To understand the role energy plays in the body, it is important to understand the physiology of the main body systems involved in the process.

 

For all cells to perform their vital functions, energy; the capacity of a physical system to perform work,  is required. Energy is required for:

 

• Muscular activity and movement

• The circulation of  blood around the body

• The action of breathing to take in oxygen and remove carbon dioxide

• The production of new cells for growth and repair

• The transmission of nerve impulses, so we can respond to changes in the environment

• The building of complex molecules such as enzymes and hormones

 

There are many forms of energy, but chemical energy is the most common. It is stored in the bonds that unite atoms and molecules e.g. glucose, with each other. The energy stored in these bonds is known as potential energy.

 

Other forms of energy include:

• Light

• Thermal (heat)

• Sound

• Kinetic

• Electrical

• Magnetic

• Nuclear

• Gravitational

 

 

 

Metabolism

Energy is essential for all the biochemical reactions that take place in the body.

 

Metabolism refers to all of these biochemical reactions. 

 

There are two types of biochemical reactions that occur simultaneously and constantly in the body. These are anabolic and catabolic reactions

 

Anabolic Reactions

Anabolic reactions are concerned with growth and involve the joining of small, simple molecules to form larger, more complex molecules e.g.

 

• The joining of glucose molecules to form glycogen, which can be stored in the liver and muscle cells

• The joining of amino acids to form proteins, which can be used for growth and repair

• The joining of fatty acids to form triglycerides

 

Anabolic reactions utilise energy during this process.

 

 

 

 

 

 

 

 

 

 

 

 

Catabolic Reactions

Catabolic reactions are concerned with the release of energy. They involve the breaking apart of larger, complex molecules to form smaller, simple molecules. When the bonds, that unite atoms and molecules, are broken, energy is released.

 

Examples of catabolic reactions in the body include:

 

• The process of digestion, which involves the breakdown of large food molecules such as carbohydrates, fats, and proteins to form simple molecules such as glucose, fatty acids, and amino acids, which are then small enough to cross cell membranes 

• The breakdown of glucose during the process of cellular respiration to form ATP, the fuel that drives all functions in every living cell

 

Adenosine Triphosphate (ATP)

ATP is a complex molecule which is critical for all life. All cells run on ATP, and it is considered the primary energy currency of the cell. ATP consists of carbon, hydrogen, nitrogen, oxygen, and phosphorus, which are all joined together to form a complex molecule.

 

Phosphates are high-energy molecules, and when phosphorus is removed from ATP, comparatively high levels of energy is released. The energy released, when the bond is broken, is then used for many of the cell functions including:

 

• The transport of substances across cell membranes

• Supplying energy for muscle contractions

• Supplying energy for the synthesis of the many thousands  of molecules the cells need to exist

• The control of chemical reactions

• Supplying energy to send messages e.g. through the transmission of nerve impulses, and the release of hormones

 

ATP is produced by a complex series of biochemical reactions, consisting of many steps and many enzymes. It involves the controlled breakdown of foods such as carbohydrates and fats to release energy, which can then be used to make ATP.

 

This process is called cellular respiration.

 

 

 

 

 

 

 

 

 

 

 

 

Cellular Respiration

Cellular respiration involves biochemical reactions that take place in the cells to convert chemical energy released from nutrients into ATP, resulting in the waste products of carbon dioxide or lactic acid. There are two types of cellular respiration; aerobic and anaerobic. The human body relies mainly on the process of aerobic respiration to produce ATP because it is much more efficient and produces more ATP molecules. 

 

Aerobic respiration requires oxygen. However, if oxygen is in short supply e.g. in muscles during prolonged or vigorous physical exercise, the cells will produce ATP anaerobically. Anaerobic respiration is less efficient and produces much fewer ATP molecules.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Aerobic Respiration

Aerobic respiration is a catabolic reaction that involves the breaking apart of glucose, by removing the hydrogen atoms from the molecule. 

 

This is achieved by adding oxygen (oxidative reaction) for the hydrogen atoms to attach to; forming water. When the hydrogen atoms are removed from the glucose molecule, the bonds holding them together are broken; releasing energy which is then used to form ATP.

 

The remaining carbon and oxygen atoms, from the glucose molecule, form the waste product carbon dioxide.

 

 

Application of Energy Laws

According to the laws of thermodynamics, energy can neither be created or destroyed, but it can be converted from one form to another. Hence, about 40 % of the chemical energy released from the catabolic breakdown of glucose is used to make ATP, the remaining 60% of chemical energy is converted to heat energy.

 

Cellular respiration is, therefore, the main heat generator in the body and explains why the human body is so hot - 37 degrees centigrade (98.6 degrees Fahrenheit)!

 

Anaerobic Respiration

Anaerobic respiration involves the breakdown of glucose, without the use of oxygen. It occurs in humans during vigorous and prolonged exercise.

 

When muscle cells use up their supply of oxygen, they have to switch to an anaerobic pathway to process energy. This is a much less efficient method and produces much fewer ATP molecules.

 

The waste product of anaerobic respiration is lactic acid. This is what produces the muscle soreness after exercise. 

 

The Process of Cellular Respiration

The process of cellular respiration commences when the glucose enters the cell. The first step is the splitting in half of the glucose molecule (glycolysis), to produce pyruvic acid. This occurs in the cytoplasm and is an anaerobic process which results in the production of 2 ATP molecules.

 

If oxygen is available, the pyruvic acid enters the mitochondria of the cell. This is further broken down by a complex process called the Krebs Cycle, which involves the gradual removal of the hydrogen atoms. This is an aerobic process.

 

On completion of the process, a further 36 ATP molecules are produced along with water, carbon dioxide, and heat.

 

If oxygen is not available when the glucose enters the cell the pyruvic acid, produced from the original splitting of the glucose molecule, is converted to lactic acid by a process called fermentation. This is an anaerobic process that takes place in the cytoplasm of the cell and results in the production of a further 2 ATP molecules.

 

Once oxygen becomes available, the lactic acid can be converted back to pyruvic acid, which can then enter the mitochondria to complete the aerobic process.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Body Systems involved in Energy Metabolism

The three main body systems involved in energy metabolism are:

 

• The digestive system: Responsible for the catabolic breakdown of carbohydrates to glucose, so they are small enough to cross the cell membrane for use in cellular respiration

 

• The respiratory system: Responsible for providing the oxygen required for aerobic respiration; the more efficient process for producing ATP. Also responsible for the removal of the waste product carbon dioxide, produced during cellular respiration

 

• The cardiovascular system: Responsible for the transport of oxygen and glucose to the cells for cellular respiration, and the transport of the waste product carbon dioxide produced during cellular respiration, back to the respiratory system for removal from the body