By Dr Ananya Mandal, MD
The energy for various functions of the human body comes from the nutrient molecules that have been metabolised. In fact, the main purpose of food intake is supply of energy. This energy comes from the fats, carbohydrates, and proteins in food. 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 from foods
Energy requirements are ordinarily expressed in terms of calories. This is actually a kilocalorie (kcal) that is defined as the amount of heat energy required to raise the temperature of one kilogram of water one degree Celsius.
Calories obtained by complete oxidation of various foods include:
- Carbohydrates yield 4 kcal/g.
- Carbohydrates have to be stored with water and each g of glycogen is hydrated with 2 g water. Hydrated carbohydrates: 1.3 kcal/g
- Proteins: 4 kcal/g
- Fat: 9 kcal/g (fats are not hydrated)
The energy requirement for a person is divided into two parts:
- Basal metabolic requirements
- Energy required for activity.
The 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 energy flow
The breakdown of complex organic molecules to yield simple molecules releases energy and the process is called catabolism.
Anabolism is the total biosynthetic processes where large complex molecules are made from small simple molecules. Anabolism requires energy that is provided by catabolic processes.
Overall, both processes of metabolism must occur concurrently because catabolism provides the energy necessary for anabolism.
While plants utilize energy from the sun in the photosynthetic process, animals and humans use the plants for food. They break down the larger and complex molecules produced by the plants to utilize them as energy sources. This maintains the flow of energy in the biosphere.
Uses of energy in cells
The body utilizes energy for a variety of functions. Energy is needed to carry out mechanical work which involves the change in location or orientation of a body part or the cell itself. This includes muscle movement. In addition, there is molecular transport and synthesis of biomolecules.
ATP of adenosine triphosphate is the energy currency in most animal cells. It carries chemical energy. In general, the energy to synthesize ATP molecules must be obtained from fuel molecules. The human body uses the three types of molecules to yield the necessary energy to drive ATP synthesis:
How is ATP synthesized?
ATP is synthesized in the mitochondria in the cells. Some of it is also synthesized in the cytoplasm. Lipids are broken down into fatty acids, proteins into amino acids, and carbohydrates into glucose.
This then undergoes a variety of oxidation-reduction reactions wherein the mitochondria degrade fatty acids, amino acids, and pyruvate. Pyruvate is the end product of glucose degradation in the cytoplasm. The final degradation leads to several intermediate compounds, as well as into the reduced electron carrier coenzymes NADH and FADH2. The intermediates enter the tricarboxylic acid (TCA) cycle or the citric acid cycle, also giving rise to NADH and FADH2.
These reduced electron carriers are themselves oxidized via the electron transport chain, with concomitant consumption of oxygen and ATP synthesis. This process is called the oxidative phosphorylation.
Each molecule of fatty acid releases over 100 molecules of ATP and each amino acid molecule releases almost forty ATP molecules. Two ATP molecules are synthesized in the cytoplasm via the conversion of glucose molecules to pyruvate.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)