When 1 gram of glucose is completely oxidised during cellular respiration, approximately how much energy is released in the form of calories that can be used by the body?

Introduction / Context:
Glucose is a primary source of energy for the human body. Nutritional science often expresses energy in kilocalories, commonly shortened as kcal. Standard values are used to estimate the energy provided by carbohydrates, proteins, and fats. This question asks about the approximate energy released when one gram of glucose, a carbohydrate, is completely oxidised, which is a common fact tested in biology and nutrition related exams.

Given Data / Assumptions:

• The substance is glucose, a carbohydrate.
• The mass considered is 1 gram.
• The options list different energy values in kilocalories.
• We assume the standard physiological fuel value used in dietetics and biochemistry.

Concept / Approach:
The physiological fuel value of carbohydrates such as glucose is approximately 4 kilocalories per gram. Proteins also provide around 4 kilocalories per gram, while fats provide about 9 kilocalories per gram. These rounded values are widely used in diet charts and calculation of caloric intake. When glucose is completely oxidised through glycolysis, the Krebs cycle, and oxidative phosphorylation, it releases energy that is mostly captured in adenosine triphosphate, with an equivalent of about 4 kilocalories per gram in dietary calculations.

Step-by-Step Solution:
Step 1: Recall the standard energy values for macronutrients: carbohydrates and proteins provide approximately 4 kcal per gram, and fats provide about 9 kcal per gram. Step 2: Identify glucose as a carbohydrate. Step 3: Use the standard carbohydrate value to estimate the energy released per gram of glucose. Step 4: Recognise that this value is 4 kilocalories per gram. Step 5: Select 4 kcal from the options as the correct answer.

Verification / Alternative check:
Biochemistry textbooks describe the oxidation of one mole of glucose to carbon dioxide and water, which yields a defined amount of energy. When converted into nutritional energy values, this corresponds to roughly 4 kilocalories per gram. This value is consistently used on food labels and in diet planning across many countries. Comparing this standard with the options confirms that 4 kcal is the most accurate answer among the choices given.

Why Other Options Are Wrong:
Option A: 6 kcal per gram overestimates the energy content of glucose and is not supported by standard nutritional values. Option C: 5 kcal per gram is higher than the widely accepted value and is not used in dietetics. Option D: 3 kcal per gram underestimates the energy of glucose and does not match the standard carbohydrate value. Option E: 2 kcal per gram is much too low for a carbohydrate and is incorrect.

Common Pitfalls:
Students sometimes confuse the energy values of carbohydrates and fats or may remember approximate numbers incorrectly. Another mistake is mixing up kilocalories and kilojoules. To avoid errors, it is useful to memorise the simple rule that carbohydrates and proteins each give about 4 kcal per gram, while fats give about 9 kcal per gram. Keeping this rule in mind makes questions about energy per gram of glucose straightforward.

Final Answer:
When 1 gram of glucose is completely oxidised, it releases approximately 4 kcal of energy.