Aerobic respiration is one of the most common processes found across a wide array of biological life. The primary definition is that aerobic respiration requires oxygen, whereas anaerobic respiration does not. Humans have a complex aerobic respiration process that requires oxygen to feed cells, the basic unit of life on Earth.
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The Basic Units of Life
The term "cell" was first used in 1665 by Robert Hooke. Since then, the cell has revolutionised our understanding of biology. When we eat, we are really feeding the cell, which is one of the insights of cell theory developed in 1839. At the atomic level, life is primarily composed of carbon. Most molecules important to life are composed of a carbon backbone plus oxygen and hydrogen. Phosphate and nitrogen are also sometimes integral.
The Beginning of Respiration
Once digestion is complete, usable energy enters the cell in the form of glucose, a simple sugar (a carbohydrate) that is commonly found in nature. During the metabolic process, glucose and oxygen form the main reactants. The byproducts are carbon dioxide, water and ATP—the main energy carrier that is used throughout the body to sustain its basic functions. Respiration begins within the cell’s cytoplasm, which is the fluid-filled area between the wall-like membrane and the central nucleus. From there, the process transfers into the mitochondrion, which is kind of like the power station in most complex cells. Prokaryotic cells, which are simpler and tend to be endemic to single-celled organisms, don't have a mitochondrion and are thus anaerobic.
In the cytoplasm, the glucose undergoes a series of reactions called glycolysis. This requires an initial investment of two ATP, which breaks the glucose molecule into two separate molecules called PGAL. PGAL then undergo a series of reactions that begins a steady process of rearrangement. Both molecules lose two electrons and a hydrogen atom, which converts NAD+ to NADH. As the molecules undergo further change, two ATPs are produced, and water is given off. The end result of glycolysis is two molecules of pyruvate.
The Krebs Cycle
As respiration enters the mitochondrion, there are two separate processes. The first is called the Krebs Cycle (also known as the citric acid cycle). Before the cycle begins, the two pyruvate molecules are stripped of a carbon atom, which releases carbon dioxide. The molecules enter the cycle as acetyl-CoA and undergo constant rearrangement to become different molecules. Eventually, the process produces four molecules of carbon dioxide, six molecules of NADH, two molecules of FADH2, and two molecules of ATP. Oxaloacetate, a molecule with four carbons, is the final outcome of the pyruvate.
Electron Transfer Chain
The final step is the electron transfer chain. Electrons and hydrogen are taken from the NADH and FADH2 that formed in the Krebs Cycle. Electrons that are transferred through a series of chains galvanise positive hydrogen ions (atoms with an electric charge) to move back and forth across the mitochondrion membrane, which drives ATP synthesis. Thirty-two ATP are produced. The remaining electrons bind with oxygen to produce water. If oxygen is not present, then electrons become backed up, stopping the process in its tracks.
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