Cells require chemical energy to function. Energy cannot be recycled, so organisms require a constant input of energy, which is why humans need to eat. The process that requires oxygen and converts food into chemical energy (in the form of ATP) is cellular respiration. Cellular respiration can be summarized by:
The general order of the processes is:
Realize that in general, the outputs of one process are the inputs of following processes. Knowing the inputs and outputs of each step is key to understanding cellular respiration!
Cellular Respiration: Process in which cells break down glucose and make ATP for energy.
Glycolysis: First stage of cellular respiration in which glucose is split, in the absence of oxygen, to form two molecules of pyruvate (pyruvic acid) and two (net) molecules of ATP.
Glucose: Simple carbohydrate with the chemical formula C₆H₁₂O₆ that is the nearly universal food for life.
Pyruvic Acid (aka pyruvate): A 3-carbon molecule that results from the splitting of glucose
ATP (adenine triphosphate): Energy-carrying molecule that cells use to power their metabolic processes.
ADP (adenine diphosphate): The molecule that results from dephosphorylation (a phosphate group is removed).
Krebs Cycle: Second stage of aerobic respiration in which two pyruvate (pyruvic acid) molecules from the first stage react to form ATP, NADH, and FADH₂.
NADH: Molecule that acts as an electron carrier in cellular respiration.
Electron Transport Chain (ETC): Series of electron-transport molecules that pass high-energy electrons from molecule to molecule and capture their energy
Anaerobic Respiration: Type of cellular respiration that does not require oxygen.
Aerobic Respiration: Type of cellular respiration that requires oxygen.
Mitochondrion (plural, mitochondria): Organelle in eukaryotic cells that makes energy available to the cell in the form of ATP molecules.
Fermentation: A type of anaerobic respiration that allows ATP to be made through glycolysis.
The first step in cellular respiration is glycolysis, an anaerobic process. Glycolysis occurs in several steps to split glucose and make pyruvate, ATP, and NADH in the cytosol of the cytoplasm. Pyruvate is used for the Krebs cycle while NADH is collected for the electron transport chain.
The next two stages of cellular respiration are aerobic processes that occur in the mitochondria.
Before the Krebs cycle, pyruvate is combined with an enzyme called CoA (Co-enzyme A) to form acetyl-CoA. Acetyl-CoA enters the Krebs cycle and combines with a four-carbon molecule to form citric acid.
A series of reactions releases energy from the carbon compounds, which is captured in molecules of FADH₂, NADH, and ATP.
Glycolysis producese two pyruvic acids, so it takes two turns through the Krebs cycle to completely breakdown the original glucose.
In the electron transport chain (ETC), FADH₂ and NADH molecules from the Krebs cycle are oxidized. The oxidation of the electron carriers (NADH and FADH₂) produces H⁺ ions and electrons.
ETC is an aerobic process, as it requires oxygen molecules to attract and combine with the electrons at the end of the ETC to make water.
Cellular respiration is very complex as it involves several reactions occurring. Because of its complexity, inefficiencies in some processes, such as the ETC and ATP synthase, often lead to less than 38 molecules of ATP produced by the end of cellular respiration (around 28 or 29 molecules).
While cellular respiration does require oxygen as a whole, glycolysis is an anaerobic process. Glycolysis is a part of both aerobic cellular respiration as well as anaerobic fermentation. The difference between the two processes is what happens to the pyruvate it makes afterwards!
Fermentation does not produce ATP, but it re-generates NAD⁺, which keeps the process of glycolysis moving. Glycolysis produces ATP, so fermentation allows for constant production of ATP. There are two types of fermentations: