9.6 Chapter 9 Summary

Nutrition and Energy Production

Animals are heterotrophs, meaning they must obtain organic molecules and energy from external sources (food) for biosynthesis (growth, repair, reproduction) and energy production. A balanced diet must supply three major classes of macromolecules—carbohydrates, proteins, and lipids (fats)—as well as essential nutrients (vitamins, minerals, and essential amino acids) that the body cannot synthesize.

• Energy Currency: The primary source of energy is glucose, derived from carbohydrates. Food energy is ultimately converted into Adenosine Triphosphate (ATP) via cellular respiration for use in all cellular functions.
• Energy Storage: When energy intake exceeds demand, excess is stored:
  • Carbohydrates are stored short-term as glycogen in the liver and skeletal muscle.
  • All caloric excess (from carbs, proteins, or fats) is efficiently converted to and stored long-term as triglycerides (fat) in adipose tissue.

Food across Cultures Optimize Nutrition

Traditional foods from a variety of cultures have been associated with enhanced nutritional value and bioavailability of nutrients. This includes

• Complementary pairing of certain foods to ensure all essential amino acids are included in the meal.
• Enhanced bioavailability of vitamins and minerals due to food preparation

Decoding Nutrition Labels: Mass vs. Calories

Nutrition labels provide information about the matter (nutrients measured in grams) and the energy (measured in Calories or kilocalories, kcals) contained in food.

• Caloric Density: Different components of food yield different amounts of energy, macromolecules such as lipids have a higher energy density than others such as carbohydrates and proteins based on their interactions with water. Other components of food (e.g., H₂O) add to their mass but do not contribute to metabolism and thus, lower the caloric density of the food.
• Mass vs. Energy Contribution: The ultimate fate of ingested food depends on the organism’s activity level.
  • If energy is extracted (catabolized) through cellular respiration (high activity), most of the matter is released as CO₂ and H₂O
  • If energy is not immediately needed (low activity), the excess matter (mass) is converted and stored in specialized tissues (glycogen and fat).

Measuring Metabolic Rate

Metabolic rate is a measure of the total energy used by an organism over a specific time.

• Measurement Technique: Since cellular respiration is the primary process consuming oxygen (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy), the total energy expenditure can be measured indirectly by tracking the Oxygen Consumption Rate (OCR).
• Basal Metabolic Rate (BMR): This is the minimum energy required to keep vital functions going at rest.
• Tissue Metabolism: Different tissues have varying metabolic demands: Nervous tissue generally has the highest constant metabolic rate, followed by active muscle tissue. The four main animal tissue types are epithelial, connective, muscle, and nervous tissue.

Metabolism in Extreme Environments

Environments that limit resources, such as the low oxygen availability (hypoxia) at high altitude, force organisms to adjust their metabolism.

• Acclimation (Individual Response): When an individual moves from low to high altitude, they undergo acclimation, a temporary and reversible physiological change. This includes an immediate increase in heart and breathing rates, followed by a gradual increase in red blood cell production over weeks to temporarily compensate for the lack of oxygen.
• Adaptation (Evolutionary Response): Populations that have lived at high altitude for many generations (e.g., Sherpas) exhibit adaptation, a permanent, heritable genetic change. Their metabolic adaptations include greater efficiency in cellular O₂ use (mitochondrial changes) and the ability to quickly regenerate ATP anaerobically via phosphocreatine reserves, rather than simply increasing red blood cell counts like an acclimating person.