In human physiology, the term "metabolism" is best defined as which of the following?

Introduction / Context:
The term "metabolism" is widely used in conversations about diet, weight loss and health, but in biology and physiology it has a specific, broader meaning. Metabolism is not just about how fast a person burns calories; it covers all the chemical reactions that occur in living cells. This question checks whether you can identify the most accurate, textbook level definition of metabolism among several common but incomplete ideas.

Given Data / Assumptions:

• The term in focus is metabolism in a human physiology context.
• The options include ideas about calories, digestion time and biochemical reactions.
• We assume the learner has encountered the terms anabolism and catabolism in basic biology.

Concept / Approach:
Metabolism is defined as the sum of all chemical reactions that occur within a living organism. These reactions include anabolism, which builds complex molecules from simpler ones (such as synthesising proteins from amino acids), and catabolism, which breaks down molecules to release energy (such as breaking down glucose in cellular respiration). Together, these reactions support growth, repair, movement, temperature regulation and many other functions. While energy expenditure and calorie use are related to metabolism, they represent only part of the overall concept.

Step-by-Step Solution:
Step 1: Recall that metabolism includes both building up (anabolic) and breaking down (catabolic) biochemical pathways. Step 2: Recognise that the definition must mention biochemical reactions and not focus only on calories, temperature or digestion time. Step 3: Evaluate option B, which explicitly states that metabolism consists of biochemical reactions involved in building cell molecules or breaking down molecules for energy; this matches the standard textbook definition. Step 4: Compare other options that talk only about calories required to avoid shivering, or time taken to digest fat, or carbohydrate utilisation, and see that each describes only a narrow aspect of body function. Step 5: Conclude that the only option broad enough and accurate enough to match the true meaning of metabolism is option B.

Verification / Alternative check:
Physiology and biochemistry textbooks define metabolism as the totality of an organism's chemical reactions. They then subdivide metabolism into anabolism and catabolism and discuss how energy in the form of ATP is created and used. None of these texts limit metabolism to shivering, digestion time or simple calorie counting. Instead, they clearly emphasise the wide range of chemical processes involved. This consistency between sources confirms that the best definition is the one that describes all biochemical reactions involved in building and breaking down molecules.

Why Other Options Are Wrong:
The number of calories required to keep from shivering on a cold day relates to thermoregulation and basal metabolic rate, but it is only one outcome of metabolism, not the definition itself. The length of time to digest and absorb fats focuses only on one aspect of digestion and does not capture the idea of all biochemical reactions in the body. A measure of carbohydrate utilisation based on calorie counting again focuses on energy intake and expenditure but ignores the many synthetic and regulatory reactions in metabolism. The volume of food eaten in one meal is simply food intake and is not synonymous with metabolism.

Common Pitfalls:
Popular media sometimes use "metabolism" as shorthand for "how fast you burn calories", which can mislead students into thinking of it only in terms of weight control. To avoid this mistake, always link the word metabolism with the idea of all chemical reactions in the body, including both building and breaking down processes. Remembering the pair of terms anabolism and catabolism will help you keep the proper broad meaning in mind when answering exam questions.

Final Answer:
Metabolism is best defined as All biochemical reactions involved in building cell molecules or breaking down molecules to release energy.