Glucose

Glucose is a six-carbon sugar with the formula C6H12O6 that serves as the central fuel of cellular metabolism and the main product of photosynthesis. It is the most abundant monosaccharide on Earth, the building block of starch, glycogen, and cellulose, and the molecule whose blood concentration the human body regulates most tightly.

Nearly every organism, from bacteria to humans, can break glucose down for energy, which makes its metabolism one of the oldest and most conserved sets of reactions in biology.

Glucose is a carbohydrate of the aldohexose class: a six-carbon chain with an aldehyde group at carbon 1 and hydroxyl groups on the other carbons. Its molar mass is about 180.16 g/mol. The biologically important form is D-glucose (also called dextrose).

In water, glucose exists almost entirely as a six-membered ring (a pyranose), formed when the hydroxyl group on carbon 5 attacks the aldehyde carbon. Ring closure creates two forms, or anomers, that differ in the orientation of the new hydroxyl group at carbon 1:

• β-D-glucopyranose, about 64% at equilibrium.
• α-D-glucopyranose, about 36%.
• The open-chain form is less than 1%, but it is the reactive form that lets glucose act as a reducing sugar.

The difference between α and β linkages has enormous consequences. Starch and glycogen are chains of glucose joined by α-1,4 bonds, which animals readily digest. Cellulose uses β-1,4 bonds, which produce straight, tightly packed fibers that most animals cannot break down without help from microbes.

Plants, algae, and cyanobacteria build glucose from carbon dioxide and water using light energy. In net form:

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

The enzyme RuBisCO fixes carbon dioxide in the Calvin cycle inside the chloroplast. The resulting three-carbon sugars are assembled into glucose, then stored as starch or exported as sucrose. Animals can also make glucose from non-carbohydrate sources such as lactate, glycerol, and some amino acids through gluconeogenesis, mainly in the liver.

Cells harvest glucose's chemical energy in stages:

1. Glycolysis: In the cytoplasm, one glucose is split into two pyruvate molecules, giving a net yield of 2 ATP and 2 NADH. Glycolysis needs no oxygen.
2. Citric acid cycle: In the mitochondria, pyruvate is converted to acetyl-CoA and fully oxidized to carbon dioxide, loading electron carriers (NADH and FADH₂).
3. Oxidative phosphorylation: Electrons pass down the electron transport chain to oxygen, pumping protons that drive ATP synthase.

Complete aerobic oxidation of one glucose molecule yields roughly 30–32 ATP by current estimates (older textbooks cite 36–38). Without oxygen, cells regenerate NAD⁺ by fermentation, producing lactate in muscle or ethanol and carbon dioxide in yeast, and get only the 2 ATP from glycolysis. The overall process is described in cellular respiration.

Human blood glucose is held within a narrow range, roughly 4–6 mmol/L (about 70–100 mg/dL) when fasting. The brain depends heavily on glucose and uses about 120 grams a day, around a fifth of the body's resting energy use.

Two pancreatic hormones keep the level stable through homeostasis:

• Insulin, from β-cells, is released when glucose rises. It promotes glucose uptake into muscle and fat (by moving GLUT4 transporters to the cell surface) and storage as glycogen.
• Glucagon, from α-cells, is released when glucose falls. It stimulates glycogen breakdown and gluconeogenesis in the liver.

In diabetes mellitus, this control fails, either through autoimmune destruction of β-cells (type 1) or through insulin resistance combined with impaired insulin secretion (type 2). Chronically high blood glucose damages blood vessels, nerves, kidneys, and eyes.

Through photosynthesis, glucose links sunlight to almost every food web on Earth. Its carbon is cycled back to the atmosphere by respiration and decomposition, making glucose metabolism the biochemical core of the carbon cycle. The oxygen released when glucose is made, and consumed when it is burned, ties the same reactions to the oxygen content of the atmosphere.