Metabolism is the process in which matter from outside an organism is transformed into energy or material for the organism. In bacteria, metabolism takes place inside the cytoplasm and along the cell membrane. In higher organisms, metabolism takes place in the cell membrane and in the mitochondria. There are two types of metabolism in the body, these are the anabolic and the catabolic reactions.

The Catabolism is the reaction that breaks the bonds of complex molecules. In the body, these reactions maybe linked to enzyme catalysts allowing for storage of the released energy in the form of ATP or Adenosine Triphospate. Polymers yield energy when they are converted to intermediates. Anabolism are reactions which synthesize complex molecules from simpler molecules using ATP as an energy source. Simple molecules become polymers.

Adenosine Triphospate is the energy medium inside the cells of the body. It is the overall currency of the human body. It is needed by the body to produce energy. ATP is produced inside the cell in an organelle called mitochondria which is the "powerhouse" of the cell. In the mitochondrial inner membrane or the cristae, a process called oxidative phosporylation occurs and it yields adenosine triphospate. The production of ATP requires glucose. The process is a coupled reaction of an exergonic reaction which involves lossing an electron and endergonic reaction which involves gaining an electron. Electrons are transferred from one complex to another by a gradient controlled by the pH of the envioronment inside the cell and produces an end acceptor which is oxygen. This means that the elctrons are transferred from one complex to the next until it binds with oxygen. This energy yeilding process is what is called oxidative phosporylation.

Energy sources are oxidized to produce energy. Excess calories aare stored in the body. In nutrition, the normal human being is composed of 85% adipose tissue or fats, 14.5% protein and 0.5% carbohydrates. A starch made in the liver and muscle which is the first fuel used to power activity or work , is called the Glycogen. Glycogen is a carbohydrate.

Fat which is called triacylglycerol is the major energy store in the body. Adipose tissue is an efficient way to store energy because fat is twice as energy dense as carbohydrate or protein, and has less water associated with it. When excess calories are taken in, only limited amounts of carbohydrates and protein can be stored unless they are converted to fats which uses energy. Fats need little processing and easily stored; this is one reason why too much fat in the diet can be a problem. In the U.S., the total amount of fat in the diet is near 45% and the goal is to reduce the fat calories into 30% or more of total calories. There are two types of fats. The saturated and the unsaturated fats. The saturated fats are the unhealthy fats and increase the risk of cardiovascular diseases.

Proteins are metabolized as an energy source in healthy, well-nourished people but there are some turnovers due to repair and renewl of tissue. The biological value of proteins is highest compared to plants. This is why the lowest fat and inexpensive form of protein sources would be a combination of animal and plant derived foods. Protein would yield 5 kiloCalories if it were completely metabolized but this would produce compounds too toxic to the body. Instead, urea is the ultimate excretory form and still has some energy left in it which can be utilized by bacteria. Bacteria can metabolize urea to ammonia. Ethanol is metabolized in the liver. It inhibits gluconeogenesis which is the cause for liver damage in too much alcohol consumption.

The total energy requirement of the body is the sum of the Basal Metabolic Rate which is the energy required for the maintenance functions of the body; physical activity and Specific Dynamic Action. The amount of energy used secondary to the increase in the metabolic rate that occurs during the digestion and absorption of food. The thermic effect of food is about 1o% of the energy consumption.When at rest, the energy source utilized by the muscles is fat. When active, it utilizes carbohydrates. The brain solely utilizes carbohydrates which is glucose but in some conditions where glucose is not available, it utilizes the fatty acids.

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