The energy released by 1 gram of glucose is

Analyze the question: The question asks for the energy released when 1 gram of glucose is oxidised (respired or burned). This is a standard recall fact about carbohydrates.

Explain the concept: Glucose is a simple carbohydrate and the main respiratory substrate in living cells. During respiration, its chemical energy is released and used to make ATP.

Energy values of the main food classes:

• Carbohydrates: about 4 kcal/g (about 17 kJ/g)
• Proteins: about 4 kcal/g (about 17 kJ/g)
• Fats/lipids: about 9 kcal/g (about 37-39 kJ/g)

Step-by-step solution:

1. Glucose is a carbohydrate.
2. Carbohydrates yield about 4 kcal per gram. A more precise figure for pure glucose is about 3.75 kcal/g, which rounds to about 4 kcal.
3. Checking the options: 6 kcal is too high, 5 kcal is too high, 3 kcal is too low, and 4 kcal matches the standard value.

Correct answer: B (4 kcal)

Exam tip: Remember the "4-4-9" rule: carbohydrates 4, proteins 4, fats 9 kcal per gram. If a question asks about the energy value of fats, the answer is always the higher one, because fats are the most energy-rich food class.