Glucose + glucose → _____ by _____.
Glucose + glucose → maltose by dehydration synthesis (also called condensation). Two glucose monosaccharides bond through an α-1,4 glycosidic linkage, and one water molecule is removed to form the disaccharide maltose.
The answer
Fill in the blanks: glucose + glucose → maltose by dehydration synthesis (condensation reaction). When two glucose monomers join, a hydroxyl (-OH) group from one glucose and a hydrogen (-H) from the other are removed as a molecule of water (H₂O). The two sugars link through an oxygen bridge called an α-1,4 glycosidic bond. The product is maltose, a disaccharide also known as malt sugar.
The formula bookkeeping works out cleanly: glucose is C₆H₁₂O₆, so two of them contain C₁₂H₂₄O₁₂. Remove one water (H₂O), and you get C₁₂H₂₂O₁₁, which is exactly the molecular formula of maltose. That lost water is the signature of dehydration synthesis.
Why the reaction works this way
Monosaccharides are the building blocks (monomers) of carbohydrates. Cells assemble larger sugars by joining monomers, and the universal method for linking biological monomers, whether sugars, amino acids, or nucleotides, is dehydration synthesis: bonds form and water is released. The reverse process, hydrolysis, splits the bond by adding water back. So maltose + water → glucose + glucose is hydrolysis, and the enzyme maltase catalyzes it in your small intestine so the glucose can be absorbed.
The "α-1,4" label tells you the geometry: carbon 1 of the first glucose bonds to carbon 4 of the second, and the α orientation points the bond downward. This same α-1,4 linkage, repeated thousands of times, builds starch and glycogen, which is why maltose is a breakdown product when starch is digested.
Ruling out the common wrong answers
- Sucrose: Sucrose is glucose + fructose, not two glucose units. Different monomers, different bond (α-1,β-2), so it cannot be the product here.
- Lactose: Lactose is glucose + galactose, joined by a β-1,4 bond. Again, the second monomer is not glucose.
- Cellulose or starch: These are polysaccharides made of many glucose units, not two. Two glucose monomers give a disaccharide, not a polymer. Cellulose also uses β-1,4 bonds, which humans cannot digest.
- "By hydrolysis": Hydrolysis is the opposite direction. It breaks maltose down using water; it does not build it. Building the bond releases water, so the mechanism is dehydration synthesis.
The bigger picture
Maltose is a disaccharide (two sugar units), not a monosaccharide. Recognizing which two monomers combine, and by which bond, lets you identify any common sugar: glucose+glucose = maltose (α-1,4), glucose+fructose = sucrose, glucose+galactose = lactose. And every one of them forms by dehydration synthesis and breaks apart by enzyme-driven hydrolysis, a symmetry that runs through all of biochemistry.
| Maltose | Glucose + Glucose | α-1,4 | Maltase |
| Sucrose | Glucose + Fructose | α-1,β-2 | Sucrase |
| Lactose | Glucose + Galactose | β-1,4 | Lactase |
Frequently asked
What bond joins the two glucose units in maltose?
An α-1,4 glycosidic bond. Carbon 1 of the first glucose links through an oxygen atom to carbon 4 of the second glucose, in the alpha orientation. This is the same linkage found in starch and glycogen.
What is dehydration synthesis?
Dehydration synthesis (condensation) is a reaction that joins two monomers by removing a molecule of water. An -OH from one monomer and an -H from the other are released as H₂O while a new covalent bond forms. It builds all biological polymers.
What enzyme breaks maltose back into glucose?
Maltase, an enzyme in the small intestine, catalyzes the hydrolysis of maltose. It adds a water molecule to split the α-1,4 bond, releasing two free glucose molecules that the body can then absorb into the bloodstream.
Is maltose a monosaccharide or disaccharide?
Maltose is a disaccharide, meaning it is made of two sugar units, in this case two glucose molecules. Monosaccharides like glucose and fructose are single sugar units and are the monomers that combine to form disaccharides.
How is maltose different from sucrose and lactose?
All three are disaccharides, but they differ in their monomers. Maltose is glucose + glucose, sucrose is glucose + fructose, and lactose is glucose + galactose. They also use different glycosidic bonds and are broken down by different enzymes.