stage 1 consists of digestion, absorption, and . Stage 2 includes the breakdown of many small molecules into a few key metabolites. In Stage 3, the complete breakdown of metabolites to water and liberates large amounts of energy.

The term describes a series of chemical reactions that either break down a large compound (catabolism) or build more complex molecules (anabolism). Cells are known as the 'work centers' of metabolism. The basic animal cell is divided into two parts--the and the , which is filled with a fluid called cystosol. The are power generators that contain many important energy-producing pathways. and their cofactors speed up chemical reactions in metabolic pathways.

Breakdown and Release of Energy

Although each energy bearing nutrient initially follows a different metabolic pathway, they all follow the and the electron transport chain. During glycolysis one molecule of glucose yields NADH, a net of two and pyruvate. In the next step of carbohydrate metabolism, pyruvate is converted to . Without oxygen, pyruvate cannot be converted to this substance. In this case, it is rerouted to form . begins when acetyl CoA combines with oxaloacetate to yield citrate. This cycle produces most of the energy-rich molecules that ultimately generate ATP. It is also an important source of building blocks for fatty acids and . The electron transport chain is the last step in glucose breakdown and occurs in the inner membrane. produced in the citric acid cycle delivers its high energy electrons to the beginning of the chain. At the end of the chain, oxygen accepts the energy depleted electrons and then reacts with hydrogen to form water. The three end products of glucose catabolism are water, and ATP.

Special States: Fasting

When your body is faced with starvation it must deal with several dilemmas. The first priority is to preserve glucose-dependent tissue: , brain cells, and the central nervous system. After carbohydrate reserves are depleted within a few hours, circulating are used to make glucose and . The second priority is to maintain muscle mass. Because little glucose can be made from triglycerides stored in the adipose tissue, the brain cells adapt so that they can use for fuel.

When glucose levels drop to baseline levels after several hours of fasting, the hormone stimulates the breakdown of liver glycogen to glucose. During the first few days of starvation glucogenic amino acids, especially alanine, furnish 90% of the brain’s glucose supply and provide(s) the remaining 10%. Eventually protein breakdown must slow down or the body will not survive for more than three weeks without food. Fat catabolism rates double to supply fatty acids for fuel and glycerol for glucose. The average person has of fat stores, then the body must turn back to protein for energy. Protein breakdown slows drastically and gluconeogenesis drops by two-thirds or more. A starving person can survive until of his or her proteins are broken down.

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