Metabolism/Energy Overview Glycolysis Citric Acid Cycle  Elmhurst College
Cell Structure Energy Storage - ATP FAD, CoQ, CoA  Chemistry Department
Energy Overview NAD+ Electron Transport  Virtual ChemBook

Energy Values of Foods

 Food Class

 Energy in Calories / Gram







Energy for the Human Body


Supplying enough energy to support the many functions of the body at work and play is one of the chief functions of food. This energy comes from the fats, carbohydrates, and proteins in the food you eat. Of the three, fat is the most concentrated source of energy because it furnishes more than twice as much energy for a given weight as protein or carbohydrate.

Energy requirements are ordinarily expressed in terms of calories. The calorie used in nutritional discussions is actually the "large calorie" - Calorie. This is really a kilocalorie - the amount of heat energy required to raise the temperature of one kilogram (about 1 quart) of water one degree Celsius.

The energy requirement for a person is divided into two parts: basal metabolic requirements and energy required for activity. Basal metabolic rate (BMR) is the heat eliminated from the body at rest when temperature is normal. An average person requires 2000-2400 Calories per day while a large man doing heavy work may require up to 6000 Calories per day.

The three main classes of food provide the following amount of energy: carbohydrates and proteins provide about 4 Calories per gram; fats provide about 9 Calories per gram. When you choose foods that furnish more energy, or Calories than you need, the excess energy is stored as fat in the body -- a gain in weight may result. If too little food is eaten to meet energy demands, the body's stored fat serves as an energy source -- a weight loss will result. Your weight stays about the same if the energy from food matches the energy requirements of the body.

Flow of Energy in the Biosphere:

All of the chemical processes of the cell are called metabolism. The breakdown or degradation of complex organic molecules to yield simple molecules and energy is called catabolism. Anabolism is the total biosynthetic processes where large complex molecules are made from small simple molecules. Anabolic processes require energy because order is being created and thus work must be done. Overall, both processes of metabolism must occur concurrently because catabolism provides the energy necessary for anabolism.

Plants utilize energy from the sun in the photosynthetic process to synthesize larger molecules from smaller ones. On the other hand, animals and humans use the plants for food. The larger molecules are catabolized to provide energy.

If you recall, the definition for an exothermic reaction is one where energy is given off. An endothermic reaction is where energy is required.

Quiz: Identify the processes (plants) and (people) as catabolism or anabolism.
Identify the processes (sun) and (plants) as endothermic or exothermic.  

Uses of Energy in the Cells

The body and cells need a constant supply of energy for a variety of reasons. Energy is needed to carry out mechanical work which involves the change in location or orientation of a body part or the cell itself. A major example is the energy required for the contraction of muscles.

Molecular transport also requires energy. The movement of molecules from an area of low concentration to an area of higher concentration requires energy since this is opposite to the normal movement of molecules. This process is also called active transport. Examples include the movement of nutrient raw materials into a cell and the movement of waste materials out of the cell.

Electrical work is also included under molecular transport since the establishment of a differential concentration of ions across a membrane is used to build up an electrical charge. The result of electrical work is the excitation and conduction of impulses in nerve and muscle cells.

Finally, energy is needed for the synthesis of new complex biochemical molecules. Biosynthesis involves the formation of many new molecules from simpler molecules. New cellular material is produced not only during active periods of growth but also in existing structures to repair and replace damaged molecules.

Quiz: For the uses of energy is this endothermic or exothermic?