A Level Biology Practice Exam

Which type of sugars are known as reducing sugars?

1. Sugars that do not participate in chemical reactions

2. Sugars that donate electrons to other molecules

3. Sugars that can only be absorbed by the small intestine

4. Sugars that contain more than one carbohydrate unit

The correct answer is: Sugars that donate electrons to other molecules

The correct answer highlights that reducing sugars are those that have the capability to donate electrons to other molecules during chemical reactions. This property arises from the presence of a free aldehyde or ketone group, which can easily undergo oxidation. Common examples of reducing sugars include glucose and fructose, which can reduce chemicals such as copper(II) ions in Benedict's solution, leading to a color change that indicates the presence of reducing sugars. The other responses do not accurately capture the defining characteristic of reducing sugars. Some sugars might not participate in reactions due to structural properties or if they are chemically stable, but such sugars wouldn't be classified as reducing sugars. Sugars absorbed only in the small intestine is a function of digestion rather than a defining chemical property. Similarly, the presence of more than one carbohydrate unit pertains to oligosaccharides or polysaccharides but does not directly relate to the ability to act as a reducing agent.