You have probably understood since you were young that the food you eat has to become "something" far smaller than that food for whatever is "in" food to be able to help your body. As it happens, more specifically, a single molecule of a type of carbohydrate classified as a sugar is the ultimate source of fuel in any metabolic reaction occurring in any cell at any time.
That molecule is glucose, a six-carbon molecule in the form of a spiky ring. In all cells, it enters into glycolysis, and in more complex cells it also participates in fermentation, photosynthesis and cellular respiration to varying degrees in different organisms.
But a different way of answering the question "Which molecule is used by cells as an energy source?" is interpreting it as, "What molecule directly powers the cell's own processes?"
Nutrients vs. Fuels
That "powering" molecule, which like glucose is active in all cells, is ATP, or adenosine triphosphate, a nucleotide often called "the energy currency of cells." Which molecule should you think of, then, when you ask yourself, "What molecule is the fuel for all cells?" Is it glucose or ATP?
Answering this question is similar to understanding the difference between saying "Humans get fossil fuels from the ground" and "Humans get fossil fuel energy from coal-powered plants." Both statements are true, but address different stages in the energy-conversion chain of metabolic reactions. In living things, glucose is the fundamental nutrient, but ATP is the basic fuel.
Prokaryotic Cells vs. Eukaryotic Cells
All living things belong to one of two broad categories: prokaryotes and eukaryotes. Prokaryotes are the single-celled organisms of the taxonomic domains Bacteria and Archaea, whereas eukaryotes all fall into the domain Eukaryota, which includes animals, plants, fungi and protists.
Prokaryotes are tiny and simple compared to eukaryotes; their cells are correspondingly less complex. In most cases, a prokaryotic cell is the same thing as a prokaryotic organism, and the energy needs of a bacteria are far lower than those of any eukaryotic cell.
Prokaryotic cells have the same four components found in all cells in the natural world: DNA, a cell membrane, cytoplasm and ribosomes. Their cytoplasm contains all of the enzymes needed for glycolysis, but the absence of mitochondria and chloroplasts means that glycolysis is really the only metabolic pathway available to prokaryotes.
What is Glucose?
Glucose is a six-carbon sugar in the form of a ring, represented in diagrams by a hexagonal shape. Its chemical formula is C6H12O6, giving it a C/H/O ratio of 1:2:1; this is true, in fact, or all biomolecules classified as carbohydrates.
Glucose is considered a monosaccharide, meaning that it cannot be reduced into different, smaller sugars by breaking hydrogen bonds between different components. Fructose is another monosaccharide; sucrose (table sugar), which is made by joining glucose and fructose, is considered a disaccharide.
Glucose is also called "blood sugar," because it is this compound whose concentration is measured in the blood when a clinic or hospital lab is determining a patient's metabolic status. It can be infused directly into the blood stream in intravenous solutions because it requires no breakdown before entering body cells.
What is ATP?
ATP is a nucleotide, meaning that it consists of one of five different nitrogenous bases, a five-carbon sugar called ribose and one to three phosphate groups. The bases in nucleotides can be either adenine (A), cytosine (C), guanine (G), thymine (T) or uracil (U). Nucleotides are the building blocks of the nucleic acids DNA and RNA; A, C and G are found in both nucleic acids, whereas T is found only in DNA and U only in RNA.
The "TP" in ATP, as you have seen, stands for "triphosphate" and indicates that ATP has the maximum number of phosphate group that a nucleotide can have – three. Most ATP is made by the attachment of a phosphate group to ADP, or adenosine diphosphate, a process known as phosphorylation.
ATP and its derivatives have a broad range of applications in biochemistry and medicine, many of which are in the exploratory stages as the 21st century approaches its third decade.
Cell Energy Biology
The release of energy from food involves breaking the chemical bonds in food components and harnessing this energy for the synthesis of ATP molecules. For example, carbohydrates are all oxidized in the end to carbon dioxide (CO2) and water (H2O). Fats are also oxidized, with their fatty acid chains yielding acetate molecules which then enter aerobic respiration in eukaryotic mitochondria.
The breakdown products of proteins are rich in nitrogen and are used for the building of other proteins and nucleic acids. But some of the 20 amino acids that proteins are built from can be modified and enter cellular metabolism at the level of cellular respiration (e.g., after glycolysis)
Summary: Glycolysis directly produces 2 ATP for every molecule of glucose; it supplies pyruvate and electron carriers for further metabolic processes.
Glycolysis is a series of ten reactions in which a molecule of glucose is transformed into two molecules of the three-carbon molecule pyruvate, yielding 2 ATP along the way. It consists of an early "investment" phase in which 2 ATP are used to attach phosphate groups to the shifting glucose molecule, and a later "return" phase in which the glucose derivative, having been split into a pair of three-carbon intermediate compounds, yields 2 ATP per three-carbon compounds and this 4 overall.
This means that the net effect of glycolysis is to produce 2 ATP per glucose molecule, as 2 ATP are consumed in the investment phase but a total of 4 ATP are made in the payoff phase.
Summary: Fermentation replenishes NAD+ for glycolysis; it produces no ATP directly.
When insufficient oxygen is present to satisfy energy demands, as when you are running very hard or lifting weights strenuously, glycolysis may be the only metabolic process available. This is where the "lactic acid burn" you may have heard about comes in. If pyruvate cannot enter aerobic respiration as described below, it is converted into lactate, which itself doesn't do a lot of good but ensures that glycolysis can continue by supplying a key intermediate molecule called NAD+.
Summary: The Krebs cycle produces 1 ATP per turn of the cycle (and thus 2 ATP per glucose "upstream," since 2 pyruvate can make 2 acetyl CoA).
Under normal conditions of adequate oxygen almost all of the pyruvate generated in glycolysis in eukaryotes moves from the cytoplasm into organelles ("little organs") known as mitochondria, where it converted into the two-carbon molecule acetyl coenzyme A (acetyl CoA) by stripping off and releasing CO2. This molecule combines with a four-carbon molecule called oxaloacetate to create citrate, the first step in what is also called the TCA cycle or the citric-acid cycle.
This "wheel" of reactions eventually reduced the citrate back to oxaloacetate, and along the way a single ATP is generated along with four so-called high-energy electron carriers (NADH and FADH2).
Electron Transport Chain
Summary: The electron transport chain yields about 32 to 34 ATP per "upstream" glucose molecule, making it by far the largest contributor to cellular energy in eukaryotes.
The electron carriers from the Krebs cycle move from the inside of mitochondria to the organelle's inner membrane, which has all sorts of specialized enzymes called cytochromes ready to work. In short, when the electrons, in the form of hydrogen atoms, are taken off their carriers, this powers the phosphorylation of ADP molecules into a great deal of ATP.
Oxygen must be present as the final electron acceptor in the cascade occurring across the membrane for this chain of reactions to occur. If it is not, the process of cellular respiration "backs up," and the Krebs cycle cannot occur, either.
- BC Open Text Biology: Comparing Prokaryotic and Eukaryotic Cells
- Molecular Biology of the Cell (4th Edition): How Cells Obtain Energy from Food
- European Bioinformatics Institute: Enzymes of Glycolysis
- Georgia State University Hyperphysics: Cellular Respiration
- CK-12 FlexBook 2.0 Biology: Glucose and ATP