Broadly speaking, you can divide metabolic pathways into two different categories: catabolic pathways, which break down larger molecules to release energy or provide starting materials for synthesis, and anabolic pathways, which synthesize larger molecules from smaller starting materials. Just like those of all other mammals, your body synthesizes glucose via an anabolic pathway called gluconeogenesis.
Energetics
Glucose has a much higher energy content than the smaller molecules of pyruvate your body uses to synthesize it. Just like all other anabolic pathways, glucose synthesis requires energy input. That energy is delivered in the form of ATP, the "energy currency" your cells spend to drive otherwise unfavorable reactions. Your body gets the energy to make ATP from glucose catabolism. Since no biochemical process can be 100 percent efficient, energy is lost as heat during both glycolysis and gluconeogenesis. Using energy from glucose to synthesize glucose entails a net loss of energy.
Uses
Synthesizing glucose is wasteful, but your body still has a couple compelling reasons to do it. Your brain subsists mostly on glucose -- in fact, it consumes some 120 g each day. During periods of strenuous exercise, your muscle fibers release lactate, a potential source of energy your brain cells cannot directly use. Consequently, your liver converts lactate back into glucose, which can serve as energy supply for the brain and other parts of the body. Various amino acids from dietary protein are also employed in glucose synthesis for much the same reason.
Process
Many of the steps in gluconeogenesis are the same as those in glycolysis, the pathway for glucose catabolism. The two pathways differ at three key points. Three of the steps in glycolysis are highly exergonic, meaning they release a considerable amount of energy, and they are essentially irreversible. In gluconeogenesis, the cell bypasses these three steps through reactions that require expenditure of energy. The first of these is the synthesis of phosphoenolpyruvate from pyruvate; the second is the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, while the third and final is the conversion of glucose 6-phosphate to glucose.
Plants
Glucose anabolism in plants is another critically important process, because humans ultimately depend on sugar synthesized by plants. Plants harness light to make ATP and then spend this ATP on the manufacture of three carbon sugars called triose phosphates. These triose phosphates are exported from the chloroplast, the organelle where photosynthesis takes place, and converted into fructose and glucose. The plant then links fructose and glucose together to make sucrose, the sweet-tasting compound humans call table sugar. Cells in plant leaves export sucrose for use by cells elsewhere in the plant.
References
- "Lehninger Principles of Biochemistry"; David L. Nelson and Michael M. Cox; 2008
- "Essential Cell Biology"; Bruce Alberts, et al.; 2004
