Living organisms require energy to survive. Even at rest, the body needs energy for life functions such as breathing, blood circulation, hormone regulation and cellular growth and repair. Living cells are able to harvest energy from food molecules through cellular chemical processes that produce high-energy adenosine triphosphate (ATP) molecules. When ATP production occurs in the presence of oxygen, it is called aerobic respiration, and according to the BBC Higher Bitesize educational website, this process happens in small compartments in the cell called mitochondria.
Food and energy
The sun supplies the energy necessary to sustain life. Through a process called photosynthesis, plants convert the sun’s thermal energy into chemical energy and store it in the form of carbohydrate molecules. When carbohydrates are consumed in the diet, they are broken down into glucose, a simple sugar that is the major energy source for most of the body’s cells. Through aerobic respiration, the body’s cells harvest the energy stored in glucose molecules and package it into the more usable energy form of ATP.
The body stores chemical energy from food in the chemical bonds of ATP molecules. ATP consists of a 5-carbon sugar, a nitrogen-containing base and three phosphate groups. According to Dr. Mike Farabee, professor of biology at Estrella Mountain Community College in the USA, most of the energy supplied by ATP is locked in the bond between the second and third phosphate groups. When the third phosphate group is removed from ATP, energy is released and used by the cell to do work. The remaining adenosine diphosphate (ADP) molecule is reused to produce more ATP.
During aerobic respiration, glucose and oxygen react to produce carbon dioxide, water and energy. The process occurs in three stages: glycolysis, the citric acid cycle and electron transport phosphorylation. Glycolysis does not require oxygen and occurs in the liquid part of the cell, or cytoplasm. It produces a net gain of two ATP per molecule of glucose. The citric acid cycle and electron transport phosphorylation take place in specialized membrane-bound structures within the cell called mitochondria. The citric acid cycle generates two ATP, and energy-rich NADH and FADH2 molecules. Electron transport phosphorylation uses the energy from the eight NADH and two FADH2 molecules produced during the citric acid cycle to produce an additional 32 ATP. The three stages of aerobic respiration collectively harvest 36 ATP per glucose molecule.
Aerobic respiration benefits
All cells are capable of producing ATP without oxygen through glycolysis, but the process generates only two ATP per glucose molecule and is not very efficient. According to Dr. Stan Eisen, Director of Pre-professional Health Programs at Christian Brothers University in the USA, aerobic respiration is approximately 37 percent efficient, meaning that 37 percent of the free energy originally in glucose gets stored as ATP. Fermentation, a process that complements glycolysis and permits continuous ATP production in the absence of oxygen, is approximately 2 percent efficient.
Additional energy sources
The body uses glucose as its primary energy source, and if all of the energy in a meal is not used, some of it is stored as a substance called glycogen, and the remainder is converted to fat and stored in body tissues. Hours after a meal, the body uses glycogen and eventually fat for energy. When glycogen and fat stores are depleted, as occurs in starvation, protein, such as that found in the muscles, is broken down to supply the energy required to sustain basic life functions.
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