Glucose is the ultimate source of cellular fuel for all living things, with the energy in its chemical bonds being used to synthesize adenosine triphosphate (ATP) in various interconnected and interdependent ways. When a molecule of this six-carbon (i.e., hexose) sugar crosses the plasma membrane of a cell from the outside to enter the cytoplasm, it is immediately phosphorylated – that is, a phosphate group, which carries a negative electrical charge, is attached to part of the glucose molecule. This results in a net negative charge on what has then become a glucose-6-phosphate molecule, which prevents it from leaving the cell.
Prokaryotes, which include the Bacteria and Archaea domains, do not have membrane-bound organelles, including the mitochondria that in eukaryotes host the Krebs cycle and the oxygen-dependent electron transport chain. As a result, prokaryotes do not participate in aerobic ("with oxygen") respiration, instead deriving almost all of their energy from glycolysis, the anaerobic process that also operates in advance of the aerobic respiration carried out in eukaryotic cells.
As glucose is among the most vital molecules in biochemistry, and is the starting point of perhaps the most vital set of reactions in the annals of life on planet Earth, a brief discussion of the structure and behavior of this molecule is in order.
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Also known as dextrose (usually in reference to non-biological systems, such as glucose made from corn) and blood sugar (in reference to biological systems, e.g., in medical contexts), glucose is a six-carbon molecule with the chemical formula C6H12O6. In human blood, the normal concentration of glucose is about 100 mg/dL. 100 mg is a tenth of a gram, while a dL is one-tenth of a liter; this works out to a gram per liter, and since the average person has about 4 liters of blood, most people have about 4 g of glucose in their bloodstream at any time – only about one-seventh of an ounce.
Five of the six carbon (C) atoms in glucose sit in the six-atom ring form that the molecule assumes 99.98 percent of the time in nature. The sixth ring atom is an oxygen (O), with the sixth C attached to one of the ring Cs as part of a hydroxymethyl (-CH2OH) group. It is at the hydroxyl (-OH) group that inorganic phosphate (Pi) is attached during the phosphorylation process that traps the molecule in the cell cytoplasm.
Glucose, Cell Types and Metabolism
Prokaryotes are small (the overwhelming majority are unicellular) and simple (the one cell most of them do have lacks a nucleus and other membrane-bound organelles). This may keep them from being as elegant and interesting in most ways as eukaryotes, but it also keeps their fuel requirements comparatively low.
In both prokaryotes and eukaryotes, glycolysis is the first step in the metabolism of glucose. The phosphorylation of glucose upon its entering a cell by diffusing across the plasma membrane is the first step in glycolysis, which is described in detail in a subsequent section.
- Some bacteria can metabolize sugars other than, or in addition to, glucose, such as sucrose, lactose or maltose. These sugars are disaccharides, which comes from the Greek for "two sugars." They include a monomer of glucose, like fructose, a monosaccharide, as one of their two subunits.
At the end of glycolysis, the glucose molecule has been used to generate two three-carbon pyruvate molecules, two molecules of the so-called high-energy electron carrier nicotinamide adenine dinucleotide (NADH), and a net gain of two ATP molecules.
At this point, in prokaryotes, the pyruvate usually enters fermentation, an anaerobic process with a number of different variations that will be explored shortly. But some bacteria have evolved the ability to carry out aerobic respiration to some extent and are called facultative anaerobes. Bacteria that can derive energy only from glycolysis are called obligate anaerobes, and many of these are actually killed by oxygen. A limited few bacteria are even obligate aerobes, meaning that, like you, they have an absolute requirement for oxygen. Given that bacteria have had about 3.5 billion years to adapt to the demands of the Earth's shifting environment, it should not be surprising that they have commanded a range of basic metabolic survival strategies.
The Process of Glycolysis
Glycolysis includes 10 reactions, which is a nice, round number, but you don't necessarily need to memorize all of the products, intermediates and enzymes in all of these steps. Instead, while some of this minutiae is fun and useful to know, it's more important to gain a sense of what happens in glycolysis overall, and why it happens (in terms of both basic physics and the needs of the cell).
Glycolysis is captured in the following reaction, which is the sum of its 10 individual reactions:
C6H12O6 → 2 C3H4O3 + 2 ATP + 2 NADH
In plain English, in glycolysis, a single glucose molecule is broken apart into two pyruvate molecules, and along the way, a couple of fuel molecules and a pair of "pre-fuel" molecules are made. ATP is the near-universal currency for energy in cellular processes, whereas NADH, the reduced form of NAD+ or nicotinamide adenine dinucleotide, functions as a high-energy electron carrier that ultimately donates those electrons, in the form of hydrogen ions (H+), to oxygen molecules at the end of the electron transport chain in aerobic metabolism, resulting in a great deal more ATP than glycolysis alone can supply.
The phosphorylation of glucose after its entry into the cytoplasm results in glucose-6-phosphate (G-6-P). The phosphate comes from ATP and its incorporation into glucose leaves adenosine diphosphate (ADP) behind. As noted, this traps glucose within the cell.
Next, G-6-P is converted to fructose-6-phosphate (F-6-P). This is an isomerization reaction, because the reactant and the product are isomers of each other – molecules with the same number of each type of atom, but with different spatial arrangements. In this case, the ring of fructose only has five atoms. The enzyme responsible for this atomic juggling act of sorts is called phosphoglucose isomerase. (Most enzyme names, while often cumbersome, at least make perfect sense.)
In the third reaction of glycolysis, F-6-P is converted to fructose-1,6-bisphosphate (F-1,6-BP). In this phosphorylation step, the phosphate again comes from ATP, but this time it is added to a different carbon atom. The enzyme responsible is phosphofructokinase (PFK).
- In many phosphorylation reactions, phosphate groups are added to the free end of an existing phosphate group, but not in this case – hence "_bis_phosphate" rather than "_di_phosphate."
In the fourth reaction of of glycolysis, the F-1,6-BP molecule, which is quite unstable owing to its double dose of phosphate groups, is split by the enzyme aldolase into the three-carbon, single-phosphate-group-carrying molecules glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). These are isomers, and the DHAP is rapidly converted to GAP in the fifth step of glycolysis using a push from the enzyme triose phosphate isomerase (TIM).
At this stage, the original glucose molecule has become two identical three-carbon, singly phosphorylated molecules, at the cost of two ATP. From this point forward, each described reaction of glycolysis occurs twice for every glucose molecule undergoing glycolysis.
In the sixth reaction of glycolysis, GAP is converted to 1,3-bisphosphoglycerate (1,3-BPG) under the influence of glyceraldehyde 3-phosphate dehydrogenase. Dehydrogenase enzymes remove hydrogen atoms (i.e., protons). The hydrogen freed from GAP becomes attached to the NAD+ molecule, yielding NADH. Because the initial molecule of glucose upstream has given rise to two molecules of GAP, after this reaction, two molecules of NADH have been created.
In the seventh glycolysis reaction, one of the phosphorylation reactions of early glycolysis is, in effect, reversed. When the enzyme phosphoglycerate kinase removes a phosphate group from 1,3-BPG, the result is 3-phosphoglycerate (3-PG). The phosphates that have been stripped from the two 1,3-BPG molecules are appended to an ADP to form two ATP. This means that the two ATP "borrowed" in steps one and three are "returned" in the seventh reaction.
In step eight, 3-PG is converted to 2-phosphoglycerate (2-PG) by phosphoglycerate mutase, which shuttles the one remaining phosphate group to a different carbon atom. A mutase differs from an isomerase in that it is less heavy-handed in its action; rather than rearranging the structure of a molecule, they merely shift one of its side groups to a new spot, leaving the overall backbone, ring, etc. as it was.
In the ninth reaction of glycolysis, 2-PG is converted to phosphoenolpyruvate (PEP) under the action of enolase. An enol is a compound with a carbon-carbon double bond in which one of the carbons is also bound to a hydroxyl group.
Finally, the tenth and last reaction of glycolysis, PEP is transformed into pyruvate thanks to the enzyme pyruvate kinase. The phosphate groups removed from the two PEP are attached to ADP molecules, yielding two ATP and two pyruvate, the formula of which is ( C3H4O3) or (CH3)CO(COOH). Thus the initial, anaerobic processing of a single molecule of glucose yields two pyruvate, two ATP and two NADH molecules.
The pyruvate ultimately generated by the entry of glucose into cells can take one of two paths. If the cell is prokaryotic, or if the cell is eukaryotic but temporarily requires more fuel than aerobic respiration alone can provide (as in, for example, muscle cells during hard physical exercise such as sprinting or lifting weights), pyruvate enters the fermentation path. If the cell is eukaryotic and its energy requirements are typical, it moves the pyruvate inside of mitochondria and takes part in the Krebs cycle:
- Fermentation: Fermentation is often used interchangeably with "anaerobic respiration," but in truth this is is misleading because glycolysis, which precedes fermentation, is also anaerobic, though it is not generally considered part of respiration per se.
- Fermentation regenerates NAD+ for use in glycolysis by converting pyruvate to lactate. The entire purpose of this is to allow glycolysis to continue in the absence of adequate oxygen; a shortage of NAD+ locally would limit the process even when adequate amounts of substrate are present.
- Aerobic respiration: This includes the Krebs cycle and the electron transport chain.
- The Krebs cycle: Here, pyruvate is converted to acetyl coenzyme A (acetyl CoA) and carbon dioxide (CO2). The two-carbon acetyl CoA combines with the four-carbon oxaloacetate to form citrate, a six-carbon molecule that then proceeds through a "wheel" (cycle) of six reactions that result in two CO2, one ATP, three NADH and one reduced flavin adenine dinucleotide (FADH2).
- The Electron transport chain: Here, the protons (H+ atoms) of NADH and FADH_2_ from the Krebs cycle are used to create an electrochemical gradient that drives the synthesis of 34 (or so) molecules of ATP on the inner mitochondrial membrane. Oxygen serves as the final acceptor of the electrons that "spill" from one compound to the next, starting all the way up the chain of compounds with glucose.